Doc #39 — Abrasives and Cutting Tool Maintenance

---

RECOMMENDED ACTIONS (BY ACTUAL URGENCY)

Abrasive and cutting tool stocks are not a first-month crisis. Grinding wheels last years under managed consumption, HSS tool blanks last a decade or more, and the immediate concern — keeping edges sharp — is solved by skills that already exist in NZ workshops. The government has far more pressing demands in Month 1 than cataloguing sandpaper; the actions below are sequenced to reflect that reality.

First week:

1. Issue guidance to all machine shops, workshops, and toolrooms: do not discard any cutting tools, grinding wheels, abrasive products, or sharpening stones, regardless of apparent condition. Worn carbide inserts, stub drill bits, and partially used grinding wheels are all national assets.

Months 2–3:

2. Classify experienced tool and cutter grinders as critical-skills personnel (Doc #1). These are typically the most senior machinists — people who can look at a cutting edge and know what angles to grind. This folds into the general essential-worker classification framework as it develops.

Months 3–6:

3. Include grinding wheels, abrasive products, sharpening stones, and diamond dressers in the national asset census (Doc #8). Capture by type: vitrified grinding wheels (by size, grit, bond type), coated abrasives (sandpaper, sanding discs, flap wheels), sharpening stones (oilstones, waterstones, diamond plates), lapping compounds, and honing sticks. This folds into the general asset census as that programme reaches industrial consumables.
4. Inventory all carbide insert stocks nationally — distributors (Sandvik, Kennametal, Iscar, Seco agents), workshop stocks, polytechnic workshops. This overlaps with the machine shop census (Doc #8).
5. Begin structured knowledge-capture sessions with experienced tool and cutter grinders: document HSS lathe tool geometry (rake angles, clearance angles, nose radius) for common operations and materials. Film the grinding process. Print reference cards for distribution.
6. Identify NZ geologists and mineralogists who can assess domestic abrasive mineral deposits — GNS Science (Institute of Geological and Nuclear Sciences) is the primary source of expertise.¹

Months 6–12:

7. Collect and test NZ garnet sands from Westland beaches for abrasive properties — hardness, grain size distribution, crushing behaviour. Establish whether these are suitable for loose abrasive use, coated abrasive production, or bonded wheel production.
8. Establish HSS tool-grinding training at every Te Pukenga polytechnic workshop and major machine shop, using the knowledge captured in Action 5. Pair experienced grinders with trainees (master-apprentice model).
9. Produce trial batches of NZ natural abrasive products: crushed garnet powder for lapping, crushed quartz for basic grinding, natural stone blanks shaped into sharpening stones.
10. Begin assessment of silicon carbide furnace feasibility — site selection (near sand supply and grid power), furnace design from published literature, power requirements.

First year:

11. Distribute NZ-specific tool-grinding reference materials to all machine shops and polytechnics — printed while printing capability exists (Doc #5).
12. Achieve pilot production of NZ natural abrasive products at useful scale.
13. All machine shop trainees competent in HSS tool grinding as part of standard machining curriculum (Doc #91, Phase A–B).

Years 1–3 (Phase 2):

14. Commission silicon carbide pilot furnace if feasibility assessment is positive.
15. Develop bonded abrasive wheel production using NZ-produced or NZ-processed abrasive grain with vitrified (ceramic) or resinoid binders.
16. Scale natural abrasive processing to supplement declining imported grinding wheel stocks.

---

ECONOMIC JUSTIFICATION

The cost of blunt tools

The economic argument for abrasive and tool maintenance investment is not speculative — it is immediately calculable from machining practice. A blunt cutting tool does not stop working. It works badly: cutting forces increase, surface finish degrades, dimensional accuracy drops, heat generation rises, and the tool wears faster. The machinist compensates by reducing cutting speed and depth of cut, which reduces productivity. A shop working with blunt tools might achieve 30–50% of the output of one with properly sharpened tools, while consuming more material (scrapped parts from poor accuracy) and more power.²

Person-year equivalence: If NZ has an estimated several hundred to a few thousand active machinists under recovery conditions (the actual number depends on the census — Doc #8), and blunt tools reduce their productivity by 30–50% (see above), the effective loss is equivalent to losing a third to half of the machining workforce. Restoring that productivity through proper tool sharpening and maintenance requires only a modest investment: training time (weeks per machinist for HSS grinding basics), equipment preservation (grinders and sharpening stones already in NZ workshops), and eventually domestic abrasive production.

Investment in abrasive production

Natural abrasive processing (Phase 1–2): - Labour: 5–15 person-years to establish collection, processing, and distribution of NZ garnet and quartz abrasives - Equipment: Crushing, grinding, and sizing equipment — available in NZ’s existing mining and mineral processing sector - Output: Functional but coarse abrasive products for general sharpening, lapping, and surface finishing

Silicon carbide production (Phase 2–3): - Labour: 20–40 person-years for furnace construction, process development, and initial production - Power: 3–10 MW continuous during furnace operation — significant but manageable from NZ’s grid³ - Raw materials: Silica sand (NZ has high-purity deposits at Parengarenga Harbour, Northland) and carbon (petroleum coke from existing stocks, or charcoal from NZ forestry)⁴ - Output: Synthetic abrasive grain suitable for grinding wheels, coated abrasives, and loose abrasive applications

Breakeven: Tool sharpening capability pays for itself immediately — the “investment” is primarily training and knowledge preservation, which have negligible material cost. Silicon carbide production breaks even once existing grinding wheel stocks approach depletion, which is estimated at 3–7 years depending on consumption management. Starting development before depletion avoids a capability gap.

---

1. WHAT WEARS OUT AND WHY IT MATTERS

1.1 The cutting tool ecosystem

NZ’s workshops depend on several categories of cutting tools, each with different materials, wear patterns, and resharpening characteristics:

Carbide inserts: Tungsten carbide (WC) with cobalt binder, often with ceramic coatings (TiN, TiCN, Al2O3). The dominant cutting tool material in modern machining. Hard (Vickers hardness 1,300–1,800 HV), heat-resistant (maintains hardness to ~800degC), and wear-resistant. Inserts are indexable — rotated to present a fresh edge when one edge wears — then discarded. NZ imports all carbide inserts and has no pathway to domestic production in the foreseeable future.⁵ Carbide cannot be resharpened on ordinary grinding wheels — it requires diamond or silicon carbide wheels and specialised tool geometry.

High-speed steel (HSS): Iron-based alloy with tungsten, molybdenum, vanadium, and chromium additions. Maintains hardness to ~550–600degC. The standard cutting tool material from the early 1900s until carbide became dominant. Harder than carbon steel but softer than carbide. The critical advantage for recovery conditions: HSS can be resharpened on an ordinary aluminium oxide grinding wheel, by hand, by a skilled machinist, to virtually any geometry required, indefinitely. A single HSS tool blank can be ground, used, resharpened, and reground hundreds of times until it is too small to hold in the tool post. NZ has significant stocks of HSS tool blanks and existing HSS tools in workshop drawers across the country.⁶

Carbon steel tools: Plain high-carbon steel (0.7–1.4% C), hardened and tempered. The original cutting tool material — standard before HSS was developed around 1900. Loses hardness above ~250degC, limiting cutting speed severely (roughly one-quarter to one-third of HSS speeds).⁷ Can be made from NZ-produced steel (Doc #89) if properly hardened and tempered. Sharpenable on any abrasive surface, including natural stones. The ultimate fallback — slow but indefinitely renewable from domestic materials.

Pre-European Māori cutting tools: Before iron, Māori worked with two principal cutting materials: mata (obsidian, volcanic glass) and pounamu (greenstone/nephrite jade). Mata, sourced primarily from Mayor Island (Tūhua) in the Bay of Plenty, fractures conchoidally to produce extremely sharp edges — sharper at the microscopic level than any ground metal tool.⁸ Obsidian edges were used for fine cutting work, tattooing implements, and surgical-grade incision. Pounamu, sourced from Te Wāhipounamu (the South Island west coast and Arahura River area), is nephrite jade with a Mohs hardness of 6–6.5 and exceptional toughness — it was ground and shaped into adzes (toki), chisels, and ornaments over many hours of abrasive work against sandstone, quartzite, and other hard stones.⁹ These traditions are relevant to recovery conditions in two ways: the techniques for knapping obsidian to produce sharp edges require no industrial inputs and could produce useful cutting implements; and the abrasive working methods used to shape pounamu represent a living knowledge tradition of non-metallic precision abrasive work. See Doc #160 (Heritage Skills Preservation) for the broader mātauranga Māori recovery framework.

Grinding wheels: Bonded abrasive products — aluminium oxide (Al2O3) or silicon carbide (SiC) grain held in a vitrified (ceramic) or resinoid (synthetic resin) bond matrix. These are the tools that sharpen other tools. Grinding wheels are consumed during use — the abrasive grain fractures and is lost, the bond wears away to expose fresh grain, and the wheel gradually reduces in diameter. NZ imports all grinding wheels.¹⁰

Files: Hardened steel bars with precisely cut teeth. Used for hand-finishing metal, deburring, and fitting. Files wear with use and cannot be easily resharpened (re-cutting file teeth requires specialised machinery). NZ imports all files.¹¹ Existing stocks will deplete over years. File re-cutting is a historical skill that could potentially be revived but requires purpose-built equipment.

Saw blades: Circular saw blades (for metal cutting), bandsaw blades, hacksaw blades, and wood saw blades. Metal-cutting blades are HSS or carbide-tipped. Wood-cutting blades are typically carbide-tipped or chrome-vanadium steel. HSS and steel blades can be resharpened; carbide-tipped blades require diamond or silicon carbide grinding.

Woodworking edge tools: Chisels, plane blades, spokeshaves, router bits. Typically carbon steel or HSS. Sharpenable on natural or synthetic stones — this is the application where traditional sharpening methods work best.

1.2 The depletion sequence

As imported abrasive and tooling stocks deplete, NZ’s cutting tool capability degrades in a predictable sequence:

1. First to deplete: Coated abrasives (sandpaper, sanding discs, flap wheels). These are single-use and consumed rapidly. Stocks may last months to 1–2 years under rationed use.¹²
2. Next: Specialist carbide inserts for specific operations (threading, grooving, profiling). Less common types deplete before general-purpose turning inserts.
3. Then: General-purpose carbide inserts and grinding wheels. Grinding wheels last longer per unit because they are reusable until worn to minimum safe diameter. Estimated 3–7 years under managed consumption, depending on total NZ stock and usage patterns.¹³
4. Last to deplete: HSS tool blanks, natural sharpening stones, and diamond dressers (which wear very slowly). These may last a decade or more.

The strategic response is to stay ahead of each depletion step: shift to HSS tools before carbide runs out, develop natural abrasive sharpening before grinding wheels run out, and develop synthetic abrasive production before all sharpening media are exhausted.

---

2. TOOL RESHARPENING: THE IMMEDIATE PRIORITY

2.1 HSS lathe tool grinding

Grinding a lathe tool from an HSS blank is the most important single skill in tool maintenance. A machinist who can grind HSS lathe tools can keep a lathe productive indefinitely, regardless of carbide insert availability.

The process: a rectangular HSS tool blank (typically 10 x 10 mm to 25 x 25 mm cross-section, 100–150 mm long) is shaped on a bench grinder or pedestal grinder fitted with an aluminium oxide wheel. The machinist grinds three primary surfaces:¹⁴

• Top rake (chip breaker): The angle of the top face relative to horizontal. Determines how the chip forms and breaks. Typical range: 5–15deg positive rake for mild steel, 0–5deg for harder materials, negative rake for interrupted cuts.
• Side clearance: The angle of the side face below the cutting edge. Must be positive (angled away from the workpiece) to prevent rubbing. Typical range: 5–8deg.
• End clearance: The angle of the front face below the nose. Similar function to side clearance. Typical range: 5–8deg.
• Nose radius: The rounded tip where side and end cutting edges meet. Affects surface finish and tool strength. Typically 0.4–1.6 mm.

An experienced machinist grinds these angles by eye and feel, checking occasionally with a protractor or angle gauge. A competent apprentice can learn the basics in a few days of supervised practice; developing the judgment to grind tools for different materials and operations takes months to years.¹⁵

The knowledge-capture urgency: Many NZ machinists under 40 have never ground an HSS lathe tool — they grew up with carbide inserts. Machinists in their 50s to 70s learned tool grinding as apprentices and practised it daily for years. This skill transfer must happen before the older machinists are unavailable. It is the same urgency described in Doc #91 (Machine Shop Operations), Section 5.

2.2 Drill bit sharpening

Twist drills are the second most commonly used cutting tool after lathe tools. A standard twist drill has a specific point geometry — typically 118deg or 135deg included angle, with relief (clearance) behind each cutting lip and a chisel edge at the centre. When the drill becomes dull, it must be resharpened to restore this geometry.¹⁶

Hand sharpening: A skilled machinist can sharpen a twist drill on a bench grinder by hand, holding the drill at the correct angle and rotating it against the wheel with a specific rocking motion to produce the correct lip relief. This is another skill that was routine for older machinists and is now uncommon among younger ones. The difficulty increases with smaller drill sizes — drills below about 6 mm diameter are difficult to sharpen by hand without practice.

Drill grinding fixtures: Purpose-built fixtures that hold the drill at the correct angle relative to the grinding wheel, simplifying the sharpening process. Several designs exist in NZ workshops. These fixtures are valuable because they allow less experienced operators to achieve acceptable results. Some can be fabricated in NZ machine shops.

Drill point thinning: As a drill is resharpened and the point recedes along the flute, the chisel edge (the non-cutting web at the centre) becomes wider, increasing thrust force and making the drill harder to use. Thinning the web by grinding material away behind the chisel edge restores drilling performance. This is a more advanced sharpening skill.

2.3 Milling cutter sharpening

End mills, face mills, and other milling cutters are resharpened on a tool and cutter grinder — a specialised grinding machine with precise indexing and workholding. NZ’s machine shops have tool and cutter grinders; the skill of using them effectively is concentrated among older toolmakers and tool and cutter grinders (a specialist trade within machining).¹⁷

End mills can also be sharpened on a surface grinder using appropriate fixtures, or by hand on a bench grinder for rough work. The flute geometry (helix angle, radial rake, axial rake, corner radius) is complex, and poor resharpening degrades cutter performance significantly. This is a skill where the gap between a competent practitioner and an amateur is large.

2.4 Saw blade sharpening

Bandsaw blades: HSS or bi-metal bandsaw blades can be resharpened by manual filing (for large-pitch blades) or by dedicated bandsaw blade sharpening machines. Several NZ businesses specialise in saw blade sharpening and have equipment for this purpose. These businesses should be identified in the census and their equipment secured.

Circular saw blades: Carbide-tipped circular saw blades (for wood and metal) require diamond or silicon carbide grinding wheels for resharpening. Manual saw-filing (for non-carbide blades) was a standard skill in the timber industry until recently and remains practised in some NZ sawmills.¹⁸ Carbide-tipped blades that cannot be resharpened once diamond grinding capability is lost can sometimes be adapted by brazing new carbide tips from salvaged inserts.

2.5 File reconditioning

Files cannot be practically resharpened in the traditional sense — the teeth are too fine and too numerous. However, file life can be extended by:

• Acid etching: Dipping a worn file in dilute acid (hydrochloric or sulfuric) removes a thin layer of metal, sharpening the remaining teeth slightly. This works once or twice before the teeth become too shallow. The acid must be handled carefully — hydrochloric acid can be produced from NZ salt (Doc #112), making this approach feasible long-term.
• Proper use: Files wear fastest when used with excessive pressure, on hardened materials, or without clearing chips from the teeth (carding). Chalk rubbed into file teeth reduces clogging in soft metals like aluminium.
• File re-cutting: A historical process where worn files are annealed (softened), the old teeth ground off, new teeth cut with a chisel on a dedicated filing machine, and the file re-hardened. This requires specialised equipment that NZ does not currently possess but could fabricate. File re-cutting was a significant industry in the 18th and 19th centuries.¹⁹

---

3. TOOL REHARDENING AND HEAT TREATMENT

3.1 Why tools lose their hardness

Cutting tools work because they are harder than the material they cut. Hardness in steel comes from heat treatment — specifically, heating to the austenitising temperature (typically 780–870degC for carbon steel, higher for HSS), quenching rapidly to form martensite (the hard phase), and tempering at a controlled temperature to relieve internal stress while retaining hardness.

Tools lose their hardness through:

• Overheating during use: If the cutting edge exceeds the tool’s maximum operating temperature (250degC for carbon steel, 550–600degC for HSS), the tempered martensite begins to decompose, softening the tool. This is visible as a blue discoloration on the tool tip. Once softened, the tool cannot cut effectively and must be rehardened or replaced.
• Overheating during grinding: Grinding generates significant heat. If the tool is ground without adequate cooling (or with excessive pressure on the wheel), the cutting edge can be locally overheated and softened — a “burnt” edge that looks fine but will not cut. Experienced grinders avoid this by using light passes, frequent dipping in water, and correct wheel dressing.
• Fatigue and microcracking: Repeated thermal cycling and mechanical stress cause micro-cracks at the cutting edge, leading to chipping and crumbling rather than clean wear.

3.2 Rehardening carbon steel tools

Carbon steel cutting tools (chisels, plane blades, old-style lathe tools, files) can be rehardened by a blacksmith or anyone with a forge and quenching tank:

1. Heat to cherry red (approximately 780–830degC for 0.8–1.0% carbon steel). The steel should be heated evenly — use a forge (Doc #92), a gas torch, or an electric muffle furnace if available. The correct temperature can be judged by colour: bright cherry red in dim light.
2. Quench in water or brine (for carbon steel) by plunging the heated tool edge-first into the quench tank. This produces full hardness (approximately 60–65 HRC) but also full brittleness — the tool will shatter if struck.
3. Temper by reheating the hardened tool to a controlled temperature, judged by oxide colour on a polished surface: pale straw (220degC) for maximum hardness (files, scrapers), dark straw (240degC) for lathe tools, bronze (260degC) for drills, purple (280degC) for cold chisels, blue (300degC) for springs and saws.²⁰ Tempering reduces hardness slightly but restores toughness.

This process requires only fire, water, and the ability to judge colour — no instruments, no imports. It is the heat treatment method that was used for thousands of years before thermocouples and controlled-atmosphere furnaces existed.

3.3 Rehardening HSS tools

HSS is more complex to heat-treat than carbon steel because it requires higher temperatures (1,200–1,300degC for austenitising, depending on grade) and specific cooling and tempering sequences. The high austenitising temperature makes HSS difficult to heat-treat in a blacksmith’s forge — a muffle furnace or salt bath is normally used, and temperature control matters more.

Practical approach for recovery conditions:

• Avoid overheating HSS tools during use by reducing cutting speed when tools begin to show heat discoloration
• If an HSS tool is accidentally overheated and softened, it can be rehardened by heating to bright yellow-white heat (1,200–1,300degC) in a furnace or forge with careful temperature judgment, cooling in still air (not water — HSS is air-hardening), then triple-tempering at 550–570degC for one hour per cycle²¹
• The temperature control required for HSS heat treatment is more demanding than for carbon steel. Without a pyrometer, judgment by colour is less reliable at these high temperatures. Salt bath furnaces provide more uniform heating and better temperature control than open forges and can be constructed from NZ materials (steel shell, refractory lining, barium chloride or sodium chloride salt mixture, electrical heating elements)

Honest assessment: Rehardening HSS tools is feasible but requires more skill and better equipment than rehardening carbon steel. Many HSS tools that are “soft” are better resharpened past the softened zone (grinding away the damaged layer to expose properly hardened material beneath) than rehardened from scratch. Rehardening is the fallback when grinding would remove too much material.

---

4. ABRASIVE MATERIALS: WHAT NZ HAS

4.1 Existing imported stocks

NZ’s abrasive product stocks are distributed across:

• Industrial distributors: Norton/Saint-Gobain agents, 3M agents, Flexovit/Tyrolit agents, Pferd, and other abrasive product suppliers hold warehouse stocks of grinding wheels, cutting discs, sanding belts, and loose abrasives.
• Workshop stocks: Every machine shop, fabrication workshop, joinery, and auto body shop has grinding wheels, sanding discs, and related products. In aggregate, this distributed stock is substantial.
• Retail: Hardware stores (Bunnings, Mitre 10, Placemakers) hold consumer-grade abrasive products — sandpaper, sanding discs, grinding wheels for angle grinders, sharpening stones.
• Specialist sharpening businesses: Saw doctors, tool sharpening services, and industrial grinding operations hold higher-quality abrasive products and diamond dressing tools.

The total NZ stock is unknown and must be established through the census (Doc #8). As a rough estimate based on import volumes and distribution chain depth, NZ likely has 1–3 years of normal consumption in-country at any time — less for rapidly consumed items (cutting discs, sandpaper), more for durable items (bench grinding wheels, sharpening stones).²²

4.2 NZ natural abrasive minerals

NZ has several natural abrasive minerals, none of which are currently commercially exploited for abrasive applications:

Garnet: NZ’s west coast beaches (Westland, from about Hokitika south to Jackson Bay) contain placer deposits of garnet, concentrated by wave action along with other heavy minerals (ilmenite, zircon, magnetite). The garnet is predominantly almandine variety (Fe3Al2(SiO4)3), which has a Mohs hardness of 6.5–7.5 and is widely used as an abrasive mineral internationally.²³ Garnet is used in waterjet cutting, sandblasting, coated abrasives (garnet sandpaper), and as a loose abrasive for lapping and polishing.

The NZ garnet deposits have been studied by GNS Science and earlier geological surveys but have not been commercially mined for abrasive purposes. The concentration of garnet in beach sand varies — some deposits are rich enough to be worth processing; others are too dilute. Assessment of specific deposit grade and the garnet’s abrasive properties (hardness, toughness, grain shape) would be needed before production could begin.²⁴

Processing pathway: Beach sand containing garnet is concentrated by gravity separation (the same principle used in gold panning — garnet is denser than quartz sand). The garnet concentrate is washed, dried, and sized by screening into grade fractions. For coated abrasive production, the sized garnet grains are bonded to paper or cloth backing with hide glue, shellac, or synthetic resin. For loose abrasive use, the sized grains are used directly as lapping or polishing powder.

Quartz and quartzite: Quartz (SiO2, Mohs hardness 7) is abundant throughout NZ. Crushed quartz produces a usable but relatively soft abrasive — adequate for rough grinding of non-ferrous metals, wood sanding, and basic deburring, but too soft for efficient grinding of hardened steel. Quartzite (metamorphosed sandstone, essentially compacted quartz grains) occurs in several NZ formations and can be shaped into crude sharpening stones.²⁵

Emery and corundum: Emery (a natural mixture of corundum, magnetite, and other minerals) and corundum (natural aluminium oxide, Al2O3, Mohs hardness 9) are among the hardest natural minerals. NZ’s geological literature records occurrences of corundum in some metamorphic and igneous formations, but no deposits of commercial significance for abrasive purposes have been identified. If suitable deposits exist, they would be extremely valuable — natural corundum is an excellent abrasive for steel grinding.²⁶

Sandstone: NZ has extensive sandstone formations. Shaped sandstone was the traditional grindstone material for centuries — used for sharpening axes, knives, chisels, and other edge tools. NZ sandstone varies widely in grain size and hardness. Fine-grained, hard sandstones from formations in the South Island (e.g., Canterbury and Otago greywacke) can be shaped into functional grindstones and sharpening stones, though they wear faster and cut more slowly than modern synthetic stones.²⁷

Pounamu (nephrite jade) as abrasive: Nephrite jade (Mohs hardness 6–6.5, exceptional toughness and fracture resistance) was the most prized material in pre-European NZ. The process of working pounamu into toki (adzes) and other forms required sustained abrasive grinding against sandstone and quartzite, using water and progressively finer abrasive material — a process conceptually identical to modern lapping.²⁸ The abrasive knowledge developed through pounamu working constitutes a practical tradition of controlled surface-finishing with natural materials. The sandstone slabs and quartzite rubbing stones used in pounamu working are functional sharpening tools in their own right. Pounamu itself, while too culturally significant for routine abrasive applications, is one of the hardest and toughest natural materials in NZ; its working properties inform understanding of what NZ’s harder natural minerals can achieve.

Other Māori natural abrasives: Pre-European Māori used pumice (abundantly available in the central North Island volcanic plateau) as an abrasive for smoothing wood, bone, and wood-to-wood surfaces in woodworking and waka (canoe) construction.²⁹ Pumice ranges from 6 to 7 on the Mohs scale depending on composition and is lightweight and self-renewing from volcanic activity — a renewable abrasive resource requiring no processing beyond collection. It is too soft and porous for metal grinding but adequate for wood, bone, and soft stone finishing. Tohunga whakairo (expert carvers) also used shark skin (with its denticle structure) as a fine abrasive for finishing carved surfaces — a technique that produced surfaces comparable to medium-grit sandpaper without any mineral abrasive requirement.

4.3 Natural abrasive performance gaps

Natural abrasives are inferior to modern synthetic abrasives in several respects:

• Hardness: The hardest natural mineral commonly available in NZ (garnet, Mohs 7–7.5) is significantly softer than synthetic aluminium oxide (Mohs 9) or silicon carbide (Mohs 9.5). This means natural abrasives cut more slowly and wear faster, particularly on hardened steel.
• Consistency: Natural minerals vary in hardness, grain shape, and composition even within a single deposit. Synthetic abrasives are manufactured to precise specifications. Inconsistency in natural abrasive products means unpredictable performance.
• Friability: The best synthetic abrasives are engineered to fracture in a controlled way during use, continuously exposing fresh, sharp cutting points. Natural garnet and quartz tend to round off rather than fracture cleanly, reducing cutting efficiency as the abrasive wears.
• Grain size control: Synthetic abrasive grains are precisely sized by screening and settling. Natural abrasive processing in NZ would initially produce less precisely graded products, leading to inconsistent surface finish.

These gaps are real and important. A grinding wheel made from NZ garnet will not perform like an imported aluminium oxide wheel. But for many applications — sharpening HSS tools, lapping valve seats, finishing machined surfaces, sanding wood — natural abrasives are adequate, and adequate is vastly better than nothing.

---

5. SYNTHETIC ABRASIVE PRODUCTION

5.1 Silicon carbide (SiC)

Silicon carbide is the more feasible synthetic abrasive for NZ to produce because the raw materials and energy source are all domestically available.

The Acheson process: Silicon carbide was first commercially produced by Edward Acheson in 1891 using an electric resistance furnace.³⁰ The process has changed remarkably little since then:

1. A mixture of silica sand (SiO2) and carbon (petroleum coke or charcoal) is loaded around a graphite or carbon core (the resistor) in a long, rectangular furnace
2. Electric current is passed through the carbon core, heating it to approximately 2,200–2,500degC
3. At these temperatures, silica reacts with carbon: SiO2 + 3C -> SiC + 2CO
4. After 24–48 hours of heating, the furnace is cooled and broken open
5. The silicon carbide forms as a mass of crystals around the central core, with unreacted material on the outside
6. The SiC is extracted, crushed, washed (in acid to remove impurities), and graded by size

NZ raw materials:

• Silica sand: NZ has high-purity silica sand deposits. The Parengarenga Harbour deposit in Northland contains sand with >98% SiO2, already used in glassmaking research and exported historically.³¹ Other silica sand deposits exist at various locations. The silica must be of reasonable purity — iron and aluminium contamination degrades SiC quality.
• Carbon: Petroleum coke from NZ’s Marsden Point refinery stocks (finite), or charcoal from NZ forestry (renewable). Charcoal is a feasible but less ideal carbon source — it is less dense and has higher ash content than petroleum coke, which affects furnace packing and product purity. Historical SiC production used charcoal before petroleum coke became standard.³²
• Graphite core: A carbon electrode or graphite rod to serve as the resistance heating element. NZ does not produce graphite (this is the same constraint affecting EAF steelmaking, Doc #94). Initial furnaces could use carbon electrodes made from compressed charcoal and pitch, or salvaged graphite from industrial applications (motor brushes, electrodes, crucibles).
• Electricity: The Acheson process is extremely energy-intensive. A single furnace run producing 1–5 tonnes of SiC requires 6,000–12,000 kWh per tonne of product.³³ A small production facility operating one furnace on a regular cycle would draw 1–5 MW during operation. This is significant but well within NZ’s grid capacity under baseline assumptions.

Dependency chain: Silica sand (NZ) + carbon (NZ charcoal or finite petroleum coke) + electricity (NZ grid) + furnace construction (NZ steel and refractory) + acid washing (requires hydrochloric or sulfuric acid — see Doc #113) + crushing and grading equipment (available in NZ mineral processing sector)

The acid washing constraint: Crude SiC from the furnace contains impurities (unreacted silica, iron silicides, carbon) that must be removed by washing with hydrochloric acid or caustic soda. Hydrochloric acid production requires salt and sulfuric acid (Doc #113). Without acid washing, the SiC grain contains impurities that degrade grinding performance but do not make it non-functional. Unwashed SiC is usable for rough grinding applications.

Development timeline: Designing and building an Acheson furnace, producing trial batches, refining the process, and scaling to useful production volume would take 1–3 years from the decision to proceed. The furnace itself is conceptually simple (a refractory-lined box with carbon electrodes) but scale, power supply, and process control require engineering attention. This is a Phase 2–3 project.

5.2 Aluminium oxide (Al2O3) — fused alumina

Fused alumina (corundum) is the other major synthetic abrasive. It is produced by melting bauxite or refined alumina in an electric arc furnace at approximately 2,000–2,050degC.³⁴

The problem for NZ: Bauxite (aluminium ore) does not occur in NZ in commercially useful quantities. NZ’s Tiwai Point aluminium smelter at Bluff processes imported alumina (refined from Australian bauxite). Without imported alumina or bauxite, NZ cannot produce fused alumina from primary raw materials.

Possible workaround: NZ has aluminium — in the form of existing aluminium stock (sheet, extrusion, castings) and potentially continued production at Tiwai Point if Australian alumina can be obtained via trans-Tasman trade (Doc #109). Aluminium scrap could theoretically be oxidised back to alumina and then fused into abrasive-grade material, but this is a circuitous and energy-intensive route to a product that could more efficiently be made from bauxite.

Assessment: Fused alumina production is rated [C] Difficult for NZ unless trans-Tasman trade provides alumina or bauxite. Silicon carbide is the more realistic synthetic abrasive for domestic production.

5.3 Bonded abrasive products

Raw abrasive grain — whether natural garnet, NZ-produced SiC, or imported Al2O3 — must be bonded into useful products: grinding wheels, cutting discs, honing stones, sanding belts.

Vitrified bond (ceramic): The traditional and still most common bond for precision grinding wheels. Abrasive grain is mixed with ceramic bond materials (clays, feldspars, frits), pressed into shape, and fired in a kiln at 900–1,300degC. The bond partially vitrifies (forms glass), locking the abrasive grains in a rigid, porous matrix. NZ has suitable clays and feldspars, and kiln capability exists in the ceramics industry. Developing specific vitrified bond formulations for grinding wheels requires experimentation — the bond must be matched to the abrasive grain, the intended application, and the operating speed.³⁵

Resinoid bond: Abrasive grain bonded with synthetic resin (typically phenolic resin). Used for cutting discs and high-speed grinding wheels. Phenolic resin is currently imported. NZ could potentially produce phenolic resin from locally available phenol (producible from coal tar) and formaldehyde (producible from methanol via wood gasification), but this represents an additional chemical dependency chain.

Shellac bond: A natural resin bond used historically for fine grinding wheels and polishing wheels. Shellac is produced by the lac insect (Kerria lacca), which does not occur in NZ — shellac is an import. Existing NZ shellac stocks (used in French polishing and as a sealant) are small.

Hide glue bond: For coated abrasives (sandpaper). Abrasive grain is bonded to paper or cloth backing using animal hide glue (producible from NZ cattle hides and bones — gelatin extraction is established chemistry). This is the original bonding method for sandpaper and produces a functional product, though with lower heat and moisture resistance than modern synthetic resin bonds.³⁶

Assessment: Vitrified bond grinding wheels from NZ-produced SiC grain and NZ clays are the most realistic medium-term product. Coated abrasives using natural garnet and hide glue are the most realistic near-term product. Both require development effort but use only NZ-available materials.

---

6. CONSERVATION OF EXISTING ABRASIVE STOCKS

6.1 Grinding wheel management

Grinding wheels are the highest-priority abrasive item to conserve because they enable tool resharpening — the capability that keeps all other cutting tools functional.

Use wheels to minimum safe diameter. In commercial practice, grinding wheels are often discarded with significant abrasive remaining because the wheel has become inconveniently small or because shop policy errs on the side of caution. Under recovery conditions, wheels should be used until they reach the minimum safe diameter for the machine — typically when the wheel spindle or flange would contact the workpiece or guard. Grinding wheel manufacturers recommend discarding wheels when they have worn to approximately 60–75% of their original diameter, depending on the machine and guard configuration.³⁷ Under recovery conditions, the lower bound of this range is appropriate.

Correct wheel dressing. A grinding wheel that is loaded (clogged with metal particles) or glazed (abrasive grains rounded smooth rather than fractured to expose fresh cutting points) does not cut efficiently. Dressing the wheel with a diamond dresser or star-wheel dresser restores its cutting action. However, dressing removes abrasive material and should be done only when needed — not as a routine start-of-shift practice unless the wheel is visibly degraded. Diamond dressers are irreplaceable once exhausted; star-wheel (huntington) dressers are simpler but less effective and can be fabricated from hardened steel.³⁸

Match wheel to work. Using a fine-grit finish grinding wheel for heavy stock removal wastes the fine wheel. Rough grinding should use coarse wheels; finish grinding uses fine wheels. This distinction matters more when wheels are rationed.

Speed control. Operating a grinding wheel above its rated speed is dangerous (the wheel can explode) and wasteful (faster wear, more heat generation). Operating below rated speed is safe and can reduce wheel wear for non-critical work, at the cost of slower grinding.

6.2 Coated abrasive conservation

Sandpaper and sanding discs can have their useful life extended by:

• Cleaning with a crepe rubber cleaning block (commonly available in NZ workshops) or by tapping the back to dislodge embedded particles
• Using the correct grit for the job (starting with a coarser grit and progressing to finer grits, rather than trying to do all the work with a fine grit)
• Using a backing pad on flexible sanders to distribute pressure evenly
• Storing unused abrasives in dry conditions (moisture degrades paper-backed products)

6.3 Sharpening stone preservation

Oilstones (Arkansas stones, India stones) and waterstones in NZ workshops are typically used with oil or water as a lubricant to float away swarf and prevent the stone from glazing. These stones wear with use but very slowly — an oilstone can last decades of regular use. The main risk is breakage from dropping. Sharpening stones should be treated as irreplaceable precision tools: stored in protective cases, used on stable surfaces, and never dropped or used as work surfaces.

Natural Arkansas stones are quarried in the USA and will not be available as replacements. Japanese waterstones are manufactured in Japan. NZ’s existing stock of quality sharpening stones is the national reference for fine edge work until domestic production of adequate substitutes is established.

---

7. NZ NATURAL ABRASIVE PRODUCTION

7.1 Garnet processing

The most practical near-term NZ abrasive production pathway is processing Westland beach garnet into usable abrasive products.

Collection: Garnet-bearing heavy mineral sands are collected from beach deposits. The richest concentrations occur as dark bands or layers in beach sand, typically near the high-tide line or in dune formations. Collection is labour-intensive but requires no specialised equipment beyond shovels and containers.

Concentration: The garnet is separated from other minerals by gravity methods — sluicing, jigging, or spiral concentration. These are standard mineral processing techniques used in NZ’s historical gold mining industry and still practised at West Coast alluvial gold operations. A simple sluice box can produce a garnet-rich concentrate from beach sand. More sophisticated separation (shaking tables, spirals) produces a cleaner concentrate.³⁹

Magnetic separation: Garnet is weakly paramagnetic, while associated minerals (ilmenite, magnetite) are strongly magnetic.⁴⁰ A magnet — even a permanent magnet salvaged from a loudspeaker or electric motor — can remove the most magnetic minerals from the concentrate, improving garnet purity.

Sizing: The garnet concentrate is screened through a series of meshes to produce graded size fractions. Coarse fractions (equivalent to 40–80 grit) serve for rough grinding and sandblasting. Medium fractions (80–150 grit) for general sharpening and coated abrasives. Fine fractions (150–400+ grit) for lapping and polishing. Precise grading improves product quality but even roughly graded garnet is usable.

Products: - Loose abrasive powder for hand lapping (valve seats, surface plates, mating surfaces) - Slurry abrasive for basic hand-lapping operations (garnet powder in water or oil, applied to a flat plate or lap) - Coated abrasives: garnet bonded to paper or fabric with hide glue — this was the standard “garnet sandpaper” sold commercially before aluminium oxide sandpaper replaced it. Garnet sandpaper is inferior to aluminium oxide for metalwork but adequate for woodworking and light metalwork.⁴¹ - Waterjet cutting media (if waterjet equipment remains operational — garnet is the standard waterjet abrasive)

Scale of effort: Establishing a garnet collection and processing operation is a modest undertaking — perhaps 5–10 workers at the collection and processing site, plus existing mineral processing equipment adapted for the purpose. The output in the first year would be small (tonnes, not hundreds of tonnes) but useful for high-priority applications.

7.2 Quartz and sandstone products

Crushed quartz: Readily available from NZ quartz veins and river gravels. Crushed and graded quartz produces a soft abrasive suitable for wood sanding, non-ferrous metal polishing, and surface cleaning. Not effective for grinding hardened steel. Production requires crushing equipment (jaw crusher or stamp mill), washing to remove fines and contaminants, and grading by screening through standard mesh sieves — all steps that use equipment available in NZ’s existing mining and aggregate sectors.

Shaped grindstones: NZ sandstone can be cut or shaped into grindstones for edge-tool sharpening. This was common practice before the 20th century. A foot-pedal or water-powered grindstone was standard equipment on NZ farms and in workshops. The key requirement is selecting a sandstone with the right grain size (fine enough for a reasonable edge, coarse enough to cut) and hardness (hard enough to hold its shape, soft enough that the surface does not glaze). Local trial and assessment would identify suitable formations.⁴²

---

8. ABRASIVE APPLICATIONS BY PRIORITY

Not all abrasive uses are equally important. Under rationing, abrasive products should be allocated to the highest-impact applications:

Priority 1 — Tool resharpening: Grinding wheels for sharpening HSS lathe tools, drills, milling cutters, and saw blades. This is the application that maintains all other manufacturing capability. Aluminium oxide wheels (existing stock) are used first; NZ-produced SiC wheels replace them as they become available.

Priority 2 — Precision surface finishing: Honing and lapping of critical surfaces — hydraulic valve seats, bearing journals, seal surfaces, machine tool ways. These applications require fine abrasive (150+ grit) and consistent quality. Natural garnet lapping compound can serve here.

Priority 3 — Weld preparation and cleaning: Grinding welds for structural integrity and fit-up, cleaning rust and scale for welding. Angle grinder discs are the primary consumable. Coarse NZ garnet or SiC can substitute for commercial discs once bonded product development is achieved.

Priority 4 — Woodworking: Sanding timber for joinery, boatbuilding, and construction. NZ garnet sandpaper is historically appropriate and adequate for this application.

Priority 5 — Surface preparation for painting and coating: Currently consumes large quantities of sandpaper and abrasive blasting media. Under recovery conditions, paint and coating stocks are also finite, reducing this demand. Wire brushing substitutes for abrasive blasting in many applications.

---

CRITICAL UNCERTAINTIES / KEY RISKS

Uncertainty	Why it matters	How to resolve
Total NZ abrasive product stocks	Determines depletion timeline and urgency of domestic production	National census (Doc #8) — include abrasives as Category B item
NZ garnet deposit grade and properties	Determines whether Westland garnet is suitable for abrasive use	Geological assessment and materials testing (GNS Science)
Number of experienced tool and cutter grinders in NZ	Determines knowledge-capture urgency and training capacity	Skills census (Doc #8) — specific trade category
Feasibility of Acheson furnace construction from NZ materials	Determines timeline for synthetic abrasive production	Engineering assessment — design study using NZ steel and refractories
Availability of acid for SiC washing	Affects product quality of NZ-produced silicon carbide	Linked to sulfuric acid production (Doc #113)
NZ corundum/emery deposit existence	If significant deposits exist, natural hard abrasive becomes available	Geological survey — priority search by GNS Science
Grinding wheel stock age and condition	Old wheels in storage may have degraded bonds and be unsafe	Inspection program — ring test, visual examination of all stockpiled wheels
Vitrified bond formulation for NZ materials	Determines whether NZ can produce bonded grinding wheels	Materials science R&D — coordinate with University of Auckland or Canterbury ceramic engineering

---

CROSS-REFERENCES

• Doc #1 — National Emergency Stockpile Strategy (abrasive and tooling stocks as controlled materials)
• Doc #8 — National Asset and Skills Census (abrasive product inventory; tool grinder identification)
• Doc #22 — NZ Geological and Mineral Resource Atlas (garnet, corundum, and other abrasive mineral deposits)
• Doc #34 — Lubricant Production (cutting fluids as part of tool maintenance ecosystem)
• Doc #45 — Chainsaw and Tool Maintenance (overlapping tool sharpening and maintenance guidance)
• Doc #89 — NZ Steel Glenbrook (steel for cutting tools; electrode supply for arc furnaces)
• Doc #91 — Machine Shop Operations (tool grinding as core machining skill; grinding wheels as critical consumable)
• Doc #92 — Blacksmithing and Forge Work (heat treatment of carbon steel tools; forge as rehardening facility)
• Doc #93 — Foundry Work (furnace construction for SiC production)
• Doc #94 — Welding Consumable Fabrication (grinding for weld preparation; abrasive discs for fabrication)
• Doc #97 — Cement and Concrete (clay and mineral processing overlap with abrasive production)
• Doc #98 — Glass Production (silica sand supply chain shared with SiC production)
• Doc #102 — Charcoal Production (charcoal as carbon source for SiC furnace)
• Doc #70 — Copper Wire Production (wire drawing dies require precision grinding)
• Doc #113 — Sulfuric Acid (acid needed for SiC washing and file reconditioning)
• Doc #157 — Trade Training (tool grinding as part of machining curriculum)
• Doc #160 — Heritage Skills Preservation and Transmission (manual tool grinding, saw filing as heritage skills at risk; mata knapping, pounamu-working, and natural abrasive traditions as traditional edge-tool knowledge; partnership framework for engaging tohunga whakairo as knowledge holders, §4.5–4.7)

---

---

1. GNS Science (Institute of Geological and Nuclear Sciences) is NZ’s primary geoscience research institution. https://www.gns.cri.nz/ — GNS maintains NZ’s geological maps, mineral occurrence databases, and related expertise. Their assessment of NZ abrasive mineral deposits would be the authoritative starting point for domestic abrasive production.↩︎

2. The relationship between tool sharpness and machining productivity is well-established in manufacturing engineering. See: Shaw, M.C., “Metal Cutting Principles,” Oxford University Press, 2005; Trent, E.M. and Wright, P.K., “Metal Cutting,” 4th edition, Butterworth-Heinemann, 2000. Cutting force increases of 200–500% are typical for severely worn tools, and surface finish degrades from Ra 1.6 um (sharp tool) to Ra 6.3+ um (worn tool).↩︎

3. NZ grid generation capacity is approximately 9,000–10,000 MW peak, predominantly renewable (hydro, geothermal, wind). A 3–10 MW allocation for abrasive production is <0.1% of total capacity. See: Electricity Authority NZ, “Electricity in New Zealand” reports. https://www.ea.govt.nz/↩︎

4. Parengarenga Harbour silica sand: NZ’s highest-purity silica sand deposit, located in Northland. The sand is >98% SiO2 with low iron content, making it suitable for glass production and potentially for silicon carbide production. The deposit has been studied by multiple NZ government and university surveys. See also Doc #98 (Glass Production).↩︎

5. Tungsten carbide production requires tungsten metal (not available in NZ), cobalt (not available in NZ), and precision powder metallurgy (sintering at 1,300–1,500degC under controlled atmosphere). This is beyond NZ’s foreseeable manufacturing capability. See: Trent and Wright (footnote 2), Chapter 4.↩︎

6. HSS tool steel grades (M2, M42, T1, etc.) contain 5–20% tungsten or molybdenum, plus vanadium, chromium, and cobalt in some grades. These alloying elements are what provide the hot hardness that distinguishes HSS from carbon steel. NZ cannot produce HSS from domestic materials (tungsten and molybdenum are not available). However, HSS tools can be resharpened and reused almost indefinitely, making the existing stock functionally renewable through maintenance rather than production.↩︎

7. Carbon steel cutting speed relative to HSS: Carbon steel tools lose hardness above approximately 250degC, while HSS maintains cutting ability to 550–600degC. This temperature limitation restricts carbon steel to surface speeds of roughly 5–15 m/min on mild steel, compared to 20–40 m/min for HSS under comparable conditions — hence the approximate one-quarter to one-third ratio. See: Trent, E.M. and Wright, P.K., “Metal Cutting,” 4th edition, Butterworth-Heinemann, 2000, Chapter 3; also “Machinery’s Handbook,” cutting speed tables.↩︎

8. Mata (obsidian) edge sharpness: Obsidian fractures conchoidally, producing edges with a radius of curvature of a few nanometres — significantly sharper than ground steel (typically 100–500 nm). Mayor Island (Tūhua) obsidian has been identified by geochemical characterisation in archaeological sites throughout NZ, demonstrating its use as a traded cutting material. See: Leach, B.F. and Leach, H.M. (1979), “Prehistoric Man in Palliser Bay,” National Museum of New Zealand Bulletin 21; Sheppard, P.J. (2010), “Lapita Colonisation Across the Near/Remote Oceania Boundary,” Current Anthropology, 52(6); Weisler, M.I. and Woodhead, J.D. (1995), “Basalt Pb isotope analysis and the prehistoric settlement of Polynesia,” Proceedings of the National Academy of Sciences, 92(6). For obsidian blade sharpness relative to steel, see: Bhatt, D.L. et al. (1997), “Morphological and histological evaluation of vascular damage and repair after percutaneous coronary interventions” — obsidian scalpels are used in ophthalmological and dermatological surgery precisely because of their superior edge geometry.↩︎

9. Pounamu (nephrite) toki and working techniques: Nephrite jade (Mohs 6–6.5, extremely high toughness due to fibrous crystal structure) was shaped by prolonged grinding against sandstone slabs using water and fine abrasive, progressing to polishing with finer stone. The process for a single toki could take many hours over multiple sessions. Pounamu in Aotearoa is sourced from Te Wāhipounamu — the West Coast of the South Island, particularly the Arahura River and Westland area. See: Leahy, A. (1999), “Pounamu: Jade of New Zealand,” Reed Books; Davidson, J. (1984), “The Prehistory of New Zealand,” Longman Paul; Phillips, W.J. (1955), “Maori life and custom,” Whitcoulls.↩︎

10. NZ abrasive product imports: NZ imports grinding wheels, cutting discs, and coated abrasives primarily from Australia (Tyrolit, Norton/Saint-Gobain Australian operations), China, and Europe. Total import volumes are not readily available from public sources but the product is widely distributed through industrial supply chains. Specific figures require verification from Stats NZ trade data.↩︎

11. NZ file imports: NZ does not manufacture files domestically. Industrial files are imported primarily from European manufacturers (Bahco/Snap-on, Pferd, Vallorbe) and Chinese producers, distributed through industrial supply chains (MSC, RS Components NZ, Blackwoods). Specific import volumes require verification from Stats NZ trade data (HS code 8203).↩︎

12. NZ abrasive product stock estimate: This is a rough estimate based on general knowledge of NZ’s industrial supply chain depth, not specific inventory data. The 1–3 year figure assumes that NZ’s abrasive product distributors typically hold 2–4 months of normal demand, with additional distributed stock in workshops. Under recovery conditions, demand patterns would change (some applications reduce, repair work increases), making the depletion timeline uncertain. Census data is required.↩︎

13. NZ abrasive product stock estimate: This is a rough estimate based on general knowledge of NZ’s industrial supply chain depth, not specific inventory data. The 1–3 year figure assumes that NZ’s abrasive product distributors typically hold 2–4 months of normal demand, with additional distributed stock in workshops. Under recovery conditions, demand patterns would change (some applications reduce, repair work increases), making the depletion timeline uncertain. Census data is required.↩︎

14. HSS lathe tool geometry: Standard reference for tool angles is “Machinery’s Handbook” (Oberg, E. et al., Industrial Press, various editions). Also: South, D., “Workshop Technology,” Longman — widely used in NZ apprentice training. The specific angles cited are representative general-purpose values; optimal geometry varies by workpiece material, operation, and machinist preference.↩︎

15. Tool grinding skill acquisition: The estimate of “basics in a few days, judgment over months to years” is consistent with the traditional NZ engineering apprenticeship model (4 years for full qualification — see Doc #159, footnote 17). Tool grinding was typically taught in the first year of apprenticeship and practised throughout.↩︎

16. Twist drill geometry: Standard drill point angles and relief specifications are documented in manufacturer technical guides (e.g., Dormer, Sutton Tools, Guhring) and in “Machinery’s Handbook.” The 118deg standard point is suitable for most steels; 135deg split-point drills are better for harder materials and stainless steel. Resharpening to these specifications requires either skill or a fixture.↩︎

17. Tool and cutter grinding: A specialised branch of machining that uses dedicated machines (e.g., Deckel, Clarkson, Cincinnati tool and cutter grinders) to resharpen multi-flute cutting tools to precise geometry. The skill was a distinct trade in NZ’s engineering industry. As carbide inserts replaced resharpened HSS cutters, demand for tool and cutter grinding decreased, and the trade contracted. Remaining practitioners are concentrated among older machinists and toolmakers.↩︎

18. Manual saw filing: The skill of sharpening and setting hand saws and circular saws by filing was standard in the NZ timber industry until the widespread adoption of carbide-tipped blades in the 1980s–1990s. Some NZ sawmillers and heritage woodworkers still practise this skill. The NZ Guild of Sawyers and related organisations may have members with this knowledge. Verification through the skills census (Doc #8) is needed.↩︎

19. File re-cutting: A significant industry in Sheffield, England, and other metalworking centres from the 17th to 19th centuries. Files were expensive tools, and re-cutting worn files was economically important. The process required a special annealing furnace, a re-cutting machine (essentially a chisel driven by a cam mechanism), and a hardening furnace. See: Barraclough, K.C., “Sheffield Steel,” in various historical metallurgy publications. The equipment could be fabricated in NZ but represents a niche manufacturing project.↩︎

20. Heat treatment tempering colours: The oxide colour scale for steel tempering is well-established metallurgical practice, documented in any workshop technology or metallurgy text. See: “Machinery’s Handbook,” heat treatment section; South, D., “Workshop Technology.” The colours are reliable guides for carbon steel but less reliable for alloy steels and must be observed on a freshly polished surface in even, indirect light.↩︎

21. HSS heat treatment: HSS requires austenitising at 1,200–1,300degC (grade-dependent), air cooling (HSS is air-hardening due to its high alloy content), and multiple tempering cycles at 540–570degC to achieve secondary hardening (a phenomenon where precipitation of alloy carbides during tempering actually increases hardness). See: Thelning, K.E., “Steel and its Heat Treatment,” Butterworth-Heinemann, 1984. The triple temper is standard practice for HSS tools — omitting it results in retained austenite that degrades performance.↩︎

22. NZ abrasive product stock estimate: This is a rough estimate based on general knowledge of NZ’s industrial supply chain depth, not specific inventory data. The 1–3 year figure assumes that NZ’s abrasive product distributors typically hold 2–4 months of normal demand, with additional distributed stock in workshops. Under recovery conditions, demand patterns would change (some applications reduce, repair work increases), making the depletion timeline uncertain. Census data is required.↩︎

23. Garnet as an abrasive: Garnet (particularly almandine and pyrope varieties) has been used as an abrasive since antiquity. Modern industrial garnet production is centred in Australia, India, and the USA. The mineral’s hardness (6.5–7.5 Mohs), sub-conchoidal fracture (producing sharp edges when crushed), and chemical inertness make it suitable for coated and bonded abrasives, sandblasting, and waterjet cutting. See: Olson, D.W., “Garnet (Industrial)” in USGS Mineral Commodity Summaries.↩︎

24. NZ garnet deposits: GNS Science and its predecessor organisations (DSIR, IGNS) have documented heavy mineral sand occurrences along the West Coast of the South Island. These deposits contain garnet, ilmenite, zircon, and other heavy minerals concentrated by coastal processes. The grade and economic viability for abrasive production have not been formally assessed — historical interest focused on ilmenite (for titanium) rather than garnet. See: Williams, G.J., “Economic Geology of New Zealand,” Australasian Institute of Mining and Metallurgy, Monograph 4, various editions.↩︎

25. Quartz as an abrasive: Quartz (Mohs 7) was one of the earliest abrasive materials used by humans. Crushed quartz (flint) was the standard abrasive coating for early sandpaper. While softer than aluminium oxide or silicon carbide, quartz is adequate for wood sanding, soft metal polishing, and surface cleaning. NZ has abundant quartz sources. See: general mineralogy texts.↩︎

26. NZ corundum occurrences: Corundum (alpha-Al2O3) has been reported in NZ geological literature in association with metamorphic rocks (particularly in the schist belt of Otago and the Alpine Fault zone), but no deposits of commercial significance for abrasive applications have been identified. If significant deposits exist, they would be extremely valuable given that natural corundum (Mohs 9) approaches the hardness of synthetic aluminium oxide. A targeted geological survey would be required to assess this. See: GNS Science mineral occurrence database; Suggate, R.P., Stevens, G.R., and Te Punga, M.T. (eds.), “The Geology of New Zealand,” NZ Geological Survey.↩︎

27. NZ sandstone as grindstone material: Historical NZ workshops and farms commonly used locally quarried sandstone for grindstones. The suitability varies by formation — the ideal grindstone sandstone has uniform, medium-fine grain size, moderate hardness, and even texture. NZ’s greywacke (the dominant basement rock in much of NZ) varies from too hard and fine (metamorphosed to argillite) to suitable for grinding purposes. See: NZ engineering and agricultural heritage literature.↩︎

28. Pounamu working as abrasive practice: The multi-stage abrasive sequence used in pounamu finishing — coarse sandstone for shaping, medium sandstone for smoothing, fine-grained schist or greywacke for polishing, with water as lubricant — is documented in both archaeological analysis and surviving practitioner accounts. The selection of appropriate stone for each stage was part of the specialist knowledge held by tohunga whakairo. See: Leahy (1999) (footnote 34); also ethnographic accounts compiled by Elsdon Best (early 20th century), held by the Museum of New Zealand Te Papa Tongarewa.↩︎

29. Pumice as an abrasive in Māori tradition: Pumice (Mohs 6–7, varying by composition and vesicularity) from the Taupō Volcanic Zone was used by Māori communities in the central North Island and deposited throughout the Waikato River system. Traditional uses included smoothing waka hulls, finishing carved surfaces, and polishing bone ornaments. Pumice continues to be collected from Waikato River banks as a traditional practice. See: Best, E. (1942), “Forest Lore of the Maori,” Dominion Museum Bulletin 14; also archaeological literature on central North Island Māori material culture.↩︎

30. Acheson process for silicon carbide: Patented by Edward Acheson in 1893 (US Patent 492,767). The process has remained fundamentally unchanged for over a century. Modern SiC production uses essentially the same resistance-heated furnace concept at larger scale. See: Saddow, S.E. and Agarwal, A., “Advances in Silicon Carbide Processing and Applications,” Artech House, 2004; also Gupta, C.K. and Krishnamurthy, N., “Extractive Metallurgy of Rare Earths,” CRC Press (for discussion of electric resistance furnace processes generally).↩︎

31. Parengarenga Harbour silica sand: NZ’s highest-purity silica sand deposit, located in Northland. The sand is >98% SiO2 with low iron content, making it suitable for glass production and potentially for silicon carbide production. The deposit has been studied by multiple NZ government and university surveys. See also Doc #98 (Glass Production).↩︎

32. Early silicon carbide production used wood charcoal as the carbon source. The transition to petroleum coke occurred as the petroleum industry developed and coke became cheaper and more consistent. Charcoal-based SiC production is feasible but may produce lower-purity product due to the higher mineral ash content of charcoal relative to petroleum coke. For abrasive applications, moderate impurity levels are acceptable. See: Acheson patent documentation; early industrial chemistry references.↩︎

33. Silicon carbide energy consumption: The Acheson process requires approximately 6,000–12,000 kWh of electrical energy per tonne of SiC produced, depending on furnace size, insulation, and product grade. This makes SiC production one of the more energy-intensive industrial processes. At NZ grid rates under recovery conditions (where electricity is available from renewable sources but is a shared national resource), this power consumption must be weighed against other demands. See: Gupta and Krishnamurthy (footnote 22); industrial SiC production technical literature.↩︎

34. Fused alumina production: Brown fused alumina is produced by melting bauxite in an electric arc furnace at approximately 2,000degC. White fused alumina is produced from Bayer-process alumina (pure Al2O3) at similar temperatures. Both require aluminium-bearing raw materials that NZ does not possess. See: Cichy, B. et al., “Mechanical properties of alumina based ceramic materials” and related industrial abrasive literature.↩︎

35. Vitrified bond grinding wheel manufacture: The vitrified bond is a glass-ceramic material formed by firing a mixture of clays, feldspars, and frits at 900–1,300degC. Bond composition determines wheel hardness, porosity, and strength — and must be matched to the abrasive grain and intended application. Development of bond formulations for NZ-produced SiC grain would require iterative experimentation. See: Marinescu, I.D. et al., “Handbook of Machining with Grinding Wheels,” CRC Press; general abrasive engineering references.↩︎

36. Hide glue bonded coated abrasives: The original sandpaper (patented in the 1830s) used hide glue to bond crushed flint or garnet to paper. Hide glue remains adequate for light-duty coated abrasive applications (hand sanding, wood finishing). It is inferior to modern resin bonds for heat resistance and moisture resistance but is producible from NZ cattle hides and bones using well-established gelatin extraction chemistry. See: coated abrasive manufacturing literature; adhesives technology references.↩︎

37. Grinding wheel safe operating diameters: Grinding wheel manufacturers and safety standards (AS/NZS 1788, ISO 603) specify minimum operating diameters based on the relationship between wheel speed, diameter, and centrifugal stress. A grinding wheel should not be used below the point where the mounting flanges or safety guard would contact the work, or where the reduced diameter means the surface speed drops below efficient cutting speed. See: Norton/Saint-Gobain abrasive safety publications; AS/NZS 1788 “Abrasive wheels.”↩︎

38. Diamond dressers: Single-point or multi-point diamond dressers are used to true and dress grinding wheels. The diamond is the only material hard enough to cut the abrasive grain cleanly. Diamond dressers wear slowly but are irreplaceable once exhausted (NZ has no diamond sources). Star-wheel (Huntington) dressers use hardened steel points that crush rather than cut the abrasive grain — less effective but producible locally. Rotary dressers using silicon carbide or industrial diamond substitutes may extend capability. See: grinding technology references.↩︎

39. Gravity mineral separation: Standard mineral processing technique used in NZ’s gold mining industry since the 1860s. Sluice boxes, shaking tables, spiral concentrators, and jigs all exploit the density difference between heavy minerals (garnet specific gravity ~4.0, gold ~19.3) and light minerals (quartz ~2.65) to achieve separation. West Coast gold mining heritage provides both equipment and skilled practitioners who understand gravity separation. See: NZ gold mining history and mineral processing texts.↩︎

40. Garnet magnetic properties: Almandine garnet (the predominant variety in NZ beach sands) is weakly paramagnetic due to its iron content, while magnetite (Fe3O4) is ferrimagnetic and ilmenite (FeTiO3) is paramagnetic but more strongly so than garnet. Standard mineral processing practice uses magnetic separation at different field strengths to separate these minerals. See: Wills, B.A. and Finch, J., “Wills’ Mineral Processing Technology,” 8th edition, Butterworth-Heinemann, 2015, Chapter 13.↩︎

41. Garnet sandpaper: Garnet-coated abrasive paper was the standard sandpaper product for woodworking before aluminium oxide “production paper” became dominant in the mid-20th century. Garnet paper is still commercially available (marketed for woodworking) because garnet produces a slightly finer scratch pattern than aluminium oxide at equivalent grit sizes, preferred by some woodworkers for final finishing. NZ-produced garnet paper from Westland garnet and hide glue would be functionally equivalent to this commercial product. See: coated abrasive product literature.↩︎

42. Historical grindstones in NZ: Sandstone grindstones were standard workshop and farm equipment in NZ until electric bench grinders became universal in the mid-20th century. Some NZ heritage museums display locally quarried grindstones. The skill of selecting appropriate sandstone and dressing a grindstone to maintain its shape and cutting action is among the heritage skills addressed in Doc #160. See: NZ agricultural and engineering heritage publications.↩︎