Once the magic smoke comes out, it won't work any more.
John Kasunich
jmkasunich@fastmail.fm
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Mon, 07 Jan 2008
To do CNC threading you need a spindle encoder. I dug up one this week, and figured out how to mount it. I had a couple timing belt pulleys and a belt, pulled from an old printer. It isn't really practical to belt directly to the spindle. The diameter is quite large, and there is very little depth behind the main pulley. But the stud gear that used to drive the change gears can be used instead.
Modifying one of the pulleys to fit on the stud was straighforward. Mounting the encoder is a little more complex. I decided to copy the method used to adjust the change gears - a bracket that pivots on a stud and is locked in place by a bolt in a curved slot. I drew up the bracket in EasyCad, then manually picked off the critical coordinates and wrote a g-code program to mill it. Again O-word loops and subroutines eliminated a lot of duplication.
The bracket was specifically designed to use a piece of scrap aluminum that I already had. I managed to work around 5 holes and a large notch in the side of the stock. The notch is in the upper left of the photo below, one of the holes is in the upper right, and a several small holes are on the bottom edge, below the left hold-down bolt. Another hole was inside the large hole at the right end of the part, where it will pivot.
The strange arrangement holding the part is a consequence of the short Z travel of the Shoptask. Just about every milling job needs to be blocked up in order for the quill to be able to reach it. In this case I have a couple pieces of 2" diameter steel about 3" long, topped with 1" long 1" diameter sections. The 3/8-16 hold-down studs go through everything and into the t-nuts.
The finished part, with both holes and the curved slot milled, outside profile milled, and encoder mounting holes spot drilled. I finished drilling and countersinking them on the drill press.
And finally, the bracket mounted in the machine, with the encoder, pulleys, and belt (orange). The Z axis stepper motor, belt, and leadscrew pulley are in the lower right corner, and the larger pulleys are the spindle drivetrain - v-belts and step pulleys.
Next step - wiring it, and configuring everything so I can do threading.
(posted: 07 Jan 2008 23:15) (permalink)