DIYbanter - View Single Post - Homemade 2 spindle lathe preliminary results

Don Foreman

On Sat, 06 Sep 2008 20:51:41 GMT, wrote:

On Sat, 06 Sep 2008 15:50:30 -0400, Bob Engelhardt
wrote:

Bob Engelhardt wrote:
wrote:

Another test would be to clamp a glass tube in both chucks.

I'd clamp a piece of rubber tubing in both & see if it winds up.
Bob

or a piece of stock in *each* (2 pieces) almost touching in the middle.
Make a scratch across the "joint" & see if it stays aligned.

--
Nota for President
Greetings Bob and Dan,
The glass tube idea will only work if the tube is weak enough to break
instead of forcing the spindles to keep in step. I think I have some
glass tube so maybe I'll try that. As to the rubber tubing and the
scratch I already know the spindles stay in line because of marks made
on discs at the end of each spindle. It's cool to spin one spindle
shaft by hand and watch the other keep up. It looks like the spindles
are one shaft. The discs are right next to each other with only about
.020" between the faces. So one motor spins cw while the other spins
ccw. So if the master spindle is unpowered and turned by hand the
other one keeps up perfectly. It's actually kind of creepy watching it
because I know there is no mechanical connection between the two.
Eric

There *must* be some relative torsional displacement because this is a
feedback control system depending upon an error signal. It could be
zero error at times of constant speed if the control system is PI
(proportional-integral) but there must be some error during speed
changes.

Incorporation of torque sensing could minimize this: sense torque
supplied by both drive motor and slave motor and have the control
system operate to minimize the difference.

You sorta have that now: torque difference is proportional to the
difference in encoder signals and the "twist" in the workpiece.

If the slave spindle need not provide any significant torque, then it
may just follow the load like a live center. In that case, the only
loads would be dynamic during accelerations and decelerations and the
load would drive the slave motor rather than vice versa. In that
case, if the torsional distortion in the workpiece is less than one
encoder count from drive spindle to slave motor there'd be no
excitation provided (or necessary) to the slave motor.