What Tap Drill Error Causes a Roll Tap to Strip Instead of Forming Cleanly?

A tap drill that's too small for a roll tap is what causes stripping, not too
large. It sounds backward compared to cut tapping, but roll taps form threads
by displacing material rather than cutting it away, and if there isn't enough
room for that material to go, it work-hardens and shears instead of forming a
clean thread profile.

Why roll taps need a different drill size than cut taps

A cut tap removes material to create the thread form, the same way a single-
point tool removes chips. A roll tap, sometimes called a form tap, has no
flutes to speak of — it has lobes that push the hole wall outward into the
shape of a thread, cold-forming the material into the major diameter. That
displaced material has to come from somewhere, and the tap drill hole is
sized to leave exactly enough material to flow into a full thread form
without either running out of material or having so much that the tap can't
push through it.

This is why roll tap drill charts specify a larger hole than a cut tap chart
for the same thread size and percentage of thread engagement. A cut tap for
a 75% thread and a roll tap for a 75% thread use noticeably different drill
sizes, and using a cut-tap drill chart for a roll tap is one of the most
common setup mistakes.

What actually happens with the wrong drill

If the hole is too small for a roll tap, there's too much material for the
tap to displace. The material can't flow into a full thread profile fast
enough, so instead of forming cleanly it work-hardens under the compressive
load and then shears or tears rather than deforming. That shearing is what
shows up as a stripped, torn, or incomplete thread — the crest of the thread
never fully forms because the material failed before it got there. Torque to
turn the tap also climbs sharply, which is often the first symptom before you
even see the bad thread.

If the hole is too large, the opposite problem shows up: not enough material
to displace, so the thread forms with a shallow crest and undersize major
diameter, weaker than spec but at least not torn.

What to check

• Confirm the drill chart is for roll/form taps, not cut taps. These are
  different documents from different tooling makers, and mixing them up is
  the single most common cause of this failure.
• Check the material's ductility. Roll tapping relies on the material
  flowing plastically. Free-machining or leaded alloys, and some cast or
  heavily cold-worked stock, don't flow as predictably and may need a drill
  size adjustment from the standard chart even when the chart is correct for
  the thread.
• Verify actual drill size, not nominal. A worn or slightly undersize
  drill produces a hole smaller than intended, and roll tapping has less
  margin for that error than cut tapping does because there's no flute
  clearance to absorb it.
• Check tapping speed and lubrication. Roll tapping generates more
  friction heat than cutting, since it's displacing rather than removing
  material, and a dry or slow-speed setup that's marginal for cut tapping can
  push a roll tap past the point where the material fails instead of flows.

The practical takeaway

If a roll tap is stripping, look at the drill size chart before touching
speeds, feeds, or the tap itself. A hole sized for a cut tap on a roll tap
job is undersize for what a form tap needs, and that mismatch is the most
common root cause by a wide margin.