Using threaded rod as a Rack

Mostly Printed CNC - MPCNC Hardware Development
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Was thinking about belt elasticity, and had an idea (Trying and finding if it’s feasible) : How about using a high diameter threaded rod as a Rack, and 3D printing the corresponding pinion ? I bought yesterday an M18 (wanted M16 but they hadn’t them) rod, designed and 3D printed a “hobbed gear” to go with it, and it seems to work well. Now, I’m trying and find the best way to experiment with it without demolishing my MPCNC at least for the first stage…

 

Any one has tested that ? Any interest in the idea ? Any major probmem you see ?

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It could work. If you do see stretch issues (Jamie just did some basic testing, I hope to add to it), you can simply go wider. Gears wear out and have slop/backlash/constant changing tension/not good in a dirty environment. Belts are very efficient.

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With helical involute teeth, then a larger rod is a better approximation to a slanted rack. But what you’re describing is really a worm gear, and you don’t need a huge rod for that to work. It’s not the usual way of using a worm gear but it should work the way you describe.

If your pinion diameter is relatively small you might find finer teeth/threads matter (not sure just guessing) and that might not be available on large diameter bolts. But then fine worm gears might not be easily printable either. But then again maybe you can just buy a worm gear if the threaded rod is a common one.

I would preload radially if possible to reduce the play/backlash.

Can you turn the rods to square the machine? That might be interesting.

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Souds like a clever idea.

It’s not really necessary for the MPCNC since the belts actually work fine, so I’d advise you not to go through all that trouble for what will very likely give no noticeable difference (time is better spent on your CAM-fu), but I’ll keep this in mind for other projects. :slight_smile:

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Yesterday, I was thinking that a worm gear would be almost what I wanted, but today, with your comment, I realized that it was EXACTLY what I needed. Now to model one… (Got an “almost one”, but still far from exactly what I need)

 

As for preloading, I want the rods to be completely fixed, and have something put load on the motor against the “rack”. I think that would do.

And turning the rods to square the machine is also in the plans (if I go and do it at all).

 

 

 

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Threaded rods or allthreads as they also are called are commonly available in hardware stores.

I buy them in 1 meter lengths here in Sweden, galvanized and stainless and various threads (I mostly use M8)

They are for example used on many 3d printers for Z axis…

The quality of them (precision) is not always top notch - so I run either a few nuts along the thread or even better a cutting die to smooth it out.

As many are not perfectly straight some sort of floating arrangement could be handy.

The slowest axis on MPCNC is Z (T8 Leadscrew) …your machine will be slower on X and Y compared to belts and that is direct drive from motor.

M8 is (1.25mm) * motor RPM = top speed in mm

But I just thought…using them as racks…you will not turn the threaded rod to get movement?

is this something that you want?

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I found it on youtube, working threaded rod r&p pcb plotter (on all three axes).
At first glance, it may have been working for a long time.
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