Quick answer: For 40CrMo alloy steel, choose the threading route by actual hardness, hole type, and machine rigidity. A powder-metallurgy HSS tap may be suitable in moderate hardness, but when the part is hardened, the thread is deep, or the workpiece is expensive, thread milling may be safer.
40CrMo is a chromium-molybdenum alloy steel used where strength and toughness are required. The machining behavior depends heavily on heat treatment. A soft or moderately hardened part and a quenched-and-tempered part should not be treated as the same tapping job.
Start with hardness, not only the material name
The most important RFQ detail is actual hardness. If the material is around the lower or middle hardness range for tapping, a suitable coated PM HSS tap with good toughness can be reviewed. If the material is near high-hardness territory, tap breakage risk rises quickly, especially in small diameters and deep blind holes.
That said, hardness alone does not decide the tool. Hole straightness, tap-drill size, coolant, spindle synchronization, and holder runout can turn a reasonable tap into a broken tap.
Blind hole or through hole changes the tap style
For blind holes, chip evacuation must move chips back out of the hole. A spiral flute tap is often the first style to review, but the flute weakens the tap body compared with more rigid styles. For through holes, a spiral point tap can push chips forward and may be more stable in production.
If the hole is shallow and full thread must reach close to the bottom, also check chamfer length. A tap that is correct for the material may still fail if the lead threads bottom out before full thread depth is achieved.
When thread milling is the safer route
Thread milling is worth reviewing when the material is hardened, the hole is large enough for interpolation, the workpiece is expensive, or a broken tap would scrap the part. Compared with tapping, thread milling can reduce trapped-tool risk and can machine different thread diameters with the same pitch family when the machine and programming allow it.
The tradeoff is that thread milling needs suitable CNC capability, programming, and a stable toolpath. For small threads, tapping may still be faster and more economical when the process is stable.
Tool selection checklist for 40CrMo tapping
- Material condition: Provide actual hardness and heat-treatment state.
- Hole type: Identify blind or through hole, thread depth, and available bottom clearance.
- Tap-drill size: Confirm the pilot hole is neither too small nor oversized for the thread class.
- Machine control: Check rigid tapping synchronization, holder runout, and coolant delivery.
- Fallback route: Review thread milling when tapping risk is high.
What to send HEYI for review
For 40CrMo and similar alloy steels, HEYI can review carbide tools, tapping support tools, and custom tooling options. Send the drawing, thread size and pitch, hole depth, hardness, machine type, coolant, current tap style, and failure photos through the RFQ form.