Top of brick model.
Bottom of brick model
Top of finished brick compared to real LEGO. Height is significantly off, due to a math error when I flipped the piece to mill the bottom.
Bottom of finished brick. Slightly manngled due to missing a tool change from a 3mm to a 2mm endmill. I caught the error and killed the program during the first pass, but the damage was done.
The milled brick mates snugly with a real LEGO, suggesting my tool calculations were pretty good for the 3mm endmill used to cut the bottom surface.
The milled brick mates OK in this orientation, but doesn’t really grip the green brick. I think my 2mm endmill may be a bit larger. The height difference is really obvious in this photo.
Misalignment of the top and bottom milling is really visible on this end of the brick.
Having a few hours to myself to fool around on the CNC, I found a decent parametric model of a LEGO brick and set about machining one in acetal. The surface finish is horrible, the height is off by a good millimeter and the bottom is a bit mangled by a missed tool change, but it’s fairly dimensionally accurate and mates with a real brick (the nice looking green one in the photos).
Lessons learned:
- Double check stock thickness and placement. For two-sided machining, this determines the overall height of the finished part. In this case, it was off by enough that the bottom pass poked holes in the top surface!
- Better alignment when flipping to machine the bottom. I currently drill holes and pin the stock to my spoil board. It doesn’t look like this is accurate enough.
- Double check what tool each path is expecting. I left a 3mm endmill in the collet for a finish pass computed for a 2mm bit and messed up the bottom of the part pretty badly.
- Lie to the XY finish pass. Set the endmill diameter larger than it really is. This should take care of the gouging on the sides of the part.
John Popadic 