Question about gear reduction on small belt machine

[rlrhett]

With the help of another member, I have been trying to design a belt drive system for a relatively small router table (24x36x6) but am having difficulty with the gearing. I have never seen a post with exactly how much gearing down one should do, or how to build a bracket for the motor mount and gears, etc.

If I have a 1" diameter pulley, for every full revolution the belt will advance 3.14". If I am using 1/2" x10TPI Acme screw, one revolution gives me 0.1" of advance. Even a 5 start screw will move a nut 0.5" per turn, WAY less. So I can't just turn a pulley with a motor if I want comparable gearing.

I would think there would be two problems: resolution and torque.

If I am not mistaken, a 1/8 microstep setting on your motor would give 1600 steps per revolution or about .002" resolution. Most people want more resolution. I personally think I would want more like .0005" resolution, which is a little more coarse than the 5 start screw. That would mean having my motor drive a 1" gear that drove a 4" gear. The 4" gear would be on the same post as another 1" gear that drove the axis belt. For a dual drive gantry, that would be six gears and four belts for the x-axis and three gears and two belts for the y-axis. Yikes! That is a lot of gears to buy and mount and a lot of belts to tension.

Also, even though I have not seen it discussed, if I have the usual 305oz motors and am satisfied with .002" resolution, wouldn't I have about a 10/th of the torque as compared to the 5 start screw (if I don't gear down)? For a wood router system cutting through hardwoods, would this be enough torque to push a router?

What are people's suggestions for using belts but keeping it as simple as possible? I am hoping to avoid some Rube Goldberg combination of pulleys and belts. Does anyone drive an axis without reducing? Anyone successfully drive the belt directly, without other gears and reducing pulleys? Is using belts just not feasible for a small system?

In case anyone was wondering, the reason we were looking at belts is because I am interested in building a machine that will do a fair amount of surface carving. With a lot of surface to cover, speed is important. Relatively cheap belts is also appealing (although the cost of so many gears may cancel any savings on the belts).

Thanks,

[ger21]

If 5 start acme won't get you enough speed, look at 1/2-8 8 start acme. It's about the same price as 1/2-10 5 start at Mcmaster carr. If you want at least .0005 resolution, then go with the 5 start screws. Every time I take a serious look at belt drive, the complexity always seems to steer me back to screws. One advantage belts have is efficiency, which is probably a little better than acme, but the 5 start and higher 1/2" acme is about 70% efficient, which isn't too bad.

Without changing the motor, increasing speed will usually be at the expense of resolution and force. If you're using steppers, the only ways to gain both speed and resolution would be more microsteps, or try to spin the motors faster and use less gearing or lower lead screws. Spinning the motors faster requires the right combination of motors, drives, and power supply. If you want both high speeds and high resolution, then it may be time to look at servos.