Background Most modern lathes incorporate a secondary gear reduction into their design to allow the user to select the speed required for part being machined. In some designs like those from South Bend and Myford, the reducing gear is located under the headstock cover on the rear of the lathe – thus the name “backgear”. In these designs, this portion of the gear-train is manually shifted into place using a lever and locking mechanism. The Delta Backgear and Shifting Mechanism Several years ago, I purchased a used Rockwell/Delta 11″ lathe, c.1965, from a private party. This machine had been used in a tool and die business in the midwest. The Delta lathe uses a simple, compact design where the backgear and shifting mechanism is located under the headstock. This arrangement provides for disengaging the clutch between the drive shaft and the headstock, and simultaneously engaging the backgear on a separate jack-shaft. This back gear is a single-piece casting machined with two gears – a 100T bevel gear that engages with the drive shaft and a 35T spur gear that transfers the power to a bull gear on the headstock. This arrangement provides a 12 to 1 reduction in speed at the headstock. The casting is bored for two bronze bushings and the assembly rides freely on the jackshaft. A schematic drawing of the arrangement is shown in figure 1. Damage to the Gear While making some heavy cuts in steel, the backgear failed causing the headstock to jam. After removing the headstock from the lathe bed and removing the gear/shaft assembly, it was apparent this was not the first time such a problem had occurred. In fact I found three teeth on the smaller, spur gear had previously broken off and had been welded back to the shaft. I had broken two of the three welded teeth plus an adjacent tooth. Fortunately the bull-gear on the headstock was not seriously damaged. It had a couple of minor chips from its past life, but this newest incident had not further damaged the gear. A call to Delta confirmed my worst fear that replacement parts were no longer available. Measurement of the Gear There ensued a round of measurements to determine the specifications of the original gear. The number of teeth and pitch diameter were obvious (35-T, 16-DP), but measurements to confirm the pressure angle were indirect at best. At first, I assumed the gear used the currently popular 14 1/2 degree pressure angle. It seemed to mesh with a Boston 16-DP, 14 1/2 degree change-gear. But Jon Anderson, an experienced machinist, noted a small amount of roughness indicating that tip of the new Boston gear was striking the flanks of the Delta gear. After careful study of the articles in the Machinery’s Handbook, ed. 22, a set of thread wires were used to measure and compare pitch diameter of the actual gear to that specified for 14 1/2 degree and the older 20 degree gear designs. It became clear that the gear manufactured by Delta was closer to 20 degree pressure angle. I say “closer”, because it was not precisely within the specified tolerances on all measurements. (It is not clear whether the differences were due to measurement errors, wear on the old gear, or the possibility that Delta had used a re-ground gear cutter that was out of tolerance.) Solutions Considered Considerable commentary and many excellent suggestions were offered when I requested opinions from the members of the CompuServe Handcrafts Metalworking forum. Their suggestions can be broken down into three general methods.
- Re-weld or braze the old teeth on the damaged shaft.
- Weld or braze an insert and reshape it to the correct profile.
- Replace the spur gear with a ring gear.
- The gear could be press-fit to the shaft, but cast iron is notoriously weak in tension.
- The assembly could be brazed together, but the press-fit bronze bearings would not likely survive the heat.
- The gear could be attached using Loctite.