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Etching stainless tubing to reduce wall thickness

March 12, 2008

We are building a scientific instrument that requires a special piece of thin-wall, stainless steel [304L or 316L] tubing which, I think, can only be made by etching the wall of a somewhat thicker-wall tube. However, after many calls to many chemical milling shops, I haven't been able to find anyone willing to take on this job.

The finished product is a ~ 1/8" O.D tube with a 0.002" +-0.0005" wall. I need a 100 foot length, but it can be coiled up into a 12" diameter coil [about 30 turns]. The finished tube must be hermetic, i.e. no pinholes.

The smallest wall I can get any tubing company to provide is 0.004". So, I need to remove 0.002" +- 0.0005" from the O.D. of this tubing. If anyone has a suggestion as to where we could get this done, I would be very appreciative. Otherwise, we are willing to do some trial and error development on short pieces of tubing to develop a process. But, we need some direction on what process would be the best.

Thanks for any assistance you can provide.

Scott Backhaus
Scientist that occasionally utilizes chemical milling - Los Alamos, New Mexico, USA

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First of three simultaneous responses -- March 14, 2008

It's likely that .001" could be easily removed per side by etching (dissolving) in a nitric acid-fluoride mixture. The chemistry portion is fairly straightforward and simple, the mechanics may be the difficult part.

My thinking is that you would plug the ends, then spread the coil a bit (like stretching a spring), so all the surfaces were well exposed to the nitric-F.

The process sequence might be:

-Clean in an alkaline detergent.
-Rinse.
-Etch in 25%vol nitric acid + 8 oz/gal ammonium bifluoride.
at ambient temp it might take 1/2 hour or so to
dissolve .001"
-Rinse
-Rinse

Given the relatively small size of the coil, this could be done in tanks of < 25 gallon capacity.

You'd need to experiment a bit to establish etching rate.

Maybe you can find a shop which will be interested in doing this work; maybe you'll need/want to do it in-house.

Metal removal could also be done in ferric chloride ⇦ this on eBay or Amazon [affil links] solution, or, properly fixtured, this metal removal could be done by electropolishing. EP would produce a much smoother surface.

Jeffrey Holmes, CEF
Spartanburg, South Carolina

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Second of three simultaneous responses --

It is too high a risk operation and I do not think that it can be done per your requirements. A few inches, maybe, but 100'-- I would like to know how they did it if it is done and does not have ANY areas or spots that are in your 0.0005 requirement. Most tubing is not that perfect to start with.

James Watts
- Navarre, Florida
March 14, 2008

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Third of three simultaneous responses -- March 15, 2008

Etching sounds like a feasible candidate to me but maybe you would like to contact a syringe manufacturer. They may give you some pros and cons of the different manufacturing techniques akin to your project. Another idea, there are special low melting point alloys (Cerrobend is one brandname). They're mostly used to fill tubes to bend them, then evacuate the pipes by just immersing them in boiling water. You could probably use this technique to centerless grind the OD as if it was a solid rod (provided you start with a reasonably straight tube to fill).
Good luck,

Guillermo Marrufo
Monterrey, NL, Mexico