Why Does a Rigid Tap Finish Oversized in Stainless but On-Size in Aluminum With the Same Tap?

The tap is fine. It's picking up more material spring-back and tool deflection in stainless than it does in aluminum, and that shows up as a pitch diameter that finishes larger even though the tap itself never wore or moved.

What's actually different between the two materials

Aluminum cuts cleanly at low cutting forces. The chip shears off close to the tap's geometry, deflection is minimal, and the resulting thread tracks close to the tap's nominal size.

Stainless work-hardens as it's cut and generates far higher cutting forces at the same feed. That force pushes against the tap's flutes and the workpiece elastically. When the tool retracts, the material springs back slightly less than it was displaced, leaving the finished hole a touch larger than the tap's nominal pitch diameter would predict from aluminum experience. Austenitic stainless like 304 or 316 is worse for this than a lot of alloy steels because it work-hardens hard and holds onto residual stress from the cut.

Heat plays a role too. Stainless runs hotter at the tool edge for the same parameters, and thermal expansion during the cut followed by contraction on cooldown adds a small amount to the apparent oversize. The mechanical spring-back effect is usually the bigger contributor.

What to actually do about it

Don't assume one tap class covers both materials. A tap ground and coated for aluminum, run in stainless, runs hotter and gaits faster, compounding the oversize problem instead of fixing it.

Check tap drill percentage for the material, not just the thread spec. A tap drill sized correctly for aluminum's typical engagement target can be undersized for the amount of material stainless actually removes cleanly at that thread class. Running it at aluminum's numbers is a common source of this exact symptom.

Slow the feed relative to what aluminum tolerates. Stainless needs a gentler approach to keep cutting forces from growing large enough to cause the deflection in the first place.

Use a tap geometry meant for stainless, with a different flute form and coating, rather than pushing an aluminum-optimized tap into a harder job and correcting for it downstream.

When it's not the material at all

If the oversize condition is inconsistent, some holes fine and some oversized on the same tap and program, look at spindle speed and rigid tap sync before blaming the alloy. A rigid tap cycle slightly out of sync between Z feed and spindle rotation at the bottom of the hole introduces its own axial mismatch that reads exactly like a chronically oversized thread. No amount of retuning feeds for stainless will fix a sync problem.

If it's consistent across every stainless hole and consistent across every aluminum hole, but different between the two materials, that's the behavior described above. The fix is in tooling and process parameters, not troubleshooting the machine.

Rigid tapping is unforgiving about exactly this kind of material-dependent deflection. A tap program dialed in for one alloy often needs real rework, not just a speed tweak, to hold size in a different one.