Shorten lead screw while fixed to coupler and motor shaft?

Hi everybody,

I’ve finally 3d-printed and sourced most of the parts - except the electronics -, and have started assembling my mpcnc.
Unfortunately, I’ve forgotten to shorten the lead screw of the z-axis (currently 300mm) and I’ve already fixed it to the 5-to-8mm-coupler and motor shaft. Since, I’ve applied Loctite to the small set screws of the coupler, I don’t necessarily want to loosen them again.
However, the length of the lead screw should be the default 131mm for a workspace z-height of 81mm.

I plan to carefully shorten it while leaving everything connected. Now does it have to be exactly 131mm long, or can it be a little longer (ca. 5 to 10mm)?
I’m asking because it’s nearly impossible to get an exact measurement without taking it out of the coupler.

My very hobbyist answer would be that it’s easily doable. It’ll be very simple to eyeball and cut when it’s getting inserted in the gantry. You’ll see very easily what lengths you’ll need when everything is put together right in front of you.

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Longer is not a functional issue. The only limit is that the lead screw should not extend past the bottom of the Z tubing. If it does, the lead screw will hit the working surface before the nut on your router does. Longer lengths do have one minor downside…increased mass at the core. Any mass at the core needs to be accelerated and decelerated, but an extra 5 or 10mm will not be noticed.

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I ended up making my shaft a good 10mm longer than necessary in case I messed up my threads when I cut it, so I would have some play room to trim it down if necessary. I have had zero issues with it, even when my router is all the way down.


Noted. I’ve been following some of your threads here for a while now. Very inspiring stuff!

Yes, I probably should have dry fitted the z-axis and core, to noticed that myself. :wink:

You’re right, the 300mm long lead screw adds a lot of weight to the core assembly. The shorter the better, seems to be the way to proceed.

I see. Even if you mess up the threads, where you cut the lead screw, it shouldn’t matter right, since it’s a little longer and while travelling up and down in z the machine will never reach that spot?

Thank you guys for your replies!! They’re much appreciated.

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Correct. I only needed the threading to be good enough to go through the mating nut that is screwed to the top of the core. From there it doesn’t matter because it will never get back to that nut, unless I disassembled it.

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Yes, I’ll leave the T-nut on, when I cut it, just to be safe. :smiley:

Let your machine do the work. Either mount a side grinder appropriately and move the lead screw carefully towards it, or let the Z-stepper jog the thread and mount a hacksaw in a custom clamp and let it press slightly to where you want the cut to be.