I've been perusing through my Machinist Handbook (24th Edition) and googling with no real success. I think it was on this forum that the discussion came up on the holding power of R8 collets and how they were essentially only useful for light machining. The tables found online suggest that torque for a 7/16-20 thread is in the range of 23 ft-lb or more if the material is harder. So assuming the drawbar isn't the hardest steel in the world then 23 ft-lb is used to tighten the drawbar onto an R8 collet holding a 7/8" diameter tool. Now the real question. The R8 collet is keyed so it can't turn. The tool held in it is not. What's the torque required to twist a 7/8" diameter tool held in an R8 collet torqued to 23 ft-lbs. The reason I'm asking is if I'm using a fly cutter or boring bar the torque applied to the 7/8" shaft of the cutter is considerably more. Depth of cut and RPM obviously play a part here. How does one know what the max depth of cut and RPM should be to avoid spinning the cutter in the R8 collet. For a 7/8" end mill? For 1/2" end mill? For 3" face mill on a 7/8" shaft? John
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