Coulter's Smithing Home
We didn't just buy one as we could not be sure what would fit, and they tend to be fairly pricey. This kind of project is right up our alley anyway, so full speed ahead! Here's what it looks like. Yes, the box won't win any beauty prizes, but this is the first one Doug has folded up since High School, which was a very long time ago. We used our spot welder to affix the joints at the corners, then cobalt plated the whole box so it wouldn't rust. The box is supported on 4 4-40 standoffs that are threaded into the motor bracket. The electronics are all mounted on a little piece of Radio Shack perf board which is held to the lid with the same type 4-40 spacers. In other words, it is easy to take apart, which will be important as we make changes.
We found a suitable servo motor over at Marlin Jones, and bought the timing belt and pulleys at McMaster-Carr. The rest is hardware store steel, nuts, bolts, and copper. Our rev A controller is just a pwm switcher based on a NationalSemiconductor LM3524, and is as simple as can be -- the schematic is essentially the test circuit from the data sheet, with a fet and diode added to drive the motor. (note to self, I'm going to have to find a nice schematic drawing tool so I can put these up here and have them look nice)
We plan to improve on this and add motion feedback so it will run slowly with better reliability. It is useful as is, though, and with a little holder on the mill quill for our wire welder, is helpful in making very good, uniform welds.
As the shafts involved were weird sizes, we obtained pulleys with the next smaller shaft size and drilled them here to match the actual shafts. It surely is nice to have a big lathe for things like that. An 18" belt worked out right for the pulleys we used. This can go a little faster than is really wise, so the next rev we will probably use a somewhat smaller motor pulley, which will mean the next shorter belt as well. This will make it a little harder to turn by hand, but that's life.
To avoid having to do endless tracking adjustments, we made a little belt guide out of copper flashing. Once the tracking is close, this rubs very little, but keeps the belt on no matter what. Notice we engraved the motor body with the allen wrench and nut sizes. Saves time later.
Marlin P. Jones number 14676 MD, $24.95
Motor:MFG: JAPAN SERVO
USED. 24VDC gear head motor. Approx 370RPM @ 24VDC.
WT: 2.3 lbs
The catalog lists the shaft size as 5/16 inch, but it is really the next nearest metric size. The bolts are also metric, but can be and were rethreaded for 10/32.
Pulleys and belt:
All from McMaster-Carr.
Pulleys cost more than I'd like, but they'd be pretty hard to make here. 18" XL urethane belt, trade number 180XL, McMaster number 1669 K33
40 tooth XL pulley McMaster number 6495 K727
60 tooth XL pulley McMaster number 6495 K732 REC
LM-3524 from DigiKey.
IRL-3705 fet from DigiKey.
Old Shottky diode from our stock.
24v 3a power supply from our stock.
100k potentiometer, from Radio Shack. Reversing switch from Marlin (that one no longer stocked there) Knob fabbed here from 1 inch 6061 stock. It was a rainy day.
Coulter's Smithing Home
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