How to figure needed stepper motor size?

[Hack]

I am currently in the process of building a 3 axis router with approx a 32" x 48" x 6" cutting area. MDF based with a little aluminum bracing, threaded rod for screws, basically a cheap machine. What I am have a problem with is deciding which size stepper motor I should be looking to buy to give me adequate performance. I was looking at a 3 axis kit with 116 oz steppers, but don't know what kind of speed I can expect. I am not looking for blazing performance, just dont want to listen to my PC laminate trimmer scream forever if i dont have to. Any suggestions?

[sendkeys]

[sendkeys]

[arvidb]

Originally Posted by sendkeys with a threaded rod sounds like theres going to be alot of firction to get over. i would think you have to use pullys/belts on the steppers. *snip*

It is true that threaded rod has low efficiency (therefore requiring perhaps three-four times input power compared to a ball screw for the same output power). But they also have very low lead (= many turns per inch). This acts to "gear down" the motor, thus lowering the torque demand from the motor.

A threaded rod with 20 turns per inch and efficiency of 25% (guessing) actually has about the same torque demands as a ball screw (90% efficient) with lead of 5 mm (~5 turns per inch) for the same load.

The threaded rod will only move your load 1/4 as fast, though (and with a lot less accuracy!).

Arvid

[ESjaavik]

Let's assume you can get a suitable ballscrew on CNCZONE or eBay for $100, you probably can get it cheaper. Let's also assume the alternative is free, like it was already on the machine. If you tighten down an acme screw until the backlash is as small as possible. Remember that's not zero, as there have to be space for lubricant. Then your efficiency will be 25-40%. With a ballscrew it's 90-95%. This means you can get away with a smaller motor, drive and power supply. If you can save $100 on these parts, you have the ball screw "for free".
Then what do you get? Zero backlash, makes it possible to mill a climb cut that give a much smooter result. A climb cut also lowers the needed torque. The smaller motors also weigh less = less strain on guides and again lowering needed torque. Keep on, and soon we don't need any motors!
But rather buy good used screws with preload than new without. With normal hobby use even somewhat worn ballscrews will be better than acme. I've worn down many acme screws just in hobby use, and I believe if I can keep the ballscrews free from dirt, they'll last as long as the rest of the machine(s).