Monday, September 8, 2008

MIlling tiny parts from thin brass sheet

I recently completed a project that required me to replace a steel part with a brass one for a small game-controller joystick. In the picture below, there are two brass replacement parts mounted on a small circuit board with the original steel part on the right. The part itself started life as a flat piece of metal that was bent on four axes to produce a 5-sided cube (the bottom face of the cube is open and fits into a white plastic base).

Here are is the original steel part and its brass replacement, which I machined from sheet stock. The stock is about .025" thick, which is just slightly thicker than the original steel. I began this project by unfolding the original part, and scanning it with a flatbed image scanner. I cleaned up the image in the GIMP, and converted it into a vectorized image format. Then, I created a toolpath for my CNC mill. This sounds relatively easy, but it took many hours to accomplish the whole series of tasks. I started experimenting with speeds/feeds/lube/depth of cut, etc, etc and I eventually found a winning combination:

Use stub-length micro endmills. I used a .020" dia end mill, which often broke when I breathed on it too hard. Using stub-length end mills helps prevent breakage.

The depth-of -cut should be about half the tool's diameter, so .010", in this case. I used three passes to cut through the .025" thick stock.

The chipload should be the tool diameter divided by 130. So, .020/130 = .00015. My mill is limited to 4500 RPM, so this means, I can only feed the tool at about 1.4 inches per minute (2 flute cutter). In reality, I chose to use an even slower feed rate of about .5 to .75 inches per minute.

Galling the brass was a big problem, and I found that WD-40 with occasional air blasts to clear the chips worked very well. Probably, a mist coolant system would be the best, but I don't have one.

Workholding was accomplished by using double-stick tape to adhere the brass stock to a thick acrylic spoil board.

I also tried making a fixture to hold my Dremel tool to the mill spindle. This would allow me to spin the tiny end mills at 30,000RPM (proper speed). It was a good idea, but the runout in the Dremel tool (.005") caused the .020" end mills to break very often. It just didn't work, so I would advise against using a Dremel for this kind of milling.

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