Timing belt reduction, which to use?

[phantomcow2]

Rather than go and buy a rotary table and mount a motor for a rotary axis, i am making one.
I've got some super stiff 50mm ID taper roller bearings i will be using. The setup is a lot like the headstock of a lathe, except much smaller. This is for a Sieg x1 for size, except with the longer table.
Anyways, i was going to have this thing run by a stepper motor that is 400steps/revolution. And then a timing belt reduction from the stepper shaft to the spindle of the rotary axis.

I will be using MXL timing pulleys here, but im not sure which ratio i should be after. My thoughts were an 18T on the stepper, and a 72 for the spindle itself.
Does anybody else have some thoughts?

[MrBean]

I'd go for 16t :72t
This would give you 0.2 degrees each step, rather than your 18t giving 0.225

Not that it's really much different. It just seems nicer to work in round figures.

Is your design idea similar to this:

Regards Terry.

[One of Many]

Are you planning on a direct belt drive to hold cutting forces?

Common RT's have a high ratio through the worm wheel. This also helps keep the table from being driven by the external forces applied to the work being done.

The belt ratio should be based on what you would expect in resolution for each step. A system scaler parameter can take care of this too, if the steps far exceed what is expected. The belt ratio would be of less importance in that case.

DC

[phantomcow2]

I like the 16:72 idea. I just figure, you look at a lot of these rotary tables and they have some serious resolution, so i would like that for my rotary axis.

Yes my idea is very similar to that picture. Except my bearings are super stiff 50mm. THese things look like they could support a truck, way the hell overkill. But, its what i've got