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  • How to machine a cam on a lathe?

    Hopefully, this isn't too dumb a question...I want to make a small cam for tensioning a drill press slow speed belt setup.

    Is there a good technique for drilling / boring the center offset hole in the cam using a lathe - and, say using an independent four jaw chuck? It seems to me that if I try to offset the piece in the four jaw chuck that it will not grab the piece solidly.

    Now, I could put the piece in the mill and easily offset the hole - but, I'm curious whether you'all have any slick methods of doing an eccentric hole offset with a lathe.
    Bob J

  • #2
    Chuck

    If you do not like the four jaw set up use a face plate and clamp the work in position to bore the hole. Or bore it using the milling machine.

    JRW

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    • #3
      the four jaw will grip it solidly, depends though on how much of an offset we're talking about - you'd have to go with a faceplate if the offset was beyond what the four jaw can do. sometimes its easier to drill/ream the hole then mount a stub mandrel in the 4 jaw and indicate it to the desired offset
      Last edited by Mcgyver; 11-30-2007, 11:47 AM.
      .

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      • #4
        A four jaw will do fine. An independent four jaw chuck doesn't know it has a center and will hold wherever you want.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          That's a perfectly valid setup for what you want to do. In the four-jaw, two of the jaws will set the offset and an indicator will show the amount of lift from low to high. The other two jaws should show the same indicator reading. Then drill, bore and ream to your heart's content.
          .
          "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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          • #6
            Shazaam - you guys are good - fast responses and good ideas, thanks.

            So, let's see - if I want to make, say, drill a hole with a 3/4" offset in a 2" cam - if I move the piece over in the 4 jaw chuck 3/4", won't that move the work past the flats on the chuck jaws? This then have two of the jaws holding on the work on the side of the jaw.

            Maybe I'm over thinking this setup - and, should just go try it out and see what happens.

            Again, thanks for the quick replies
            Bob J

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            • #7
              Why not just do it on the drill press or mill Alistair
              Please excuse my typing as I have a form of parkinsons disease

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              • #8
                How to machine a cam on a lathe?

                There may be a rule of thumb for this
                Originally posted by RJulian
                So, let's see - if I want to make, say, drill a hole with a 3/4" offset in a 2" cam - if I move the piece over in the 4 jaw chuck 3/4", won't that move the work past the flats on the chuck jaws? This then have two of the jaws holding on the work on the side of the jaw.
                but I don't know it. I have made some cams of this type for use in model steam engines. These were for the valve displacement and the amount of offset was pretty small compared to the diameter of the part.
                Given your 3/4" offset in a 2" circle, I suppose it depends upon the size of hole you intend to drill and the beefiness of your chuck. Personally, I suggest laying out the location and clamping the part to a faceplate. Since extreme accuracy is not needed for what you are doing, why not set up the offset in the 4 jaw chuck and center drill it with a small drill. You can then use a center in the tailstock to hold the part to the faceplate while clamping it.

                Alan

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                • #9
                  Machining eccentrics & cams on a lathe.

                  In my experience, I've made 3 eccentrics or cams in a lathe. In all cases, I used a 4 jaw chuck and a centre finder.
                  One was a valve eccentric for a 5A steam engine. The others were offset hubs that a ball bearing was pressed on to. This provided reciprocating motion to operate a hydraulic jack pump.
                  Lay out your centres, centre drill, set up in the 4 jaw and drill and ream each hole as required. Use your tailstock centre or centre finder wiggler (or Sir John's device) to locate. The 4 jaw chuck is a very flexible workholding device. You can also reverse jaws as needed if that increases the security of the work holding.
                  Don't be afraid to use a mandrel if you have to finish the OD after the offset has been machined.
                  Hope that helps....
                  Rick

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                  • #10
                    OK, I'm with you guys on this, makes sense. Thanks

                    But, what's a "Sir John's device"?
                    Bob J

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                    • #11
                      Holding on the sides of the jaws with a four jaw chuck is standard practice. I just whipped up this rather extreme cam to show how it may be done. I used acetal because I had a piece the right size handy. Don't laugh, Briggs and Stratton has been using acetal cams for many years. If using metal then a center support would be called for while turning.






                      It's perfectly ok to hold by any part of the jaws that make contact with the work. If the hold isn't particularly secure or the stickout is much then bring in the tailstock center to assist. Just make sure that the jaws are not wound out too far. There should be at least 1 1/2 turns of engagement with the adjustment screw. Also, when turning off center work you may need to run the spindle slower than normal to avoid excessive vibration.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        Depending on how far off centre you want to be you can get away with bunging a lump of packing under one jaw of a 3 jaw chuck to cause the work to throw off centre.

                        Something not dimensionally critical like a belt cam can easily be done very quickly this way.

                        .
                        .

                        Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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                        • #13
                          you can (on a unimat certainly) reassemble the 3 jaw 'wrong' putting jaws 1 and 3 in the slots for 3 and 1 to achieve the same thing. I made the eccentric for my first steam engine this way, as I only had a 3 jaw chuck. I dont know if it would work with a larger chuck as Ive got a big 4 jaw for my harrison.

                          Dave
                          Just south of Sudspumpwater UK

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                          • #14
                            Re: Dave's trick, I would think any 3 jaw could be assembled with the jaws offset in that manner as long as you can easily remove them. There is no requirement that they be the same distance from the center.

                            The main limitation with this is you are limited by the scroll's pitch for the possible offsets. You could even assemble them out of order for 1-3-2 instead of 1-2-3 for additional possible offsets. But you are still stuck with the limited number of offsets it will produce.

                            As for adding packing on a 3 jaw, that is also possible and I have done it. It will give you a continous range of possible offsets, but predicting the results can be difficult. I have tried the math and it gets complicated with the width of the jaw faces to take into account. An accurate CAD drawing would help. But you may still have to experiment to get it down to the last thousanth or so.

                            Best way is to get a 4 jaw.
                            Paul A.
                            SE Texas

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

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                            • #15
                              Sir John's device.....

                              The "Sir John's device" I refer to is a type of centre finder that uses a dial indicator and simple linkage from a centre point. It fits in your tailstock quill and seems so much more elegant & easy to use rather than various "wigglers".
                              John Stevenson posted a drawing of it on here a couple of times....Search under centre finders, maybe.
                              Thanks to the knight-machinist too.
                              Rick

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