Craftsman 115.7575 Restoration – Part 3
Thanks for your patience as I continue on this series.
Part 4: Motor restoration
The ‘guts’ of a block grinder like this one consist of a stator, a rotor, a centrifugal switch, an electrolytic start capacitor, and a power switch. The power switch feeds power into the run windings and also through the start capacitor, centrifugal switch, and into the start windings. At rest, the centrifugal switch is closed, so giving the machine power will energize both the start and run windings. The capacitor phase shifts the amperage going into the start windings and makes the motor spin up. At about 75%-80% of full rated speed, the centrifugal switch opens, cutting power to the start windings and the start capacitor, leaving only the run windings engaged. When the power switch is turned off, the motor slows, the centrifugal switch reconnects (usually with a distinct click), discharging the capacitor back into the start windings (causing some degree of braking), and the grinder coasts to a stop.
(Before we go on, remember that in the previous part in this series, I discussed removing the bearings.)
The center band is attached by four small screws. The power switch is held on by a little flat nut around the collar of the switch. My ON/OFF switch plate was missing.
I ordered a Mallory branded (USA manufactured) 72-88 MFD 110-125V capacitor to replace this one (from ebay). I think that’s pretty cool, replacing it with an OEM capacitor. I think the total cost was under $10 shipped. You can go up a smidge on the MFP rating, but you don’t want to go too high or low, or you can cause issues with spinning up the motor. Stick to the same VAC rating. These capacitors are non-polar.
I wanted to replace the wiring to the stationary centrifugal switch, but I didn’t want to re-solder it, so I modified the switch to incorporate 1/4″ quick connect leads.
Ok, now here, I’m embarrassed to say I neglected to take photos of the re-wiring. I originally did this project for a forum post, not a blog entry. I replaced the electrical cord with a new 14/3 wire cord. The green ground wire goes to a ring terminal sandwiched into a screw holding the capacitor bracket. For the other replacement wires, I used 14 gauge machine tool wire (MTW) from wirebarn.com (I had a good experience with them. They’re a small outfit in California, I believe). I replaced the black wires going to and from the lamp and the yellow wire going from the capacitor to the centrifugal switch. I clipped the three wires exiting the stator windings and shoved some heat shrink over them as far as I could get it, then crimped on quick connect terminals. I couldn’t access the interior of the windings to completely replace them, unfortunately, so I am hopeful the heat shrink will suffice. I lengthened them with MTW in the appropriate colors. I added a strain reliever and gromet for the power cord where it entered the frame. I still haven’t found a replacement light fixture, so those two wires are currently taped up and not doing anything.
The power switch, I believe I ended up replacing with a new single pole single throw switch, which was basically identical to the original. I needed a double pronged end for one terminal on the power switch, so I bought a double male 1/4″ copper quick connect chair adaptor like this one. You can find them anywhere – I think most car places have them, or you can look online. I think I used one for the capacitor as well, so that I could get three wires into one side.
So, at this point, I had rewired as much as I could, I put in a new switch and start capacitor, I modified the centrifugal switch to be quick disconnect, and I was ready to reassemble the motor. So that will be the next post.