Where can I send 3.5" flues for swaging? Anderson use to offer this but no longer.

Hi Bob:
I believe that the swager at the Nevada Northern shop will do 3-1/2".
It is an interesting machine which has a series of rollers which rotate around the flue to compress the metal.
Should you go this route, I would suggest doing some practice pieces as it would appear that getting the nice smooth transition from the original O.D. to swaged O.D. requires a fine touch.
Good luck!
J.David

The most impressive tube place I have visited was Philadelphia Pipe Bending. I do not know if they do swaging, but maybe they could advise who does if they do not. There web site says they have been in business since 1880. Probably always in the same building. Maybe someday they will get around to putting a floor in the shop. Take a look at their web site. A most interesting company.

http://www.philapipebend.com

Scepko Tube Swaging
http://tubeswaging.com/home.html

Located in Rancho Dominguez, CA

Scepko website states, "Swaging capacity is from 1/6” up to 2.75” O.D." O.P. is looking to have 3.5" flues swaged.

An article that covers the basis of rotary swaging is here:

https://www.sciencedirect.com/science/article/pii/S1877705814016099

Note that you can download the entire conference proceedings in PDF.

The principle is a bit different from what J. David indicated, as the rollers act on multiple-part dies that produce the tapered shape progressively. The combination of dies and shims allows for adjustment of geometry and accommodation for hot or cold working. A profiled mandrel can be inserted into the bore to form or preserve shape, and it may be possible to preform ridges into the tube periphery that would facilitate subsequent prossering.

I don't have firsthand experience with rotary swaging, but it does occur to me from what I have seen of the equipment and diagrams that a machine capable of working 3.5" tube of flue cross-section would be able to use longer dies to work typical tube as well, e.g. if any kind of profiling after safe-ending were desired.

It may make sense for someone to acquire a rotary-swaging head or machine of appropriate capability, and make up the appropriate tooling to do this job 'optimally' with the appropriate speed and process efficiency for large numbers of flues at one time. I would nominate Kelly Anderson as a leading prospective source.

Make a die and press the material through. Here is a 5.5 sample we did in about 2012