Lathe – techniques – invisible joints

For the purposes of a magic trick it was necessary to make two types of joint that would be “invisible”. It happens that both are made using the lathe.

The trick is, essentially a tube about 150mm long and 20mm in diameter. This tube consists of two parts that screw together in the middle of the trick. One end of one part is solid. The end of the other part has a false bottom.

The first is that two pieces of aluminium tube are joined together in the middle with a screw joint that is invisible. The second is that one end of this has a false bottom that can be taken in/out and yet the join is, again, invisible.

The invisible screw joint

A piece of solid material is drilled out and on the open end is turned down and threaded with a M16 x 1 die. This part is slightly longer than is needed so it can be held in the chuck and parted off when finished.

fig part with solid end

It is most important that the surface is flat and at right angles. It is also important that the thread is relieved where the die cannot reach the shoulder.

This is removed from the lathe. The other part is fitted in the lathe. It is drilled all the way through. On one end it is drilled and tap to take the other part, ie M16x1. Here, again, it is important that the shoulder is flat and very, very slightly cut as shown.

fig part with hollow end

The other end is drilled and reamed to take the false bottom. There is a shoulder inside so the false bottom will on go in the required amount.

the two parts can be screwed together.

fig the two parts screwed together

Notice how the two parts do not fit together very well

They are held in the chuck by the spare piece on the end of the closed part. The whole assembly can now be turned along the whole length of the outside.

fig the two parts turned down

Notice that the join is invisible.

 

The false bottom

The trick in hiding the false bottom is that the eye has trouble seeing a join if it is in a corner. The false bottom is patterned. This might seem ornamental but the pattern conceals the join.