Machining by Material: Complete Guide to Cutting Parameters for Every Workpiece Group

Match cutting parameters to the ISO 513 machinability group first: steel (P) and gray cast iron (K) run carbide at roughly 150–300 m/min, austenitic stainless (M) drops to 60–100 m/min, titanium and superalloys (S) fall to 30–60 m/min, hardened steel (H) needs PCBN at 80–200 m/min, while aluminum and free-machining brass (N) climb to 300–800 m/min. The single largest lever is matching speed and tool grade to the material's dominant wear mechanism — abrasion for cast iron, built-up edge for stainless and non-ferrous, diffusion for titanium, crater wear for hardened steel — because the same end mill that lasts an hour in one group can fail in seconds in another.

Every cutting-parameter decision in metalworking traces back to one question: which of the six ISO 513 machinability groups does the workpiece belong to? ISO 513:2004 sorts every engineering material into application groups P, M, K, N, S, and H based on the dominant wear mechanism the material imposes on the cutting edge — and that wear mechanism, not bulk hardness, sets the speed band, the carbide grade, the coating, and the coolant strategy. This pillar guide maps all six groups (plus the non-ISO plastics and composites family) against the parameters used in the eight material-specific satellite guides, so you can pick a starting point in seconds and then drill into the detailed guide for the workpiece in front of you. For the tool-side view of how grades and geometries are chosen, see the cutting tools complete guide; for the speed-feed-depth optimization framework that sits underneath every group, see the CNC machining optimization guide.

The Machinability Framework: ISO 513 and the Three Drivers

ISO 513 sorts all cutting materials into six application groups — P (steel), M (stainless), K (cast iron), N (non-ferrous), S (superalloys), and H (hardened) — by the dominant wear mechanism, which is why a P-grade insert that excels in steel underperforms in the abrasive flank-wear environment of cast iron. The group, not the hardness number, is the first variable to fix because it determines whether the tool fights crater wear, built-up edge, abrasion, diffusion, or thermal damage.

Within each ISO group a two-digit number (e.g., P10, P30, K20) indicates the hardness-versus-toughness trade-off: lower numbers are harder and more wear-resistant for finishing under stable conditions, higher numbers are tougher for roughing and interrupted cuts. ISO 513 §4 states explicitly that an application group is not a grade-equivalence standard — a Sandvik P25 and a Kennametal P25 share the application range, not the chemistry — so the group is a starting filter, not a substitute for vendor data.

The machinability index expresses relative material-removal rate at constant tool life against a reference of AISI 1212 free-machining steel rated at 100%, published in Machinery's Handbook 31st Edition (Cutting Speeds section, page ~1095). On that scale gray cast iron rates roughly 95–120%, free-machining brass C36000 sits at 100 on the parallel CDA scale, austenitic stainless falls to about 45–55%, titanium alloys land near 20–30%, and white cast iron drops below 10%. These figures vary ±15% between sources and are comparative starting points, not absolute predictions.

Three intrinsic material drivers explain the entire spread between groups:

• Cutting force / strength. Higher yield strength and work-hardening rate raise the force per unit chip area, which raises temperature and accelerates the dominant wear mode. Stainless and superalloys sit at the high-force end.
• Thermal conductivity. Materials that carry heat away through the chip (gray iron ~50 W/m·K, free-machining brass ~115 W/m·K) tolerate high speeds; low-conductivity titanium (~7 W/m·K, roughly one-sixth of carbon steel's ~40 W/m·K) concentrates heat at the edge and forces speed down by a factor of 2–4×.
• Work hardening / adhesion. Austenitic stainless, pure copper, and aluminum harden or weld at the cutting interface and produce built-up edge below a critical chip thickness of roughly 0.020–0.025 mm.

Carbon and Alloy Steel (Group P)

Carbon and alloy steels (ISO group P) are the machinability baseline — soft steel below 300 BHN runs general-purpose carbide at 150–300 m/min in turning with P20–P30 grades, and its dominant failure mode is crater wear on the rake face driven by chemical diffusion at temperature. Steel is the reference against which every other group is judged because its continuous chip, moderate cutting force, and predictable Taylor behavior make it the most forgiving common material.

The Taylor tool-life exponent for soft steel (<300 BHN) with carbide runs n ≈ 0.14 (small feed) to 0.25 (large feed), rising to n ≈ 0.20–0.33 for alloy steel above 300 BHN, per Machinery's Handbook 31st Edition Table 5b. The practical consequence: at small feeds a 10% speed reduction can extend tool life roughly 2.1×, while at large feeds the same reduction yields about 1.5× — so speed is the dominant lever, but its leverage shrinks as feed grows. CVD multilayer coatings with their 8–20 µm thermal barrier are preferred for continuous steel turning because the thick Al₂O₃ layer resists the crater-forming diffusion wear. In a typical CVD stack the inner TiN layer is used to bond the coating to the carbide substrate, the MT-TiCN intermediate layer provides abrasion resistance against the steel chip, and an outer TiN flash is added as a gold-colored wear-indicator that makes flank wear visible to the operator; thinner PVD coatings suit interrupted milling where edge sharpness matters more. The detailed speed-feed-depth and Taylor-calibration workflow for steel lives in the steel machining parameters guide.

Austenitic Stainless 304 and 316 (Group M)

Austenitic stainless (ISO group M) runs coated carbide at 60–100 m/min for 304 and 50–90 m/min for 316, with built-up edge as the dominant failure mode controlled by holding feed per tooth above the 0.025 mm work-hardening floor. Stainless is roughly half the machinability of free-cutting steel not because it is harder in bulk (both 304 and 316 are typically 150–185 HB annealed) but because it work-hardens rapidly and its low thermal conductivity (~15–17 W/m·K for 304) keeps the chip-tool interface in the adhesion-wear regime.

The grade difference is a consistent 10–15% speed reduction: 316's 2–3% molybdenum raises the work-hardening rate and cutting forces by roughly 15–20% over 304. The Taylor exponent for M-class stainless is flat at n ≈ 0.08–0.12, meaning a 10% speed reduction extends tool life only 6–8% — so the safer correction is to reduce speed before feed, since dropping feed below the floor triggers the very built-up edge you are trying to avoid. M15–M25 carbide with TiAlN coating (oxidation resistance to 800°C) is the default; AlTiN or AlCrN (oxidation onset near 1,100°C) are preferred at higher speeds or in 316. The full 304-versus-316 parameter set, BUE elimination checklist, and spiral-point tap selection are in the stainless steel 304/316 machining guide.

Cast Iron: Gray, Ductile, and White (Group K)

Cast iron (ISO group K) is selected by graphite morphology, not bulk hardness — gray iron with flake graphite tolerates K10–K20 carbide at roughly 150–300 m/min dry, ductile iron's spherical nodules force a 30–40% speed reduction to 80–180 m/min with tougher K20–K30 grades, and white iron above 500 HB requires Si₃N₄ ceramic or CBN. The dominant wear mode across the group is abrasive flank wear from hard carbide and pearlite phases, which is why K-grades trade toughness for wear resistance as the number rises.

Gray iron's flake graphite acts as an internal crack initiator that fragments the chip and lubricates the interface, lowering friction by an estimated 20–40% versus equivalent-hardness steel — this is why gray iron is the one common material that production shops machine dry by default. Al₂O₃ (aluminum oxide) is the dominant CVD coating layer for cast-iron turning grades because its low thermal conductivity (~30 W/m·K) provides a chemical diffusion barrier above 200 m/min in gray iron. White cast iron's 30–45% cementite volume fraction (800–1,100 HV) exceeds the room-temperature hardness of WC-Co carbide, so coated carbide survives only below 30 m/min. The complete gray-versus-ductile-versus-white parameter tables and glazing diagnostics are in the cast iron machining guide.

Aluminum and Non-Ferrous (Group N)

Aluminum and non-ferrous alloys (ISO group N) run the fastest of any group — 6061-T6 aluminum cuts at roughly 500–800 m/min surface speed and free-machining brass C36000 at 300–600 m/min — but built-up edge is the dominant failure mode, controlled by low-friction DLC coatings and sharp positive-rake geometry rather than by hardness. The N group inverts steel-machining logic: you want the lowest coefficient of friction against the workpiece, not the hardest coating, because the failure mode is adhesion, not abrasion.

For aluminum, DLC (diamond-like carbon) coatings cut the friction coefficient against aluminum from 0.30–0.45 (uncoated) to 0.05–0.10, a 3–6× reduction that suppresses the cold-welding behind built-up edge; TiAlN and AlCrN are avoided because their aluminum-bearing layers react with the chip and accelerate BUE. For brass and copper the parameter spread is wider than between any two steel grades: free-machining C36000 (CDA rating 100) tolerates 300–600 m/min with uncoated K10, while oxygen-free copper C10100/C10200 (CDA rating 20) drops to 80–200 m/min with polished DLC tooling because pure copper's ~390 W/m·K conductivity keeps the chip too cool to suppress welding. See the aluminum high-speed machining guide for 6061/7075 flute-count and spindle-limit detail, and the brass and copper machining parameters guide for the full alloy-by-alloy breakdown.

Titanium and Superalloys (Group S)

Titanium and nickel-based superalloys (ISO group S) run the slowest of the metals — Ti-6Al-4V end milling holds 30–60 m/min with K10–K20 carbide — because low thermal conductivity concentrates heat at the edge and drives diffusion wear above roughly 700°C. The S group is defined by heat: parameters that work in steel destroy tools in titanium within minutes, and the controlling rule is to keep speed low while holding feed above the 0.025 mm work-hardened-layer floor.

Titanium's ~7 W/m·K conductivity (about one-sixth of carbon steel) forces a 2–4× speed reduction versus steel, and high-pressure flood coolant (>20 bar where the machine supports it) is the production minimum because it lowers the chip-tool interface temperature by approximately 100–200°C — enough to shift the dominant wear mode from diffusion back to abrasion. AlTiN coatings are preferred over TiAlN for titanium when interface temperatures approach 800°C because the higher aluminum content forms a more stable protective oxide. Nickel superalloys such as Inconel impose similar notch-wear and thermal constraints at comparable or lower speeds. The detailed titanium parameter bands, chip-thinning math, and wear-mode diagnostics are in the titanium alloy machining parameters guide; the superalloy-specific strategy is covered in the Inconel and superalloy machining guide.

Hardened Steel (Group H)

Hardened steel (ISO group H, above 45 HRC) is finished with PCBN at roughly 80–200 m/min for continuous turning above 55–60 HRC or with TiAlN/AlCrN-coated carbide at 60–100 m/min for milling 45–58 HRC, where the dominant constraints are abrasive flank wear and crater wear from the hard carbide precipitates in the workpiece. Group H is the only metal group where the workpiece bulk hardness can exceed the room-temperature hardness of the cutting material, so tool-material hardness must be matched to workpiece hardness directly.

PCBN's 4,000–4,500 HV hardness resists workpiece carbide phases at 1,500–2,000 HV that defeat coated carbide (TiAlN at 3,000–3,500 HV) above 58 HRC. The Taylor exponent for carbide in hardened steel runs n ≈ 0.20–0.33, so a 15% speed reduction can extend tool life roughly 1.5–2× under stable conditions; flood coolant is avoided on PCBN because thermal cycling reduces its life 30–50% versus dry cutting. Tool changes are scheduled against the ISO 3685 flank-wear criterion of VB_B = 0.3 mm. The parameter-by-hardness tables and CBN-versus-carbide economics are in the hardened steel machining guide; the upstream finishing-process decision (when to hard-turn versus grind) is covered in the hard turning vs grinding guide.

Plastics and Composites (Non-ISO)

Engineering plastics and fiber-reinforced composites sit outside the ISO 513 metal-cutting groups because their dominant failure modes are thermal (matrix melting) and mechanical (delamination), not flank wear — neat plastics run polished 2-flute carbide at roughly 200–500 m/min while CFRP and GFRP require PCD or CVD-diamond tooling at 100–250 m/min. The mental model is closer to woodworking than metalworking: sharp high-rake tools, polished flutes, and low feed at exit boundaries.

For thermoplastics the controlling limit is cutting-zone temperature relative to the melt point — crossing the glass transition is acceptable for chip formation, but crossing Tm fuses the chip back to the workpiece, so halving cutting speed roughly halves localized temperature and is the most effective lever against gummy finish. For composites the limit is delamination: carbon fibers at 2,000–2,500 HV exceed uncoated carbide's hardness and cause measurable wear within 10–30 m of cut, while PCD (7,000–10,000 HV) sustains tens of meters; the dominant delamination lever is backing the exit face, which can drop the delamination factor from 1.5 toward 1.05–1.10 with no parameter change. The full plastic-by-plastic and composite parameter set is in the plastics and composites machining guide.

Quick Selection by Material

This table is the fast entry point: find the workpiece group, read the starting speed, grade, coating, and coolant strategy, then open the linked satellite guide for the detailed bands. All numbers are reference starting points for general-purpose carbide tooling at typical production rigidity — tune downward 15–25% for thin walls, long reach, or marginal machine condition.

Machinability % is relative material-removal rate at constant tool life versus AISI 1212 free-machining steel (=100), per Machinery's Handbook 31st Edition; values vary ±15% between sources and by specific alloy chemistry.

Sources

• ISO 513:2004 — Classification and application of hard cutting materials for metal removal
• ISO 3685:1993 — Tool-life testing with single-point turning tools
• Machinery's Handbook, 31st Edition — Cutting Speeds and Feeds section
• Boothroyd & Knight, Fundamentals of Machining and Machine Tools, 3rd Edition