I've got a nice little CNC Milling Machine, it's a converted Proxxon MF70.
After doing some shopping around, I found that if I bought it from Germany and getting it shipped over, it was about £200 cheaper than buying it in the UK. I got a CNC conversion kit from ebay (Just search MF70 CNC, it'll pop up I'm sure!) and a Smoothieboard interface/driver.
The Smoothieboard is a 3D printer driver at it’s core, but it does work equally well as a CNC driver, with some minor adjustments.
All in all, a nice little piece of kit. I have, however, been having some problems with it. First of these is setting the correct Z datum. For those that don’t know, a datum is a reference point, so a point of known position. In this case, the Z-datum is height zero. For subtractive manufacturing processes, like CNC milling, operations are usually carried out below the datum, so heights are negative. For additive, like 3D printing, the process is carried out above the datum, it builds up the design on the table/platter.
My problems started when I tried to set the Z datum. I’d lower the bit down to the surface of the material (just enough to grab a piece of paper, but not to make a hole in it), then enter the set home command, and the bit would move back up the Z axis by 1mm, and decide that this was Z=0. This meant that whenever I cut anything, I had to add 1mm onto any depths in order to get the correct cut depth. I could not figure this out, until I found the “Home Offsets” command.
The home offset GCODE command is M306, and it works like this:
First, move the mill to it’s home position using “G28”. Then, move it down to the top of your cutting material, tiny increments at a time, like .1mm at a time. When it is just enough to catch the paper, enter “M306 Z0” which sets that as the Z0 home offset. Now you can go ahead and enter G0 Z0 to see if Z0 is correctly set at the material height. If it is set to the correct height, enter M500 to save everything, and you’re good to go!
