My experience suggests that 5,000 rpm is just about slow enough to work with plastic IF the drill bit is sharp. If it's even a little dull, the plastic is liable to melt. The various sanding drums, grinding burrs, etc. also are likely to be troublesome at that speed.
A couple of weeks ago, on the thread "Is Dremel worth it?" in this forum, I sounded off (probably at too much length) on this subject. I think Dremel is missing the boat, as far as the hobby market is concerned, by putting so much emphasis on speed and power. The ideal Dremel-type tool for a modeler would be small in diameter and easy to hold near the tip, and would have a speed range from 0 to about 5,000 rpm. Dremel used to make a "table-top speed control" (I still have and use mine) that, in conjunction with a single-speed Moto-Tool, would let me put the motionless drill bit exactly where I wanted it and ramp up the speed gradually. At the moment, my favorite power tool is a scaled-down version of the Dremel made by a German company called "We-Cheer." It's about an inch in diameter, uses Dremel chucks, and, though it's single-speed, works well with the old Dremel speed control. I got it from Woodcraft (on the web at <www.woodcraft.com> for about $25.00.
Otherwise, for most plastic applications I'm a fan of the original, small-size Dremel "Mighty-Mite." The truth of the matter is that for most modeling applications, the full-size, 110-volt Dremel tools have far more power and speed than will ever be of any practical use.
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