When tapping with a tension/compression holder on a machining center, sometimes the hole does not accept the No-Go gauge (oversize internal threads). However, any chipping and welding on the tap is visible.
This problem usually occurs because of an over- or under-feeding. The observation of the rake surface on the tap’s flutes gives an idea about which case you are dealing with (see pictures below).
Case 1: over feeding
When the tap proceeds with more than one lead of the thread by one rotation, abnormal cutting marks appear on the leading flank.
Case 2: under feeding
When the tap proceeds with less than one lead of the thread by one rotation, abnormal cutting marks appear on the back flanks.
To overcome oversize cutting, therefore, the feed rate of the tap must match with the thread lead, i.e., with the distance a tap travels in one revolution.
Practical tips to adjust the feed rate:
- When possible, use a machining center with fully synchronous feed system and a rigid holder.
- Otherwise, adjust the weight balance of the main spindle: in the case of over feeding, strengthen the tension spring; in the case of under feeding, strengthen the compression spring.