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Lathe Too Small

Marking Your Boiler

Many model engineers have a lathe of 7 inches swing (referred to as a mini lathe or desktop lathe) as that appears to be the standard many manufacturers have adopted these days, although you can buy smaller ones as well as ones much larger.

This in turn makes it awkward from the novice's point of view regarding knowing what a lathe of that size is capable of being used for.

Generally speaking, a lathe of this size has a bore of 20mm (3/4+ inches) through the headstock, and this accomodates the rear axle of many model engines up to 2 inch scale for the road going versions, with 3 and 1/2 scale for the trains. But this in turn, owing to the many varied designs, makes it awkward when it comes to marking out a boiler tube in a lathe.

Fine, you know the swing will accomodate the tube, but is it long enough for the boiler length?

Most of them are barely big enough, especially when the manufacturers are spouting the between centres length, so what you want is a way of making a mark along the tube that runs parallel to the centre line to give you the top and bottom of the tube.

This is easily done without a lathe, in that sitting a piece of angle iron on the tube, running along its length will give you a line along one edge. The opposite side can be worked from the paper method, where a sheet of paper is wrapped tightly round the circumferance and the overlap point marked.

All that's needed now is to fold that paper edge in two between the marks and use that fold point to mark the opposite side of your tube.

In the same way, the first line could be made by rolling your tube up against a piece of angle iron sat on your bench and the mark made, with the second being done in the same way as before using the paper method.

Simple really, isn't it?

George.


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