Ned Simmons wrote:
... I'm with Jon here in thinking you're way overestimating the
required speed range. I just checked my lathe ...
The usable feed range is about .15 IPM to 4 IPM,
less than 30:1, ...

Thanks - actual data! My numbers were strictly based on personal
assumptions & my lathe (e.g., its finest feed is .003).

As to your lathe, let's see: say you are finish turning a 10" cast iron
chuck backing plate, 75 fpm gives spindle speed of 75*12/pi*10 = 29 rpm.
Then .15 ipm / 29 rpm = .005 feed (in/rev). About what I assumed.
Then say a 1/2" diam AL rod roughing at 400 fpm, spindle speed of 3000
rpm. 4 ipm / 3000 rpm = .001+ feed. Not what I'd call a roughing feed.
In fact, 4 ipm seems slow for roughing (a minute to rough 4").

I'm definitely keeping this question open: 1000:1 is out, as is 500:1,
based upon max practical carriage speed. But I think that I'd like more
than 30:1.

I've never found the available range to be a problem.

Personal experience is way better than theory! And I'll keep it in mind.
I'll also keep in mind that if it's trivially easier to have a greater
range, then I'll do it. For example, the amp that I bought can handle
12 1/2A at 190v - that's a lot of power & a given (not something that's
going to cost more).

The motor is a 75W PM.

That is a definite advantage.

...
Most any PM brush DC motor should work for you as long as you can fit
the proper feedback device to the motor. ...

Good!

I don't think you'll find many motor/tach combos on the surplus market.

That was my assumption. I have a small collection of DCPM motors & my
plan was to get a tach to use on them, as experiments.


Thanks for the good info,
Bob


--
Nota for President