Long expanding mandrels

I need to hold thin wall tube in the lathe and machine the OD along its entire length.

Material: nickel silver

ID: 19mm

Wall thickness: 0.5mm

Length: 200+mm

Does anyone know if expanding mandrels are commercially available in longer lengths? I've spent some time looking online but among all the ones available, the bit that does the actual 'holding' is far too short. I need the inside of the tube to be supported along its entire length.

Any ideas? Would something that does the same job be easy to make?

Maybe an expanding reamer could do the job?

Not as long, but have supported thin round material on a rod with sliding "spacers" interspersed with "O" rings to grip when the spacers are compressed. An idea you might be able to adapt.

You may get away with setting up a piece of 3/4" (19.05) between centres and polishing a couple of thou off it and maybe a thou or so taper. Then assuming the inside diameter of the tube is OK warm with a heat gun and slip it on the mandrel. Turn to size then warm to remove, you may need to wrap some emery round it to help get it off when warm.

Just a suggestion, why not turn up a piece of solid steel and use loctite or superglue to hold the tube onto it while you are turning it. When finished heat to release the tube.

I thought nickel silver could be a bit difficult to machine but wasn't sure so checked - opinions vary but machinability ratings can put it next to phos bronze. There seems to be a version with added lead that is a lot easier.

Maybe you should make an expanding mandrel. Bar slit into 3 collet style with a cone or ball pulled into each end to expand it and cone into the end of the bar. The hole up the centre would need to be as small as possible subject to drill length to keep it stiff but maybe you could curve the shape of the bar a bit so that it's a couple of thou bigger in the centre.

There used to be a company in the UK that handled telescope tubes in this sort of fashion. Long gone.

Not that I have ever tried this sort of thing. Only other thought was something metal that could be poured in and melted out plus plugs in each end for centres.

John

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Thanks for the great suggestions guys

I ought to have clarified: it's important that the bore doesn't get damaged / scratched in any way, so an expanding reamer might have to be modified somehow to prevent it from reaming the bore in the event of some kind of slippage.

Also, in the future it's likely I'd have a whole batch of tubes to do, so the benefit of the expanding mandrel is that it's quick and easy to set up.

It's worth taking the time to make some sort of mandrel, because it will save quite a lot of time in the long run, not to mention minimise the number of tubes that might otherwise get ruined ...

Something made from decent wood, or plastic? with a steel centre

Edited By Ady1 on 26/02/2016 23:50:19

I've seen O ring suggestion from KIWL used with short spacers made flat one side with a tapered edge on the other to expand the interposed O ring. Was told that if you have tapers both sides of the spacer things tend to jam up but with alternate flat and tapered ones any sticky O ringgs are pulled off down the taper so long as the taper sides come out first. Memory says spacers were of the order of 1/4" long.

Similar thing can be done with short lengths of thick wall rubber tube bulged out between flat spacers. Basic clearances need to be minimal to reduce risk of eccentricity so tube needs to be a close sliding fit on the centre rod and spacers need to be a nice sliding fit on rod and in tube. Thick wall rubber tube lengths around 10 to 15 mm with intervening spacers half as long sounds reasonable. I've done similar with a single inch or so length of rubber to hold some thick wall steel tube for a quick and dirty job but I wasn't overly concerned about minor eccentricity.

Clive

How about a shaft thro the tube (centred each end) and 'plugs' each end turned concentric with the shaft and a snug fit on the tube. Fill the tube with CEROBEND which is a low melting point (85 - 90 deg C) before fitting the plug at the other end.
BobH

If it was just a one-off job, then superglue, cerobend, etc, would certainly get the job done. My issue though is that I want to make a tool that can be used for many years to come to work through a whole batch of tubes, and not have to worry about cleaning off every last bit of superglue etc from inside the bore.

I really like the idea of this O ring mandrel as suggested by KWIL and expanded upon by Clive - it doesn't sound like it would be very difficult to make, and with centres in both ends it would be easy to set it up to machine a batch of tube.

I am understanding this right? I'm picturing a series of 8 or so 19mm OD O rings on a shaft, with 19mm spacers either side. The tube is slid over this assembly, and then all the spacers are nipped up by means of a nut at one end. This ought to expand all the O rings slightly to the point where they grip the tube firmly from the inside. Loosen the nut, slide the tube off, slide the next tube on, tighten the nut again... and repeat. Sounds to me like that will work very nicely indeed.

Start as BobH suggests

"How about a shaft thro the tube (centred each end) and 'plugs' each end turned concentric with the shaft and a snug fit on the tube."

Then turn between centres using a travelling steady and very light cuts.

Eric

To make hot caps for hot air engines, I have made mandrels from steel scrap. First bore the cap to size, bore and depth, make the mandrel to fit, bore a hole through it to let the air out, p[ut a drop or two of Loctite on, and push the cap on. Turn to size, heat and remove. On thin walled tube, any expanding mechanism might distort the tube.

Ian S C

Rubber sleeve just short of the overall length of the tube with a compression washer at each end over a bar with one end threaded for a nut. Sleeve o/d marginally down on tube bore. Sleeve bore and bar need to be lubricated.

Regards Ian.

I need to hold thin wall tube in the lathe and machine the OD along its entire length.

Material: nickel silver

ID: 19mm

Wall thickness: 0.5mm

-----------------------------

And length 8 inches plus...

Going to be a tuff job on anything less than a good lathe...

Accurate between centres work tests the mettle of any turner

Thanks very much for all the great suggestions.

A further idea:

Take a solid round bar that fits the hole, and mill a long tapering flat on the side - starting from nothing-removed and finishing with everything-removed. Then make another identical. Remove burrs, polish the flat surfaces and the edges. Push these into your tube from both ends, flats together. This should allow you to hold the bar between centres, or leave an extra bit on one end and hold in a collet.

The bar might be metal (eg aluminium alloy) or plastic (eg Delrin), depending on how rigid it all has to be.

You could, perhaps, make a more sophisticated version with a series of flats at a rather steeper angle ...

Never done it for really long tube, but it works for two or three inches.

Regards, Tim

How about a large number of Belleville washers back to back on a central mandril with a nut to compress the stack . When compressed the diameter would increase slightly. I don't know if you can buy accurate Belleville washers. You may have to make or modify them.

Les.

I would advise a test run along the tube with a dial indicator before cutting metal, just to ensure concentricity and tailstock alignment before cutting metal with such a small margin for error.

Neil

Yes indeed Neil !

Whichever method I employ, I would ideally want it to be something I can clock up in the 4 jaw.