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Round peg in a round hole (but that same size)

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  • Round peg in a round hole (but that same size)

    Silly question, but baffling to an aspiring machinist. I am working on Chris from Clickspring's Gyroscope project. You can see it where I am at below on an aluminum test piece (before I try it on expensive brass).



    In his plans the axle is 3/16", as well as the hole in the flywheel (which I did with a reamer). So if I have a 3/16" piece of SS rod, how would you approach getting that to fit into a 3/16" hole? Going up 1/64" on the flywheel hole would be way to big. I guess I could turn down the 3/16" rod, but Chris didnt seem to do that. I have the feeling I am missing something obvious

  • #2
    Heat the aluminum, and freeze the rod in dry ice. I've seen it done many times on larger pieces when both parts were stainless steel, but not on anything this small or on a mix of materials. The higher expansion rate of aluminum should make it easier, though.
    Last edited by winchman; 03-11-2017, 02:11 PM.
    Any products mentioned in my posts have been endorsed by their manufacturer.

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    • #3
      Size to size is a light press fit. Just press it in. A slip fit is with the hole .001 bigger than the rod. A medium press fit is with the rod .001 bigger than the hole. No heat or cold needed on any of these unless both the parts are soft and would gall during a press fit. Some lube helps when press fitting parts.
      Kansas City area

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      • #4
        Does the "axle" you are putting in need to spin free or does it need to be tight in the bore?

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        • #5
          Originally posted by Toolguy View Post
          Size to size is a light press fit. Just press it in. A slip fit is with the hole .001 bigger than the rod. A medium press fit is with the rod .001 bigger than the hole. No heat or cold needed on any of these unless both the parts are soft and would gall during a press fit. Some lube helps when press fitting parts.
          Great info here. When light press fitting, would I need a real press? I don't have one. Also the ends of the axle will have been turned to a sharp point (bearing in cone of sorts).

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          • #6
            Originally posted by oxford View Post
            Does the "axle" you are putting in need to spin free or does it need to be tight in the bore?
            The plans call for using lock tight. See https://youtu.be/j6ExQ4utHEo?t=540. You can see he just slips the axle in by hand, which seems to defy the specs in the plans.
            Last edited by Croy9000; 03-11-2017, 04:49 PM.

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            • #7
              Originally posted by Croy9000 View Post
              Great info here. When light press fitting, would I need a real press? I don't have one. Also the ends of the axle will have been turned to a sharp point (bearing in cone of sorts).
              This sort of matching sizes push fit can be done easily with even a drill press. It won't even take all that much pressure.

              You don't want to push against the tip of the axle if you've already sharpened and polished the end to fit into the cone bearing. To cover this off I'd make up a "push button" from a bit of scrap. Drill it with something like 5/32 then follow it with a short counter bore of 3/16 drill. That'll slip over the conical end of your shaft and push against a ring of contact out around the edge where a slight bruise won't matter.

              In time you will likely want to either buy a medium size arbor press. Or a smaller home shop size press could be done with a length of threaded rod and nuts in the frame. A suitable home shop size screw press could be done in a manner that it breaks down for storage too.

              I also often use my bench vise for smaller jobs using an array of stuff as pushers or supports.
              Chilliwack BC, Canada

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              • #8
                Might be time to invest in a set of over and under reamers... Drilling rarely gives a good fit when you need to hit a particular number and nominal sized reamers may not match your shaft size.

                As above, you can use your bench vise to press parts. An arbor press is better.
                Last edited by lakeside53; 03-11-2017, 10:06 PM.

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                • #9
                  everything has tolerances of course. Reamers can cut slightly under nominal, and stock can be larger than nominal leading to a heavy interference when you expected close locating fit. Adjustable hand reamers are very handy for getting the exact fit you want.

                  Probably better advice, when it really matters, don't rely on bar stock dimensions. SOP is Drill/ream or bore then turn the male to the fit you want.
                  Last edited by Mcgyver; 03-11-2017, 10:30 PM.
                  .

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                  • #10
                    If the SS is just "Mill stock" it can be all over the place, and not even very round. If you don't want to make your own, use TGP material ("turned, ground and polished" - that can be extremely accurate.

                    Measure what you have in several places and when rotated 90 degrees. Don't be afraid to polish in your lathe it with 220-400 grit emery to get to desired size.

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                    • #11
                      Lakeside has it right with SS barstock. I personally don't have any experience with 3/16 but the can tell you that larger OD will in fact be all over the place and may or may not be round. Checking with a mic may not give you the whole story on what is actually going on with it.

                      If you don't have a press I would start it with a hammer and "gauge" how tight it is. If it seems like it will go with out much trouble continue to drive the axle in. If it seems too tight take it back out and heat the aluminum up your axle should drop right in.

                      If looking for a fit that requires locktite (which you should be using a retaining compound not a thread locker) most of the time using a drill bit of the size of the stock that is going in the hole will work. Drill bits will usually drill slightly over size.

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                      • #12
                        Originally posted by Croy9000 View Post
                        The plans call for using lock tight. See https://youtu.be/j6ExQ4utHEo?t=540. You can see he just slips the axle in by hand, which seems to defy the specs in the plans.
                        He specifically states that one should slide the wheel on the axle to center it in the frame, then add Loctite. It has to be somewhat easy to move the way he has centered it.

                        If I were building this project, I'd put the axle in the lathe collet with slightly more than half exposed, and then very slowly work it down with fine wet/dry paper (500 grit?) until the wheel tightly slides to the desired position. "Slowly, slowly, catches the monkey"

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                        • #13
                          I've used super glue and/or a press fit. If you don't have an arbor press, large c-clamps can be rigged up. With the size of yours I would recommend loctite 609. Note it's not just the "gween," kind. "609," or similar will work. I've used it to put together the snap ring and impeller's ring fit many times. Heating the outside material, the "gyro" and possibly cooling the spindle. Not really necessary if they are the same size, just makes it easier. Wipe the excess loctite off of course. Here is a link, http://www.loctite.com.au/3320_AUE_H...=8802648195073 of the loctite.

                          Sent from my RCT6513W87 using Tapatalk
                          Last edited by zachary.s.hodge; 03-12-2017, 03:36 AM. Reason: Added picture of my "1/2" size gyroscope.

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                          • #14
                            Also know that reamers come in almost any size you want. It is common to find common dimensioned ones that are on size, 0.001" under size, and 0.001" oversize. This is how a shaft that is "on size" can be either a slip fit, a force fit, or a shrink fit.

                            Actually some sources will have reamer sizes by thousandths so you can have any number of whole thousandths you want. And others are down to fractional thousandths, like a 17/64" reamer would be 0.2656". You are the machinist and you determine the fit of the hole/shaft combination.
                            Paul A.
                            SE Texas

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

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                            • #15
                              If you have drilled a hole and the rod still won't slip,it can't be more than a few thou undersized.Make a "D" bit reamer from the SS stock you are using.Since the Aluminum is quite a bit softer than the SS rod,the rod will be hard enough to ream a few thou out of the bore without much trouble.Use cutting oil or WD-40 of course to prevent galling.
                              I just need one more tool,just one!

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