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From: Richard Smith <null@void.com>
Newsgroups: rec.crafts.metalworking
Subject: Re: rod-mill project - "mains" electric motor advice
Date: Tue, 22 Apr 2025 08:44:18 +0100
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"Jim Wilkins" <muratlanne@gmail.com> writes:

> "Richard Smith"  wrote in message news:m1wmbd29er.fsf@void.com...
>
> Hi everyone
> I have been shown how to do this with power-electronics and
> "off-the-shelf" gearbox, etc.  With all speed-control needed.
> A friend had made a small welding positioner.  Hence adapted it:
> * VFD - 1ph-to-3ph
>   v
> * 3ph induction motor
>   v
> * worm-drive gearbox
> You turn a knob setting the output Hz.  Adequate speed range.
>
> Having seen this - that it works and does everything wanted - will
> simply do it this way.
> Have friend's equipment on "unspecified loan' in my car to take home and
> play with more.
>
> -----------------------------------------
>
> That is an excellent way to learn what works. The potential problems
> are cost and custom machining. In that instance I'd look up and save
> the components' data sheets and see how their max power ratings
> compare to what was needed and what you intend to do. I've been burned
> by guessing that something was good enough, my sawmill left a trail of
> broken ball bearing innards until I switched to a more reputable and
> expensive brand for the blade guides.
>
> I hardly ever find the same device twice as second hand and must
> decide on the spot if what I found is suitable, which may depend on
> its power rating. People dispose of what they can't use, typically
> because they are either inadequate or broken.
> I tested that contactor for pull-in voltage and contact resistance.

I will go with "off the shelf".  Now seen what's needed.
Time si too valuable.

* have the thing working (hopefully!) and making inroads here in
  Cornwall

* free to get on with other things I have to do - would cost me more in
  what I have not done than I could save not "placing orders" for what
  is spec'd for the job.

One "blessing".
Gearbox must for sure have a torque rating.
Derive torque.
Found it's gloriously simple - relation of power, torque and revs.
P=tau.omega

P=power (Watts)
tau = torque (Newton.metres)
omega = rotation-rate (radians/s)
Latter makes total sense - well it does for me :-)  Radian is where a
radius is wrapped around the circumference.  Very often gives vast
simplifications (compared to working in angular Degrees or Revs Per
Minute, etc.).
Prompted that way, I can easily see from first principles how
"P=tau.omega" can be derived, giving total satisfaction in applying it.

So I can look through spec.sheets for "off-the-shelf" gearboxes.

I have a 30:1 gearbox on loan.
However, reckon 7:1 would make the drive "synchronous" (if ran motor at
mains frequency, would give the right drive speed).
Of course you need speed adjustment to make the mill(s) work exactly
right - but what you are asking is very achievable - say +-25% - which
an induction motor can do no problem.
[welding positioner - is so low power draw for small parts that eg. 10% of
the mains synchronous speed it is designed for is not a problem]
So going to have to get a 7:1 ish gearbox for mill.

Speed control should be wide enough to run rod-mills and ball-mills on
the same rollers-on-a-frame, no mechanical changes needed.

I calculated for the "metallurgical" part of the rod-mill - everything
which is and is within the "shell" ("drum"):
* 65RPM of shell rotation rate
* 127W of power draw