Today, I discovered something that is probably pretty obvious to most,

Today, I discovered something that is probably pretty obvious to most, but still might help others like me. I use Mdf board as a sacrifice sheet, and level when I install it. Then one piece will last me for a bunch (20 - 30?) of cuts. However, I now realize that over time, its surface warps – not enough to be visually apparent, but enough to cause a 0.5 - 0.8 mm differential between high and low spots, which is enough to cause problems when cutting PCB’s. (I had thought the highs and lows were the result of cheap PCB board.) Since I like to cut with a 0.6mm drill at 0.5mm depth, multiple passes are required. Today, I figured out that if I level the Mdf immediately before cutting, one pass is all it takes. Beyond this difficulty, I am able to produce nice, double sided, fairly large (about 4" x 6") PC boards. TinyG, ChilliPeppr and EagleCad are really great.

Edit – I meant 0.05 - 0.08 mm height differential, and 0.05 depth of cut.


Below are pics of the top and bottom sides of a board I just produced. Only one pass, at 0.05mm, was necessary for each side. I think the cuts are very uniform, to which I credit having pre-leveled the sacrifice board, as I describe above. No auto-leveling of the board was necessary. Also of note, in my opinion, are how well centered the holes are on both sides of the board. Not surprisingly, the holes are perfectly centered on the top side, but because my machine’s Y-axis is not perfectly parallel to its X-axis, I had a problem getting the holes to properly meet the bottom-side traces; instead the lack of perpendicularity in the y-axis caused the differential to double. Initially, I wanted to solve that problem by physically adjusting my machine, but there is no easy way to do that – it would have been, at best, a trial and error process, or I would have had to significantly redesign the machine. So, instead I wrote some code to adjust the G-Code produced by ChiliPeppr to move the x-axis coordinates in proportion to the y-axis coordinate (the further from 0 that the y-coordinate is, the more pronounced the x-coordinate error; and the relationship is highly linear; As a matter of geometry, a small y-coordinate adjustment is also needed, and it would be best to adjust the top and bottom so that everything is truly perpendicular, but this works well enough.
This pic is the top side of the board – looks like I must post the bottom side separately:

missing/deleted image from Google+

Here is the bottom side:

missing/deleted image from Google+