Machining Graphite

Discussion in 'Tips and Tricks' started by kiwi2, Aug 4, 2018.

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  1. Aug 4, 2018 #1

    kiwi2

    kiwi2

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    Hi,
    I'm going to revisit a Stirling engine I couldn't get to work. I'm looking to swap the aluminium piston with one made of graphite. Any tips on machining graphite? I'm going to need to turn it and put a slot in it with a slitting saw and maybe thread it as well.
    Regards,
    Alan C.
     
  2. Aug 5, 2018 #2

    Cogsy

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    The only tip I can give is to mount up the nozzle of a shop vac close to the cutting tool and run it while you're machining. It helps keep the mess to a minimum. Apart from the mess I like machining graphite, it holds detail well, is easy to machine and finishes up great.
     
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  3. Aug 5, 2018 #3

    el gringo

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    Very well said...
    Ray M
     
  4. Aug 5, 2018 #4

    kiwi2

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    Thanks for the comments. I'll set up my shop vac as suggested and spread newspaper over the surrounding areas as well.
     
  5. Aug 5, 2018 #5

    Brian Rupnow

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    Use a very sharp HSS cutting tool. Graphite cuts very very easy, but it doesn't like any side thrust on it from dull tooling. When you get close to final diameter, leave it .002" to .003" oversize and work it down to final size with the reverse side of a sanding strip.
     
  6. Aug 5, 2018 #6

    doc1955

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    As Brian states but for the final size I use news paper to get to the finish size and I get her a little closer than .002 or .003 I go for about .001
     
  7. Aug 6, 2018 #7

    olympic

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    Great tips! And just what I need, as I'm about to try a graphite piston in Dr. Senft's "Poppin", which I haven't been able to make run since I built it about two years ago!
     
  8. Aug 6, 2018 #8

    deverett

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    Newspaper has also been used for final sizing/polishing.

    Dave
    The Emerald Isle
     
  9. Aug 6, 2018 #9

    plipoma

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    I've done that, it works well but is messy.
     
  10. Aug 6, 2018 #10

    BaronJ

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    Hi Guys,

    No problem machining graphite, however be aware that the dust is conductive and will pass through the filters in your vacuum cleaner. The result will be a damaged motor ! A vac that is half full of household debris will capture most of the fine particles. For the same reason don't use a vac to clean up spilt toner. You can get vacs with special filters for this use.
     
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  11. Aug 6, 2018 #11

    Anatol

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    Newbie question
    why would you choose graphite for a piston? Self lubrication? Thermal insulation?
    Is in hard enough to withstand steam? How would you attach said piston to a wrist pin or piston rod? Can you bond it (graphite) to a metal plate, ie with epoxy? (but then epoxy might let go in heat...)
    Is this 'graphite' pure, or is it glued together with some kind of binder?
    thanks!
    (always learning)
     
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  12. Aug 6, 2018 #12

    Brian Rupnow

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    Graphite is very lightweight and is self lubricating because in many external heat applications such as Stirling engines you don't want to use lubricating oil because it "gums up" from heat and restricts the running of the engine. The correct type of graphite for making pistons is very dense, and does not have the appearance of a sintered material. It machines very easily with sharp HSS tooling.
     
  13. Aug 7, 2018 #13

    BobsModels

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    My granddaughter just finished her Stirling engine last May. She made a graphite piston - OD .282 x .3 long for her piston on a Sherline. Plus 1 for dense graphite. She needed to bore it .186 x .252 for the sleeve to hold wrist pin for the crank. The other end needed a recess for a 0-80 flat head screw to hold the sleeve. Without some really dense graphite it would have crumbled. She just kept working the OD until it slid into the cylinder. At the end she used a sheet of plain white paper and some patience to get the fit just right. I forget who told me that trick. By the way I have never made a graphite piston so she was sort of her own. Unfortunately I did not get my usual pictures as I was the vacuum holder!

    Here was the result, nice and smooth. I had posted it last May.

    https://youtu.be/TYlPWvfav4o

    I got the graphite at McMaster.

    Bob
     
  14. Aug 7, 2018 #14

    kiwi2

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    Thanks for all the great information. I got the graphite from China so I'm not sure of the quality. A case of suck it and see I suppose.
    I'm think I might try getting the engine to go using the existing aluminium piston but lubricating it with graphite powder instead of oil. Mr Pete on YouTube restored an old Stirling engine a while ago and showed how to trouble shoot the problems which stopped it from going. I'll work through that process as well. If there is still no joy, I'll proceed with the graphite piston.
    Thanks again,
    Alan
     
  15. Aug 10, 2018 #15

    jim333us

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    The Graphite works amazingly well on these relatively low power motors. I really doubt you would get similar results using graphite as a lubricating medium! You should first concentrate on getting the cylinder polished up as high as you can get it. I used a bronze cylinder in mine with a mirror bore polish in it, then I fit the piston to it. The piston fit should be air tight, and friction free! Those two terms usually don't go together well. Graphite is actually perfect in this situation. Little friction even in very tight tolerances, it also expands very little with the amount of heat necessary to run these engines. My Stirling motor was from a Home Shop Machinist article years ago. The plans called for an aluminum piston and bronze bushings for most of the bearings. I used ball bearings wherever possible, and also used graphite for the displacer shaft bearing. Mine runs all day long on a smallish alcohol lamp flame! I've been displaying it for years at Cabin Fever Expo, and the NAMES show in Michigan. Around a dozen people said they made the same one from the article, but their's won't run without a blow torch on it, and only for very short periods of time. I didn't even waste my time making an aluminum piston for mine. Because going to those shows and asking a lot of questions to the people who had very nice running Stirling's, I kind of knew it would never run as I wanted it to if I had followed those original plans! The graphite machines extremely well, so go for it!!! Polish the cylinder first! The fit your looking for is just a few ten thousandths smaller than the bore! The piston should hold in place with the cylinder air tight on the bottom (as in the palm of your hand), when you break the seal the piston should fall right to the bottom! Hope this helps! Jim
     
  16. Aug 10, 2018 #16

    olympic

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    Good thoughts there, Jim, especially about polishing the bore. I'll try that, too, once I find some graphite (no McMaster available here in Canada).

    Obviously (at least to me) it takes very little friction to prevent these things from running. I used ball bearings instead of sleeves for my Poppin', tried three pistons so far (2 aluminum, 1 steel), and even though it appears to coast freely it will not run at all when the flame is applied.

    Fourth time lucky?
     
  17. Aug 10, 2018 #17

    kiwi2

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    Thanks Jim.
    One of the first things I'll check will be to ensure I'm getting a pressure variation at the cylinder when the displacer moves back and forth. Tubal Cain has a video where he troubleshoots an old Stirling engine which won't go.

    He has a clever way of using a thumb cut from a vinyl glove at 24minutes in, to check whether there is a pressure swing at the cylinder.
    Regards,
    Alan
     
  18. Aug 10, 2018 #18

    Anatol

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    Jim, while the graphite is thermally stable, what about the bronze cylinder? Do the tolerances open up a little when the engine gets hot? At those tolerances I guess this would matter. I seem to remember high school physics demo about heating an iron washer - the ID increased, much to the surprise of students.
     
  19. Aug 10, 2018 #19

    jim333us

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    The cylinder should also be very accurately parallel with very little bell mouthing on either end! My Stirling can get very hot after running for 6 to 8 hours, years ago when I first made it after it got hot it would run slower & more finicky, I even at times had to let it cool down a bit. This is only a guess but it might have been within 1 to possibly 3 ten thousandths. I used regular loose leaf paper to sand or burnish the piston down! Again only a guess but it could be closer to 4 or 5 tenths now. It now runs well no matter how hot it gets, so I'm guessing that was from the bronze expanding the way it used to run. Jim
     
  20. Aug 12, 2018 #20

    kiwi2

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    Hi all,
    I've finally got round to facing the graphite rod I got out of China. I used a tangential tool steel cutter and the graphite peeled off and fell down the face of the cutter onto a sheet of paper beneath so there were no issues with graphite dust going everywhere.
    The interior of the rod seems to have voids in it.
    I think I have uploaded a picture (the new system is a lot more confusing than the old one).
    Is this usual? I was expecting a smooth finish.
    Regards,
    Alan Graphite.jpg
     

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