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A Ringbom engine

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Darren:
That's an interesting engine.... :scratch:

DavesWimshurst:
Roger,
I followed Dr Senft's procedures in his book "Miniature Ringbom Engines".
There are two critical fits involved, the power pistod and cylinder and the displacer rod and bushing.  The power cylinder is brass and the final finish was by lapping using a non embedding compound (Timesaver brand) and homemade expanding lap:



The piston is graphite as used for EDM machining, it was turned to a very close fit.  The displacer bushing is also graphite bored to size and the outside grooved for epoxy to hold it in its housing.  The piston rod is stainless steel drilled to make a thin walled tube, I believe there is a piece of scrap where the drill came out the side from not clearing the drill often enough. :bang:
The outside is turned and polished to a very close fit in its graphite bushing.  In fact if it runs for more than about five minutes the rod has heated enough to expand and stick stopping the engine.  After cooling it's fine again.  It"s a great little engin to take to our weekly "Techie dinner".  The restaurant is used to us by now! :lol:

Some time I'll get my act together enough to do a project, IVe only had my camera for a few mounths and am still not used to having it in the shop on a daily basis, maybe I need  :poke:

Dave

rleete:
Thanks for the info.  That lap is an interesting idea, I might have to try it.

One more question: where did you get the graphite?

sorveltaja:

--- Quote from: DavesWimshurst on July 25, 2009, 04:05:23 PM --- In fact if it runs for more than about five minutes the rod has heated enough to expand and stick stopping the engine.  After cooling it's fine again. 

--- End quote ---

One possible way to fix that would be to make bushing and piston rod from the same material, so that their heat expansion rates are almost equal.

I've built similar engine, and used brass for bushing.
Thin walled brass tube (from an old telescope antenna)for piston rod. It's much lighter, and has already smooth surface.

DavesWimshurst:
Roger,
I got the graphite from McMaster Carr.  I bought it years ago and the catalog listing has varied but I think the material is called high temperature conductive graphite.  Some have said that certain grades of EDM electrode material is abrasive to machine but I had no trouble with what I used.  There may be some highly engineered grades that we don't want to use.
I hope this helps.
Dave

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