Rebuilding the Carriage Motor and Worm Gear

I tore apart the carriage controller and found the old selenium rectifiers burned out. I called the local Hardinge service rep, Jim Granquist, and he made a house call for free. I think he may have felt sorry for me after inspecting the lathe. I bought two modern bridge rectifiers from him and a set of carriage motor brushes. (He tried to give them to me.)

Anxious to see what would happen, I patched in the field and armature rectifiers with alligator clips. The carriage worked! The motor made a lot of noise, but it went back and forth and the power cross feed worked as well. Emboldened, I flipped the speed switch to "high". Smoke poured out of the motor. I pulled out the armature and found it utterly destroyed. I sent the armature to a motor shop.

The main drive still rattled. The spindle motor platform bounced up and down. I replaced both belts, the motor platform pivot bushings, counter shaft ball bearings and the main motor bearings. At last the drive ran smoothly!

I cranked the speed up and down marveling at the smooth quiet operation. My wife came out to see what I was doing: "Try it out", I said, inviting her to crank the speed up and down. She did. There was a loud thump followed by grinding noises! "Arrg! Stop Stop!", I cried. She fled the garage in dismay.

The countershaft is kept centered in a pivoting bracket by two teflon bushings. These had worn short, allowing the shaft to work out of the ball bearing on one end.  I just happened to have some teflon pipe that fit the shaft. I made new bushings and reassembled the counter shaft. This time all was well.

The armature came back from the motor shop and I installed new ball bearings. The armature worm drives a worm wheel and shaft which also needed new bearings and seals.  Trying it out on the bench, I found it ran smoothly but with a clicking sound in one direction of rotation. I discovered a divot on the drive worm. After much effort trying to polish out the defect, I gave up and called Hardinge. The worm was no longer available for my machine. After some calculating and reading in Machinery's Handbook, I set up my old Dalton lathe to cut a new worm. This is non-trivial because the worm needs to have transcendental tpi to drive the 30 dp worm wheel. I finally found a compound gearing that made a satisfactory approximation.


Turning the worm blank on a taper mandrel

Grinding the worm form tool

Form tool close-up

Finished armature with new windings, worm and bearings

This time the carriage drive seemed to work smoothly, but I found that one of the clutches could not be adjusted. I tore down the carriage again and found that a pinion had not been pressed on to a clutch shaft far enough. This defect had been around a long time because someone had installed an extra spacer in the clutch to try and make it work. After assembling the carriage again (a messy job because of Permatex) and filling with transmission fluid, all was well.