Machinability Chart — ISO 513 / VDI 3323

Annealed AISI 4140 falls under ISO 513 group P (steel) at VDI 3323 subgroup P6 — low alloy steel, annealed, ~180-200 HB. Quenched-and-tempered 4140 at typical conditions (28-32 HRC, ~270-310 HB) shifts to P7 or P8 depending on the tempered hardness. If you re-harden 4140 above ~45 HRC for tooling applications, it moves to ISO group H (hardened steel) at H38.1 or higher.

304 and 316 stainless are both ISO 513 group M, subgroup M14.1 (austenitic stainless, ~180 HB). Both are difficult to machine compared to free-machining steels because they work-harden rapidly and have low thermal conductivity. 304 is generally rated slightly more machinable than 316 — adding molybdenum to 316 (for chloride resistance) increases gumminess and tool wear. For best results in either alloy, use sharp positive-rake tooling, generous coolant flow, and avoid dwelling. Both grades sit in the same VDI 3323 row, so you can use the same vendor speeds-and-feeds line for either, but expect 316 to need slightly slower SFM and more aggressive coolant.

ISO 513 sorts workpiece materials into six color-coded groups by machinability behavior: P (blue) = steel, including non-alloy, low-alloy, and high-alloy steels; M (yellow) = stainless steel, austenitic and duplex grades; K (red) = cast iron, gray, ductile, and malleable; N (green) = non-ferrous metals, aluminum, copper alloys, plastics; S (brown) = heat-resistant superalloys and titanium, like Inconel and Ti-6Al-4V; H (gray) = hardened steel and chilled cast iron, typically above 45 HRC. Tooling vendors use these letters plus a VDI 3323 subgroup number (P6, M14.1, K15, etc.) to publish speeds-and-feeds tables that apply to whole material families instead of individual AISI grades.

VDI 3323 is a German engineering standard (Verein Deutscher Ingenieure 3323) that subdivides each ISO 513 letter group into numbered subgroups based on alloying, heat treatment, and hardness. Per the formal ISO 513 / VDI 3323 reading (Iscar, Tyson Tool): group P (steel) covers VDI 1 through 13.2 — including ferritic, martensitic, and martensitic-PH stainless steels, which behave more like alloy steel than austenitic stainless; M (austenitic stainless) covers 14.1 through 14.4 only; K (cast iron) covers 15 through 20; N (non-ferrous) covers 21 through 30; S (superalloys / Ti) covers 31 through 37; and H (hardened) covers 38.1 through 41.2. Some tooling catalogs (notably Haas HEPM) simplify by lumping all stainless under M — so always cross-reference with your specific vendor’s chart.

AISI 1212, 1213, and 1215 (resulfurized free-machining grades) and 12L14 (leaded) are the most machinable carbon steels. AISI 1212 is the historical baseline for machinability ratings, with other materials rated as a percentage of its cutting performance. In ISO 513 terms, these grades sit in group P, subgroups P1-P2 (non-alloy steel, low carbon, free-machining). Among low-alloy steels, leaded variants (41L30, 41L40, 86L20, 86L40) and resulfurized 4140 / 4150 grades cut significantly faster than their standard counterparts and are commonly used when high cutting speed and good surface finish are required. Standard 1018 cuts moderately well in the annealed state, and 4140 in the annealed condition runs faster than its quenched-and-tempered form.

First, identify your workpiece material and its heat-treatment state (annealed, normalized, quenched-and-tempered, etc.). Look up that grade in the table on this page using the search box or by filtering the ISO group. Note the VDI 3323 subgroup number (e.g., P6 for annealed 4140). Then open your tooling vendor’s speeds-and-feeds chart — Haas, Sandvik, Iscar, Kennametal, etc. — and find the row that matches that VDI number. The vendor’s recommended SFM, IPT, and depth-of-cut values for that row apply to your part. If the same grade appears at multiple VDI numbers in the table, pick the row whose hardness range matches your actual material; vendor cutting parameters are very sensitive to hardness.

A machinability rating chart expresses how easy a material is to machine as a percentage relative to a reference grade — historically AISI B1112 / 1212 (resulfurized free-machining steel) at 100%. A rating above 100% means the material cuts faster than 1212; below 100% means slower. Free-machining grades 1213, 1215, and 12L14 score similarly to 1212; standard 1018 cuts moderately well; alloy and stainless grades cut more slowly. Machinability rating charts and ISO 513 / VDI 3323 classification charts (like the one on this page) serve different purposes: ratings give a single relative number, while ISO 513 groups materials so a tooling vendor can publish one set of speeds-and-feeds for an entire family. Most modern tooling catalogs use the ISO 513 system rather than percentage ratings.

Among wrought aluminum, 6061-T6 (ISO group N, VDI 22) is the workhorse — predictable chip formation, low cutting force, and good surface finish. 2011-T3 and free-machining 6262 cut even faster thanks to lead/bismuth additions, but 6061 is more commonly stocked. 7075-T6 cuts well but is harder (~150 HB) and produces stringier chips. Avoid 1100 and pure aluminum if surface finish matters — they tend to gum up. For cast aluminum, A380 and A356 cut cleanly at VDI 23-24, but watch for high-silicon hypereutectic alloys like A390 (VDI 25) which contain primary silicon particles that wear carbide rapidly — diamond-coated or PCD tooling is recommended for those grades.