On 11/9/22 19:59, John Dammeyer wrote:
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?
IDK John, but that was the driving force for me to mount all my taps
small enough my spindle could turn them, in a 7/8" brass hat with a grub
screw on all 4 faces of the tap shank, running in a 7/8" r8 collet with
a notch cut in the face to engage a 4mm cap screw in the outer face of
the brass hat, thereby keying the tap to the spindle. Since the go704's
reworked spindle driver limits it to about 2 hp and low gear is about 4x
too fast, I'm stuck pecking the tap about 1/4 turn per peck for holes
above 8mm. It does get the job done, but I hear Jons pwm-servo current
limiting at about 18 amps, the motor iron quietly chirps...
John
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