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How would you machine a bayonet mount?

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  • How would you machine a bayonet mount?

    Someone recently asked me about making a bayonet mount similar to that used on some SLR camera lenses. The diameter would be about 2.5 to 3.0 inches.

    It seems this would be much like machining a short length of a very fine pitch multiple-start threads, and then cutting away sections so they could be engaged and locked.

    I don't see much problem machining the rings to make them interlock, but I can't figure out how to make such a short length of multi-start threads. The only thing that makes sense is setting the lathe up to cut the threads, and then turning the spindle manually one quarter turn back and forth as I advance the bit. That just doesn't seem very practical.

    Is there a way to do it on a milling machine with a rotary table?

    Any ideas?

    Roger

    Any products mentioned in my posts have been endorsed by their manufacturer.

  • #2
    Well, for one thing a "bayonet mount" doesn't have threads. That's why they call them bayonet mounts. They're just a big washer with the cutouts (on the lens side).
    What camera?

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    • #3
      That was how it was described to me, and I'm not familiar with cameras using that type lens mount.

      I guess a more accurate terms would be interrupted thread, or maybe breechlock. The idea is to get some tightening of the joint as the parts are rotated toward the locking position.

      The Posiedon missle had several large-diameter joints like that, but I don't know how the mating rings were machined.

      Roger

      [This message has been edited by winchman (edited 04-24-2004).]
      Any products mentioned in my posts have been endorsed by their manufacturer.

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      • #4
        Here's a pic of a Cannon Lens mount. It is a "bayonet mount". The silver ring around the outer edge is the mounting ring. You can see the cutout areas (three). These cutouts match tabs on the camera body. The tabs go into the cutouts then the lens is rotated about 1/16 turn and locks. Thers's a locking pin that engages. To remove, a button is pressed on the lens to disengage the lock pin.



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        • #5
          Forgot to mention. As stated before, the ring is nothing more than a large washer with cutouts. The ring sits down in a resess and held in place by three tiny screws (around the outside of the lens). The back side of the "Washer" could be made with slopes between cutouts to give you your tightening needs. I think I would still put a lock on it.

          [This message has been edited by CCWKen (edited 04-24-2004).]

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          • #6
            Yeah, the lock not only locks the lense in position but ensures the mechanism for depth of field preview and auto exposure is lined up properly.
            Free software for calculating bolt circles and similar: Click Here

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            • #7
              Many artillery pieces had interupted thread breech blocks. I believe they where just single start threads with carefully indexed sections milled out so that the male and female milled sections would slide together to full depth and then a partial turn would lock them together.
              THAT OLD GANG 'O MINE

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              • #8
                A bayonet mount is a pair of interlocking flanges one internal and one external that when assembled axially and rotated a partial turn may be engaged to form a rigid assembly but readily disengaged. Usually there's a locking feature to hold the assembly engaged and oriented. Generally there one diameter fit and one flange fit to ensure radial and axial alignment.

                Making a bayonet mount is first a lathe job accomplishing the flange thicknesses and diameter fits then a rotary table job on a mill (or circular interpolation on a CNC mill) to machine the lugs. None of this is difficult machine work but it can be intricate and the steps have to be planned well.

                Interrupted threads and timed lugged theads are another matter. Here the machinist can exercise his greatest creativity (or meet his Waterloo) in the grandest and most spectacular fashion.

                There is no flop sweat like accompanying a breech ring out onto the ship and have a gang of guys check fit it to the tube and breech door. There's about 20 guys in a compartment where there's room for maybe six, outside mechanics, riggers, bosses, the BuOrd rep, and assorted spear carriers.

                Dire predictions are made "I thought there was supposed to be a wide lug next to the hinge" "Hold it! The threads are on the worng hand.""Acme threads? The breech has square" Haw Haw Haw.

                Things are positioned and coaxed. Everything is snug so the threads will only engage if perfectly positioned. The breech has multiple theads so naturally it gets mis-oriented a couple of times. Finally it goes together and the breech door is closed through the breech ring's lugs. A partial additional 1/16 turn and the ring lugs lock over the door lugs. Silence while the ordinance guy goes over it with a feeler. "They're all hittin'"

                "Yeah well that went OK. The inside guys didn't serw it up. That's a first." "You son bitches wouldn't know good machine work if it bit you..." "You was bone lucky we were there to hold your hand."

                No respect.

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                • #9
                  I mounted a lens for use on a Minolta SLR camera some years ago and it had a bayonet mount. The lens had a simple interrupted washer like flange as described in the other posts and the camera had a more complicated arrangement that provided a spring tension on the lens flange to hold it tight. And there was a small notch at one point for a latch to hold it secure. I've seen the interrupted thread mounts but can't remember which brand of camera they are on.

                  I machinned all of the lens parts except the bayonet and the iris. I purchased those two parts, the iris came from Edmund Scientific and the bayonet was an adapter that I purchased at a good camera store for a few dollars. It was well worth the price.

                  There are universal lenses that are made to fit any brand of camera and the camera stores that sell them will have the adapters for the various brands. The adapters will be a perfect fit for the camera and will usually have a simple thread for mounting it to the lens. I'm sure you could find the appropriate adapter for your camera. Much easier than machining it.

                  If you are determined to roll your own, first you need to determine the exact thread used and if it is really a multiple start thread or just a single start. There is no inherent reason why this kind of mount has to be multiple start and I suspect a single start would be better from a mating point of view since it would only lock in one position. If it's really a multiple start, they are cut by engaging the lead screw at different places on the threading dial. The exact technique depends on the TPI and your lead screw pitch. I've never done one but I have read about it.

                  Paul A.
                  Paul A.

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

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                  • #10
                    I think if I had to do a piece like that, I'd turn the piece to major diamaters and cut the appropriate grooves on the lathe. Then I'd need a rotary table mounted on the vertical mill table, on a sine plate or or a rotary that could do compound angles on it's own to cut the ramps for the bayonette part. I would think it would need some sort of locking pin to maintain alignment once the bayonette mount was fully engaged.

                    I'm no expert, but this is what I'd try (if I had a rotary table and a sine plate).

                    John

                    Edited a it seems I was typing more slowly and with much less expertise than Forrest. Glad to see him posting and glad I was at least thinking in a similar vein.

                    ------------------
                    Pursue Excellence and the rest will follow.

                    [This message has been edited by Excitable Boy (edited 04-24-2004).]

                    [This message has been edited by Excitable Boy (edited 04-24-2004).]

                    [This message has been edited by Excitable Boy (edited 04-24-2004).]
                    Pursue Excellence and the rest will follow.

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                    • #11
                      Thanks to all of you, and especially John for mentioning the RT and sine plate.

                      If the part is mounted on the sine plate on top of the RT, it will cut a ramp as the RT is turned. With a small angle on the sine plate and a small rotation of the RT, the result will probably be acceptable. The trick will be making a fixture which will position the part over the axis of the RT with a way to index it for cutting several (hopefully) identical ramps.

                      As usual, it's not how to remove the metal--it's how to hold the part while you do it.

                      The other option appears to be machining a short length of multi-start or regular threads, and then removing segments as needed to make the interlocking joint.

                      Thanks,
                      Roger



                      [This message has been edited by winchman (edited 04-24-2004).]
                      Any products mentioned in my posts have been endorsed by their manufacturer.

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                      • #12
                        If you have a table, you can make the ramps. Heck, you don't even need a rotary table. No need for a sine plate either. The ramps only need to be about .003-.004" difference between the low and high side. (Dependant on your skills to make the recess behind it and the tabs on the male side.) A shim will work for the slope angle. Then, use a 1/2-3/4" end mill.

                        Don't make it more complicated than necessary. If this isn't production stuff or to seal poision gas couplers, why invest in a bunch of equipment for a couple of rings. The pros here have bazillions in equipment and machines and can provide all the trig calcs to make it; the masters could make it with a file.

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                        • #13
                          "As usual, it's not how to remove the metal--it's how to hold the part while you do it."

                          Don't make the "washer" first. Make the slopes and cutouts first.
                          1. Take a comfortable length of tube stock and face off both ends.
                          2. Mount tube, cut your slopes.
                          3. Make your cutouts.
                          4. Drill your holding pockets.
                          5. Part-off "washer".
                          6. Debur.

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                          • #14
                            The old threadded mounts were a "Pentax" mount. I have made lens extenders a few times for my old Fujica and cameras like that (Pentax). The extenders, when set up right, really help in focusing for extremely close work.
                            CCBW, MAH

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