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  • Press Fit Help Requested

    The engine I am currently building is going to use 3/8" cold rolled shafting for the crankshaft. This stuff measures 0.374" with a micrometer. I want to buy an undersize reamer to ream for a press fit of the shafts into the 3/4" thick brass flywheels. The question is, how much undersize do I want? there seem to be an endless supply of different undersize reamers, but at $25 each, I don't want to guess wrong and buy the wrong undersize reamer. The piece of crankshaft that gets pressed into each flywheel is only about 3" long, so shaft deflection shouldn't be a problem. Any knowledgeable help would be much appreciated.---Brian
    Brian Rupnow

  • #2
    Anything from 0.0004" to 0.001" interference should work, with a strong preference for the lower end of that range.

    Just a suggestion: you could machine the shaft from oversize material, size the pressed in parts to suit a standard reamer, and size the bearing journal for a 10mm ID bearing.

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    • #3
      First off I highly recommend you use something other than CRS for the crankshaft. It tends to be undersized, out of round and not very straight.

      Instead I'd go with some drill rod as it will be much more accurate and consequently be easier to work with during assembly.

      You probably want to go with .0005" to .001" undersize on the reamer. Knowing exactly what size you need is a bit like black magic. Some of it depends on the rigidity of the machine you're using, quality of the reamer, cutting speed & feed rates.

      You may want to consider a shrink fit where you need to heat the flywheel and freeze the shaft before assembly. Then once together finish machine the flywheel so it is concentric with the shaft.

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      • #4
        Sorry, dumb suggestion for the bearing size. I thought this was for the conrod bearing shaft connecting the flywheels.

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        • #5
          Why ream it. Just bore the hole in flywheel in lathe. Measuring the bored hole is tricky; so to get the size "right" I'd create an undersize test piece. That is, lathe a portion (0.001") off a 3/8 bar. This you can measure with a mic.

          If taking a 0.001" cut is too tricky for you; then cut down something larger. A couple of steps in the test piece is often useful to gauge how the boring is progressing.
          Last edited by Norman Bain; 06-25-2013, 02:36 PM.

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          • #6
            Brian; unless you've done some successful pressings in the past I suggest you ask the lurkers here for some tricks as to how to get best result (straightness of the shaft into flywheel) during the actual press itself.

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            • #7
              Of course; we could think outside the square. Bore the hole in flywheel to something less than but close to 3/8". Then machine the shaft in a couple of steps so that there is a portion that will slide readily into the hole and a portion that will "press" into the hole.
              Last edited by Norman Bain; 06-25-2013, 02:56 PM. Reason: typo

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              • #8
                If your stock is measuring .374 I would shoot for .373 or even .3725 since there will be some misalignment in the chuck,tailstock etc. causing a bit of over shoot.
                I just need one more tool,just one!

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                • #9
                  Okay--Faint heart never won fair lady!!! After reading all the responses on this forum and the other forum I hang out on, the general consensus is about .0005 to .0015 interference for a press fit on a shaft this size. Since the shaft I want to use is 0.374", I have ordered a .3735" chucking reamer, which is .0005 undersize. That should get me close to where I want to be. I will leave enough material on the flywheel for a "skim cut", and true it up after pressing by holding the shaft in the chuck jaws of my lathe and taking a very light cut on the flywheel.----Thanks guys, for your help.---Brian
                  Brian Rupnow

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                  • #10
                    Hi Brian-

                    New reamers often cut a little oversize. I would recommend you drill and ream a few holes in some scrap and measure them before doing the actual part. In general, a dry reamer will make a bigger hole than with cutting fluid. Higher RPM makes a bigger hole than slower.
                    For small parts I often use the mill vise to press parts together as it has high clamping force, smooth jaws and accurately parallel jaws. If you turn a short length of the pin (1/8"?) to .001 under the reamed hole it will help get everything started straight.
                    For future reference - .001 clearance makes a nice slip fit. Size to size makes a light press fit. .001 interference makes a firm press fit.

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                    • #11
                      As mentioned : Drill rod for the axle and bore the hole. Reamers are a shot in the dark and will not make the hole in your fly wheel true to the center line or perpendicular if you are not extremely careful. You could also use loctite and fixture for accurate alignment.
                      Toolznthings

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                      • #12
                        Although not the right way, I would not have been afraid to use a .375 reamer then put a knurl on the shaft and set it with some 603 Loctite. It is amazing how well that stuff holds.

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                        • #13
                          I thought I should come back and post a fitting end to this thread. The undersize reamer I purchased had a diameter of .3735". The cold rolled shafts I had originally chosen to run as crankshaft ends in my engine were .374" diameter--- a thou under the optimum .375" which was my planned target diameter. They did press fit into the reamed holes, but with only .0005" interference, it was a pretty wimpy press fit. You couldn't push the shaft through with your fingers, but it certainly didn't take much grunt with my small arbor press. My next move was to get some nominal 3/8" drill rod, which comes in oversize at .3755". Now, with an interference of .002" it really took a lot of "grunt" with my small arbor press to assemble the crankshaft ends and flywheels. So---If you ever wonder, a really good press fit for a 3/8" shaft is to ream the hole .002" less in diameter than the measured diameter of the shaft going into it.--Brian
                          Brian Rupnow

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                          • #14
                            Good post Brian, need more of this procedures posted here.

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