Thin-Walled Tube Manufacturing 4

[flash3780]

I'm wondering if anyone out there might have suggestions for manufacturing techniques for an extremely thin-walled tube requiring very tight tolerances on the ID.
The application is an inconel cylinder encasing a piston which requires very tight tolerances on the (~2") ID; the wall thickness is on the order of .010".

The part is currently machined, but is very expensive. I was thinking that a more consistant method of manufacture might be to expand a thicker-walled tube to the proper size by running a solid cylinder through it and then stress relieving it.

Suggestions?

 

[TVP]

I previously investigated a precision tube application (titanium not inconel) where a company recommended an unusual manufacturing technique: creep expansion (high temperature) of a smaller tube blank to the desired size by using a cylindrical plug of a higher expansion material. As the two components are heated to a high temperature, the plug expands more than the tube, increasing it's diameter by a known amount. Apparently this can be very accurately controlled, but I don't have any first hand experience with this method.

Your description is essentially the Drawn Over Mandrel process that is the foundation for precision tube manufacturing around the world. Welded or seamless tubes are drawn to reduce the diameter and/or the thickness, while also increasing strength, reducing surface roughness, etc. Not sure what your tolerances are, but you may be able to get a precision drawn tube to meet your requirements. Here is a link with some additional information:

http://www.mrw.de/downloads/Steel_tube_and_pipe.pdf

 

[btrueblood]

Gotta second TVP on tube drawing, though creep forming sounds interesting too. Drawn tube will be cold-formed to some extent, increasing its yield strength and hardness, whereas the creep-formed material will likely be closer to annealed material after finishing.

 

[unclesyd]

Here are two other possibilities for making your part. I have used both processes for different thin wall components.
The first is the Magneform machine. There are several incarnations of this process. The second is the flow forming process. Flowform is the one I used.

Depending on your exact requirements there are several other process that maybe able to accomodate your part.

http://www.magneform.com/apps.html

Flow forming

http://www.flowform.com/flowforming/index.php

http://www.pmfind.com/

 

[swall]

When I worked in jet engine fuel controls, we had some small Inconel tubing, perhaps .20" O.D. with maybe .015" wall (about like a soda straw). It was O.D. ground to get the thin wall.

 

[flash3780]

Flow forming (seems similiar to metal spinning) looks like it may be a good option. Seeing as the cylinder would be used to house a piston, extremely tight tolerances are required. Would flow forming be capable of producing a 2" ID within +/-.0002" or so? Perhaps a finish-grinding operation would be required for the final .002-.004" of material.

 

[ornerynorsk]

Machining or grinding brings up insane difficulties on wall thicknesses of what you have described. I can't imagine the pressure being very high on your requirement. Have you checked into electroforming, basically electroplating over a mandrel or form of the correct size. Not sure if it's feasible, but just a thought.

 

[TVP]

flash3780,

If you are looking for a final tolerance of + 0.0002 inches, then no, flow forming will not be capable of that. Take a look at the following link, which shows a typical tolerance of + 0.002 inches for the ID of a 2 inch tube:

http://www.flowform.com/flowforming/cap_sizes_tolerances.php

It sounds like flow forming may be a good option for you to pursue, with the caveat that a secondary operationmay be needed if insane tolerances are required. I thnk honing of the ID would be more appropriate than grinding. Another possibility to consider would be abrasive flow machining (extrude hone) of the ID.