Hi All, first post here. Have been CNC for a few years now and decided to build myself a new machine from scratch. Have been educating myself and in the area of motors and drives, had an unexpected observation. Like many others, I too am attracted to the import lead screws for their economy. In the speeds for me, 5mm and 10mm lead screws need to turn at 500-1000rpm. So I start studying stepper torque curves and see the dramatic reductions in torque and then see that many DIY builders incorporate timing belt drive in an attempt to operate the stepper in a range of higher torque. So I started designing a drive to incorporate such but as I was comparing the (torque) benefit belt vs direct drive, I was surprised to find none.

I considered 3:1 timing pulley gain from stepper to screw. The 3:1 ratio means the torque at the screw is now 1/3 the stepper shaft just due to pulley ratio. It turns out, my stepper torque was about 1/3 at 450rpm, so there was absolutely no (torque) benefit in drive screw motive force compared to direct drive. In some cases, the belt driven screw still offers some packaging convenience, but not performance, unless possibly you have more dramatic gain/reduction through the transmission. For grins I checked at 6:1 and there was still little to no gain with either approach. In fact, the reduction instepper torque seemed to track the inverse gain/reduction through the timing pully ratio throughout the speed range. The same held true if you are stepping down the stepper speed. Although you now get an increase in torque delivered at the slower turning screw, the stepper suffers a similar reduction in torque at the higher operating speed, so little to nothing is gained.

Now, if you have a (more) constant torque motor/servo, all bets are off, and this of course is not the case. This doesn't consider any benefits (or detriments) for resloution, but with microstepping there are many options and the affects seem somewhat subtle as far as torque goes. I've also been cautioned about the potential for lead screw whip at higher speeds but it's hard for me to see that as much of a concern with 20mm screw andmax unsupported length of say 750mm.

I guess the moral of the story is: Power is the rate at which you make torque and once you set the speed of the lead screw you are not going to alter the motor power with a transmission, and in the case of steppers, my observation is neither the delivered the torque, yet I see all these DIY CNC routers equipped with belt drives for this reason. What am I missing?

It was sort of a deflating revelation for me. I've opted for 10mm lead screws and at the moment am planning to direct drive everything except possibly one axis, and that would be for packaging, not drive performance.

Best,
Kelly