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Wobble-Free Z Knob

Posted: Tue Apr 21, 2015 4:12 pm
by pyronaught
I've always been annoyed with how the Z knob wobbles as it spins due to the set screw being driven between the threads of the ACME screw and pushing the thing off center. No matter how I try and adjust it, I could never get it to sit perfectly centered on the screw. I've tried redesigning the knob with a longer neck that fits down inside the recess to try and get more of the screw to be in contact with the knob to fight the cockeyed forces being applied by the set screw, but no luck. It helped a little, but still wobbled as it spins. It just doesn't look right to have this machine that is built from all these precision parts and then there's that damn knob making it look like the shaft is bent.

So what I finally did was ditch the collar and set screw altogether and just made an ACME thread inside the knob that exactly fits the screw. Since you only ever need to turn the knob clockwise, it doesn't matter that turning it counter clockwise would remove it. I just had to get the threads tight enough, otherwise the knob builds up enough angular momentum during the platform drop at the end of a print that it can spin itself off and dance around the M2 like a top.

Because the threads are intentionally tight, you will need to wedge a long object like a wrench between the build plate carriage and the top bearing block when screwing the knob on in order to keep the screw from spinning and raising the platform. It's not a good idea to raise the platform all the way to the top and let it crush against the limit switch when doing this, just stick some object in there to brace against.

Since the lock key is the kind of thing that will always get lost if left to loosely lay around, I designed it to snap onto the lock nut that sticks out the side of the bearing block.

The attached zip contains the original CAD model in Rhino 4 and IGES formats, with two versions of the knob (one with the lock key holes on top and one without), and also a version of the knob prior to joining all the geometry into one object so that individual aspects can be tweaked without having to rebuild the whole object. The ACME thread plug is also isolated separately so that you can Boolean it into your own knob models.

Print Settings for ABS:
coast and wipe turned off
support turned off
50% infill
Layer height = .2
Outline direction = inside-out (important because you are printing threads without support)
Extrusion multiplier = .95
Extrusion width = .46
(882.63 KiB) Downloaded 150 times
(4.69 MiB) Downloaded 130 times
z_knob.jpg (188.35 KiB) Viewed 3507 times
z_knob1.jpg (433.73 KiB) Viewed 3512 times
z_knob2.jpg (334.2 KiB) Viewed 3512 times

Re: Wobble-Free Z Knob

Posted: Tue Apr 21, 2015 4:45 pm
by PcS
Thanks ! Grabbed um both !

Re: Wobble-Free Z Knob

Posted: Tue Apr 21, 2015 5:44 pm
by Jules
Yup! Me too! Many thanks! :D

Re: Wobble-Free Z Knob

Posted: Tue Apr 21, 2015 6:32 pm
by Bratag
I thought I was the only one this bothered. Thankyou for this.

Re: Wobble-Free Z Knob

Posted: Wed Apr 22, 2015 12:08 pm
by PcS
Thanks again.

Re: Wobble-Free Z Knob

Posted: Thu May 14, 2015 9:14 pm
by TS3g
I just bought an M2 kit, got it all put together last night, will be firing it up tonight. I noticed the wobble as well, so this may be my first functional print after I get it dialed in. Thanks!

Re: Wobble-Free Z Knob

Posted: Tue Jul 28, 2015 8:44 pm
by loiphin
Thank you Pyronaught!!!

This annoyed the hell out of me, as the rest of the machine is built so precisely, and then Makergear do a crappy job on the knob and it catches my eye every time I look at the M2.