Re: Converting a Bridgeport Clone

Terry, I hope you don't mind, I just joined and the first thread up was your's and just what I am interested in doing as well.

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

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? 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.

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.

Thank you for the help, and again sorry Terry for barging into your thread, it's just your situation is identical to mine, just a little bigger.

on the Problem is I want to be able to use it

--- In CAD_CAM_EDM_DRO@yahoogroups.com, CNC 6-axis Designs <gcode.fi@...> wrote:
>
> Either will work but ... as it depends (grin .. like always).
> The limiter will be your acme screw efficiency.
>
> Neglecting screw efficiency:
> Lbs = pi * TPI * in-oz / 8
> 3 Nm is about 400 oz-in, btw.
>
> Lbs = pi * 5 (tpi) * 400 / 8 = 785 Lbs = 365 kg.
> At 80% (mechanical losses x ballscrew effiency) this is 280 kg force.
>
> Using the smaller motors, for example with 3 Nm motors, at 1:3 gearing,
> and a 80% efficient screw the thrust produced is 280 kg.
> I use this size, 3Nm, nema 23 motors, at 1:3, to move my milling table.
> Table is 160 cm wide, 50 cm deep, 6 cm thick (on ribs), solid (F1
> calibrado) tool steel.
> I have a 0.750 inch ballscrew, 0.200 inch rise or 5 tpi, at about 90%
> efficient, and the table is on linear rails.
> (Linear rails are about 98% efficent, and HTD belts are about 97%
> efficient).
> Table can be moved by one finger (under 10 kg force required).
> Table mass is 240 kg give or take.
> Acceleration is excellent as is accuracy (better than 0.01 mm).
> Gecko251s, 48 V dc.
>
> HOWEVER, the acme screw is going to be about 10-20% efficient, and you
> will have a lot of static start-up friction.
> Likewise, you are not on linear rails.
>
> The force achieved at 1:6 (very big secondary pullies at 1:6, btw), with
> the smaller motor, will certainly move it.
> But I think you wont have a lot of margin.
> As in you will get slow acceleration if you program for worst case, and
> missed steps otherwise.
> Acme screw stiction varies a lot with how clean it is, lubrication, if
> you leave it to stand, cold/warm etc.
> I would definitely use the bigger motor for this app.
>
> However, I would not use belts but a worm gear.
> It is much more efficient, much more accurate, wont break, wont
> backdrive, is a better fit.
> IMO.
> A 10:1 -20:1 worm will work fine with the smaller motor, and will be
> much more accurate.
>
> I did my (stock) M-head quill with belts, and will now re-do it with a
> worm, fwiw.
> Probably also preload it, upwards, get rid of backlash.
>
> > It has been a long time since I posted on this group and I notice
> > there is not so much activity these days. Anyway on to my question:
> >
> > I have a Bridgeport clone and want to fit a stepper to the knee and I
> > am unsure of the size of stepper motor to do the job properly. I was
> > thinking of buying the motors from Arc Euro Trade here in the UK along
> > with their drivers:
> >
> > http://www.arceurotrade.co.uk/Catalogue/Stepper-Motors/Stepper-Motors
> >
> > If it takes 11.5 foot lbs of force to lift the knee and there are
> > 135.581795 newtons centimetre to one foot/pound, and I am thinking of
> > a 6:1 reduction ratio on a toothed belt, what size motor would I need
> > for reliable operation? I am confused about holding torque and moving
> > force.
> >
> > Would I need the stepper 160-010-00500 @ 4.2A 650Ncm or will
> > 160-010-00450 @ 4.2A 350Ncm do the job?
> >
> > Regards, Terry
> >
>
>
>
>
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>