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At Chris's suggestion, I bought new acme leadscrews and dumpstercnc anti-backlash leadnuts. This was to replace the old triangular leadscrews and brass nuts, which measured huge backlash, .010" or more on X. This was due at least in part to wear on the nuts: fine brass dust was visible around the leadnuts after I ran a stall test of 1000 inches or so at 1.5ips; I had earlier measured the backlash as only about .005".
The main work consisted of drilling and threading the holes for each new leadnut, and slightly boring out the existing couplings to accommodate the acme thread (.249") which was bigger than the triangular thread (.241"), with secondary tasks being the disassembly and reassembly of the machine, and preparing the new leadscrews (cut down from a 3' piece and grind a flat).
In one case (the X-axis leadnut) there was not enough room in the full mounting flange. Cutting off one side (leaving 3 mounting holes) fixed this problem, though.
Now the machine has little enough backlash that I don't feel I can measure it (.001 to .002" if I had to pick a number, but I feel there's at least that much "give" in the way I've mounted my dial indicator for testing). It also seems stall-free at 2ips on X and Y; I'm actually running at 1.5ips now.
I have a couple of boards floating around in my head to work on--a board for an at90usb chip with some LEDs and a bunch of headers so I have a chance to play with that technology (QFP packages on milled boards would be a first for me), and a board to adapt the mesa 50-pin header to the xylotex 26-pin header to test the hostmot2 step generator personality (though I could just as well do that with jumper wires; that's how the pluto-p is currently hooked to the xylotex stepper driver, after all).
Future work on the mill includes the Z-axis home switch, a new tool-length
sensor mount (the current one has too great a tendency to move around),
and dowel pin holes to help with alignment of double-sided boards. Of course,
a vacuum table would be a nice addition, too.