X & Y Axis - Use belts or use threaded rod???

I am thinking about my design and have a few questions...

I presume the by turning the threaded rod it is a finer adjustment than the belt drive system. Is this correct?

Is there any reason or disadvantages why the X & Y axis cannot use the method of turning the threaded rod (similar to the way the Z axis moves) to move the carriage?

The Mendel design uses a complicated set of bearings to reduce friction on the round bar whereas the MakerBot just uses a sleeve (PTFE?). The sleeve seems a simpler and cheaper mechanism. Is the Mendel system better?

Why I have asked these questions is that I was wondering if I could use a simpler mechanism for each of the X & Y axis. So each axis would have 2 rotating threaded rods and each rod would have 2 nuts to the carriage, so the carriage would have 4 points which would be fastened to the threaded rods by 4 nuts. Do you think this would work successfully?

TIA

I believe the belt drive for x and y is for speed. Threaded rod would be very slow.

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I presume the by turning the threaded rod it is a finer adjustment than the belt drive system. Is this correct?

Yes. With threaded rods you easily get better than 0.01 mm accuracy, with a belt in Mendel's design about 0.15 mm.

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Is there any reason or disadvantages why the X & Y axis cannot use the method of turning the threaded rod (similar to the way the Z axis moves) to move the carriage?

With threaded rods (1 to 1.5 mm pitch) you typically can achieve 1000 mm/min or 16 mm/s. With belts, you get about 25 times more speed (and 25 times less pushing force). Good extruders, as far as I can interpret postings in this forum, can feed up to some 40 mm/s. So with threads you're limited by the max. rpm of your stepper motor, with belts by the capabilities of your extruder.

To get the best of both, you'd have to use ball screws with 5 or 10 mm pitch, but these are expensive.

As always, the choice depends on your preferences :-)


Edit finds I should hit "send" _after_ completing the message.

Edited 1 time(s). Last edit at 08/14/2010 11:11AM by Traumflug.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

The real issue with X & Y being rod driven is inertia. Think of it this way, if you are doing a .5 infill with square pattern, you need your X & Y to be able to move at 25mm/s + for .5 seconds, reverse in less than .1 second, then go 25mm/s the opposite direction for another .5 seconds, thousands of times with out EVER loosing even 1 step.

Now pick up your drill, try to do the same thing with your cordless drill.

Don't be scared of the belt, it's cheap, we have scripts that can generate your gears.

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Thanks for your answers guys. I am learning a lot here.

The Darwin design used sleeves instead of ball bearings. They recommend using printed sleeves, but I found that nylon bushings work best. They are plenty low friction and they don't wear down. PTFE would be better, but cost more.

The Darwin has larger motors than the Mendel and the Makerbot pushes around less weight than the Mendel. So, I'm not sure if you will be able to handle the extra friction created by sleeves with the typical Mendel motor recommendations.

There are some improved Mendel designs that reduce the number of bearings. I'm not sure where they all are, but here are the Z-axis parts: [www.thingiverse.com] [www.thingiverse.com]

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
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