Re: [CAD_CAM_EDM_DRO] Building a small 4th axis

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