Removing a dent in 1/4" stainless tube

wehnelt · Sep 5, 2020

I have a bit of a hell-project.

* I have a 1/4" diameter, 6 foot long piece of stainless steel tube between two flanges

* It has a tiny 1/16" dent in it.

* It must be worked in situ (welded to a $30,000 piece of scientific equipment). No turning.

* It must slide against an o-ring sliding seal (no abrasives can embed themselves in the surface).

* The sliding seal separates the atmosphere from a chamber filled with a rare isotope of Helium. (can't goof this up.)

* I can't work from the inside of the tube since it's filled with wires (No ball-sizing.)

How do I make the surface round again so it'll seal against the o-ring? I think I should fill it with silver solder, but what then? A little 600 grit sandpaper to finish it will be fine as I don't expect too much grit to detach and embed, but I don't think making a fully splittable lap and filling it with lapping paste is acceptable. I'm mostly concerned with doing the majority of the material removal. How do I file it round without mucking up the surrounding stainless? Is there some kind of filing guide I could make? Someone on the machinists facebook group suggested I scrape it using a v-block. That sounded promising.

Any ideas, folks?

moonlight machine · Sep 5, 2020

sp1nm0nkey said:
I have a bit of a hell-project.

* I have a 1/4" diameter, 6 foot long piece of stainless steel tube between two flanges

* It has a tiny 1/16" dent in it.

* It must be worked in situ (welded to a $30,000 piece of scientific equipment). No turning.

* It must slide against an o-ring sliding seal (no abrasives can embed themselves in the surface).

* The sliding seal separates the atmosphere from a chamber filled with a rare isotope of Helium. (can't goof this up.)

* I can't work from the inside of the tube since it's filled with wires (No ball-sizing.)

How do I make the surface round again so it'll seal against the o-ring? I think I should fill it with silver solder, but what then? A little 600 grit sandpaper to finish it will be fine as I don't expect too much grit to detach and embed, but I don't think making a fully splittable lap and filling it with lapping paste is acceptable. I'm mostly concerned with doing the majority of the material removal. How do I file it round without mucking up the surrounding stainless? Is there some kind of filing guide I could make? Someone on the machinists facebook group suggested I scrape it using a v-block. That sounded promising.

Any ideas, folks?

I've never seen silver solder you did not need heat to use, melted wires will result. Sounds like it needs to go back to the manufacturer for repair.

wehnelt · Sep 5, 2020

The wires have either a metal sheath or high temperature enamel outside. They should be fine seeing the heat.

eKretz · Sep 5, 2020

I don't think the wires will be fine at silver soldering temp. Maybe soft solder... As far as refinishing the surface, I'd mask around the repair with tape, then file it in close to size. The tape will protect the original surface. Use something thin and you'll be able to get very close. Remove tape and finish with a smooth file and some emery cloth/sandpaper. It doesn't have to be perfectly round if there's an o-ring. The o-ring should conform to the surface as long as it's smooth with no abrupt steps or fissures.

adammil1 · Sep 5, 2020

If the goal is to get an o-ring to seal well around the tube what about build up with epoxy like JB weld what type of pressure and service will this see.

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Phil in Montana · Sep 5, 2020

Machine a die to clamp around the tube to make it round, use eproxy to fill in the dent and sand to shape.Shot the sob that made the dent....Phil

Tony Quiring · Sep 5, 2020

Get 2 chunks of steel and mill one side of each flat so they can be bolted together.

Clamp them and drill holes through both parts to allow bolts to clamp them together.

Now Chuck in 4 jaw lathe and drill hole through where they meet same size as tubing.

Now you have a forming die same size as tubing.

If it is dented it is not round.

The die will make it round.

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wehnelt · Sep 5, 2020

I was gonna use Harris Stay-Brite, which melts at 430 F (221 C). If some ABS wire sheath was pressed against the wall, yeah, it would probably get a bit mushed, but I think these wires should be fine. I could also switch to a low melt indium solder or something.

bigjon61 · Sep 5, 2020

Proofing alloy would be a consideration once you get any high spots off of the tube. Make a die to wrap around the tube with the appropriate diameter thru hole, with a pour spout and subgate. A little bit of emery cloth and Bob's your uncle

Milland · Sep 5, 2020

bigjon61 said:
Proofing alloy would be a consideration once you get any high spots off of the tube. Make a die to wrap around the tube with the appropriate diameter thru hole, with a pour spout and subgate. A little bit of emery cloth and Bob's your uncle

I think proofing alloy is made to easily separate from the base metal, no?

If this was my job, I'd want a truly permanent repair and would bring in a micro-TIG welder, as in for small mold repair. Do a slow buildup using SS filler, letting the tube cool between every little dab. build up a little proud, use a file on a guide (think split bushing, flat milled on side to just above the tube OD, pierce in area of weld buildup to allow file to cut), then finish off with 600-1200-polishing paper.

Then an in-situ passivization, and a microscope inspection before trying it out.

wehnelt · Sep 5, 2020

could you say a little more about the split bushing? That sounds like a really nice idea.

Cole2534 · Sep 5, 2020

Isn't He an escape artist of a gas? This project does not sound fun.

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wehnelt · Sep 5, 2020

Something like this maybe? I guess I'd try to accommodate a few pins and screws to get it to come together right.

wehnelt · Sep 5, 2020

Yep. It's a lighter (has an easier time working through cracks) isotope worth about $500,000 per liter when liquefied, and this probe serves to dip an experiment into a dixie cup sized volume of it.

David_M · Sep 5, 2020

I, too, thought of a split die, but of a different design. Actually it would involve three split dies. The first die would have a transition in it. It would be .25" round on both ends and in the center it would be an ellipse. The ellipse dimensions would be determined by trial and error on an exact copy of the tube including the dent.

To use you would line up a vertex of the ellipse directly over the dent and squeeze lifting the dent up in the process. Then use a round die to return the tube to round.

You may have to hold the ellipse shape in the tube for a small area at the dent (with a short die that is ellipse shaped only) and slowly work toward it with the round die working on alternate sides of the dent being very careful to keep things smooth .

Chip Chester · Sep 5, 2020

Anything magic about the wires inside the tube? I would think that unsoldering and re-soldering some wiring would be closer to a standard repair procedure (sensor or PSU replacement) than some of the tasks outlined above. With the wires out of the way, the ball method seems it would be easiest. Or, a 'dent wizard' type formed tool, or and internal pushing tool activated with a tapered inner rod pushing against a ball that's smaller than the overall tube diameter.

wehnelt · Sep 5, 2020

The wires are epoxied at the other end of the tube. Pretty fantastic.

eKretz · Sep 5, 2020

If it's that expensive, the best plan would probably be to replace the tube. Is there any reason that's not possible?

Milland · Sep 5, 2020

sp1nm0nkey said:
View attachment 298598

Something like this maybe? I guess I'd try to accommodate a few pins and screws to get it to come together right.

If that's my filing guide, yes - although, no need for two pieces, one section rotated 90deg (body only, slot and portal stay as is) rests on top of the tube, file is guided to prevent scarring of tube surface.

As you want a cylindrical final shape, the guide is pulled and cleaned after each section is filed flat, then reset at a slight angle to the prior session to give a faceted "near cylinder" that is finished off with sanding and polishing. Don't rock the guide, if you do you'll almost certainly scratch the tubing where it contacts.

Passivation is used to truly clean the stainless and restore the oxide layer, use a microscope to double-check for scars or pinholes in the surface.

Presumably it's Helium-3 you're dealing with? I love that stuff, I do the best "Donald Duck" imitation ever with it...

wehnelt · Sep 5, 2020

Like so? When you say pulled, do you mean that this is used to draw file?

Removing a dent in 1/4" stainless tube

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