Lead screw for X and Y

I've been looking at doing a new build with extruded aluminium, but have some other ideas. It was while looking at the OpenBuilds site that I saw all the nice trolley connected to 8mm lead screw builds. I can see both benefits and drawbacks to this design.

Is anyone using a lead screw for X and Y? Can the NEMA's cope? Can you get decent mm per second? I've not seen any geared implementations, they all seem to be directly coupled to the shaft.

... screws are much more precise than belts, but much slower too ... my fastest CNC-mill/repstrap/laserbase has 16x5mm screws (16mm diameter, 5mm travel per turn) and with NEMA-23 sized servo-motors with 2000-ish encoders it can reach 150mm/s max. with 0.0025mm resolution ...

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You can pretty easily find lead screws with ~25mm travel per turn, which would make the speed comparable to using belts. (see roton.com) Lead screws are more precise because they are stiffer. However, the added stiffness can play havoc with steppers because they transmit more vibration which can cause the steppers to stall sooner. On the other hand, lead screws work really nicely with servo motors. Leadshine makes a closed-loop stepper controller which essentially turns a standard stepper into a servo motor. I've also built a closed-loop stepper controller board discussed here. Using a closed-loop stepper is nice because it is a drop-in replacement for a standard stepper. In some of my testing using lead screws, I was able to go about twice as fast with closed-loop control v. a standard stepper driver. Using belts, though, there wasn't a big difference in the max. speed obtainable.

I need to look into servos, just to better understand them. My experience of servos is radio controlled... things... where they move through a defined arc, I had no idea they could turn fully, repeatedly. With the 20/20 hindsight, now I think about it, they were geared inside, and the gearing should of suggested they could go round more than once, but I still have that, limited range thing going on.

I'm thinking belts are still the way to go in the short term for me. I'm changing a lot of variables at once and need to limit that until I've got greater stability. I'm actually printing pretty well. It's reliable (until I fiddle with it), which I think is testament to the simplicity of Reprap.

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UkIan
I need to look into servos, just to better understand them. My experience of servos is radio controlled... things... where they move through a defined arc, I had no idea they could turn fully, repeatedly.

Hobby RC servos are a little different from what I'm talking about. They are limited to less than 1 rotation (unless you disable the position feedback). I'm talking about servos that use a continuously rotating encoder for position feedback so that you get unlimited rotation.

you will want to run a setup like what is mentioned here with a dsp controller, or software such as mach3, or emc. the smooth acceleration required for velocity requires precise control that the current generation of firmware and reprap software can not deliver. the software and firmware has improved dramatically but it is focused on belt driven systems for how it is optimized in performance. there are micro pauses that the firmware can compensate for by decreasing extrusion, with assurance that inertia force with keep the motion linear. with lead screw there would be micro stops, and possible jamming. speed will greatly be reduced, and cost quite expensive because you will want materials that do not have high thermal expansion, or wear. otherwise the benefit of precision is a myth. then of course you have accuracy within the extrusion, that is only so precise, and the theoretical limit of 0.1mm resolution accuracy in x,y.