Replacement Change Gears for 12” Geared-head Lathe

[Paul Jones]

This not exactly “homemade” because I started with manufactured spur gears and modified these to fit my lathe. I have the situation where replacement parts are no longer available and I have to either make or modify the parts.

I have a used 12” x 37” geared-head gap bed lathe that came with the wrong change gears for cutting imperial or metric threads. The lathe has two spur gears located in the compartment behind the spindle that must be changed when switching between cutting imperial and metric threads. When I acquired the lathe it had an odd set of 32 tooth and 46 tooth spur gears for cutting only 11 ½, 23, 46 and 92 TPI and nothing else. Missing were the two 40 tooth gears required for imperial thread cutting and a 30 tooth gear for cutting metric threads. Luckily, the lathe still had the 120/127 combo gear essential for either imperial or metric thread cutting.

My geared-head lathe was built in Taiwan in 1987 but it is very similar design to the 12” swing Grizzly lathes. I called Grizzly but they could not provide the specs to the gears other than the number of teeth. I took a chance and the ordered replacement gears. The Grizzly gears did not fit because they use a 14 ½ degree pressure angle and I needed 20 degree PA and a diametral pitch of 20. Boston Gear makes steel spur gears in their YA series that fit my specs and I ordered the gears with 0.500 bores.

I machined the gears to be 10 mm wide on the teeth and 10 mm wide on the hub using an expandable mandrel with a slotted brass tube to fit the in between mandrel size. Next I bored to 18 mm ID by chucking the hub section and not touching the gear teeth. Finally I broached a 5.01 mm wide x 2.51 mm deep keyway to fit my lathe shafts.

I made a “broaching” tool using a HSS cutting tool blank ground to cut the keyway using a boring bar and having the lathe chuck locked in place (and turned off). I used a digital height gage to align the horizontal direction and height of the broaching tool. After creating the keyway slot, a 0.1970" (5.004 mm) gage pin was used to check the slot width before removing the gears from the chuck.

These are not exactly “homemade” from scratch but the gear modifications allowed me to install the exact parts I needed for this lathe. Now the lathe has all the necessary change gears and a quick change gear box combinations to cut a wide range of threads from 3 3/4 to 112 TPI and 7.5 to .25 mm pitch (see attached threading chart showing the change gear sizes and placement options). The two 40T change gears normally drive the 120T gear for most imperial threading and the lower 40T gear is reversed on its shaft to drive the 127T for metric threading.

The change gears are lubricated using a high press grease and run almost silently. The grease product is "Dart LUBE CMD Extreme Pressure Lubricant" (see https://www.amazon.com/Dart-LUBE-Ext...sure+Lubricant ).