M8x1.25 rigid tap in 304 stainless alarms on the reverse about one time in eight. The hole is drilled 6.8 mm and is 18 mm deep. The tap is a spiral flute. The program uses a standard rigid cycle with the same feed as the pitch.
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The reverse alarm in stainless is often the tap still loaded with a chip when the spindle flips. A short dwell is the wrong fix. Use a tap with a back taper and pull out before the hole is packed, or peck the thread if the control allows it.
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Check that the feed is exactly the pitch at the rpm you really run, not the rpm in the header comment. A 3 percent mismatch feels fine on the way in and faults on the way out.
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6.8 mm is a common drill for M8, but in 304 a 6.9 mm hole gives the tap less to do and fewer alarms. The thread gauge still has to pass, so gauge the first one before you change the lot.
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