need advice on threading optical tubes or rings

[Ted Coffey]

Looking for advice on threading very thin aluminum or brass tubes for optical instruments. Tubes tend to deflect too much under cutting pressures and need some type of supporting core or mandrel. Does anyone know how the commerical companies do this?
How are lens retaining rings made?

[Evan]

Since a commercial company will be doing a production run they will make custom tooling, plugs, mandrels etc. I have cut lots of threads like that. I make some of my instruments and adapters from black abs pipe which deflects easily. I make a bore plug from either wood or a plastic like nylatron. For thin wall metal tubing I use an aluminum plug with a very slight taper. In some cases the deflection can be used to advantage. I made a 4" dew cap for my small Meade telescope from an end cap. after cutting off the end (a nice piece of flat round material for the stock bin) I gripped it in the four jaw on the outside, deforming it slightly. I then turned it to the correct ID. When released this gives a tube with four contact points making for an easy friction fit on the scope tube.

[Spin Doctor]

Another thing a commercial company doing production runs will do is allow a certain amount of material to be excess so as to allow to be held in work holdinding fixtures and chucks. Things like lens retaining caps are probably turned on a CNC lathe with a stock puller or feeder from the appropriate alloy of aluminum or steel tubing. Just how thin is the wall thickness of the tubing.

[Paul Alciatore]

I've mounted some lenses for telescopes and cameras in the past. Fun work.

Rings can be turned from a solid disk that's about 1/2" or more longer than the ring's finished thickness. Chuck it in 3 or 4 jaw, face, and turn OD. Cut threads on OD while it's still solid. Then use boring bar to bore ID a bit deeper than the ring requires and part off.

If you need internal threads in a ring, bore the internal diameter first and thread it, then do the OD and part off. Use the bulk of the disk to support the work in progress.

You might also cut any notches or drill any holes (for wrenches) before threading. If you are doing several with same thread, you can turn and thread enough length for all at one pass and then bore and part off individually.

But if you are doing a lot of identical ones make tooling and try to cut them from thick wall tubing. Less work and less waste.

For internal threads in a thin wall tube, you can make a ring to fit over the end with a single saw cut slit. Turn the ID for a slight interference fit and expand it to insert the tube. This will stiffen the work for cutting the thread and if turned with a accutate OD it will also provide a support surface for the steady rest's jaws.

Paul A.



[This message has been edited by Paul Alciatore (edited 04-03-2004).]

[Ted Coffey]

The tubing can be as thin as .030 inches but as the diameter increases up to say 2" or more the relative stiffness of the tube decreases significantly. I now assume that retaining rings are probably threaded tubing on a mandrel and then parted off to length.