CNC Tip:

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If your machine does not have a spindle encoder, you should be using a "floating" tapping holder. The holder will extend or compress to make up for mismatches in speed and feed.

The spindle takes longer to reverse directions than the feed. A machine with a spindle encoder will recognize this, and adjust the feed to match (rigid tapping). If you don't have an encoder on your spindle, a floating tap holder is the way to go. I have heard of and seen people get away without one by running very low spindle speeds. But if you tap many holes at all, the holder is worth the money. If you go with the floater, you can kick your spindle speeds back up to normal. Just remember a couple things:

(1) The spindle is still going to "coast down", even more so with higher spindle speeds. On most of our machines this translates into about 1 turn for every 100rpm. Program the depth a little short to keep from coasting too deep, especially on blind holes.

(2) Since the holder can extend, it's possible for the tap to have not made it completely out of the hole before the machine moves to the next location (snap). Use a larger R-plane to help avoid this.

A couple of the controls we own have features to compensate while "flexible" tapping. Fadal can use a P-code to adjust feed on the way out of the hole. P5, for example, reduces the feedrate by 5% on the way out. Incon allows a D-code to dwell (feed) at the bottom to wait for the spindle to catch up. If you have the spindle encoder, nothing beats M29 rigid tapping.