Machining Graphite

I have hopes of building a simple Stirling engine with my son. The plans for the engine we would like to build calls for a graphite piston.

Are there any special considerations? What type of cutting tools should I use? My only familiarity with graphite is in pencils.

Mike

Surface grinders work well for machining graphite. It also mills and turns easily. Keep in mind that the dust is less than healthy to breathe.

Norb

Graphite is some nasty stuff. About the only upside is that the dust is considered a lubricant rather than an abrasive. That would not stop me from putting down towels and the like to catch the stuff.

I should add that graphite scraps are usually abundant in mold shops. A fine grade like Poco 2 or 3 should hold up well to both machining and use in the engine. Cheaper grades are less dense and are made of a coarser grain of material. While being lighter in weight, they tend to chip more easily. Any high speed tool will work, however the best surface finish is obtained by grinding.

Norb

In one of my previous jobs I used to build machines that made carbon fiber/graphite sheets and tubes. The individual fibers are very conductive so our electrical control boxes that had relays and switches were pressurized to exclude the odd scrap that was created.

Are you sure they mean EDM graphite? That stuff has practically no strength in any direction, and is extremely brittle in thinner sections.

I've also been thinking of building that Stirling engine. I haven't ordered the plans yet though. What I'm wondering is if its really necessary to have that graphite piston. The guy sells the little Material Kit but couldn't most of those things just be replaced with more common materials?

I don't imagine the first ever stirling engines were made with delrin and graphite and such.

No graphite is not required. You could use a leather piston with washer backing. A good chrome tanned leather will make a fine long running piston. Use thin washers to keep the weight down.

If you are planning on a low temperature Stirling engine of the sort invented and described by Jim Senft in his books, graphite is much the best to use in combination with a glass cylinder. Any form of fluid lubricant provides so much viscous friction that the engine won't run, all lubrication must be dry, and the clearances on the piston must be very small so lubricant powders are out. I've used acrylic and delrin and they will work for a while but they gradually deposit a tacky layer on the cylinder that brings the engine to a halt. Some of my engines have run at 150rpm for years, perched on video monitors, video recorders or other source of waste heat. Teflon also works but it has a high expansion coefficient and either grips the cylinder or when heated flows plastically so that when cool it is too loose. I lap and polish glass tube to be circular and parallel within a micron or so, which is fairly easy if you have experience with lapping and optical work (making a telescope mirror for instance) but Jim recommends using dashpot pistons and cylinders, which are available off the shelf in various sizes - Google for his books to find out where, they are self published under the Moriya label - and don't need any special skill to use.
EDM grade graphite works very well and takes a high polish, I use mylar drafting film to "sand" it to final diameter, but a local graphite supplier told me they polish it with newspaper.

- Mike -

Thank you for all the info. The plans I have call for a graphite piston in a bronze cylinder. The glass sounds like a better idea, but I'll have to learn about lapping glass bores starting from scratch.

I have just started learning about Stirling engines. A fascinating topic, and a technology whose time seems to have finally come; very much like Boolean algebra came into its own with computer technology.


Mike

I've built a number of hot air engines with graphite pistons. I'll tell you what works for me. It might not be the best way, but it keeps me happy.

I use ordinary HSS cutting tools and run the lathe at 6-700 RPM. I always hold the nozzle of a shop vacuum cleaner to collect the chips and dust as they fly off the cutting edge.

For roughing, take a deep cut of at least 1/8" and use an agressive feed. That way, the graphite will come off as little pellets instead of a fine dust.

For finish cuts, use a very sharp cutting edge. I always touch up my tools with an India stone or a diamond hone. Use a fine feed and you'll get a good finish. You'll be making dust, so be sure to use the vacuum cleaner.

I don't know what it is about graphite, but either it or the binding agent is abrasive. It will be necessary to touch up the cutting edge from time to time. Again, an India stone works well. If you can see a reflection coming off the cutting edge when held in a bright light, the edge is not sharp enough.

After arriving at your final diameter, polish the piston with a clean sheet of typing paper, newspaper, or whatever else works for you.

Jerry Howell's Web site has some useful tips.

http://www.jerry-howell.com/

This page covers hot air pistons.

http://www.jerry-howell.com/Pistons.html

Like Jerry, I occasionally remove the piston and clean both it and the cylinder. I use a clean paper towel and a bit of de-natured alcohol to remove whatever residue that may have formed during operation.

Regards,

Orrin

Poco graphite had several examples of small pistons on display in their facility when I toured it in 1998. They also have an extensive amount of information available on feeds, speeds, SFM, etc. for each grade of graphite that they make as well as comparisons with their competitors. Most of this information is available for free and I'm sure they would make a recommendation to you over the phone. Also, you're only talking about a small amount of material that may be available as a sample. Poco Graphite

Orrin's approach is very good, but I would recommend using carbide tooling if possible. It's really not difficult to machine overall and the speeds and feeds aren't that critical for one or two piece runs.

This form of Graphite is abrasive and is not a lubricant. The dust itself is inert and won't cause you any health problems other than the possibility of the particles irritating your lungs. Like cast iron dust, it seems to go everywhere it isn't wanted!

Try to find a piece of Poco 3... Even better is Poco AF5, it has a very fine grain structure, you can even flex thin sections without breakage...

Thanks again guys!

My knowlege of graphite has at least quadrupled today!

Mike