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Need to "interface" 1/8in key stock to 1/2in round stock

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  • Need to "interface" 1/8in key stock to 1/2in round stock

    I need to interface a small DC gear motor to the end of some 1/2in solid steel round stock which is part of a kind of bench top motor dynomometer.

    The motor has a square drive "socket" that nicely fits 1/8in square key stock (see attached).

    Don't have any means of broaching.

    My first inclination was to square up the end of the round stock to fit the motor "socket", with my Bridgeport, until I discovered that the key stock fit the "socket" so nicely as-is.

    Now I'm thinking of drilling a hole in the end of the round stock on my lathe, of the appropriate diameter, for an interference fit, and pressing the key stock in and possibly tack welding it.

    Can anyone advise of any clever alternatives for mating the 1/2in round stock with the 1/8in square key stock?


  • #2


    • #3
      If you have a press, you can make a broach out of 1/4" HSS tool bit. I've done it a few times. After you drill the hole, press the bit into the hole. It will just be cutting the corners so grind the center of bit to expose corners. I welded a button on the back end of the bit so I could use my slide hammer to extract it. And yes, you can weld HSS. I've made rectangular dies with them too.


      • #4
        Woodruff Key. If I understand what you are trying to do. 1) Does the 1/2 round stock fit n the hole when the square stock is removed? 2) If it fits, does it fit tightly? If not turn the shaft to fit and use a Woodruff Key cutter to provide pocket in the shaft.
        Depending upon the load on the key I would probably use a #403 or #404 key with a matching cutter.

        After a second look at the photo, I can not tell if the square is centered in the hole or as it first appeared to me that it is on one side of the hole.
        Last edited by Stepside; 02-02-2016, 08:30 PM.


        • #5
          Drill a hole in the 1/2" round for a close slip fit of the square, then drill and tap through #10-24 or 1/4-20 for opposing setscrews.
          Kansas City area


          • #6
            Drill a 1/8 inch hole in the round stock. A few strokes of a file at each corner will make it square enough.

            Alternate: Drill to slight interference fit. Fill with epoxy of your choice. Press (pound, hammer, etc) the shaft into place.

            At the end of the project, there is a profound difference between spare parts and left over parts.

            Location: SF East Bay.


            • #7
              If you've got a mill then the obvious fix is to square the end of the round stock. Get yourself a square 5C collet block and a 1/2" 5C collet and you'll be done in a few minutes. All these other ideas seem kinda mickey mouse to me...
              Just one project too many--that's what finally got him...


              • #8


                • #9
                  Drill and ream the 1/2" shaft to insert a square hole sleeve.



                  • #10
                    Turn a series of diameters on the end of the key stock, so the end is 1/8" diameter, with perhaps a slight undercut to provide a sharp cutting edge with some rake. Possibly also make some grooves for chip clearance. In other words, make the end of the key stock into a square broach. Drill a 1/8" hole in the shaft, and press into place. Something like this:


                    Paul , P S Technology, Inc. and MrTibbs
                    USA Maryland 21030


                    • #11
                      you could also make an round/square adapter and glue, press or pin in place. depending on the load.


                      • #12
                        Excellent ideas everybody.

                        As stated, I originally intended to do as Keith said in post #7 (squaring off the round stock with my Bridgeport), but was concerned about the sharp "corner" at the transition being a stress riser. I'm an amateur machinist and I'm not sure which end mill to use to make the radius. A ball end?

                        I had forgotten about the epoxy option. I've used good 'ol JB Weld to glue tool bits into sockets before. Works great. Getting JB Weld down a deep skinny hole might be a bit of a trick.

                        Thanks for the ideas.


                        • #13
                          Slot the 1/2" deep enough to be 3 sides of the square hole. Solder a piece in to be side #4 and if needed/desired, turn to round it off.

                          The square end is a good idea, and the stress riser likely is no issue unless there is more power involved than it sounds as if there is.

                          Keep eye on ball.
                          Hashim Khan

                          It's just a box of rain, I don't know who put it there.


                          • #14
                            If you're worried about a sharp corner, side mill it.
                            Location: Long Island, N.Y.


                            • #15
                              Originally posted by becksmachine View Post
                              Drill and ream the 1/2" shaft to insert a square hole sleeve.

                              Yep, my thought too.

                              My assumption is there's a need for concentricity here.