How are speeds and feeds different when flex tapping as oppossed to ridged tapping?
Let's say 1/4-20 tap going into cast stainless steel
In materials that can be tapped fast, rigid tapping allows much higher RPM to be used. As mentioned in previous posts, the lack of synchronization in floating tapping will not allow high spindle speeds to be used. At high spindle speeds the the spindle is still turning at the programmed speed when the Z reaches the programmed depth. At that point the spindle turns off and decelerates to a stop. During that deceleration, the tap continues to pull itself into the part. The faster the spindle turns, the more revolutions it takes to stop. Then when the spindle starts up in reverse it has to reach 80% of the commanded speed before the Z axis begins moving up. During this reverse acceleration the tap is backing out of the hole yet the Z is not moving.
In rigid (synchronized) tapping, the control begins decelerating the spindle as the Z approaches the programmed depth and both the spindle and Z stop at the same time. When reversing, the Z begins moving out as soon as the spindle starts to rotate.
Depending on machine controller, the feed programmed for rigid tapping may need to programmed in the pitch of the thread rather than a calculated feed based on RPM X pitch.
I have a machine than can rigid tap up to 6000 RPM. Just for giggles I did program some 6-32 x .375 deep in aluminum at 6K. It worked great, but the machine never made it to the full 6K before having to stop and reverse. Probably hit about 4K though. In any case it was fast.
For 1/4-20 in cast SS, you are probably only going to run ~400 rpm and likely slower depending on what alloy you are working with. At those speeds most machines are not going to have much difference in cycle time between rigid and floating tapping.