I thought people here might be interested in some 3d printed Lathe change gears that my friend Richard Oakley made for me.
We discussed the manufacture of these gears at the last Moray Firth makers meet in Findhorn. The gears were drawn in Inventor professional and then printed in high impact resin with a Formlabs Form 2.
A little bit more detail on the gears and the reason for their manufacture. I have a Harrison M250 lathe which has a full set of change gears for cutting metric threads. This was fine until recently, when I inherited a 1919 vintage Drummond B type lathe from my Uncle. In order to refurbish this machine I suddenly needed to cut Whitworth threads.
So I needed a set of change gears to enable me to cut imperial threads. Harrison made gears for this machine to cover the full range of imperial tpi’s, but they are prohibitively expensive. The usual recourse is to make one’s own. As can be seen by the pictures in these posts, the gears have a very elegant 6 pointed spline to key them to the lathe shafts. This is quite difficult to reproduce well.
I thought that 3D printing would be just the job for doing this. So we took some measurements for the spline and off we went. The gears are module 1.5, and the principle dimensions of the gears can then be found by plugging the number of teeth required into standard design equations.
The photographs below show some more of the gears and a view looking straight down the splines showing the quality of finish and the accuracy achieved. The involute tooth form can be seen in the close up picture.
The main drawback to this process is the cost of production. Making these gears in the high impact resin comes out at about £1 per tooth. The low impact resin is perfectly acceptable for sympathetic use, but I did a very simple impact test (put one in a vice and hit it with a small ball pein hammer) of one of the low impact gears. It broke easily across the central axis, showing a classic brittle glassy fracture. They would be unlikely to see this sort of loading in service, but I just wonder whether we could actually sell these gears in the low impact resin given the caveat that they must be handled with some care.
I’m currently looking at other options, for example having the gears laser cut or possibly EDMing them.
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I would have thought witworth would be useful to you for your rocket engine?
Whitworth is quite a coarse thread. I am using metric threads in the main for the rocket engine. What you might be thinking of is Bsp threads, which share common dimensions with Whitworth, but are finer. I use taps and dies for these threads, but having these gears does now open the way to screw cut them if required.
Witworth threads work really well, but are a pain to make, I think BMW still use them in high stress parts of racing engines.(but don’t quote me on that)
Opinion 3 put holes in it to stop it the cracks
