[dyingbreed]

We are designing a custom build tapping machine and were thinking of using a AC servo motor to drive the spindle. Is the application approriate? We will be tapping a 1 11/16-12 thread with a lead screw unit. Material is Zinc casting.
Motor will be cycling (reversing) about 12 times a minute.

[Al_The_Man]

You do need fairly tight control of the spindle in tapping applications and a servo can be used, but it also depends on what system you are using.
Some systems do not require servo type control, as long as rpm can be controlled fairly accurately through a digital tach/encoder on the motor itself and an encoder on the final spindle.
The Z axis will then synchronize to the spindle rpm through the final encoder.
Al.

[dyingbreed]

if I am driving the tap pitch with a lead screw, I shouldn't need to sychronize the Z axis. The feed will be controlled by the lead screw pitch, or am I forgeting something?
Also my concern is the reliability of the motor holding up to millions of reversing cycles. We currently drive these spindles with a hydraulic motor. They are fairly reliable , but I would like to illiminate the hydraulics on the new machine.

[Al_The_Man]

Obviously you do not want the spindle rpm to vary widely, but as I mentioned, controls that are designed to perform this will synchronize the Z axis feed servo to the spindle shaft encoder, If the spindle fluctuates slightly with load the Z servo will feed accordingly.
It is similar to the threading cycle on a Lathe.
Al.

[novacustard]

I can't really comment on the reliability of servos v. hydraulic motors, but if you were going to start and stop a normal three phase synchronous motor (ie a cable hoist), then you would normally have to derate the motor to allow for the heat that builds up during start up to dissipate. The compumotor servos I have used make recomendations that they are mounted to suitable aluminium heatsinks, in order to get the most performance out of the motors (although I haven't seen much sign of people actually doing this).
So, assuming that you have done the power calculations correctly, it might be sensible to derate the motor that you are using, ie fit a more powerful one. Finally, if you are purchasing a new servo, why not talk to the suppliers technical support line for their advice?

Richard.