Our CNC Building Blocks are one of the easiest ways to assemble a CNC machine. Follow this forum for information and sharing plans, blue prints and other ways to make your machines using these parts.
By ecurtz
#675
I'm starting to assemble my machine and have received my 3/4" shafts (from Misumi). However they are an extremely tight fit with the bronze bushings in the z-axis. I can move them by hand but it certainly requires more push than seems ideal. My initial impulse was just to take some 2000 grit sandpaper to the inside but a quick search seems to say that's a no-no for oil infused bronze. Any suggestions for this?
By frodeheg
#676
Is it still difficult to move after you oil the shafts? I Guess a thin lubricant would help most?
By bobw63
#679
Sanding the ID of the bearing is generally a bad idea. You may introduce the abrasives into the bearing surfaces which would be bad for the shaft. Machining the bearing can be done. You would have to remove it, fine-finish machine it in a lathe with a sharp tool so as not to close the pores, and not alter the concentricity while doing so. If you can move it by hand you are only talking about a few 0.0001" to make it smooth.

Many bushing-style bearings have their ID influenced by the interference of the press fit. A lighter press fit should equal a larger ID - to a point. Reaming the aluminum with a plus-size reamer to reduce the interference when the bearing is pressed seems like an option. That assumes the press fit will allow some material removal and still retain the bearing afterwards.
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By AJ Quick
#687
Good advice Bobw63.

Do you have the specs on the shafts you ordered? Are they metric equivalents?
By ecurtz
#691
Misumi 3/4" shaft (unplated)

http://us.misumi-ec.com/vona2/detail/110302148420/

Tolerance is given as
0.7490 - 0.7495

EDIT: Apologies, it looks like it's mostly slop in my build at this point. If I take it apart for testing it gets a little smoother each step until it's pretty reasonable on just a single shaft (still not nearly as smooth as the vertical slide on two shafts, but definitely in the realm of acceptable.) :oops:

Any good advice on dialing in something like this? I'm not used to worrying about thousandths of an inch.
By bobw63
#692
When aligning the shafts you have two primary considerations. The first is to have exactly the same center-to-center distance on the shafts as the z-axis block. Second, it should not change or fall out of adjustment easily (ideally, never).

The center-to-center problem is huge with a wood structure. You just can't hold the accuracy of the holes required and even if you did, changes in humidity, temperature, or mechanical load on the gantry could certainly change the center-to-center distance enough to cause problems. Traditionally, router builders would open one hole on either side to "float" one of the two shafts and/or install larger steppers to overcome the additional friction caused by "moving the wood". A simple fix to holding accurate center-to-center distances on a wood-based router is an alignment plate for each side. It would be 1/4" to 3/8" thick, could sit in a nifty pocket milled into the gantry side, and be held in with a few wood screws.
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Now the bad news about this solution. I am a supervisor in a shop with CNC vertical machining centers. The AQuick z-axis block is a good piece of machining that I would not do myself given the attractive price. Having said that, any small deviations in alignment or concentricity from the left and right bearings on either shaft will cause the center-to-center distances to be ever-so-slightly different. My alignment plates caused a slight binding on one side when the z-axis got to within 2" of the left gantry alignment bracket. Thinking it was my machining accuracy, I swapped the left and right alignment plates. The problem remained on the left side. It is only a few 0.0001".

The ultimate solution would be a set of alignment plates where each plate consisted of overlapping joints. The top half would be permanently attached. The bottom half would be locked into place against the top half using the z-axis itself as an alignment fixture. No measuring, no fuss.
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The AQuick rod holder thingies I see in the pictures of the plastic version would allow adjustment for small deviations from side to side. The benefit of the above alignment plate halves being locked together should keep the center-to-center distance the same even if the wood does not cooperate.

Note - I am aware the bracket drawn does not allow for lead screw clearance. I did something different for the lead screw mounting. Slight modifications would allow for conventional mounting of the screw.
By ecurtz
#693
Thanks for the excellent feedback. Luckily my machine is aluminum so not quite so complex. I think something is probably just minutely out of alignment, so I'm planning on getting some better tools and precision shims and making sure I get everything square and true.

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