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Turning Questions

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  • Turning Questions

    I am making a Stirling engine kit that I bought from LMS. As part of that kit, I need to make a finned part that cools the cylinder. Do I want to turn down the workpiece to the nominal diameter, then cut the fins with the cut-off tool? Or do I want to cut the fins first, then turn down to diameter? Which is more accurate? I also need to bore out the center hole, which is the most dimension critical portion of this piece. Should I be doing that first, then cutting the fins?

    Also, I was playing with my lathe the other night and I had turned a piece of 6061 to a nice finish. I took it out and turned it around to face off the other end. After facing, there seemed to be a slightly raised edge on the turned part. Is this caused by my cutting tool (carbide tools from Enco) being dull or my speed incorrect?

    Having fun with my tools. Thank you for your help.

    - Tom

  • #2

    If this is not the actual cylinder, then the dimensions are probably not all that critical. I would imagine it does need to fit on the cylinder fairly close to transfer the heat so the inner bore may be the most critical dimension.

    But in determining the order in which to machine it, what I would consider is the rigidity of the part while machining, not the tolerances on the dimensions. I have never cut fins but this sounds like a fairly difficult operation to me. Cutoff tools are somewhat problematic. You will probably wind up with a fairly thin wall inner cylinder on this part after it is complete and that will be hard to support for further cutting so I would leave the central hole till last. Do the ends and the fins first so the solid core can help support the work while this work is being done.

    As for the fins, as I said, I have not done this kind of cut, but it would worry me. I think I would use two cutoff tools, one to make a starting cut that is not the full width and a second wider one to finish it off. Or use a single narrow tool and make three cuts: center then left and right. I think this will make the job a lot easier and the finished fins a lot nicer looking.

    As for the turned up edge, this happens with softer metals, like some aluminum alloys. A slightly dull tool will definitely contribute to it. Tools should be dead sharp. You can give them a few strokes on a fine stone before making a cut. I use a quick change tool holder to allow me to remove the tool during cuts and stone it if needed. With the QC holder, you can return the tool to the same position after stoning and only a thousanth or two of adjustment is needen in the cut to make up for the material you stoned off.

    Also a good cutting fluid can help prevent this. For a difficult cut like this, I would try to use a non water based fluid like Tap Magic. They make a version for aluminum. WD-40 also works very well for aluminum, but not for other metals.

    However, they do sell deburring tools for a reason and even with all precautions some deburring is often needed.
    Paul A.
    SE Texas

    Make it fit.
    You can't win and there IS a penalty for trying!


    • #3
      Tom, welcome to the board. It sounds like you have started on an interesting and challenging project.

      I would start with the bore first, the OD next and the fins last. You might need to make a mandrel to hold on the ID of the bore for the next operations.

      As for the raised edge, it could be a result of the carbide tooling. Turning can be a combination of cutting and displacing material. There is no shame in cleaning up the edges with a file. Just make sure you have a proper handle on the file. I recommend getting away from the use of carbide, it is not usually needed in most HSM applications. It does take a bit of practice and experimenting to properly grind high speed steel, but it will have to be mastered at some time and you might as well start now.
      Jim H.


      • #4
        It depends upon how your holding the piece. So lets guess at that.
        You need a finished piece about 2 inchs long and have a piece of stock 3.5 long. chuck the stock in about 1 inch. Face off the outside end, measure off this finished surface. Cut your fins with the parting tool from finished surface twards chuck. Drill undersized hole for piston. Bore cylinder. Part off piece.

        I made mine that way and it worked perfect. I would cut the fins before the final bore because your removing stresses and the piston to cylinder clearance is a few tenths 0.0002. i.e. cylinder must be straight.

        Now that the bore is done, finished to whatever size. Make a piston slightly oversize to match. I'm assuming that the piston is carbon, almost friction free. Then polish the piston down to size with 1000 grit paper keeping it cylindrical.

        oh, the raised edge is just a burr. just touch it with a file or for an ID burr use a de-burring tool.