Re: [CAD_CAM_EDM_DRO] Re: Bridgeport retrofit servo questions

I would be very careful about certain ametech type motors.While I have never
sold the following type motors some Ametek type motors have motor power +
connected to the motor case.
I have personally seen a couple of them. I had a guy email me that he
bought some motors off of ebay. He bench tested them one at a time and all
was fine. Then he installed them on his machine. KAPOOOWWWEEEEE!. It blew
his servo amps. He wanted to know what went wrong. After a few emails I
figured out that he had motors where the + lead was connected to the case.
He checked with an ohm meter and sure enough they were. The short circuit
path was from one motor through the machine to the servo amp. I don't
recall how many of his amps blew but I'm sure he was upset to find out
that his $25 Ebay motors destroyed his $$$ servo amps.

Here are some things that most newbies forget to ask or check when they buy
PMDC motors for servo systems.
1. Is either motor wire connected to the case?
2. What is the end play of the motor shaft ( if more than .005 could cause
damage to the encoder)?
3. What kind of bearings are installed?. Should be ball bearing
4. If the motor doesn't have a shaft for the encoder can the shaft be
drilled? Some shafts are hardened and can't be drilled in a lathe
5. If the motor has a second shaft can you get an encoder that fit the shaft
length and diameter. If not can you sleeve or machine it true to another
size?
6. Is there a way to mount the encoder base to the motor frame?
7. If the motor does have an encoder what is the current rating? Gecko's can
only handle 50 ma. Other brands may be more or less.
8. What is the encoder count. Remember that most servo amps use a quadrature
encoder. So if you get a motor with 2000 lines it will be 8000 counts per
rev. That coupled to a .1 leadscrew would require 80000 steps per inch. You
PC/software may not be able to produce pulses fast enough unless the servos
have a step multiplier.
There are many other item to check as far as frame size, shaft size,
continuous torque, voltage, current, inductance and most importantly have
you calculated the torque requirements for the ballscrew, leadscrew, rack
and pinion etc.

Dan