Although this mill has produced some good results, it has also made some garbage. It would be naive of me to expect this mix of dumpster parts to perform as well or reliably as a proper mill. I don't want to make this sound too discouraging, but I'll try to describe some of the weak points of this project.

The X and Y axes behave exactly as they did in the printer. One motor moves the print head along a shaft using a toothed belt, while the other turns a roller that moves the object via friction. I have not had any real problems with the print head axis, but the paper axis is not as reliable. There were no problems when I was simply drawing on nice, flat paper. When milling, however, I produce lots of chips, dust and bumps in the surface. Usually these are able to go through the rollers just fine, but sometimes something gets caught and the board slips slightly. Unfortunately, even a slight shift can completely mess up the result. Here is a picture of a pcb for which there was a little slip that caused the drilled holes to be off. As a result, this pcb is no longer functional.

I also had a problem with friction wearing out the plastic "bearing" that guides the bit. It is not actually a bearing, but simply a tight hole in the plastic base of the Z-axis. While milling parts for the stargate project, the mill had to work for rather long periods of time. After a while I noticed that the bit had enlarged the hole in the plastic. It probably melted slightly from friction. I started applying a little oil each time I use it, but the enlarged hole has thrown the precision way off. Of course this could be fixed by replacing the plastic part and remembering to oil it.

A third problem is with cut depth. At this point I have to calibrate the cut depth each time by eye. I've looked at zeroing methods used on other designs, but they would not work here due to slop in the Z-axis. I'm usually able to eyeball a reasonable depth, but sometimes it is just a little too shallow, causing poor electrical isolation, or too deep, causing too much loading on the bit that results in poor precision.

This project has been very educational and a ton of fun. I'm pleased with the way it has turned out, considering the cost and parts used. But with these problems, pcb production has been a long and wasteful process. I'm looking for an alternative and these are some of the options I'm looking at:

Do you have any other ideas?