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  1.  
    While building the "Paddle Wheel Engine" in the recent issues of Model Engine builder, one of the parts requires two offset holes.

    The main shaft is .250 diameter and each of the holes to be drilled is to be offset by .0625 (1/16").

    The designer and author suggests the following "Grip in a 3 jaw chuck with packing under on jaw. The packing should be 0.081"

    My question is how is the packing thickness calculated?
    • CommentAuthorgbritnell
    • CommentTimeAug 3rd 2009
     
    That would really be a tough thing to calculate. If you put a .081 packing under one jaw then the other 2 jaws would also move .081 but being as the other 2 jaws would be moving at 60 degrees from the centerline they wouldn't be moving .081 from the centerline but .0405. Now you have to figure the width of the flat on the chuck jaw. Depending on the diameter of the part you are holding it might not touch on the centeline of the jaw but rather on the corners of the jaw so that would give you another mathematical equation. It sure would be easier to do it in a 4 jaw chuck. I made a sketch with measurements from my 3 jaw chuck and I would get an offset of .054.
    gbritnell
    •  
      CommentAuthorgphunt
    • CommentTimeAug 3rd 2009
     
    The easyest way would be trial & error. Chuck up a scrap peice with the shim, turn it till it is just cutting all the way around and meshure it. If the offset is to small increse the shim thickness, if it is to big decrese the shim thickness. This is how I allways done it when I was working. It is a lot faster than doing all the math and gives you just as good a result!
    George
  2.  
    Thanks for the input.
    Dennis
  3.  
    Hello all.....When I need to make and install a off set bushing, Itry to figure out how much I need to off set the bushing hole.....After I figure the amount, I then put the offset amount plus .001 / .002 under the bushing and a single jaw......It seems to me if you want to off set some thing by a set amount, if you put the same amount under one jaw you will have the off set that you need at that one point of the bushing........Thanks...Dale