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What Could Have Moved Here???????

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  • What Could Have Moved Here???????

    The other day I was working on my balancing hub for my surface grinder. I have a 4" round, I turned one end to make sure it was round because you all know that these rounds aren't really round. So after I did that I chucked it up gently tightened the chuck as I wiggled the round to feel when all the jaws are contacting it and then snugged it down.
    I turned the OD to exactly 3.75 and then proceeded to carving out the groove for the weights. The groove is .313 deep by .438 wide.
    It's a real PIA to cut a groove like that as I had to pick away at it with a .062 wide grooving bit a little at a time. When I was close to finished dimension I went in and cleaned up the sides and bottom with a carbide tipped square tool bit that was a little under .250 wide.
    You have to be careful when bottoming out with the square tool bit as it'll chatter like hell. I avoided all that.... learned from past experience.
    Anyway............ After I did that and proceeded to cut the inside hole out I noticed what I thought was the OD running slightly out of round.
    I didn't pay much attention to it as I thought my eyes were playing tricks on me. I left it chucked up with intentions of finishing it up this morning. I decided to check the OD for run out because it bothered me. To my surprise the OD was out of round by almost .010.
    I had to trim .005 off the OD to get it back to round. after that I checked it, I had about .0002 run out which is consistently normal for anything I turn on my lathe. My first thought was after carving out the groove the OD had moved because of stress being relieved from cutting the groove, nothing got hot.
    I then went ahead and cut the center out. When I finished that I checked the OD again and found it to be out about .002.
    Is the round moving in the chuck????? I know it's hanging out about 5" but it's a 4" dia. round. The chuck it still tight,
    If I push on the round when indicating it I only get about .001 deflection, and I know my bearings are good.
    What could have caused this??? Never had it happen before.

    JL..............





    This is the 3" OD one I made for the T&C grinder, I didn't have any problems with this one and this was a 3 1/4" dia. and it was hanging out a further than the 4" round.

  • #2
    Could be a few things. Is the face of the round against the chuck square and smooth/deburred so as to prevent any 'walking' of the round in the jaws? Not a very likely cause, but an easy thing to get out of the equation. As far as stresses being relieved maybe change your order of opps and see if that helps.

    Comment


    • #3
      My suspicion would be chuck jaws that are slightly bell-mouthed. It wouldn't take much, only a couple tenths, to let the work move and be telegraphed out to the end. One way to explore the possibility of shifting would be to indicate, then tap the work lightly with a soft hammer and re-indicate.
      .
      "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

      Comment


      • #4
        if you get too much runout, just make a balance hub for it.....

        off hand, a thou seems like quite a bit of flex. I've never used a 6 jaw so am the opposite of an expert on them......but.....for solid shapes they always seemed to me a risky business vs a three jaw where you know the work is contact with three jaws - that are 120 degrees apart. I'd thought their use was for thin walled stuff that would be damaged by a 3 jaw.....and hence would get deflected into contact with the 6 jaws. that's going to happen with a solid shape. Try it in a three jaw or better still 4 jaw

        I like the idea for the large dia hubs....I made three last weekend but had an od of 1 3/4 (with 1/4" balls). Didn't seem to do much with a hand test and I was disappointing....At some point I'll measure it more scientifically with and without the hubs....you think if the principal is sound is just a matter of getting enough moveable weight in there at the right distance. what size balls are you using, and any sense of the best percentage of fill?
        Last edited by Mcgyver; 10-18-2016, 02:22 PM.
        in Toronto Ontario - where are you?

        Comment


        • #5
          You got a lot hanging off the chuck there to be finish-turning to good tolerances.

          Instead of starting with the back I would rough-machine the front including the wheel arbor and under-size the through-hole then part/saw it off. When you have as many as you need roughed out hold each them by the arbor OD in the 4-jaw and indicate the outside. Turn the ID taper and other rear features. Finally put some stock in the 3-jaw chuck and taper-turn it to mirror the spindle including a thread to clamp the wheels. Now mount each wheel in turn on the taper never removing it from the chuck. You should end up with all of the important features very concentric.
          Peter - novice home machinist, modern motorcycle enthusiast.

          Denford Viceroy 280 Synchro (11 x 24)
          Herbert 0V adapted to R8 by 'Sir John'.
          Monarch 10EE 1942

          Comment


          • #6
            Originally posted by softtail View Post
            Is the face of the round against the chuck square and smooth/deburred so as to prevent any 'walking' of the round in the jaws?
            This was my first thought due to only hearing him say that he turned the other end but did not hear him mention he faced it,,,

            there also looks like there's a slight gap between the part and the chuck,,,

            facing the part when turning and then flipping it around and butting it up against the chuck adds great stability - no longer at the mercy of "floppy jaws"

            it is a must for precision work when at all possible to do so - and especially extended work...

            Comment


            • #7
              Maybe uneven jaw pressure allowed slight movement while machining the grooves on the face. Like this?

              The "more even grip" aspect of 6 jaw chucks has a certain element of machinists myth about it. The whole point of a 3 point gripping system is that its inherently force equalising in that the work floats between the 3 supports until each exerts equal force. Hence any scroll or other errors in a 3 jaw self centre chuck don't affect the radial gripping force. Run out and longitudinal ( along the jaws ) errors are a different matter. A 6 jaw chuck over constrains what its holding so any scroll or jaw errors directly affect the grip exerted by individual jaws. Analysis is more than a little complex....

              http://www.practicalmachinist.com/vb...6/#post1393592

              Comment


              • #8
                You did not show that "square bit". I know from experience that, when working on a groove on a face, you need extra clearance on the outside (left) edge of the tool bit you are using to form the walls of the grooves. This is due to the radius of those edges. So, if you used a standard square bit, if may not have enough clearance and it may have been rubbing. That rubbing can easily pull the work piece out of position in the chuck. When making a similar feature that had an even larger diameter, I used three tools, first a roughing one with clearance all around, and then I finished up with left hand and a right hand tools to do the actual side walls of the groove as well as the bottom. I used both of those finishing tools on the bottom and met the cuts in the middle. I made extra sure that the tool that formed the wall with the ID had more than enough clearance.

                If your square bit does not have extra clearance on that ID edge, then it is very suspect.

                You say you did one that stuck out even more than this one. Did you use different tools? Or perhaps it had a larger diameter? Or did the chuck just grab better from dumb luck? Hard to say. That six jaw looks nice, but I would have used a four jaw. With six jaws it is possible for the highest four to actually do the gripping and you have no way of knowing which three those are. The other three may be in contact, but not providing much gripping power. If the three that are actually gripping are distributed two adjacent jaws on one side and a single jaw opposite one of those two, then it could be easy for the work to rock in their grip. You may get some clues if you examine the areas where the jaws were tightened against the work piece with a good magnifier. Look for patterns in the scratches to see how it shifted.

                I have never completely understood what the proper use of a six jaw chuck would be. With a three or four jaw, you are almost completely assured that all the jaws will be gripping with a good amount of force. And those areas where they grip are properly and evenly distributed around the work. With six, it is a crap shoot. I would think a six jaw is not only limited to very round work, but also to light cuts. And even there, I don't see any significant advantage over a well made, accurate three jaw. Five jaws may be a better choice. Three out of five produces a better grip pattern than some cases of three out of six.
                Paul A.
                SE Texas

                And if you look REAL close at an analog signal,
                You will find that it has discrete steps.

                Comment


                • #9
                  Originally posted by Paul Alciatore View Post
                  Y, I don't see any significant advantage over a well made, accurate three jaw..
                  for thin walled stuff needing the clamping force distributed
                  in Toronto Ontario - where are you?

                  Comment


                  • #10
                    Originally posted by softtail View Post
                    Could be a few things. Is the face of the round against the chuck square and smooth/deburred so as to prevent any 'walking' of the round in the jaws? Not a very likely cause, but an easy thing to get out of the equation. As far as stresses being relieved maybe change your order of opps and see if that helps.
                    The back of the round is just band saw cut but it's not touching the chuck. About a 1/16" of clearance.

                    The chuck isn't bell mouthed, and it didn't loosen up.

                    Paul, I did re-cut the side relief of the square tool bit from the factory 3 deg taper to about 15 deg. Plenty of side clearance.
                    the only thing I can think of is the few times it started to chatter, and it wasn't much, the vibration could have shifted the round a little.
                    I was almost tempted to hold it on center for extra support when I cut the groove but I was working cramped as it was.

                    JL..............

                    Comment


                    • #11
                      Originally posted by davecutter View Post
                      Maybe uneven jaw pressure allowed slight movement while machining the grooves on the face. Like this?

                      The "more even grip" aspect of 6 jaw chucks has a certain element of machinists myth about it. The whole point of a 3 point gripping system is that its inherently force equalising in that the work floats between the 3 supports until each exerts equal force. Hence any scroll or other errors in a 3 jaw self centre chuck don't affect the radial gripping force. Run out and longitudinal ( along the jaws ) errors are a different matter. A 6 jaw chuck over constrains what its holding so any scroll or jaw errors directly affect the grip exerted by individual jaws. Analysis is more than a little complex....

                      http://www.practicalmachinist.com/vb...6/#post1393592
                      I know what you mean with the uneven jaw pressure. I always rotate and wiggle stuff a little as I close up on it and then you can feel when all the jaws are seated.

                      JL.........

                      Comment


                      • #12
                        Originally posted by Mcgyver View Post
                        for thin walled stuff needing the clamping force distributed
                        Makes good sense - but what also makes sense is a 6 jaw on ridged material can be worse than a three for holding - any three jaw regardless of it's scroll offset...

                        three jaws are guaranteed equalized pressures 120 degree's apart - 6 jaws can easily have the unwanted combo of two opposing jaws taking up the load force 180 degree's apart due to either material deviation and or scroll offset, this leaves the material basically un-clamped in both 90 and 270 degree areas - or technically 60-120 and 240-300.

                        Comment


                        • #13
                          Originally posted by Mcgyver View Post
                          but.....for solid shapes they always seemed to me a risky business vs a three jaw where you know the work is contact with three jaws - that are 120 degrees apart.
                          Originally posted by A.K. Boomer View Post
                          three jaws are guaranteed equalized pressures 120 degree's apart - 6 jaws can easily have the unwanted combo of two opposing jaws
                          hark, I think I hear an echo

                          we're in agreement...I'm thinking the same thing, with only .002 tir it might wobbly in the chuck the .001....but its so little you can really feel it or see it easily
                          Last edited by Mcgyver; 10-18-2016, 03:12 PM.
                          in Toronto Ontario - where are you?

                          Comment


                          • #14
                            Originally posted by JoeLee View Post
                            The back of the round is just band saw cut but it's not touching the chuck. About a 1/16" of clearance.
                            Joe I thought I seen a gap - this is not good practice - could be this and the combo of the 6 jaw hard to tell but whenever possible always face your backside whilst turning and when installing butt it up tight and clamp - this adds a major amount of rigidity to your piece esp. when extending...

                            Comment


                            • #15
                              Originally posted by Mcgyver View Post
                              hark, I think I hear an echo

                              we're in agreement
                              Absolutely was just expanding on it - makes good sense Mcgyver

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