ACME Anti-backlash nut

discussion of design changes / improvements / suggestions
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samc99us
Posts: 269
Joined: Mon Nov 11, 2013 5:20 pm

ACME Anti-backlash nut

Post by samc99us » Tue Mar 17, 2015 5:08 pm

This is a pretty simple mod that eliminates the 0.005" of backlash common in the ACME z-axis setup.

Required parts:

1) Qty. 2x ACME Delrin nuts
2) Qty. 2x M5 screws, I think I used ~25mm long fasteners
3) Qty. 2x compression springs-I bought a whole variety pack from my local Ace Hardware for $7.

Attach both delrin nuts together in the orientation they would get installed on the ACME rod w/ some tape. Drill a hole on each side of the ACME threaded hole. On one nut this will be a through-hole, and the other you will want to stop shy of the attachment holes that normally connect the nut to the z-axis carriage. Untape the two delrin nuts from each other. Open up the holes to allow a M5 fastener to go clean through the nut you drilled through-holes in. Tap the other nut to allow the M5 fastener to thread into it. Screw both Delrin nuts onto the ACME drive screw. You want some space between the nuts, about 1/4" or so. I put the nut with the through-holes on top of the nut with the tapped holes. Go down to your local hardware store and pick up a package of springs ($10). Drop some M5 bolts down through the top nut, and insert a spring of medium-light compressive resistance between two Delrin nuts, allowing the fastener to pass through. You want one spring and one fastener on each side. Tighten up the fasteners gently-this takes some tweaking, too tight and there is too much friction, too loose and effectively the second Delrin nut isn't doing anything.

Now re-assemble the z-axis. Only one of the delrin nut gets attached to the z-axis carriage, as per before. The trick is getting enough clearance in between the spindle mounting locations to allow for the second delrin nut. I had to move one of my spindle mounts lower and use those holes for the delrin nut, so there was enough vertical clearance.
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Modified Shapeoko 2: 1500mm fully supported y-axis, 1000 mm 4060 x-axis, ACME Belt Drive Z-axis, Dewalt DNP611, full aluminum t-slot table, 4x NEMA23 180 oz/in stepper motors driven by a G540

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