On Thu, 04 Sep 2008 22:07:21 -0400, Bob Engelhardt
wrote:

Jon Elson wrote:

But, I "think" you need to rethink this whole thing. Do you really need
to set speed to 0.1% of the setting? How will you ever know? If the
max speed is 100 IPM, then the minimum speed, or setting resolution will
be 0.1 IPM. Do you ever make cuts on your lathe at 0.1 IPM? Would you
know the difference between 10 IPM and 10.1 IPM?
snip

What I meant by 1000:1 was the range of speed, not the resolution.
E.g., 2 rpm to 2000 rpm. Picking a speed will be by what looks good,
not by a particular rpm. I figured the range like this: the minimum
will be for a finish cut (.003) with lowest spindle speed (30 rpm) (i.e.
large cast iron work). That figures to 2 1/4 rpm. The max lead screw
rpm will be a rough cut (.050) at high spindle speed (2200) (e.g., small
diam aluminum). That speed is 2750 rpm. A range of 2.25:2750 = 1:1222.

Wait - I just did the ipm for the fastest speed & it's 1.8"/sec! That's
probably too fast to be practical. I'll need to do some trials to get a
better number for that. So, my required speed range could be 1:600,
1:300, ... ???

Though I don't think it's a reason to stop experimenting with the
servo, I'm with Jon here in thinking you're way overestimating the
required speed range. I just checked my lathe, which is a Feeler clone
of a Hardinge HLV-H. The usable feed range is about .15 IPM to 4 IPM,
less than 30:1, and the spindle runs from 15 to 4000 RPM. (Hardinge
specs 5/16 to 7 IPM; 22:1) I've never found the available range to be
a problem. The motor is a 75W PM.

Re the servo, I've used many Copley and AMC amps, and while they have
very similar lines, I've never run into identical products. I do
prefer Copley, mostly because their literature and support is better.
Most any PM brush DC motor should work for you as long as you can fit
the proper feedback device to the motor. Tight speed control without a
motion controller in the system is the only situation I can think of
where tach feedback is appropriate these days, so I don't think you'll
find many motor/tach combos on the surplus market.

--
Ned Simmons