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7x12 mini lathe in fetching blue!

7x12 mini-lathe - Largest Job?

What is the largest job you can turn on a 7x12 mini lathe? Well, the clue is in the name. 7x12. This means that the distance from the bed of the lathe to the centre of the chuck is 7 inches. Therefore it is possible to turn something that is 7 inchs in diameter.

The observant among you will have noticed that the average mini-lathe only comes fitted with a 3 inch chuck. Unless you are machining a sponge, it is very difficult to fit a 7 inch ljob into a 3 inch chuck. The solution, as with many mini-lathe related problems, is to employ a bit of cunning.

How to fit a 7 inch job in a 3 inch chuck

I am working (occasionally) on a fairly long term project to make an equatorial mount. One of the components required for this is a pair of 6 inch diamter worm wheels. Nobody wanted to supply me with 6 inch disks of aluminium, however, a nice supplier did give me some 6 inch sqaure by half inch thick aluminium plate.

large job on a mini-lathe Here is the square bit of plate. The first thing I did was draw a 6 inch diameter circle on it using my compasses. Then, using nothing more sophisticated than a simple hacksaw, I chopped off the four corners. Then I drilled a hole down the middle using a mini-mill, and proceeded to mount this large plate on my mini-lathe.

large job on a mini-lathe Fitting the 6 inch plate to the mini-lathe requires a few stages. The chuck jaws themselves hold a 2.5 inch aluminium disk. Bolted to this disk is a 4 inch aluminium plate. I attached the two using four self tapping screws done up as tightly as possible. I then faced the plate in the lathe. I never take this appart - I've used it several times now. Next I drilled 4 3mm holes in the 4 inch plate. In the 6 inch plate I drilled 4 2.5mm holes and threaded them using an M3 tap. I was then able to bolt the 6 inch plate to the 3 inch plate using M3 hex head bolts. Again, done up very tightly.

large job on a mini-lathe Finally I was able to start turning the job. My manipulating the tool holder angle it is possible to make a right handed cutting tool reach the outside edge of the plate. With the lathe turning quite fast I slowly squared the circle. You must only advance the tool by a maximum of 5 thousandths each pass, otherwise the tool will dig in and jam. It is a long slow processes. Do not try to speed it up or you will break your mini-lathe! The chips of alumnium fly out everywhere, very fast and very hot. Keep you hands and eyes covered.

large job on a mini-lathe After the long process of squaring the disk I finally had it smooth all the way around. The next stage is to face off this big disk. Again, cunning manipulation of the toolpost is needed. Wind the cross-slide all the way out, then angle to tool so it is about 1mm beyond the edge of the job. You will then have enough travel on the cross-slide to reach the middle of the disk in one pass.

If you want to face off anything bigger than 6 inchs, you do not have enough cross-slide travel to do it in one go, so I must be done in sections, adjusting the tool post holder between sections.

My series on making telescope worm wheel and gear sets shows a practical application of this approach.

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