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Cutting gears on a lathe:

A few years back I bought a Drill/Mill from Harbor Freight Tools.

This is a very small machine, made in China.  It is not the best small milling machine but good enough and after one is used to working on it, lots of jobs can be done on it. Certainly, it is well worth the US$ 495.00.

The mill has a 7/8 HP DC variable RPM motor which drives directly, no v-belts to change, very convenient to work with. Major draw-back is the vertical feed which you have to learn to work with; it's far from steady; you have to know your machine. Nevertheless, if I would need one, I would buy it again.


With the exception of the motor's steel driving-gear, all other gears are plastic-gears.  After years of good use one day I used the machine beyond its capacity which resulted in the destruction of the driven plastic-gear. I phoned and ordered replacement gears, 3 of them, for about $5.- each. 

The problem was about the delivery of the gears since they would have to come straight from China.  When the promised 3-week delivery was more than twice over, I decided that I would have to do something about this as soon as possible, I needed the machine.


I remembered that a great many years ago my biker buddy's NSU 350 single cylinder motorcycle's first-gear broke and I had made a rudimentary gear for him on a lathe, in my grandfather's steel fabricating shop.  I decided now for my small milling machine, to make a gear the same way I had done it then, so many years ago.


It started with buying a 50 mm diameter by 150 mm (2"od. x 6") piece of bronze bar. First I turned off the raw dimensions of the gear, measured off the broken one. Next, I used a parting tool, of the same thickness as the bottom of the gap between two teeth, installed horizontally in the tool holder of the lathe.

My Atlas lathe is over 60 years old. On its back gear it has 60 holes around the perimeter for dividing purposes. There is also a pin on the base casting which can be pushed into the chosen holes to fix the positions of the chuck for the next cut.


The depth of the teeth had to be measured and after the zero-adjustment on the cross-slide I knew that the end of each cut had to be at 0.122 thou. If I'd make it again, I'd make a stop and fix it to the point  where the cross-slide has to stop. Cutting was done by taking 2-3 thou at the time and pushing the tool past over the face of the raw gear by hand-turning the wheel of the main carriage. This was a very tedious job, about 50 times for each of the 30 valleys.

For the next cut I relied on the divider-hole on the back-gear of the lathe and that was a big mistake. I used every other holes of the 60 holes to make my 30 cuts. The back-gear's holes are worn out and were not holding the chuck accurately and errors were accumulating. By the time I got to the last cut, the last tooth was narrower than the rest of them. My mistake!!!


At this point there was nothing I could do except to start from scratch. I figured, once I came that far I would finish the gear and if it did not work, then I would re-do the whole thing.


For the final cuts, a tool had to be ground to the shape of the valley between two teeth and two valleys would shape a tooth. I ground the tool to keep fitting it into the valley of the broken gear. I should have prepared a grinding wheel for this job and done an even closer approach to the shape than I managed.  Next time I would do that....


Not wanting to use the same divider-holes again, I attached a 60 tooth gear to the end of the lathe's spindle, a moveable arm to the end of the screw-shaft, a pin on the arm to fit the valley of this divider gear. The tension of the arm was provided by a couple of small bungee-cords.

Now, I had accurate divisions through the whole perimeter of the bronze gear.


The cutting procedure was the same as it was for pre-cutting the gear.

First the tool touched only a corner of the square teeth so these cuts could be larger, 5-6 thou for 2-3 cuts, then gradually less and less as the tool went deeper and cutting off more bronze each time. Towards the end, a cut of maximum 2 thou was possible.  When I arrived at the last cut, to the last tooth, the error was very visible. Almost half of a tooth was missing. My luck was that, this side of the tooth was not the loaded side.


What was left to do was the key-way in the center bore of the bronze-gear.

Basically the procedure was the same as it was for the previous cuttings. Cuts of 2 thou a time; takes patience but it works.

After the key-way was completed nothing else was left but parting the finished gear from the rest of the bronze-bar.


Installing the new gear came with varying feelings. Finally everything was up for running again. The new gear was working but with a bit of noise. In a few days the noise reduced but was never gone completely.

Finally after 6-7 months the plastic replacement wheel arrived. During all that time the bronze-gear worked very well and served the purpose without any complaint.


Since then, I bought spares, more than one copy for every gear inside this little milling machine. Now, hopefully I will not have to make another one. It took 3 days, over 5000 stroke-cuts and a sore hand to make this bronze-gear.