ACME screw upgrade for Shapeoko Z-Axis

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WillAdams
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Re: ACME screw upgrade for Shapeoko Z-Axis

Post by WillAdams » Tue Aug 05, 2014 4:26 pm

Interesting.

Obviously we need to re-work the current Acme Z-axis page so that it's two:

- one an initial overview / landing page discussing all Z-axis upgrade options
- a second for the Acme-specific discussions

(added as a To-do on the wiki)
Shapeoko 3XL #0006 w/Makita RT0701 Router w/0.125″ and ¼″ Elaire precision collets
Nomad 883 Pro #596 (bamboo)

tra
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Re: ACME screw upgrade for Shapeoko Z-Axis

Post by tra » Wed Aug 06, 2014 2:45 pm

cvoinescu wrote:What do you mean, deflects? The Z MakerSlide holds the Z axis in position in the horizontal plane -- the machine does not rely on the screw for that, so the leadscrew should never see any load that would cause it to deflect. The load on the nut should be only axial. If you are referring to, for instance, deflection of the tool tip, there are other parts of the machine that bend, twist and give quite a bit, significantly more than the small amount of backlash in the Z nut.

Assuming you're concerned strictly about the amount of backlash of the Z nut, you can try a few things. The simplest is to deliberately install the nut at a small angle to the leadscrew, as much as you can with the screw still turning relatively easily. The mounting holes should be wide enough to permit some mis-alignment like that, and it should reduce backlash substantially, at no cost. It would require more careful alignment of the screw, though.

If you don't mind modifying your nut, you could make a cut from one of the smallest sides all the way to the Acme threaded hole. This would also cut one of the M5 threaded holes in half; drill out one half to 5 mm, leaving the threads intact in the other half. Mount the nut with the drilled out hole toward the plate. The screw in the other hole is for attachment, the one in the cut hole is for anti-backlash adjustment. You could even use a longer screw with a spring on it.

There's at least one other way of cutting the nut that would give it useful anti-backlash properties, but it's more complicated to describe, and I'd need to test it to make sure it would work.
The lead nut actually moves and wiggles on the acme screw. I can jiggle the spindle up and down and the movement comes from the lead nut threaded over the acme screw. The whole z-sled moves up and down. It's as if the threads inside the nut were tapped for something other than this screw.

tra
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Re: ACME screw upgrade for Shapeoko Z-Axis

Post by tra » Wed Aug 06, 2014 2:48 pm

cvoinescu wrote: If you don't mind modifying your nut, you could make a cut from one of the smallest sides all the way to the Acme threaded hole. This would also cut one of the M5 threaded holes in half; drill out one half to 5 mm, leaving the threads intact in the other half. Mount the nut with the drilled out hole toward the plate. The screw in the other hole is for attachment, the one in the cut hole is for anti-backlash adjustment. You could even use a longer screw with a spring on it.
I'm having difficulty imagining this. I appreciate your help!

cvoinescu
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Re: ACME screw upgrade for Shapeoko Z-Axis

Post by cvoinescu » Wed Aug 06, 2014 3:32 pm

First idea:
nut cut.png
nut cut.png (4.8 KiB) Viewed 2280 times
(described above).

Second idea:
nut cut 2.png
nut cut 2.png (5.46 KiB) Viewed 2280 times
Drill the side hole first, not very deep (6-8 mm), and tap it M3 or M4 with a tapered tap. Cut the nut. Put a M3 or M4 screw (a grub screw, ideally) in the threaded hole, and use it to stretch the nut open. Adjust to take up all the slack that causes backlash.

Ideally, you'd start with a taller nut, so there's more room for the cut. Also ideally, to avoid cracking, drill a hole where the cut ends (before you make the cut). There's no room on the standard nut for that, though.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

tra
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Re: ACME screw upgrade for Shapeoko Z-Axis

Post by tra » Wed Aug 06, 2014 4:17 pm

Genius! Thank you!

cvoinescu
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Re: ACME screw upgrade for Shapeoko Z-Axis

Post by cvoinescu » Wed Aug 06, 2014 5:29 pm

I forgot to say: I have not tested these modifications, and you're chopping up your precious nut at your own risk. :)
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

tra
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Re: ACME screw upgrade for Shapeoko Z-Axis

Post by tra » Thu Aug 07, 2014 6:29 pm

cvoinescu wrote:I forgot to say: I have not tested these modifications, and you're chopping up your precious nut at your own risk. :)
No problem. I think I'm going to go with a modified version of one of the designs located in Will's post. I found some Acme nuts for a couple bucks each, and I have some HDPE laying around. I'm going to take some measurements and get cracking on this by early next week. I'll share what I come up with.

Improbable Construct
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Re: ACME screw upgrade for Shapeoko Z-Axis

Post by Improbable Construct » Fri Aug 08, 2014 2:54 am

I have been working on a design very similar to Catalins second diagram.
If you want to try out some experimental designs I have some "out of spec" nuts I can to send you for experimental use.
Shoot me a PM and I will set it up.
Shapeoko #Classified some of the bolts may be original parts.
Shapeoko 1 # ???? Stainless plates, still in the box.
Shapeoko 2 # 3926 not stock
Shapeoko 3 # 0003
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phinsil6
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Re: ACME screw upgrade for Shapeoko Z-Axis

Post by phinsil6 » Mon Sep 29, 2014 1:41 am

I'm not sure if this is the appropriate place for this but here goes:

Last week I bought the ACME upgrade from the Shapeoko.com Store. When I went to install with my self-machined Nema 23 motor plate and appropriate (1.25" spacers) and 50mm M5 screws, it seemed as though the ACME screw is TOO long. with the screw resting on the bearing at the bottom bearing plate, I went to slide on the Nema23/Motor Plate/Bearing/Coupler/Z shim assembly on top of the screw but the whole assembly hovers above the makerslide top by ~1/4".

I also tried adding the original/thicker z Shim but the assembly still hovered above.

Also, I'm not sure if this is normal or not but the ACME delrin nut seems to have a LOT of play both vertically and horizontally when it is on the screw.

Maybe a quick and easy way of knowing if my screw is too long would be to know what the dimensions of the screw are (bottom machine, main body, top machined).

Any help would be appreciated, my machine is down for the count until I get this figured out.

Improbable Construct
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Re: ACME screw upgrade for Shapeoko Z-Axis

Post by Improbable Construct » Mon Sep 29, 2014 3:33 am

The total length of the screw should be 237mm. The bottom should be turned down 7 mm and the top should be turned down 30 mm with 200 mm of threads in the middle. You may need to cut ~10 mm off of the top of your ACME screw to keep it from hitting the motor shaft inside of the coupler. This is not a critical cut and can be made with a hacksaw or cutoff blade in a Dremel.
The ACME nut, once installed and adjusted, should not have any backlash, even if it feels loose on the screw. If this is not the case please contact me by PM or email and I will send you a replacement nut.
Shapeoko #Classified some of the bolts may be original parts.
Shapeoko 1 # ???? Stainless plates, still in the box.
Shapeoko 2 # 3926 not stock
Shapeoko 3 # 0003
Store:
http://ImprobableConstruct.com
Twitter:
https://twitter.com/ImprblConstruct

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