Re: [CAD_CAM_EDM_DRO] Re: Converting a Bridgeport Clone

Hope I can help ;)

Conclusion here at top, thoughts below.
Dont change to an acme screw, either dont change at all, or go ballscrew.
For your use, I would not change the screw.

Current screw will last, and work, to better than (any) milling machine
capabilities.
Changing the screw (to a ballscrew) will not improve your ultimate results.
It will allow much higher acceleration (about 3x), unnecessary.
It will allow higher top speeds (2-3x), unnecessary.
It is a lot of work, needs new angular contact bearings, is expensive,
for no real gain.

>
> So while the subject is up I figured I would ask Hvenermo.
>

One option:
Where the hand crack attaches, make a spacer from a worm gear, (or a
HTD pulley) and re-attach the handle, or leave a nut or fixment for the
handle when needed (better I think).

Because the table is heavy, lash should never be an issue. the mass will
always preload the assembly.
I would recommend a worm gear. A worm gear will allow you better
precision, much better than 0.01 mm, and cannot ever be backdriven. It
will need a smaller motor.
The total dynamic range is much better for a stepper.
I think any worm rated for a 3-4Nm stepper, that can be driven from a 3
amp g251 (which is what is in a G540).
About 10:1 - 20:1 transmission.
Pick any 3-4 Nm modern nema 23 stepper, I think, at 48 volts.

>
>
> What do you think of powering a knee on an old Clausing 8520 off a
> Gecko G540 channel, if so which motor would you recommend. I am
> estimating the weight at 120#'s (knee with table) and currently it has
> a 7/8" .100" pitch screw with a 2:1 reduction bevel gear going to the
> hand crank.
>

I want to keep the manual capabilities as well, but due to the tight
spaces there are not many mounting options. The bevel gear has .0003" of
lash and I gain the 2:1 gearing if I drive the front crank stub, that or
I was going to try and belt drive the screw with a 3:1 or 4:1 reduction.
(probably couldn't fit a larger gear since space is tight).

If I swap the screw out with a .200" lead ball screw do you think the
stepper can keep it up without struggling once the stiction is gone?

Yes, but slowly.
Ie you still need to work in the "sure" zone of the setup, rather than
the "best" zone, so you will be using 1/3 the optimum resolution, speed
or acceleration.
This is the same problem when using cheap integrated chinese stepper
drives rather than gecko 251/203/540.

> This also effecting the ability to use it manually, if it wants to
> back-drive without the steppers running. What are your thoughts on
> higher efficiency "precision acme" screws. This way I could keep the
> lower pitch and increase efficiency to ~60% but with less tendency to
> back-drive. This is just a hobby machine and I am not looking for
> accuracy in the tenth's or high rapids, rather just a solid design
> that will not give problems.
>

Ballscrews work VERY well on manual setups.
When you connect the stepper mechnically it acts like a slight break,
when the drive is disabled (pin 10 on geckos).
The stepper keeps the ballscrew from backdriving. You need switch to
disable the drive (203s and 251 use diffrent 5v/grounding for this
disable) for manual use.

>
>
> Secondly, I saw you mention a M-Head on your, which is the same head
> that has been adapted to my 8520. Do you have any pictures of your
> current setup, as I am curious which is a reliable method if I choose
> to just power the quill instead of the knee.
>

I am just painting the members on my table (version 2).
When assembled, I will be doing photos.

My quill setup right now is a HTD 5/15 mm pulley, 48 teeth, being driven
by a stepper and a 16 teeth pulley).
A handle extension is put to the side with a manual handwheel.
This is not a good choice, re: pulleys.
Insufficient resolution, power and thus precision.

This is because I dont have space near the mount point for the M-head,
for normal handle operation (but it is much more rigid and much better
otherwise, this was an excellent modificatiion (head mount version 2).
From the edge of the mill bridge to the M-head it is now about 10 cm,
it was about 25 cm.

I will be going to a worm gear on the quill.
I expect to order the worm wed., and mount it within a week or so.
I plan to use angular contact bearings for the worm gear shaft, so it
cannot move.
I will probably leave the manual handle (I like the way it turns when in
use). However, with a worm, I would need some other way to manually use
the mill (disengage the worm)..

I will use the same 3Nm nema 23 motors I have, from
Motioncontrolproducts in the UK, about 30EUR landed.









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