On 2008-09-03, Bob Engelhardt wrote:
DoN. Nichols wrote:
On 2008-09-02, Bob Engelhardt wrote:
DoN. Nichols wrote:
Bob Engelhardt wrote:
... If you
just want a nice finish, yes, the servo amplifier and the servo motor
with built-in tach would be sufficient.
What I really want is to be able to adjust the feed with the turn of a
dial. Fast feed for roughing: too slow? - turn it up. Slow feed for
finish: still too rough? - turn it down some more. I have this with
VFD's on my drill press & lathe spindle and I've gotten spoiled by it.
O.K. No threading on this one, so that should work.
I would suggest either a logarithmic taper pot (so you can make
very fine adjustments at slow speeds and coarser but quicker ones at
high speeds. The alternative would be a 10-turn linear pot which should
give you sufficient fine control at the low end -- but you would need
lots of turns at the high end to make a noticeable change in speed.
... but you still
need a good tach generator sharing the motor's shaft to get stable
speed control.
Understood. It brings up yet another question: whether I use a tach or
optical sensor. Is it dictated by the amp? Or do amps generally accept
either?
A tach generator is what your really want for very slow speeds.
You could use an encoder with a circuit to produce a voltage
proportional to the pulse rate, but that would probably make your speed
bumpy at the low end.
And are you planning to put this on a ball screw? You get more
reliable speed with that than with half nuts. At really slow speeds you
will have stick-slip behavior with Acme threads.
I wasn't planning on a ball screw. Hadn't even thought about it. If
that's what it takes, I'll probably dump the idea: that would be too
much work & expense for the convenience of dial-it-in feed rate.
There are separate turning & threading lead screws. Well, the turning
feed uses a shaft with a key slot in it - not really a "lead screw". I
guess that avoids the stick-slip problem (?).
That should do. You've also got extra gearing in the apron to
give you a torque advantage. I was assuming that you were also planning
to thread with this as well as turn.
snip a bunch of good stuff
Thanks again - I'm off to search eBay on "servo (amp*,drive,controller)"
You'll get lots of audio amplifiers -- and automobile mounted
stereo systems with that. Try this instead:
+servo +(amp*,drive,controller)
The '+' means that the entry has to be present.
Your original one gave 3,546 hits.
My first mod only took it down to 3,545 hits. Hardly worth the
effort. :-)
Hmm ... add "+DC" to the list so you don't come up with
AC/Brushless servos and amplifiers as well.
That takes it down to 461 hits.
Add "-car" and we get 424 hits
Add ' -"hard drive" ' and it is 365 hits.
Add ' -turntable -"tape drive" ' and it is 321 hits
This seems to be mostly reasonable things -- some model airplane
stuff, and some audio stuff still sneaking through. :-)
Be careful to avoid the ones marked:
"New SERVO 1150 RPM 38 VDC AMETEK motor, wind generator"
because they have no tach generator.
Hmm ... add ' -"lost plane finder" '
This one is a nice Baldor DC servo motor -- 260279397093 -- but
out of your proposed budget I think.
Good Luck,
DoN.
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