Thread: Backlash
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Unread 03-06-2008, 21:56
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Re: Backlash

You can make a pocket in your gears on a lathe, and tolerances to .001 or better should be easy with just a little care. .001 interference in aluminum is a pretty tight fit, .0005 for a 1.5" OD bearing would be plenty. You can fiddle this a bit with sandpaper or a file.

For clearance between teeth, you must be sure there is no negative clearance (too close together). The higher the speed, the closer the tolerance, the harder the material, the closer they can be. For moderate speeds (1000 RPM-ish) in aluminum, .0005-.002 is fine, be sure to run the gears in for several hours with light lubrication, in both directions and light load.

When making intermittent cuts (like turning hex to round) don't use carbide bits, regular tool steel will manage that better. Carbide tends to shatter, especially the ones with the little triangular inserts.

An arbor press is convenient, but not required. The reason to avoid using a hammer is the shock loads being transmitted. Better use a vice or some clamps with metal blocks than a hammer, but if unavoidable, go slowly with the hammer.

Cheap bearings are OK for low speeds (600 RPM-ish) and sloppy tolerances. ABEC bearings are great for higher speeds (10,000 RPM) and longevity, as well as tighter tolerances. I buy bearings on eBay all the time, but if you have a bunch of useable bearings, use them.

Old trick if the pockets are accidentally a loose fit: Make a plug for the hole from scrap metal, then drill three (or more) evenly-spaced 1/8" or so holes in the edge of the bearing pocket, with the holes protruding maybe 0.010 into the pocket (and hole plug). Fill the 1/8" holes with metal rod, file them down on the inside a little bit to get your press fit.

Don
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