Re: [CAD_CAM_EDM_DRO] Building a small 4th axis

Hi Tony,

Not exactly the same thread, but I like these "symbiotic" ideas! A few
days ago I saw a Sherline rotary table w/stepper being used to drive the
Sherline spindle on a lathe for threading (and what not). Probably on
Bryan's web site. Seems to me that a gearhead stepper motor (some
Vextra's I picked up come to mind) could be used quite easily the same
way. This would allow any CNC controller program to turn the spindle in
direct ratio to the lead screw, which is what's wanted. The other way
is to add an encoder to the spindle, "Sync" and drive a leadscrew off of
that via software.

Comments?

Alan KM6VV
(new Sherline CNC mill)


Tony Jeffree wrote:

>
> At 13:37 04/06/2003 +0000, you wrote:
>
> >I have been enjoying milling small 3D parts on my Proxxon very much
> >and have been thinking about making a 4th axis for it.
> >
> >I have a 4th axis stepper driver so that is no problem but need a
> >small rotary table. For starters and to learn the process I will
> >probably try milling some foam or wax mounted with double sided on a
> >simple plate mounted directly to the stepper motor shaft. I will
> >only have 400 "degrees" resolution but it will get me going.
> >
> >Ultimately I want to build a tiny rotary table possibly still without
> >chuck that will give finer angular resolution. I have some Nema17
> >and Nema23 steppers that would seem to suit the task on this tiny
> >mill but no real ideas for the rotary part.
> >
> >My questions:
> >
> >1. What sort of angular resolution is recommended for general as well
> >as perhaps gear cutting processes?
>
> Commonly used worm ratios in dividing heads/rotary tables are 40:1, 60:1,
> 72:1, and 90:1. With a 400 (half-) step per rev motor, 90:1 gives you
> 1/100th degree per step. Given the choice, I would go for 72:1 0r 90:1;
> along with the higher resolution, the greater mechanical advantage means
> more usable torque, meaning heavier cuts are possible without stalling the
> motor.
>
> >2. How much of an issue is backlash? Can this be avoided by rotating
> >in one direction only?
>
> The short answer is "it depends". Most rotary tables provide a means of
> adjusting the engagement between the worm & wheel, thus allowing most (if
> not all) of the backlash to be adjusted out. In reality, a small amount of
> backlash will still be present unless you take steps to remove it
> completely by other means (there are antibacklash worm drives to be had,
> for example, where the worm wheel is split & one half is spring loaded
> against the other). However, for many purposes, a small amount of backlash
> is not a problem. For example, I cut clock wheels on a home-built CNC
> dividing head that has a small amount of backlash, but not enough to affect
> the resulting part. See:
>
> http://www.jeffree.co.uk/Pages/divheadmk2.html
>
> >3. Is a simple worm gear ok?
>
> Works just fine.
>
> >4. Are there any gearboxes already available at a low price that will
> >fit the bill?
>
> You can use standard reduction gearboxes sold for use with electric motors
> - these come in a variety of reduction ratios. Generally they don't provide
> any backlash adjustment though.
>
> >I think that's all I need to know but any tips appreciated.
>
> If you're going down the self-build route, I would recommend using ball
> races for the worm shaft and the table spindle, rather than plain bearings
> as are often used in the cheap end of commercial rotary tables. I have a
> Sherline 4" rotary table built that way that only takes about 50 oz-in to
> drive it; I also have a similar sized Vertex table that has plain bearings
> everywhere, and needs 150-200 oz-in to be usable.
>
> Regards,
> Tony
>
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