Wednesday, April 4, 2012

Low Cost CNC Machine Part II - A half-finished translation stage

Following up on my previous post regarding a low-cost cnc machine I have begun the design process and have a half-completed linear stage.  This will form the x-axis for my machine which the z-axis will be mounted to gantry-style.  The y-axis will mount to the machine bed, keeping the total amount of moving metal minimized in the interests of stiffness and acceleration.

Here is the (half-finished) result.  The carriage will run on 8mm linear shafting.  This is pretty light-duty, but cheap and easy for prototyping.  It will probably need to be replaced on the final machine with 12mm or 20mm shafting.  The shaft mounts are made from 1/2"x1.5" bar stock and clamp using one of the two mounting bolts.  This allows the shafts to be removed without totally loosening the mounts.  I started by prototyping on the Makerbot at VHS, and finally got the bar-stock cut at Metal Supermarkets, doing the drilling with the drill-press at VHS.

I have been very happy with my choice of mounting all components with 1/4-20 bolts on 1" centers.  The base stock is 1/4" aluminum plate, with drilled and tapped mounting holes.  It would also be possible, and much more accurate, to use optical breadboards, however these would severely limit the acceleration and feedrates attainable, since they are generally 1/2" thick and twice as heavy.  For now I'm sticking with the 1/4" plate.I

The bearing mounts are 3D printed for now.  They will eventually be made from 1.5" square stock cut in 1" lengths, which is just long enough to hold the LM8UU linear bearings.  The bearing is mounted 1" from the base, following the convention for the shaft-mounts, in order to provide a half-inch of clearance for bolt-heads. I had originally intended to make these out of aluminum, buying the square stock and having it cut, however when I got home I had a terrible surprise:

The prototype mount is shown with the cut aluminum, guess the pieces were cut incorrectly.  In the bag are the other 11 cut pieces that won't fit.  I don't know if it's my mistake or Metal Supermarket's, but either way the pieces are useless to me and I have a lot of them.

I may eventually switch to brass bushings similar to my Shapelock linear bearings to keep the noise down when 3D printing, but will use the LM8UU's until this is actually a problem.  The bearing mounts clamp to the bearings the same way that the shaft-supports do, and as always, mount to the carriage with 1/4-20 bolts on 1" centers.

The motor-mount breaks from the mold a bit.  I will probably make the final mounts from angle or C-channel, but for now just have the 3D printed part.  This fits a NEMA23 stepper very nicely, but is not particularly stiff.  The stiffness is not so big a concern, since I will be adding support bearing to keep the threaded rod in tension, which should handle any axial stresses, however I'm worried that the less than rugged mount might break during repeated trips to VHS in my bicycle paniers.

All that remains to finish the prototype stage is to get a piece of plate for the carriage, print the (already designed) lead-screw support bearings and lead-nut mount and drill/tap about a dozen holes.  The result should be a reasonably stiff, reasonably accurate linear stage.  If all goes well, I can then look into sourcing reasonably priced leadscrews, since the stand-in 1/4-20 rod I'm using won't allow the 100 mm/s feeds I'm looking for.  Currently I can only achieve about 30 RPS (~38mm/s) with the steppers, which needs quite gradual acceleration. Switching to a 1/4", 2- or 3- start leadscrew would give me the feeds I'm looking for, but sourcing these in Vancouver is troublesome.


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