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Getting My CNC Ass Kicked

A project log for 16 Inch 7-segment LED

This will be a giant LED numeral made with neopixels.

David WheelerDavid Wheeler 05/27/2019 at 18:450 Comments

I designed this thing with three plates to be CNC machined.  All the LED segments mount on a base plate and there are similar plates for the front and back of the case.  I've been trying to make them with polycarbonate on my DIY MPCNC machine but I keep running into problems.

On my first attempt, back on April 7, I made the first base plate with a few mistakes.  First, I didn't set the Z position properly and at one point, the CNC plunged through my workpiece and jig, gouging the work.  Second, I found the table was out of level so cuts on right side of the workpiece were too deep and results were rough.  Worst of all, though, on the last operation to cut around the perimeter of the piece, the machine missed some steps in Y and the toolpath at the end was off 2 or 3mm from where it started.  So, the beginning and end of the cut ended up in different places.

Following that session, I leveled the table again and made a couple changes to the G-code.  (Instead of cutting a slot for the perimeter, I wanted to try adaptive milling a narrow band around the piece.)

The second attempt to CNC a base plate was on May 3.  With the bed level and more care on homing the tool, things went well until I got to that perimeter operation again.  It skipped some steps and ended in the wrong position again but this time I saw what happened.  The router's power cable and the Z stepper cable shifted over the right side of the gantry while the machine worked on the left side of the perimeter.  Then, when it moved over to the right, those cables were pinched between the router head and the right side of the machine.  As it moved in -Y on the right edge of the workpiece, those cables went tight and didn't allow the gantry to move.  Eventually it unstuck itself but by then the machine was out of position.  So that's another day and another piece of stock wasted.

Preparing for another run at this, I tied those troublesome cables in a way they can't get stuck again.  I also made some changes to the CAM setup in Fusion 360:

I noticed, though, that the table the CNC is mounted on has become very wobbly.  The legs aren't clamped tightly anymore.  Not sure if this will hurt accuracy.  (It can't help.)

So then, on the third attempt today, I clamped a new piece of stock on the fixture, CNC cut the holes along the perimeter and noted that the holes in the workpiece don't line up with the holes in the jig now.  There seems to be a really bad skew in the X-Y axes that is different than it was when I made the jig.  This is a bit of a head-scratcher.  I tried to just move forward by shifting the workpiece and screwing it down but even that didn't really work.  (And, anyway, I'd have to solve this skew problem before cutting the remaining operations.)  So this workpiece is destined for the scrap heap, too.

I'll need to re-do the square calibration on the machine before I can try again.  I also need to address the Z nut since the screws that fasten it down keep stripping.  And I'm not sure when I'll have time to do those so this project will probably end up on hold for a while now.

One thing I need to consider is whether it makes sense to have these laser cut by Ponoko instead of using my CNC.  I'd have to choose a different material but that might not be bad.

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