SamKnuth - how would one change from one bit to the next without it being offset. if i have to use the drill press to drill holes for the other 3 i will but would like to figure this out. just because it should do it.
Hi Sam, I use two techniques for getting precise alignment with bit changes.
First, I use an three axis touch plate to re-zero between bit changes. I use this with all my carving. With thin material, you do need to use a half inch of spoil board to raise it high enough off the waste board to use the touch plate.
You can mill your own with a few bucks of aluminum and time, or buy one for $50-$70. The wiring is not too bad... you just need to connect one pin from the arduino to the touch plate, then use an alligator clip to connect the bit to ground. The only annoying thing is you have to take off the g-shield to access the pin.
CharleyThomas made a great excel spreadsheet that generates the g-code needed to run the calibration. You'll need one script for each different bit diameter you are using.
Second, I cut a calibration pattern for drilling into the PCB if I have enough room on the sheet, or I use a piece of scrap off to the edge of the workpiece but using the same zero coordinates. My calibration pattern is just a line of circles cut to the size of my pads, then after the bit change and zero, I run a check to drill out the test pads. This calibration check is done at slow speed, so I can pause the work if the first pad is off, go to zero and tweak my setting, then re-run, watching the result on the next pad (it will re-drill the first, then move to the second, and so on). Before I bought the zero touch plate, I had to use quite a few circles when cutting to tight tolerances, but with the zero touch-plate, I rarely have to tweak anything.
For just a few holes, use the drill press unless you want the practice. I would only use this technique if you are producing a lot of boards or a complex board.