several things may impact how "flat" you're able to machine the tooing plate. first, how much end play (axial movement) does your Dremel-like spindle have? for example, measuring my Dremel, I get 0.011" which is way to much play for any serious expectation of a flat surface. However, assuming yours is 0.002" or less, the next concern is how well it is trammed to the table in both XZ and YZ planes. Also does moisture in the air, etc effect the MDF table's thickness/flatness? If the spindle is not perfectly trammed to the table, you will get a stepped surface. Next concert is how far the endmill sticks out of the spindle/chuck. Make it as short as possible, even if you must cut some off the back end of the end mill. When ordering endmills, you can get "stub" length, with short flutes. The less "stick-out" the less tool deflection. High speed steel endmills will often cut aluminum better than carbide, however, in this case, the rigidity of carbide is a plus. If you take 80% width of cut (0.100" step over) and 0.010" depth-of-cut (DOC) and your spindle has enough power at 6000 rpm, then try feeding 11 in/min. For 8000 rpm, try 15 ipm. 10k rpm, try 19 ipm. I recommend spaying it with WD40 or some other cutting lube, and /or blowing the chips off ... you don't want the aluminum to build up on the cutter, nor do you want to recut chips. I'd use a conventional cut direction. Its going to take a while to surface the entire plate... you may be able to rev the spindle rpm's even higher, and feed faster, however if the bearings in your spindle get hot and grow, the DOC may change; one side of the plate may be cut deeper than the other. My biggest concern would be spindle quality, power and rigidity. At 0.100 WOC, 0.010" DOC at 8k rpm / 15 ipm, figure about 3 watts. If your spindle has the power, you may want to make, buy, beg, borrow a mini-flycutter, with a high positive rake, honed-sharp HHS cutter, a 0.750" flycutter at 0.600" WOC, 0.010" DOC, 4k rpm / 23 ipm, would need about 23 watts. The surface finish would likely be smoother / flatter, bearing temps would be lower / more stable, and the time to machine would much less. hope this helps.
Respects,
Tom AMS-MotoMachine & Supply
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