Re: [CAD_CAM_EDM_DRO] Re: Is this too nuts to consider ?

I have a Boxford 125TCL CNC lathe. On the back of the spindle is a large
diameter steel disc with a circle of small holes around it - I haven't
counted how many. One of these holes is formed into a notch out to the edge
of the disc. Two optical pick ups are mounted so that one senses the row of
holes while the other only sees the notch. The one sensing the holes
provides accurate spindle speed, while the other gives spindle 'starting'
position.

When screw cutting, the controller uses the spindle 'start' position pulse
as a reference. It also knows the actual spindle speed so can keep the
x-axis stepper motor moving correctly with reference to the spindle. Screw
cutting is one of the 'canned cycles' provided with the Boxford CNC sofware
- in my machine this runs on a BBC computer.

This is going to be of no use if you want to cut a very 'fast' thread, or
spiral groove. However, this problem was dealt with by Prof. Chaddock in
constructing the Quorn Tool & Cutter Grinder. The vertical column holding
the head has a spiral groove 3/32" x 3/32" at 1/2" pitch. Chaddock cut this
in a 3.5" lathe by setting it up to screwcut 32 TPI but putting the change
wheel train back to front. The spindle cannot drive the leadscrew because
of the very high forces involved, so he drove the whole thing backwards by
turning the leadscrew. The slot was cut with a 3/32" dia. slot drill in a
cross-slide mounted milling spindle. The leadscrew was power driven by a
worm meshing with an extra 50 tooth gearwheel fitted on the leadscrew. He
reckoned that this was necessary as it took two passes to cut the slot and
each pass took 78 minutes!


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