Tubing slip fit woes

Machine Shop
#1

I’m making a telescoping assembly with (multiple sets of) three pieces of brass tubing; 2mm OD, 3mm, and 4mm. I was expecting a friction fit but the assembly is more slip than fit. Without benefit of using the pin gauges, I’m guessing the smaller tubes are a few thousandths less diameter than they need to be.

Any suggestions? I’m open to changing either the smaller or larger tube somehow. I don’t need the entire length changed if one end is a friction fit.

Here’s what I have tried - all of these were on the inner piece of tubing to try to increase its OD at one end:

  • A little bead of solder (and then turning it down to size). I couldn’t control the sizing well enough (because the solder didn’t go all the way around the tube).
  • Knurling near one edge. Since I’m using a tiny jeweler’s knurl it didn’t raise the OD enough to make a difference.
  • Bending the end ever so slightly so it would rub on the other tube. I was able to get a friction fit this way but very slightly changed the alignment of the tubes.

I’ve considered flaring one end of the inner tubing but don’t have a suitable flaring tool.

All suggestions are welcomed!

#2

You can flare the ends of the inside tubes with properly sized round dapping punch and keep the tubing straight in bench block.

Much bigger than Chris Marlowe scale but here is the idea . There are dapping punches in jewelry.

Also, a rotary tubing cutter will make a crimp or bur near the end than can be used to make up small gaps in tubing . You have to tighten it slowly and rotate it evenly to make the crimp without making the cut. It helps of the tubing cutter is worn or cheap Chinese that fatigues the tubing apart weather than cutting it . I have used this to fabricate or modify custom watch spring bars for unavailable sizes.

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#3

Coats of varnish on the outside of inner tube?

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#4

Those are a couple great ideas to try.

I have fairly small dapping punches at home that I can try.

I was thinking about the tubing cutter for something else, but I really like what you’re suggesting. Miniature tubing cutters are always worn or cheap or Chinese so that should do what you’re suggesting. The locations where I would use the tubing cutters will be covered by castings that will be soldered in place, so any cosmetic damage (i.e., groove) from the tubing cutter won’t show.

@Bill - varnish is a good idea too, especially because it will prevent brass oxidation. I would have to figure out possible timing in conjunction with the fact that there’s a lot of soldering that needs to be done.

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#5

Wonder if a tube flare like so

Screenshot_20240313_193945
Screenshot_20240313_1939451440×1771 193 KB

might do the trick. Probably have one in automotive…

#6

Beading seems even better but i don’t know if they get as small as 4mm.

#7

You could try custom swaging tooling.

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#8

That was another think was going to suggest . Anneal the brass to a malleable state and swage it to size on a mandrel and die . It will work harden and crack so frequent annealing would be necessary

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#9

The tubing cutter worked perfectly! It was the solution requiring the minimum effort. Thanks everyone for the great suggestions.

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#10

Yes it is a great trick. An old watchmaker mentor of mine showed me the technique . Glad to help.

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