George Langford
January 21, 2005
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The Atlas
Press Co. 10 inch lathe that I purchased for my son had a form of
metallurgical cancer in the Zamak aluminum-zinc alloy gears.
Several teeth fell right out of the main bull gear on the headstock,
and closer examination revealed swelling and cracks everywhere on the
gear. So I had to machine a new one to make the lathe
functional. Even though Clausing is extremely cooperative and
helpful, a new wrought aluminum gear from them was way outside my
budget. Problem was, the gear was so large that I could not swing it on the Atlas index head and still get a gear cutter over the top. So the gear had to be machined horizontally on the Atlas's meager rotary table, which has no indexing capability whatsoever. My solution is shown at left. The bad gear is mounted flat on the rotary table. Above it is the new gear, with about three-quarters of its teeth already cut. In the foreground is the Atlas lathe's second back gear, removed from the gear cluster for the time being. The index pawl is at the far right. The pawl engages the teeth of the intact second back gear, and that gear's teeth average out the errors and bridge over the gaps in the defunct bull gear. |
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The new gear
teeth are shown above the bad bull gear's teeth at left. I
offhand ground the fly cutter, so these are not the quietest teeth on
the planet. But they work great. After a little running in,
the meshing quieted down. I did take a lot of trouble to make the
fit of the bore of the gear blank very close to the seat on the outside
of the spindle and to mount the blank concentrically on the rotary
table, and that worked out well. The idler gear rotates on a shaft mounted on the I-beam; I left it free to rotate, held only by the index pawl. Of course, once I had indexed to each new tooth, I took up the slack and then tightened the clamps that kept the rotary table and the gears bolted to it from rotating. So everything was nice and tight when I started each cut. There's lots of impacting from that fly cutter. Note that I fed the knee upwards to get a smooth motion without lifting the knee or saddle. The principal limitation of this technique is the reach of the fly cutter. It would be more rigid to use the Atlas's overarm to support the outboard end of the flycutter. One might make a gear a foot across; I haven't checked what the exact limit might be. |