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Press Fit Dimensions

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  • Press Fit Dimensions

    I made a bushing out of W-1, 1/2" diameter, 3/8" long. I need to put this in a holder that is an old barrel piece, 416 stainless. It is .716" diameter. I drilled a dimple in the side of the bushing before I hardened it, and will drill and tap a hole in the holder to fit in the dimple. But I'm thinking of making this a press fit also, not really press though, freeze the bushing and heat the holder. When I bore the hole in the holder, do I make the hole the same as the bushing, or a bit smaller? Also, would a propane torch heat the holder enough or should I use my oxy/propane torch and get it red hot?

  • #2
    Standard shaft hole fits, you want Class V:

    When you heat the holder, or cool the bushing, or both, you need enough temperature difference so that the expansion/contraction is at least the amount of interference (from the standard fits charts above) so the parts slip together when heated/cooled and lock together when they (very quickly) come to an equilibrium temperature.

    The change in dimension will be dL = L*C*dT where L is the original length, C is the coefficient of thermal expansion, dT is the change in temperature. Generally for steel the coefficient of thermal expansion is about 0.000012/C (C) (or 12E-6/C). So if you have 0.001" of interference (dL) and you want the parts to slip together at 0.5" (L) to assemble, you need the parts to differ in temperature by at least dT = dL/(L*C) = 167 C or 333 °F. You can do this by heating the outside, or cooling the inside, or a combination of both that adds to to the required temperature difference. If you need 0.002" interference at 0.5", then double this temperature, etc.

    Note that the heated part will immediately start to cool when you remove it from the heat source, and the cooled part will immediately start to heat when removed from the freezer, and the speed to come to thermal equilibrium will increase dramatically when the parts are in contact with each other. You need to go past this "at least" temperature by a fair bit (20%-50%) and work FAST when assembling. If you haven't left yourself some extra heat and expansion, or if you dawdle when assembling, then you'll end up with parts that are locked together when only partially assembled.
    Last edited by DrMike; 11-01-2020, 06:22 AM.
    SE MI, USA


    • #3
      Don't bother with the freezing: a typical freezer's 0* will give a dT of 70*, but with heating, you can get dT of 500* easily. Freezing also involves running back to the shop from the kitchen while trying to keep the part cold.

      Unless you have dry ice.


      • #4
        Watch your hoop stress if the female part is thin.

        Last edited by Doozer; 11-01-2020, 10:14 AM.


        • #5
          I use the electric stove and the freezer in the kitchen.
          Put the part to be heated on a bit of flat plate on the stove top, put the part to be cooled in the freezer.
          Cool the part in the freezer for about an hour before heating the part on the stove, pick up the oven gloves, when the part on the stove is judged to be hot enough ( Have never got anything red on the stove but certainly hot enough) quickly assemble, leave to cool naturally and have a cup of tea.
          Incidentally, wife fully approves and will assist if needed.
          Regards David Powell.


          • #6
            If your bushing is thin walled, about 80% of the press fit will end up being a shrinking of the ID of the part. If your ID is a critical tolerance plan on bringing it to size AFTER press/shrink fit.