Tool Steel CNC Machining: AISI Groups A to W

We offer tool steel CNC machining across the AISI groups A, D, H, M, O, S and W. This page explains what each letter means and why tool steel parts are usually machined before they’re hardened. It also covers which finishes need care on hardened steel and how to write grade and hardness on the drawing so the quote matches the part.

Which tool steel groups do you machine?

We machine all seven groups by CNC milling and CNC turning. The exact grade is confirmed in your quote.

Tool steelA series, D series, H series, M series, O series, S series, W series

Plastics and other materials on request. The exact grade is confirmed in your quote.

Tool steels are grouped by what they’re used for (cold work, hot work, high-speed cutting or shock) and by how they harden, in air, oil or water (ASM International). The letter on your drawing tells you which group the grade belongs to.

What do the tool steel letters mean?

Each letter names a group by its use or its hardening method. The table pairs each group with a labelled example grade and the kind of part it’s chosen for.

Group	Meaning	Example grade	Covered by	Best for
A	Air-hardening, cold work	A2	ASTM A681	Dies, punches and gauges
D	Cold work, high carbon	D2	ASTM A681	Wear-resistant cold-work tooling
H	Hot work	H13	ASTM A681	Tools and parts that run hot
M	High-speed (molybdenum type)	M2	ASTM A600	Cutting tools
O	Oil-hardening, cold work	O1	ASTM A681	General tooling and gauges
S	Shock-resisting	S7	ASTM A681	Parts under impact, such as punches and chisels
W	Water-hardening carbon tool steel	W1	ASTM A686	Simple, low-cost tooling

Group names follow ASM’s categories (ASM International). ASTM A681 covers alloy tool steels made into tools, dies and fixtures, ASTM A600 covers high-speed steels, and ASTM A686 covers carbon tool steels (ASTM International). Example grades are labels, not a list of what we stock.

Why is tool steel machined before it’s hardened?

Tool steel is supplied soft, in the annealed condition, so it can be machined; high-speed steel to ASTM A600, for example, is supplied annealed (ASTM International). The part reaches its working hardness only after heat treatment, and fully hardened tool steel is far harder to cut.

A hardened tool steel part usually follows this route:

Machine in the annealed condition
Most of the shape is cut while the steel is soft, with extra stock left on surfaces that must finish to size.
Heat treat to the drawing hardness
The part is hardened and tempered to the hardness range on the drawing.
Finish the critical surfaces
Surfaces that must hold size after hardening are finished, for example by grinding.
Inspect and pack
Dimensions are checked against the drawing before the parts ship.

What that means for your part:

Hardness on the drawing. Write the hardness range the finished part needs, such as 58 to 60 HRC (an example callout). Tell us the hardness, and the quote confirms how the part will be made and treated.
Size after hardening. Parts can move during heat treatment. Mark the surfaces that must hold size after hardening, so they’re finished afterward.
Already hardened stock. If you need a part cut from pre-hardened stock, say so with the drawing, and we confirm the route in the quote.

Which finishes suit hardened tool steel?

The finish depends on whether the surface is a working face. Plating hardened steel needs care, because plating can put hydrogen into the steel and make it brittle.

Finish	Why you’d choose it on tool steel	Watch for
As-machined or ground	Working surfaces and dies	Bare tool steel can rust; oil it in storage
Bead blasting	Even matte surface	Mask working edges and ground faces
Zinc electroplating	Corrosion protection on soft or lightly hardened parts	Not to ASTM B633 on steels above about 46 HRC, because of hydrogen embrittlement
Nickel or chrome electroplating	Wear resistance or a bright finish	Ask about baking after plating on hardened parts
Powder coating and painting	Color on non-working surfaces	Masking needed on working faces

ASTM B633 says steels above 1,700 MPa (247 ksi), about 46 HRC, shouldn’t be zinc plated to it (ASTM International). ASTM B850 covers baking steel after plating to reduce the risk of hydrogen embrittlement, and notes that baking reduces the risk rather than removing it.

Our full list of finishes:

As-machined
Bead blasting
Anodizing Type II
Anodizing Type III (hard anodizing)
Powder coating
Brushing
Passivation
Electroplating (nickel, chrome, zinc)
Painting

How do you call out tool steel on a drawing?

Write the grade, the hardness range and the product standard if one applies. For example (an example callout, not a recommendation): “Tool steel D2 to ASTM A681, harden and temper to 58 to 60 HRC”. For heat treatment requirements, some drawings also cite SAE AMS2759 (SAE International).

What to put on the drawing or in your message:

Grade, such as A2, D2 or H13. “Tool steel” alone isn’t enough to quote.
Hardness, as an HRC range, and where on the part it’s measured.
Surfaces that must hold size after heat treatment.
Finish, with any plating standard and the faces to mask.

Every order comes with a mill certificate that traces the metal to its batch, along with QC inspection and ISO 9001 documentation.

Standard QC inspection
Material certification (mill certs)
ISO 9001 documentation

Questions buyers ask about tool steel CNC machining

Is tool steel machined hard or soft?

Usually soft. Tool steel is supplied annealed for machining (ASTM International), then heat treated to the drawing hardness, with critical surfaces finished afterward.

Which tool steel should I choose for dies and punches?

Cold-work groups A, D and O are made for dies and punches, and the S group for parts under impact (ASM International). Tell us how the part is loaded, and our DFM notes suggest a grade.

What does the letter in A2 or H13 mean?

It names the group. A is air-hardening cold work, H is hot work, D and O are other cold-work groups, M is high-speed, S is shock-resisting and W is water-hardening (ASM International).

Can hardened tool steel be zinc plated?

Not to ASTM B633 if it’s harder than about 46 HRC, because of the risk of hydrogen embrittlement (ASTM International). Choose another finish, or ask about the options in your quote.

Do you heat treat tool steel?

Tell us the hardness the drawing needs, and the quote confirms how the part will be made and treated.

Will I get a material certificate?

Yes. A mill certificate comes with every order, so the grade traces back to the batch the metal came from.

For parts that don’t need hardness, compare mild steel CNC machining, see stainless steel CNC machining for corrosion resistance, read what CNC machining is or see all the metals we machine.

Sources

ASM International, Tool Steels: Standard Designations and Compositions (ASM Alloy Center), https://www.asminternational.org/tool-steels-standard-designations-and-compositions-asm-alloy-center/, accessed 2026-10-20.
ASTM International, A681, A600, A686, B633 and B850 (named; homepage linked), https://www.astm.org/, accessed 2026-10-20.
SAE International, AMS2759 Heat Treatment of Steel Parts (named; homepage linked), https://www.sae.org/, accessed 2026-10-20.