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  • Press fit dowell pin

    Dowel pin press fit problem
    Two parts of a mechanism to be guided by a pair of 0.125" dowel pins. One part to be press fit the other sliding fit.
    With the two parts held in alignment the holes were drilled through with a #31 bit (0.120") , the sliding part reamed with a 0.127" reamer and the pins slide fine.
    Question: what is the options for getting a "reasonable" size press fit hole with no reamer in the size range??
    I'm thinking that the 0.005" interference is going to be way too much and the next size bit is 1/8" so it may not be
    tight enough. (no metric bits and even then the only one would be, I think 0.122") So will one of the "Locktite"
    materials be adequate to lock a steel dowel pin to an aluminum part and will the 1/8" bit be appropriate???
    The pins are 1" and the sliding block is 1/2" thick. The press fit is to be 1/4" deep in the other. (Is that sufficient?)
    That will allow the sliding block to move a quarter inch or a bit more safely.
    Opinions or preferably "knowledge" appreciated. Thanks
    ...Lew...

  • #2
    Lew - Iv had to use dowels for allot of power transfer mechanisms in things iv built over the years, and recently so with .125",,,
    it's a coercing of an endmill and would recommend practicing on some same but scrap material first , but - iv been able to achieve press fits in pieces with same size endmill by just drilling really close to size with a bit first - then using the endmill as a finisher,,,
    the other piece - the engagement piece that is - I don't drill nowhere near as close and sometimes depending on the material not at all - then the dowel seems to be more of a slip fit,,, I think the spiral flutes in endmills "fan out" some when under more pressure and create this effect just like a drill bit does...


    the one good thing is when you get the fit you want in the scrap material you know what you just did to achieve it and if you get one you can usually get a half dozen or whatever...

    Comment


    • #3
      Hi,

      Been there. My first reaction is to make a D-bit. Then size is what ever you choose.

      If something like that isn't an option, then I have drilled a matching size hole - i.e.1/8" drill for 1/8" dowel, (do a test drill first), then I either simply LocTite the dowel in. If the hole is a touch to big, I have scarred up the inside of the hole and then LocTited the dowel in with good results. But, it can be a YMMV situation.

      Dalee
      If you think you understand what is going on, you haven't been paying attention.

      Comment


      • #4
        Is the actual part a small one?

        Originally posted by dalee100 View Post
        Hi,

        Been there. My first reaction is to make a D-bit. Then size is what ever you choose.

        If something like that isn't an option, then I have drilled a matching size hole - i.e.1/8" drill for 1/8" dowel, (do a test drill first), then I either simply LocTite the dowel in. If the hole is a touch to big, I have scarred up the inside of the hole and then LocTited the dowel in with good results. But, it can be a YMMV situation.

        Dalee
        IF the part is a small one, and can be held on the mill in such a way that you can heat it with a propane torch without damage to the mill,or vice then you could heat it, not to red but to black before red, without causing permanent distortion, then drill with a good 1/8 in drill. On cooling you should find the dowel a light press fit. (The actual part where the hole is to be must NOT be stopped from expanding by vice jaws or clamps, sometimes not easy to arrange) I have done things like that, sometimes works, but really is only for those Sunday nights when you HAVE to get her done. Hope this hint helps someone. Regards David Powell.

        Comment


        • #5
          You're right. pushing the pins into that .120 or thereabouts size hole would not be good. If you really don't want to spring for a .123 or .124 size reamer then one other option would be to follow down with the pilot hole with a .125. Then with a 3/16 pin punch swage the mouth in a little with some "love taps" until the pin won't fit. Or you could make a few marks around the hole with a center punch to swage in the mouth for a good fit. Then goober up the hole and pin with a high strength version of Loctite or use some medium viscosity CA glue. Tap the pins down until they center in the pilot. Between seating in the pilot to some degree and the peening at the mouth it should hold the pins in place decently. Then the Loctite or CA will kick off and hold things that much better for the long term.
          Chilliwack BC, Canada

          Comment


          • #6
            To get a "good" fit you need to use the proper reamer. Another option is to open the hole up with a needle eye lap...
            Keith
            __________________________
            Just one project too many--that's what finally got him...

            Comment


            • #7
              The D-bit reamer idea is a good one,use one of your dowel pins for the blank.Chuck the dowel in the lathe and hit it with a few passes from an oil stone,check the diameter with a mic.Maybe shoot for .1245" try a few test holes before jumping in with both feet.
              I just need one more tool,just one!

              Comment


              • #8
                Since a rule of thumb for press fits is one thou per inch of diameter, for a 0.125" dowel you don't need much. Many dowel pins are made a couple tenths oversize, so if you drill a 3/32" hole and finish with a good 1/8" drill, and your dowel pin is actually 0.1252" it may be about right. I like the suggestion to warm the block a bit before drilling.
                ----------
                Try to make a living, not a killing. -- Utah Phillips
                Don't believe everything you know. -- Bumper sticker
                Everybody is ignorant, only on different subjects. -- Will Rogers
                There are lots of people who mistake their imagination for their memory. - Josh Billings
                Law of Logical Argument - Anything is possible if you don't know what you are talking about.
                Don't own anything you have to feed or paint. - Hood River Blackie

                Comment


                • #9
                  true not all dowels are created equal either and that could be key in getting a proper fit,

                  the heating before boring is a great idea esp. with aluminum

                  last week I just went through this with 17-4 H-900 SS

                  first holes I bored I simply plunge cut with the .125 endmill - no pilot hole, they were so loose they were not even acceptable as a slip fit, next holes I center drilled and then pre-drilled to within .010" and then used the endmill,,, had to use a ball peen hammer to get them in and was seriously worried I was going to break a dowel -- barely made it .250" in

                  the receptor part needed to be slip fit for ease of assembly and disassembly,,, so I center drilled and then used a small pilot hole like about .050 or so - ended up being perfect, no play and yet ease of mating and disconnect...

                  The endmill I had seemed new but might have been worn some --- usually using an endmill of same size the dowel always seems to go in at least without a fight so maybe combo of 2/10ths larger dowel and an endmill slightly undersize

                  Comment


                  • #10
                    we get dowel pins in h6 and m6. if you have a good hole (H7) the former will be a medium sliding fit, the latter a light press fit. alternatively i play with oil and alcohol when drilling or reaming. the diffence usually does exactly what you want. as a matter of fact i will be doing it this aftenoon with 3 mm pins. i have no reamer (would not live long i guess) so it will be a drill +oil/alcohol.

                    edit: i predrilled 2.5 mm and its done. one side of the pins is pressed in, the other slides.
                    Last edited by dian; 12-21-2016, 10:35 AM.

                    Comment


                    • #11
                      I wouldn't trust a drill to give an on-size hole, particularly when looking for a press fit. I'd ream that hole with a .125 under reamer, .1245 is I had one, .124 if not. .001 press on a .125 dowel pin is a pretty good press. (Lew, since you're local feel free to borrow the reamer(s) from me.)

                      Comment


                      • #12
                        This is why there are "over" and "under" reamers. The loose side at 0.126 or 0.127, and the tight side at 0.124.

                        Drills are totally unreliable for press fits. In a pinch, you can measure you drills, find ones of suitable sizes, round off the "outer corners" on the cutting lips, and use that for a reamer. The rounding helps them self-guide and not cut oversize.

                        But if you use drills, Locktite is your friend.
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          Yep - a bearing retainer variety of loctite (can't remember the number) would probably work if you drill the hole. But I suggest buying the right size reamer OR... another trick not yet mentioned... stuff a tiny bit of oil soaked rag down one flute of an undersized reamer. It will cut oversize. I've never tried it on such a small reamer, but it works great with larger sizes. It's hard to control the exact dimension this way but if you just need a snug press fit or a nice sliding fit (i.e. not doing anything by numbers), this method can work in a pinch.

                          Comment


                          • #14
                            For dowel pins, .0005 under is a medium press and .001 under is a tight press. You don't want tighter than .001 under. A nice slip fit is .001 over. With a DRO you can put the holes in one part, then do matching holes in the other part and they will be a perfect fit. Make sure to use a spot drill to make the drills start on center though. If all you have is drill bits, you can drill to a close undersize, then use an on size. I like LocTite #648 retaining compound (green) for this type of work.

                            As J Tiers says, drill bits vary in size. Use a micrometer to find one that fits the situation. If you only have one of that size, you can still mike it to know what you have.
                            Kansas City area

                            Comment


                            • #15
                              I have a number ofhand reamers and taper pin reamers.

                              Originally posted by J Tiers View Post
                              This is why there are "over" and "under" reamers. The loose side at 0.126 or 0.127, and the tight side at 0.124.

                              Drills are totally unreliable for press fits. In a pinch, you can measure you drills, find ones of suitable sizes, round off the "outer corners" on the cutting lips, and use that for a reamer. The rounding helps them self-guide and not cut oversize.

                              But if you use drills, Locktite is your friend.
                              Hand reamers are( usually) slightly tapered and so If you do not go in full depth you can get a slightly undersize albeit tapered hole, getting tighter the deeper you pound the dowel in. The taper pin reamers have a steeper tape but can be used to give a start into an undersize hole. If you spring for the proper reamers and drill only slightly undersize with good drills the reamers will work for hundreds of holes . IF you have a spare reamer of the nominal size you can take a couple of tenths off it by reaming a hole in a block of steel, putting a little fine grinding paste on the reamer and running the reamer in the hole while gently pulling it to one side. The exact size you get is unpredictable. Remember, only the ends of machine reamers cut ( in theory!) Hope these further bodges help get someone out of a problem. Regards David Powell.

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