CNC Machining Service on Tool Steel Machining
This guide is for engineers and buyers who need CNC service on tool steel parts: mold inserts, punches, dies, jigs and wear plates. It covers grade selection, hardness limits, what to machine before and after heat treatment, tolerance and finish, and the questions to ask a supplier before you release the order.
In this article
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Key takeaways
Common tool steel grades and what they mean on the shop floor
Hardness values are typical supplier ranges. Confirm the required range on your drawing before ordering.
| Grade | Typical use | Machinability in annealed state | Notes for CNC planning |
|---|---|---|---|
| A2 (air hardening) | Punches, dies, wear plates | Good | Low distortion on hardening; stable for long parts |
| D2 (high Cr) | Cutting dies, slitters | Fair | Abrasive; expect shorter tool life and slower feeds |
| O1 (oil hardening) | Small punches, gauges | Good | Dimensions move more in quench; leave stock |
| H13 (hot work) | Die casting, extrusion dies | Fair | Tough and gummy; keep coolant on the cut |
| S7 (shock) | Chisels, shear blades | Good | Machines similar to medium-carbon steel |
| P20 (mold steel) | Injection mold cavities | Very good | Often supplied pre-hardened near 30 HRC |
| M2 (high speed) | Cutters, form tools | Poor | Best shaped before hardening, then ground |
Milling, EDM and grinding compared for hardened tool steel
Use this to decide which operation should touch which face.
| Operation | Best for | Hardness limit | Watch out for |
|---|---|---|---|
| 3-axis milling | Open pockets, faces, profiles | Up to about 45 HRC | Tool wear climbs fast above 50 HRC |
| 5-axis milling | Complex 3D forms, angled holes | Up to about 45 HRC | Rigidity of thin sections |
| Wire EDM | Through profiles, sharp corners | Any hardness | Cutting speed; taper limits |
| Sinker EDM | Blind cavities, sharp internal corners | Any hardness | Electrode wear; recast layer |
| Surface grinding | Flat faces, thickness, parallelism | Any hardness | Heat burn if the wheel is too hard |
| Jig grinding | Precise bores and slots | Any hardness | Setup time per hole |
When to send the drawing
If the part is hardened above 45 HRC, or has thin walls and tight functional faces, send the drawing with the hardness range and we will mark which operation should finish which face before quoting.
What tool steel machining actually involves
Tool steel machining is rarely a single setup. Most parts move through at least two states: soft machining in the annealed condition, then hardening and tempering, then a finishing pass on the faces that control fit. If a supplier quotes one operation and one price, ask which faces stay soft and which get finished after heat treatment. That answer tells you more than the price does.
Annealed tool steel cuts like a medium-alloy steel. It chips cleanly, holds a good finish, and accepts fine details such as 0.5 mm corner radii and narrow slots. This is the stage where you remove the bulk of the stock and put in the geometry. Leave 0.2–0.5 mm of stock on surfaces that will be ground after hardening, and more on long thin features that tend to move.
After hardening, the same part behaves differently. At 58–62 HRC, carbide tooling wears quickly and cutting forces rise. Deep pockets, sharp internal corners and thin walls become slow or impractical to mill. Wire EDM handles sharp internal corners and through profiles. Surface grinding handles flat faces and parallel requirements. Jig grinding handles precise bores. A supplier who plans all three from the start will hold your tolerance.
The practical rule: decide the final hardness on the drawing first, then work backward to the process. Hardness drives tooling, feeds, number of setups and inspection method. It also drives lead time. A part that is finished soft and then hardened to 60 HRC will not hold ±0.005 mm on its own; something has to be ground after the furnace.
- 1Bulk removal in the annealed stateFaster cutting, better finish, lower tool cost.
- 2Heat treatment in the middleHardening and tempering set the final hardness and relieve stress.
- 3Finishing after hardeningGrinding, jig grinding or wire EDM on fit and wear faces.
Choosing a grade for your part, not for the catalog
Grade choice should follow the failure mode you expect. Wear-dominant parts, such as slitter blades and wear plates, favor high-chromium grades like D2. Impact-dominant parts, such as chisels and shear blades, favor shock grades like S7. Hot work, such as die casting inserts, favors H13 because it resists softening at temperature. Mold cavities that only need moderate hardness often run P20 in the pre-hardened state and skip the furnace entirely.
Machinability changes the cost of the part, but it should not be the first filter. D2 is abrasive and slows the cut, so a part with deep pockets in D2 can cost noticeably more than the same part in A2. If the wear requirement is moderate, A2 or O1 machines faster and hardens with less distortion. If the part runs hot, H13 is the practical answer even though it is gummy to mill.
Thin walls and long slender features are the hardest case for any tool steel. A 1.5 mm wall in a 200 mm long part will move during hardening no matter which grade you pick. Design symmetrical stock allowance, avoid one-sided pockets, and keep section changes gradual. If the geometry cannot change, plan a finishing operation after heat treatment and accept a longer route.
When you send a drawing, include the grade, the required hardness range and the surfaces that matter. A supplier can then choose between soft machining, pre-hardened stock and post-hardening finishing. Without the hardness callout, a shop may machine everything soft and ship a part that measures well and performs badly.
- 1Wear-limited partD2 or similar high-chromium grade, hardened and ground.
- 2Impact-limited partS7 or O1, tempered to a lower hardness for toughness.
- 3Hot work partH13, with hardness checked after tempering.
- 4Moderate mold workP20 pre-hardened, machined and finished without a furnace step.
Tolerance and finish: where the money goes
Blanket tolerances on tool steel parts raise cost quickly. A drawing that calls ±0.005 mm on every surface forces the shop to grind or EDM everything, and inspection time grows with the feature count. In practice, only a few faces control function: locating faces, shut-off faces, wear faces and mating bores. Mark those and let clearance faces carry a looser tolerance such as ±0.05 mm.
Surface finish follows the same logic. Ra 0.2–0.8 μm is achievable on ground and polished faces, but it is not needed on a relief face or a clearance wall. Ra 0.8–1.6 μm covers most mold and die work. Ra 1.6–3.2 μm is fine for structural faces. Tell the supplier which finish each face needs, or the shop will either over-finish and charge you, or under-finish and fail the fit.
Hardened tool steel adds one more variable: the recast layer from EDM. Wire EDM leaves a thin affected layer that can crack under load. On fatigue-critical or high-impact parts, plan a light grind or polish after EDM to remove it. This is a small extra step that prevents a much larger problem later.
Inspection should match the tolerance. For ±0.005 mm features, a coordinate measuring machine report or a ground standard is reasonable. For general dimensions, calipers and micrometers are enough. Ask for the inspection report on the critical faces before shipment, especially on the first article of a new tool steel part.
- 1Mark only functional facesLocating, shut-off and wear faces get the tight tolerance.
- 2Set finish per faceGround and polished where it slides or seals; as-machined elsewhere.
- 3Remove the EDM recast layerLight grind or polish on fatigue and impact faces.
Lead time, quantity and the supplier questions that matter
Tool steel parts usually run in small quantities: one prototype, a few spares, or a set of inserts. A supplier with no minimum order quantity lets you start with one piece and scale to a 10,000+ part run without changing the process. That matters when a design change is likely after the first trial. Setup cost is real, but a supplier who quotes a single part honestly is easier to work with than one who hides setup inside a large minimum.
Lead time breaks into three parts: quoting and DFM feedback, machining, and heat treatment with finishing. Quotation and free DFM analysis within 12 hours is a reasonable benchmark. Production can start within 24 hours once the drawing and material are confirmed. Parts typically ship in 3–5 days for straightforward geometry, with heat treatment and post-hardening finishing adding time on top. Ask for the route and the dates separately so you can see where the days go.
Certifications tell you which management systems are in place. ISO 9001:2015 covers general quality. IATF 16949:2016 matters for automotive and EV work. ISO 13485:2016 matters for medical devices. ISO 27001:2022 covers information security, which is useful when your drawings are sensitive. None of these replace a process audit, but they shorten the first conversation.
Ask five questions before you release the order. Who machines the soft state? Who controls the heat treatment and tempering? How is hardness verified, and on how many pieces? Which faces are finished after hardening? What is the inspection plan for the tight faces? Clear answers to those five cover most of the risk on a tool steel job.
Confidentiality belongs in the same conversation. Uploads are secure and confidential, and an NDA is available on request. For mold inserts and die details, the geometry itself is often the valuable part, so treat the file handling as part of the technical review.
- 1No MOQFrom one prototype to 10,000+ parts without a process change.
- 2Quote and DFM in 12 hoursProduction can start within 24 hours after confirmation.
- 3CertificationsISO 9001:2015, IATF 16949:2016, ISO 13485:2016, ISO 27001:2022.
- 4ConfidentialitySecure uploads and an NDA available on request.
Step by step: how to place a tool steel machining order
Follow this order and most surprises show up before the chips fly.
- 11. Fix the hardness and the functionWrite the required hardness range, for example 58–62 HRC, and list which faces control fit, wear or shut-off. Everything else can stay general.
- 22. Pick the grade against the failure modeWear to D2, impact to S7 or O1, hot work to H13, moderate mold work to pre-hardened P20. State the grade on the drawing, not only in an email.
- 33. Set stock allowance for heat treatmentLeave 0.2–0.5 mm on faces to be ground after hardening. Add more on long thin features and on one-sided pockets. Keep sections symmetrical where you can.
- 44. Confirm the process routeSoft machine, heat treat and temper, then finish the critical faces by grinding, jig grinding or wire EDM. Ask which operation touches which face.
- 55. Agree on tolerance and finish per faceTight tolerance ±0.005 mm on functional faces; ±0.05 mm on clearance. Ra 0.2–0.8 μm on sliding and sealing faces; Ra 0.8–1.6 μm elsewhere.
- 66. Define inspection before shipmentName the faces to be measured, the instrument, and whether a report is required. 100% inspection before shipment with reports on request is the baseline.
- 77. Confirm quantity and dates in writingNo minimum order quantity. Ask for quote and DFM timing, production start, and shipping separately, with heat treatment shown as its own line.
- 88. Close the confidentiality loopUpload drawings through the secure channel and sign an NDA if the geometry is sensitive. Confirm who inside the shop can open the files.
Tool steel machining questions engineers ask
Can you mill tool steel after it is hardened?
Yes, but only up to a point. Milling is practical up to about 45 HRC with carbide tooling and a rigid setup. Above 50 HRC, tool wear and cutting forces rise sharply, and deep pockets or sharp internal corners become slow.
For higher hardness, plan grinding for flat faces and bores, wire EDM for through profiles, and sinker EDM for blind cavities. Those processes do not care about hardness, so they hold tolerance on 58–62 HRC parts.
How much stock should I leave for heat treatment?
Leave 0.2–0.5 mm on faces that will be ground after hardening. Long thin features and one-sided pockets need more because they move further, and a 1.5 mm wall on a long part can distort well beyond that.
If the part has no functional face that must be ground, you may be able to machine it oversize in the soft state and harden to final size with a controlled process. That route needs a heat treater who can hold the distortion, and it should be agreed before the first cut.
Which tool steel grade is easiest to machine?
In the annealed state, P20 and A2 machine well, and O1 is close behind. D2 and M2 are abrasive and slow the cut, so deep pockets and small tools cost more in those grades.
Pick on function first. If wear resistance is the requirement, D2 is still the right call even though it machines slower. If the requirement is moderate, A2 or P20 will give you the same result with less cutting time.
What tolerance can you hold on hardened tool steel?
We work to ±0.005 mm on functional faces, which are finished after hardening by grinding, jig grinding or wire EDM. Clearance and relief faces usually run at ±0.05 mm and do not need the same treatment.
The tolerance you can hold depends on the feature, not only on the machine. A precise bore in a thick block is easier than a thin wall on a long part. Send the drawing and we will tell you which faces can hold the tight number.
Do you have a minimum order quantity for tool steel parts?
No minimum order quantity. You can order one prototype insert and later run 10,000+ parts on the same process. The machining route stays the same, so the first article is a real check on the production part.
Setup cost is part of any small order, and we show it honestly in the quote rather than hiding it inside a large minimum. If the design is likely to change after the first trial, that is the cheaper way to start.
How do you handle heat treatment and finishing in one order?
We plan the route as soft machining, heat treatment and tempering, then post-hardening finishing of the critical faces. Hardness is verified after tempering, and the finishing operations are scheduled on the measured result.
Grinding, jig grinding, wire EDM and surface finishing can all sit in the same order, so you are not shipping the part between suppliers and losing traceability. Ask for the route in the quote and the dates for each stage.
Start your tool steel machining quote
Send the drawing with grade, hardness range and functional faces. You get quotation and free DFM analysis within 12 hours, with no minimum order quantity.
12-hour quoteNo MOQ100% inspectionNDA on request
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