V-wheel binding and gantry parallelism

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V-wheel binding and gantry parallelism

Postby krumly » Fri Dec 20, 2013 8:14 pm

Apologies in advance if this has all been covered in earlier posts. I am assembling a Hadron from an RRD kit, which BTW was very well packaged and seems like a good value. We'll know for sure when the electronics set up and it's running.

My biggest concern right now is the tolerances tuning of the v-wheels and bearings. It is pretty well impossible to get all the lateral play (wiggle) out of a wheel mounted on a standoff without causing the bearing to bind. This is true for both the fixed wheels on nylon spacers, or the adjustable wheels on the steel eccentrics.

My guess is the problem is that the internal web machined into the v-wheels is a bit thicker than the internal spacer washers. When you tighten the screws enough to clamp the assembly rigidly to a carriage plate, you end up pressing the opposing inner races together and axially loading the bearings. Replacing the nylon bushings with steel will make the assembly stiffer laterally, but it won't eliminate the binding. And bearing friction can't be good unless it's needed as a poor man's dampener, right? Any thoughts?

I also found assembling the x-axis gantry per the Wicki to not work so well. Setting the 2 carriage plates on a surface plate and pressing the extrusion down flat to insure parallelism did not result in a gantry that remained parallel to the bottom rail of the machine frame. It was a bit high on the non-motor end. And I was very careful in tuning each gantry carriage to the Makerslide and insuring the machine frame was dead square. My solution was to set up the right and left carriages for the gantry to roll smoothly and without play on the Makerslide. I then attached the carriages firmly to the gantry Makerslide and set it on a pair of machinist parallels that are equal length. Tighten each carriage completely after this and check that it rolls smoothly without binding. Voila - dead parallel to both the main frame and to the Y carriage.

IMG_1488_lores.jpg


Now onto revising the leadscrews. The 8mm screws are slightly bowed. Even with the screw ends and flex couplers concentric with the motor shaft and Z-nuts, the thing binds horribly. Way more load than I want to subject those steppers to. Plenty of documented fixes for that.

Thanks,

Chris Krumm
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Re: V-wheel binding and gantry parallelism

Postby evil2002usna » Sat Dec 21, 2013 4:04 am

Check these leadscrews out. I made some adapters as well and posted them to thingiverse.

http://www.thingiverse.com/thing:207387
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Re: V-wheel binding and gantry parallelism

Postby cvoinescu » Sat Dec 21, 2013 11:08 am

For the binding V-wheels, first make sure the bearings are seated all the way in. If they still bind, disassemble them, and put one layer of tape (aluminium tape if you have it) on the shim washer, and try again.
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Re: V-wheel binding and gantry parallelism

Postby dave3d » Sat Dec 21, 2013 2:43 pm

I got my Hadron kit from ATI and slightly thicker washers were supplied to go in between the bearings for the V wheels. You could see the difference between them and ordinary washers when put side by side.

Threaded rod is notoriously difficult to get straight. I bought two longer lengths of 8mm stainless rod so I could fit z wobble bearings. However, even though secured at the top, one has a very slight banana shaped wobble. I am in the process of changing it to an Acme or rather trapezoidal thread.

Good idea about using the blocks to get everything square.
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Re: V-wheel binding and gantry parallelism

Postby krumly » Sat Dec 21, 2013 10:01 pm

Thanks for the suggestions on the v-wheel bearings and z - rods. I do have some foil duct tape -sounds like a great solution.
With regard to the lead screws, I think I'll try the 'z-bobble' fix by tmcafferty with my current m8 rods and nuts, and if that doesn't pass muster, go to precision acme rod and nuts. I have a lathe and mill, so re-working stuff is mostly just time.

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