NavList:

Hello Brad,

I had thought of this too: using shim rings between the inner sliding tube with scale, and outer tube with scale. The cursor can be clear sheet wrapped around as a slide fit for the simplest solution; or, another metal/plastic tube with shims to give a good fit.

There is a problem with this, however, (and with all Bygrave rules) that the two scales need to be made to the same scale factor for accuracy, and hence the same diameter as near as is possible. They should in theory be exactly the _same_ diameter, though obviously one has to be slightly less than the other to slide in and out of each other.

So, the object is to make the two scaled tubes as near as is physically possible the same diameter, which brings us back to square one. The thinnest metal tubing possible for the outer scale tube to keep the two scales as close together as possible, and the closest fit as a sliding fit as possible is the specification we need.
This is where as a manufacturer, one would have the luxury of going to a tube manufacturer and ordering exactly what you want to within a few thou', but unfortunately we cannot do that.

The locking device is relatively easy: I intended to use a disk at the top of the tube as an end-piece, with a centre hole through which a threaded rod protrudes down into the cylinder, and a threaded disk on the top which when turned pulls the rod up. Exactly as in the German version. At the other end, a tapered cone-shaped piece on the rod, engaging with side pieces in an assembly fitting into the tube (and assembly locked into the tube), such that pulling on the rod pushes the side pieces outwards against the walls of the tube distorting it into an oval shape - locking the tubes. The smallest distortion will lock the tubes without damage to them. The same principle as is used in wall-fixing 'rawlbolts'.

I have all the ideas to make one successfully, and the workshop equipment - but not the nicely fitting tubes. I'm still looking.

If there were enough people interested, it might be possible to have a tube made by a manufacturer to an exact spec as required to slide in and out of a standard sized tube; and cut it up to send to those interested. The cost being shared.

Douglas Denny.
Chichester. England.
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