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Help offsetting stock in 4-jaw

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  • Help offsetting stock in 4-jaw

    I need to offset a piece of round stock in a 4-jaw so that I can bore an off-center hole.

    What I did was first center the stock, then displace it upward by loosening the top jaw and tightening the bottom one. An indicator at the top showed the desired displacement (zeroed before). Positive reading with stock "up" and negative with stock rotated 180 & "down".

    What I'd like help with is adjusting the side jaws. With the stock displaced, the side jaws have to be tightened from their centered position. With the side jaws aligned with the indicator they should have the same reading. The problem is that the reading is very sensitive to the alignment - a little rotation from perfectly aligned changes the reading a lot. (Unlike the top & bottom readings which self align by finding the min or max reading.)

    I eventually got close-enough with seemingly endless diddling, but I'm thinking that there has to be an easy way to do this. Some trick that's passed down from master to apprentice that I'm not aware of.

    Note - marking the offset center on the bench & then centering on it is not an option. I want to use the precision of an indicator to do the locating.

  • #2
    I'm going to post here so that I can see the answers you get. Currently, I stand the stock on end in the mill vice, then use the DRO in the X axis to shift the part over by the required amount, then drill with a countersink tool. Trouble is, when you flip the part to do the other end, there is no guaranteed way to keep the shaft from rotating a bit and throwing everything out of whack.---Brian
    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

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    • #3
      If the item is round stock then which jaws create the offset doesn't matter as long as the offset is correct. So don't worry about the side jaws centering as long as the total min to max is correct for your required offset. Or do you need the offset to be "clocked" so it's in a very particular direction? If it's something like that you might be wiser to do it in the mill.

      It would be tough to read the side to side because with any sizeable amount of offset you'd be trying to zero out any side to side on very sloping parts of the round item. And that would be tough because just the slightest rotational mismatch will show up as large variations on the dial. So basically you can't trust that and you're going to look for simply the max and min and it doesn't matter if the side jaws are matched or if you get the offset needed from some combination of high and low and shift side to side of the side jaws. As long as the max to min is right amount.

      When I've done offset holes like this in the past I was generally working with a measurement from center or the OD and center punch mark. The reference for center being the deliberately left center pip on the end face. The pip diameter being left at around .05'ish and measured and accounted for in the marking out. Or if measuring in from the OD I'd scribe a diameter line through the center and then measure in from the OD using my calipers. Then prick punch and center punch.

      The center punch was indicated for zero using a floating centering rod with the aid of the tail stock. This centering aid I made is pointed on one end and center drilled other end and about 5 inches long. It sits with the pointy end in the center mark and cup end on the tail stock center. And a short parallel portion for the dial indicator is turned at the same time as the pointy end is formed on the one end so it is centered with the point and not a possible source of error. This centering aid is the only "old machinist's trick" I've used for this sort of thing.

      With the centering aid as I recall it wasn't any harder to set than a regular centering. Likely because I wasn't trying to measure and adjust for side to side using very sloping portions of the part. As I recall it wasn't any more difficult to center the mark with this setup than it was to dial in any item for center. Just the usual looking for runout in the two cross axes and tweak them out. But the last time I did this was in the last shop which is something over 10 years ago now.
      Chilliwack BC, Canada

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      • #4
        You don't have to have the offset in line with the jaws when using round stock. An indicator with more travel than double the offset would make the adjustment easy though. If the offset is to be 1 unit of length from the centre, then adjusting the chuck until the indicator reads a total movement of 2 units when the jaws are tight will be accurate.

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        • #5
          Brian, you posted while I was typing.

          If I had to make both marks on the ends so they were along the same offset axis like you're after I'm thinking I'd clamp the round stock in a "V" block and mark out both ends using height gauges on a surface plate or reasonable flat substitute. Or depending on the size of the stock you're doing perhaps use a V block with clamp and set the round stock with clamp against an angle plate and clamp it all down. Or if short enough just clamp the round stock and V block directly in the mill vise. The point is that the V block would provide the needed flat reference to ensure you're on the same axis on both ends. Doesn't matter if it's used for marking out only and then holding in the lathe or if the V block is kept clamped and used as the reference plane in the mill.

          Or some other similar "consistent but temporary flat reference" might occur to you from this idea.
          Chilliwack BC, Canada

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          • #6
            I couldn't see any mention of turning the work around in the chuck to bore the ends separately, so I have assumed all the boring would be done from one end.
            Getting both ends lined up when the bores are eccentric would involve some sort of clamped on alignment fixture, which is much more difficult.

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            • #7
              Okay--I can see using a V-block and a surface plate or cast and planed machine surface. Even so, you can only get "eyeball close" when you go to pick up that second end if you are picking up lines to center punch and put a countersink hole in.
              Brian Rupnow
              Design engineer
              Barrie, Ontario, Canada

              Comment


              • #8
                Originally posted by Bob Engelhardt View Post

                Note - marking the offset center on the bench & then centering on it is not an option. I want to use the precision of an indicator to do the locating.
                Use the centering rod as suggested by BCRider, and set the indicator against the rod. If you use the flat tip accessory for the indicator it will not slide off the side of the rod. The indicator will then give you the total runout minus a few percent for measuring some distance from the face of the work. When the movement reaches zero, it's centered.

                Dan

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

                Location: SF East Bay.

                Comment


                • #9
                  Rock the spindle, this will tell you where the high spot is on each side.

                  Comment


                  • #10
                    Brian, doing it in the mill I think you'd want to center the OD of the work with a swept DTI on the spindle for each end and then offset using the dials. The V block in the vise or held to the angle plate would only be to ensure the offset stayed on the same plane when you swap ends.
                    You're right in thinking that trying to line up a center punch mark on the mill would be a bit of a disaster.

                    If working to a slightly lesser degree of accuracy where careful center punching would be OK and doing the drilling in the lathe I'd rely on that centering aid rod in conjunction with the dial gauge.
                    Chilliwack BC, Canada

                    Comment


                    • #11
                      I get into this game making eccentrics and crankshafts.
                      So far I have not found an :" easy" answer.
                      However, I do make things a bit easier for myself by using a " Pusher" from the tailstock, usually a drill chuck with jaws retracted into the body and a piece of wood faced with rubber or soft plastic.
                      By gently pushing the work against the chuck face I can stop it from falling down when I am trying to get the offset on jaws 2 and 4 and then , once I am near GENTLY tighten 1 and 3. Then I can go round tighten all chuck jaws and check the offsets.all this without the annoyance of the job merrily falling into the swarf tray !
                      This scheme will, of course not help when picking up a centre punch mark.
                      Incidentally I number my chuck jaws with large magic marker numbers. This seems to help.
                      I do not make parts for anyone else nowadays, but I do like my crank and eccentric throws within a couple of thous of intended dimensions.
                      Hope this helps someone . Regards David Powell.

                      Comment


                      • #12
                        Remember, aluminium or copper shims let work slide easier on the jaws.
                        if you are doing say a pin that needs turning around, just try to get one narrow turned spot on the far end, then clock ofc of this when you turn piece around.

                        Comment


                        • #13
                          Originally posted by brian Rupnow View Post
                          I'm going to post here so that I can see the answers you get. Currently, I stand the stock on end in the mill vice, then use the DRO in the X axis to shift the part over by the required amount, then drill with a countersink tool. Trouble is, when you flip the part to do the other end, there is no guaranteed way to keep the shaft from rotating a bit and throwing everything out of whack.---Brian
                          I don't think that will work on my horizontal mill. If I had a vertical mill I would just do the offset bore on it.

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                          • #14
                            Bob, did you want to bore from both ends, or just from one end?

                            Comment


                            • #15
                              Originally posted by BCRider View Post
                              If the item is round stock then which jaws create the offset doesn't matter as long as the offset is correct. So don't worry about the side jaws centering as long as the total min to max is correct for your required offset.
                              Yes - thank you. In fact, it might be easiest to have the offset between 2 jaws - instead of #1 positive, #3 negative, and 2 & 4 equal, have, say, 1&2 positive and 3&4 negative.

                              Or do you need the offset to be "clocked" so it's in a very particular direction? If it's something like that you might be wiser to do it in the mill.
                              No clocking & no (vertical) mill

                              ... When I've done offset holes like this in the past I was generally working with a measurement from center ... Then prick punch and center punch. ...
                              Yeah ... that's what I'm trying to avoid - locating & punching.

                              Your point about the offset not having to line up with a jaw pair is what I needed. Thank you.

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