hello world.
Im building a cnc rotary table. Im gonna use a stepper motor I guess. didnt know if anyone who had done this had any suggestions or preferances on weather or not to make it a direct drive or a belt drive and where if I chose a belt drive. could I purchase the timing gears and belts. anyone know a good  link.
thanks

Check out these folks. Stock Drive. They even have a design wizard. They are also listed in the links under machine parts.
http://www.sdp-si.com/

Brett

anyone have any thoughts on what is a good ratio for the gears. 3to1? and a good stepper motor size for an 8'' rotary table?

The gearing is going to depend on the resoloution you want. Are you building a table from scratch or do you have one you are going to hang a motor on? If so, how is it geared now? Most of the ones I have seen have 90-1, 100-1 neghborhood. If this is the case, I would gear it up so you don't get into trouble with the fact of 45KHz limit. Depends a lot on how fast you want it to rotate and if you are microstepping.

Brett

wellll right now. I have a 8'' phase2 rotary table that Im thinking of converting. thats about as far as Ive gotten. I want to cut steel and aluminum witht this thing. doesnt have to be insanely fast as Im not running any kind of production. just has to be accurate and reliable. Id like the resolution to be a little on high side just becsause im gonna be using it to cut helix's  and not so much for indexing. so if you have particular recommendations as to a particular motor or setup. were dealing with a blank slate hear.
thanks.
oh yeah the gearing is 90-1

if i read this right, the table has a worm drive with 4 degrees to one revolution of the input, assuming 200 steps per rev thats a resolution of just over one arc minute or 50 steps per degree (full step). if your drivers do 1/2 step that 100 steps per degree.  i think thats about the small same as the sherline rotary table. just a guess but i.d say direct drive cause the slop in a belt system is more that your reaolution.

does that make since

hmm sounds about right.  Im looking at gecko drivers now. I see some that do half steps. not sure what micro stepping is. is that a finer step than half stepping?

yes in full step the driver only turns on one coil poll causing the rotor to align dead on that poll (one full step between polls).
in half step the driver can turns on two coils at a time which would allow the poll to align midway between two coils (HALF way between). Microstepping is a subset of that.  Instead of turning on two coils with equal force it might turn n one coil at 25% power and the next at 75% power causing the poll to align off center of the half step position.(even better resolution.

Thats a simple explanation but you get the idea.  there is a point of diminishing returns (system friction may be too much to overcome on the smallest increment).

Basic engineering principle - everything is a tradeoff - you just gotta decide what to trade..