Friday, April 11, 2014

Project CNC - Increasing cut accuracy on CNC mill

New Setup

After some initial test cuts I realized that I needed to replace the sacrificial bed (MDF bed leveled to machine by milling the cutting area to an equal level).  This means that I'll have to redo the whole T-nut setup for work holding.

Leveling out the bed, this time, was pretty quick since I was able to use some wide cut, 1/4" shank, router bits.  I ended up having some issues but in the end I achieved a level bed and cutting this time was about 2 hours - instead of 20!

Issues I had with this bed leveling:
  • when I ran the code (too anxious to do a trial run with no endmill) initial cut started to far in along the x and y axis
  • X-axis lead-screw separated from its connection to the motor in the middle of cutting
    • changed connection to lead-screw back to pipe clamp
  • Z-axis also ended up separating from the stepper while trying to restart bed leveling
    • top edge of rotary tool was bumping into the stepper motor mount for the Z-axis
      • cut mount shorter so that rotary tool is able to move past it
In addition, I ended up retooling the mount for the cutout tool - cutting new brackets out of MDF to secure the tool and moving the mounting spots up 1/2".

After these fixes I ran some more test cuts of mounts for stepper motors that I designed in Inkscape and generated G-code for in MakerCAM:
Progressive improvement in cuts
Earliest cut is the upper left and more recent bottom right.  Cuts are progressively better, in the first row circles are uneven but improving due to changes in cutting parameters (feed and plunge speeds, as well as cutting direction).  Bottom row more significantly improved due to stiffening up the supports for the x and z axis.
Added support for x and z axis
While watching cuts and checking the stability of the machine I realized that there was too much wobble in the supports for the x and z axis (vertical supports).  As a result, I ended up adding steel angle brackets to the vertical supports and when there seemed to still be to much wobble I secured these supports to a large square of plywood.  This seemed to fix the problem which why the bottom row the cuts all look much smoother.

All seemed perfect... until I tried to use one of the brackets with a NEMA 17 stepper and the mount was way to big.  I re-checked all my measurements for designing the mount and it seemed that I had done everything right.   Continuing to research a solution I found that I didn't realize that I needed to change the Default Resolution for imported ".svg" files - automatic default is 70DPI but it needs to be reset to 90DPI for Inkscape.  Below shows a comparison of the results of the 2 settings when using a file imported from Inkscape:
70DPI on left - 90DPI on right

Much better cut with new settings, though you may see a deviation in the larger circular cut on the right at the top.  This deviation ended up being one of my next problems - the z-axis guide rails were backing out of their mount pockets.  For a quick and dirty fix I wrapped the tape around the z-axis guide rails and their supports.  So far this has helped a lot (if you look closely at the first picture you'll see the white tape).

On to the next issue - everything looked great but when I tried to secure a stepper to the mount I found that now the dimensions were just a tad too small.  So... now I'll go back to recalibrating all the axis (I did swap out all the stepper motors with ones I ordered off ebay to build a 3D Printer - they ended up having higher torque than my initial motors and I'll now use them in my 3D Printer build since they won't have to as hard as they did in my CNC mill).