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Making a piston & stub arbor

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  • Making a piston & stub arbor

    After recovering from the excitement of building the vertical steam engine, I got back to work on the John Deere.
    The piston is next.

    I mentioned previously these castings are very close to the finished dimensions. This can make some setups and machining difficult at times. The piston casting was only 0.080" over final diameter, and had nothing to hold in the chuck when machining.

    I started by boring the ID of the skirt to a diameter to clean it up and get it centered with the OD;


    I made a stub arbor by turning a piece of stock to a nice snug fit in the skirt. I milled a flat to clear a 3/16" dowel pin;



    Returning the arbor to the lathe, I installed the piston and pin on the arbor. I used the live center to back up the piston to keep it in place. A scrap with a 60* center spot was used to protect the piston head. Giving the piston a slight turn will lock the pin up, creating a good solid mount for machining;




    The piston was turned to size. I left it about 0.003"-0.005" oversize as I will hone & lap the cylinder bore to size;



    The ring grooves were plunge cut with a cutoff tool and the wrist pin hole drilled & reamed in the mill;





    [This message has been edited by JCHannum (edited 01-29-2006).]
    Jim H.

  • #2
    Nice work JC. I like the trick with the dowel pin. I'll remember that one.
    Free software for calculating bolt circles and similar: Click Here

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    • #3
      JC,
      ditto on the neat trick. that ones a keeper.
      but here's what gets me: if that pin is working the way i think it working, then there has to be a little 'slop' in the mating surfaces (arbor to sleeve) for that to "lock", right?

      i mean, if your arbor was a tight fit (snug fit) in the sleeve, why wouldn't the pin just roll around with the sleeve?

      this is the first i've ever seen this, i'm not claiming to know how its supposed to work. just trying to figure it out. i really like it.

      did it marr the inside surface at all?

      couldn't you have just gripped the ID in chuck? maybe a bullnose center to keep it safe?

      again, nice trick.
      -tony

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      • #4
        Same question, obviously from the results it works, but I just can't figure why the pin doesn't rotate. Sort of like a roller or sprag clutch? Or the different rates of rotation create a friction lock? Thanks for the tip and the pics!!


        <font face="Verdana, Arial" size="2">Originally posted by knucklehead:
        JC,
        snip&gt; if your arbor was a tight fit (snug fit) in the sleeve, why wouldn't the pin just roll around with the sleeve?

        this is the first i've ever seen this, i'm not claiming to know how its supposed to work. just trying to figure it out. &lt;snip
        -tony
        </font>

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        • #5
          Yes, it works like a sprag clutch. The work is snug on the arbor, but can turn. The flat is a tad deeper than the diameter of the pin. Turning the work locks the pin up. Nothing real precise involved, and there may be some scuffing of the bore, but it works.

          An other advantage is it is similar to turning on centers. The part can be removed and returned without loosing concentricity if care is taken in the fit of the arbor.
          Jim H.

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          • #6
            JC,

            Love the trick, falls into the "why didn't I think of that" category.

            Did you know that custom racing pistons aren't round until warmed up in an engine? It seems that because of the complexity of the machining some areas of the piston expand more than others so the good manufacturers account for this. Also the pistons are loose in the bore until the engine warms up. Be careful to leave enough piston to bore tolerance for thermal expansion of the piston vs. the bore.

            Side note, seems you are a fan of 6-jaws, are they much more accurate than a 3-jaw? I often have work that falls in between a 3-jaw and 4-jaw for accuracy hence my reason for making soft jaws for the 3-jaw chuck.

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            • #7
              Nice trick.

              One question... does that not introduce a small amount of out-of-round due to the single concentrated load of the dowel on that side? Those can develop a lot of radial force from a small rotational effort.

              1601

              Keep eye on ball.
              Hashim Khan

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              • #8
                Damn neat trick that.

                Allan

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                • #9
                  JC I use the same trick except that I mill the flat befor I put the manduarl in the lathe.

                  For instance I need a 1" mandural and the stock is 1 1/8 and a 1/8 pin,I mill a flat 1/4" deep plus a few thousands. Than put in the lathe and turn the mandural to the 1" dia. This way it does not have to be moved after turning. Sometimes a file is needed to deepen the flat.

                  I use this way to machine the cylinder for the .6 crusader engine. Machine the mandural almost to the point that you have to twist the project as you slip it on the mandural.
                  Don\'t ask me to do a dam thing, I\'m retired.
                  http://home.earthlink.net/~kcprecision/

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                  • #10
                    This type of arbor probably does deform the part somewhat, and produce an out of round condition. But most workholding devices do the same to some extent. A part from a three jaw chuck will resemble a three leaf clover if you could measure it.

                    Turning the arbor in place as Charlie mentions insures it is concentric with the chuck. Removing and replacing might throw it off. I am using a Buck 6 jaw Adjust Tru, and that is an advantage. It will rechuck to close tolerances. I witness mark the part and replace it at the same location.

                    The 6 jaw offers some benefits over the three jaw, better gripping power and repeatability being the main ones. The soft jaws turned to exact diameter are excellent also for that diameter.

                    I got the chuck basically for nothing, so I do use it. It replaced a three jaw. The Adjust Tru feature is the best part of these.

                    I know pistons are cam ground, and well remember the muscle cars and earlier cars of my youth. The sound of forged pistons rattling around in a W40 442 on a cold morning stays with you. Or a solid lifter small block with a Duntov cam winding out through a LaSalle transmission.
                    Jim H.

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                    • #11
                      JCHANNUM: Could you post a side view photo of the tool holder you were using in your photo? Is that tool holder store bought or did you make it? Gary P. Hansen
                      In memory of Marine Engineer Paul Miller who gave his life for his country 7-19-2010 Helmand Province, Afghanistan. Freedom is not free, it is paid for with blood.

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                      • #12
                        Gary, the toolholder is shop fabricated. I wrote an articlee that was in the June-July issue of Machinist's Workshop detailing it.

                        Here's a photo of the front of it;

                        Jim H.

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                        • #13
                          Neat trick,I got to try that one.Just how muh torque will it transmit?
                          I just need one more tool,just one!

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                          • #14
                            Weird once that baby is locked it will stall the machine or in jCs case it will brake the piston skirt.
                            I used that same set up to completely machine the outside of the cylinders for the .6 crusader engine.
                            Don\'t ask me to do a dam thing, I\'m retired.
                            http://home.earthlink.net/~kcprecision/

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