Alternative Z-Axis Screw Replacement

discussion of design changes / improvements / suggestions
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Hans
Posts: 290
Joined: Tue Dec 25, 2012 5:19 am
Location: Connecticut, USA

Alternative Z-Axis Screw Replacement

Post by Hans » Mon Feb 18, 2013 11:26 pm

I was struggling to do 3D carvings with the stock Z-axis because I would miss steps occasionally if I ran a total feed rate greater than 35 ipm or an acceleration of greater than about 20 mm/s^2. I didn't want to pay $75 for Improbable Construct's Z-axis replacement and I found a 1/2"-10 screw with bearings and jam nuts for just $22 on ebay http://www.ebay.com/itm/9-1-2-10-ACME-l ... 0995882296 so I gave it a whirl. A 1/2" screw won't fit where the old screw did, so I had to run it between my dual X-axis slides and the offset makes the assembly less stiff. I cut my own top and bottom plates from 1/2" cutting board after running a simulation that indicated I'd only have about 0.0006" deflection for every 10 lbf. load in the Z direction. I had extra slide left over because I've lengthened my Y-axis so I cut it to make a new Z slide, but I think this screw could be used with the old slide with proper plate design. The new nut, which is nice and snug, is bolted to a plate on the bottom of the X-axis trolley. I had to drill the screw through some steel wool and lubricate it with some bike chain lube that leaves behind a dry film of PTFE to get really smooth action.

Anyway, I went from 35 ipm to 90 ipm (2.6x) top speed and 20 mm/s^2 to 400 mm/s^2 (20x!) max acceleration. Video demonstration of before and after speeds: http://youtu.be/PRgJdWQm3lc. That's quite a substantial improvement and I'm very happy with it. The assembly seems as stiff as the simulation predicted, though it doesn't really matter because there's still the ol' twist about the X-axis to deal with. Normally, this design would allow one to cut out 0.25" from between the Z axis slide and the plate it mounts to because the screw no longer runs between them, a modification hopefully reducing said twist. But, I can't do that yet because of the way I bolted together my X-axis trolley (big 3/8" bolts with their heads in the way).

WillAdams
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Re: Alternative Z-Axis Screw Replacement

Post by WillAdams » Tue Feb 19, 2013 11:55 am

Interesting.

The same vendor has 3/8" now, in 9 and 12" lengths. was there a specific reason to go up to 1/2"?
Shapeoko 3XL #0006 w/Makita RT0701 Router w/0.125″ and ¼″ Elaire precision collets
Nomad 883 Pro #596 (bamboo)

Hans
Posts: 290
Joined: Tue Dec 25, 2012 5:19 am
Location: Connecticut, USA

Re: Alternative Z-Axis Screw Replacement

Post by Hans » Tue Feb 19, 2013 1:12 pm

It was an impulse buy, I hadn't yet noticed that it wouldn't fit. It's possible that the larger bearing surface in the nut leads to smoother operation under acceleration, but the top speed is a function of tpi and controller pulse rate. At this point, my limiting factor is the grbl's max instruction rate. Only about 10 lines are interpreted per second (or at least i think that's the problem), giving slow, jerky motion in small areas of fine detail. TinyG would have been a better choice for my application for higher feeds, proper full-current dual y drive, and better fine detail smoothness.
Shapeoko #1,123

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