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Wednesday, November 20, 2013

CNC mill conversion - Part 9

Stepper motor first test
Today I received the long awaited male-male DB25 cable from Jameco.com, and I was finally able to make the connection between the motor drivers and the computer.

That was the easy part. Setting up the software was a lot more challenging, and although I am not finished with it yet, I've gotten far enough to be able to run the motors, at last.

So, I took a quick video of the motors cutting an imaginary circular hole 2" wide by 0.950" deep (5 cm x 2.4 cm).

"Mach 3" running the stepper motors


I finally got it all put together, and started cutting some soft scrap plywood as a first test. 

First test cut with the 2 axes CNC mill

Later in the day, I found that the X axis stop-nut locking bolts were completely loose, and the resulting gap manifested itself as an appalling 0.040" (1 mm) backlash. At the time of the test cut, I had used the backlash compensation provided by the software just so that I'd have something to try cutting on.

While the software compensation is a good thing, it is not designed to offset such a huge amount, so the slot turned out a few thousands short in the X axis, and that's why I had to use a mallet to tap the metal piece into the slot in the video.

After tightening the loose bolts, I was able to measure an X axis backlash of 0.0035" (89 microns), and a Y axis of 0.0025" (63 microns), both of which are easily handled by the compensating code.

My plan for shrinking the backlash even further to nearly zero, is to acquire ball-nut bearing balls that are 0.0015" and 0.001" oversize, and use them to repack the ball-nuts. 

Trying a few more test cuts


This time I used the mill to surface an aluminum plate that will become part of the Z axis assembly. Things did not work out correctly right off the bat, but in the end I was able to get the mill to work as I was hoping it would. I tried cleaning up the noise as much as possible from this video without removing my voice, but the sound still sucks. 

First cuts on aluminum

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