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Making a telescope worm wheel and gear set on the mini-lathe. Part 13, Lapping the worm and wheel

Telescope worm wheels and gears on mini-lathe series index

The end is in sight! The teeth of the worm wheel have been cut by the worm hob to the required depth. All that remains now is to lap the worm section and the worm wheel together to smooth off any imperfections.

There are a couple of contesting factors here. With lapping, it nice to start off with course lapping grit and move up to ever finer grades. However, in this case, our lap is actually the worm wheel - and it cannot be thrown away between grades of lapping paste. Small amount of the lapping paste will become embedded in the worm wheel and are very difficult to remove, so we have to start with a polishing grit and just lap for a very very long time!

There are a couple of schools of thought here. On the one hand, we can use something like Autosol metal polish - whilst this works fine, proper diamond lapping paste works better. However, as I said above you cannot use course grit lapping paste.

I normally use a 15000 grit paste, followed by a 30000 grit paste. The finer grit is around 0.5 micron particle size. This is about 0.0005 mm, and if we look at the page about tolerances for telescope worm wheels we see that this is of about the correct order of polish. If you can find something finer, then be my guest. Do not try using Cerium Oxide ceramic/glass polish - this just seems to dull the finish and not cut well on metal in my experience.

After the hobbing is complete, the first job is cleaning up the worm wheel. I try not to remove the worm wheel from the turntable, because it is a pain to re-indicate it. Instead, I remove the turntable from the mini-lathe cross slide and dip the whole thing into a bath of white spirit and scrub it with a soft brush - a toothbrush is ideal. Spend some time making sure all the grit is out of the teeth before cleaning up the lathe and getting ready for lapping.

Getting lapping paste on the lathe is considered bad form. This isn't too much of a problem with the fine grades we are using, but it is best practice to protect the lathe bed from lapping compound - I often use a plastic sheet with a few holes cut in it to cut all the lapping and polish gunk off the lathe .

At this point, all you need to do is fully engage the worm wheel with the worm gear and apply lapping paste and oil, and let the thing spin around until the steel worm gear gleams like a mirror with no visible imperfections.

You will soon realise that the worm gear is longer than the area of engagement between the worm wheel and the worm gear. Because you don't know exactly which part of the worm wheel will engage with the worm gear on your final ultimate construction, it is wise to keep moving the carriage around so the majority of the worm gear gets lapped. Keep clear of the ends of the worm thread, this is liable to trash your worm wheel.

It is also wise to run the lathe in both directions. After an hour running the lathe forwards, switch to reverse and run the lathe backwards for an equal time.

When you first start, it is important to lubricate the lap with lots of oil, because the worm will be quite rough and generate a lot of heat. It takes several hours of lapping to get the worm mirror smooth.

As I said, I start off with the 15000 grit and keep going for as long as I can before I get bored. Then I clean up the worm wheel again in spirit and lap with the 30000 grit paste.

I don't know whether "metal polish" like Autosol gives a smoother finish than 30000 lapping paste, but my experiments on a scrap bit of metal point to the lapping paste doing a better job. Ideally, we want some 60000 grit lapping paste, but I haven't got any yet! When lapping is complete, clean up the worm and wheel - the worm should be a highly polished jewel.