Posted by Bill Simpson on November 27, 1999 at 21:53:00:
In Reply to: hobbing problems posted by Michael Kelly on November 27, 1999 at 17:38:46:
4140 preheat treat is tough on cutters. You want to use a good cutting oil on this job, and watch the hob for sharpness- don't let it get too dull, or the top cutting edge/corners will burn back quickly.
How are you determining runout? Your bores must be close to size with the arbor, or the blank will "settle" before tightening. If checking runout after mounting, be sure to check concentricity of OD with bore, or you'll be measuring eccentricity of OD and runout together. One you've determined that the blank's OD and bore are concentric, you should tighten the blank, put the center in place, then measure runout. Here you might see runout due to spacer faces that aren't parallel. Spacers that aren't "perfectly" parallel will cause a straight arbor to deflect when tightened. If your center is able to pull it back to even partially true, then your arbor/spacer/workpiece unit is not stiff enough.
Three important things about reducing runout: Perfectly clean and parallel blank and spacer faces, perfectly concentric blanks so you can truly indicate runout after mounting workpieces, and close bore and arbor sizing.
Back to the material. I would suggest taking more than a .010 finish cut - .050 would not be too much. And the cutter must be sharp. Shift it often to bring good teeth into the cutting region, and sharpen often to prevent cutter damage. Again, flood the part/cutter with a good grade cutting oil.
Table feed is going to depend on (you guessed it) the stiffness of your setup and machine. You want to use .050 to .080 to get the best cutter life, since cutter wear will increase for very slow feeds. Slowing the machine speed down to reduce the cutter rotational speed is good too if the cutter drive has little backlash.
Production in this setup is going to be unavoidably slowed because of the hard material. If you are tempted to speed the machine up, try increasing table feed instead. If the machine doesn't struggle and pound with the higher chip load, then cutter life is better off than it would be by increasing its rotational (surface) speed.
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