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Making – cylindrical squares

A cylindrical square is simply a cylinder made of metal where the sides are parallel and the bottom is square to the sides.

Though it is very simple it needs to be made carefully. If this is done a really useful precision tool is made.

If a cylindrical square is placed on the milling table then its sides will be at right angles to the milling table’s surface. This is not very interesting. But if two are made and they are placed on the milling table then it is possible to hold a workpiece against both of them so as  to form a vertical surface which is also at right angles to the milling table’s surface.

fig a pair of cylindrical squares

On a larger miller machine as shown a suitable size for these would be about 150mm long and 50mm in diameter.

Suppose we start with some round mild steel stock. A hole is drilled right through it. The diameter of this hole should be such that the size of bolt normally used on the milling table will go through it.  It is held in a three jaw chuck and a center is turned in one end and both corners are chamfered. The size of the center drill has to be such that allowing for the hole that is there, there will still be a significant amount of conical surface cut by the center drill.

fig the center at one end

At the other end the center of the end is recessed. A center is turned in this recess and both corners are chamfered. All the chamfers are important. Firstly they look good, secondly they make the part feel good, or, rather, a part with sharp edges does not feel nice,  and thirdly they reduce the chance of a ding being made on the corner.

fig the center in the recess

The accuracy of these centers is not critical. A catchplate if fitted to the headstock spindle. The part is now fitted with a dog and this assembly is fitted between a dead center in the headstock spindle and a live center in the tailstock.

The outside it turned to clean it up as far as the dog. It is turned round to finish the whole length.  It is then turned both ways to give it a good finish. In between turning it round the cross slide should not be touched. The two surfaces should overlap without the overlap being visible. The diameter is checked as accurately as possible at both ends. If these are not the same the tailstock is shifted across till the diameters are the same. What the diameter actually is is not important all that matters here is that the sides are parallel.

When the sides are parallel, the raised part of the end with the recess is just skimmed so that it is perfectly flat. It is only possible to do this because the end is recessed. Doing this will mean it is at right angles to the sides.

The same is done for the second cylindrical square. In this case not only does it not matter what the diameter is but it does not even have to be exactly the same as that of the first one. Though the length is not critical a pair of these that did not appear to be the same length would look odd.

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