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trueing up a 3-jaw

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  • trueing up a 3-jaw

    i have my 3-jaw apart a couble of times to see if i could find the problem with this thing running out .o15, have used the 4-jaw since i have more time than money. the chuck doe's not look like it has been abused. have run dial ind. in all spots that i can think of. I hate to put this in the scrap pile. any help would be nice.

  • #2
    If you look at the post for Metric Translation gears by Doc, there is a link to Logan lathes (gear charts).

    I think I seen an article on that site about truing a 3-jaw (didn't read it)

    Hope it helps

    Also, you might check the backplate to make sure it is true if it is a plain back chuck (Bet you've already done that, but it is a thought)


    Update -- just went to logan site using link. click on Tip Index at bottom.

    Neat fixture

    [This message has been edited by uute (edited 06-10-2003).]


    • #3
      .015 is a bunch, are the jaws in the right slots?
      It's not enough out to have them in the wrong order, probably.

      Is there a lot of play in the scroll around the center "post"? If so, you can shim it and that will help.

      It isn't clear how it could go 0.015 out suddenly, after being ok. Are you sure there isn't anything else outside of it (chips in bad places, etc) that is causing it?


      • #4
        Do you have a set of outside jaws for the chuck? If so, is it out the same amount with them? Also, is it out the same amount with various different sizes of stock in the chuck? If all the above are so I would check the mounting on the backplate first.

        Put the four jaw on the lathe and accurately center a a chunk of solid round stock in the 4 jaw. Put the three jaw on that round stock backwards, clamping it on with the jaws. Use the dial ind. to see if the chuck body OD has runout in relation to the jaws. Try this with different sizes of stock in the four jaw to clamp the three jaw onto. If you see large differences in runout depending on the size of stock then you have a poor scroll and it can't be fixed. If not then remount it on the back plate using a piece of stock in the jaws as a reference instead instead of dialing to the OD. If you are sure your tailstock is accurately aligned to the spindle you can clamp the chuck to a morse taper blank in the tailstock to facilitate mounting it. Keep in mind that most 3 jaw scoll chucks have certain spots on the scroll that are more accurate than others and so will exhibit a variation of centering error depending on the size of stock being held.
        Free software for calculating bolt circles and similar: Click Here


        • #5
          I agree, 15 thou is a lot, but not enough to be the jaws in the wrong slots. Is that error over the range of diameters that can be clamped, or only at one size? If it's over the range, I would try to modify the chuck so an eccentric shim can be put between the scroll and the part of the chuck it turns on. That eccentric could probably be turned just right using the chuck before the mod. One thing to know first, does the outer diameter of the scroll bear against the inner part of the chuck, or does the scroll spin on a stub within the chuck- where would the potential mod have to be done? I'm thinking that if the outer diameter of the scroll is true, and the error is over all clamped diameters, the eccentric shim could be fitted for the outer diameter of the scroll to bear against. Just some ideas.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


          • #6
            I bought an old Hendy Cone head 12" x 5' lathe that had a "nice" Cushman 8" 3 jaw chuck. As I started to us the lathe I got run out from dead on to .010" off. As I looked into it the jaws were very loose in the chuck body. If you mounted the part "on center" it would clamp up on center. If you mounted the part off center and tightened it up it would run way off center.

            My solution was a new 8" Bison TruCenter chuck that is big bangs for the buck.

            Make chips and be safe.

            Pete B.


            • #7
              Go over to Networking, there are several suggestions for your problem of chuck run out. John


              • #8

                In the wrong SLOTS, there might not be much error, maybe not even 0,015, if they are in the right ORDER.

                The slots are close to identical, but the three jaws differ because they hit the scroll at different points.

                Get the ORDER off, and there will be a tenth, or more, as in 0,100 or more, error.

                Chips or bellmouth are possibilities, but I like to have a reason before I grind on a piece of "precision" tooling. So I would want to know why I am grinding, and if that's appropriate, first.

                Some looeness in the scroll, plus a chip on a jaw tooth could set up 0.015.

                After grinding carefully, it could produce some hard swearing too, in a day or two when chip falls out and the 0.015 comes back, now in the other direction!

                Other's mileage may differ, some settling may occur with aging.


                • #9
                  Oso's right. Getting a jaw(s) in the wrong slots but in the correct order can throw it off a few thou. Putting one in the wrong order will be obvious... really obvious.
                  Before doing any grinding, make sure it's CLEAN!

                  Forgot to mention. Make sure the scroll plate (guide) is tight. This is the "axle" for the scroll. Even though it most likely sets down in a resess, It must be tight.

                  [This message has been edited by CCWKen (edited 06-10-2003).]


                  • #10
                    good advice all

                    Another thing I would do is check runout on test bar close to chuck, then check it again 6 or 12 inches out from chuck. If it is not the same (especially if it is worse farther out), look at the backplate as first suspect.



                    • #11
                      the jaws are in the right place. the thing that really made cunfused was that about 10% of the time they would run true. same piece taken out time and time again, just to see if i could see where the problem was. finally just decided to 4-jaw in. I use a 12 volt crane to change chucks it really takes the worry out IF i was to drop it on the ways. even though it is safer and esier it forced me to learn the 4-jaw. much faster and everything starts out at 0 all the way around. thanks for help so far.


                      • #12
                        forgot to say that the 4-jaw is 80lbs. and the 3 120lbs. goes on a 16"x60" reed-printice.


                        • #13
                          Oso, you got me thinking. What I meant was getting the jaws in the wrong order, not in the wrong slots, would throw it out a lot more than .015. Re-reading my post, I see that's not what I said. You raise a point, though, I've never tried putting the jaws in other slots, still in order, since I have etched marks in the jaws and on the chuck, so each jaw always goes in the same slot. I'm going to try the shifting around, and measure runout at each change. I'll post my results, heck, maybe I'll get an improvement.
                          Back to hambone's problem, another factor is which pinion you use to tighten the jaws. You may find that as you tighten these, the last one you do will set the final position of the jaws, or that if you stick to just the same particular one, the accuracy is the best, and mostly repeatable. A somewhat loose scroll could be made to lock up almost right on, or out by a mile, depending on your tightening procedure.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                          • #14
                            When dealing with heavy chucks it pays to make youself a piece of board to fit on top of the ways and under the chuck. Sometmes they have a couple of battens underneath either side of the ways to stop the board being dislodged. With the chuck sitting on the board, you can put a piece of tube through the chuck and into the hollow spindle, lift the tube until the chuck slides on.
                            Having said that, if you can use a crane, so much the better, though the wood still acts as a safety.


                            • #15
                              even though using the crane "makes me very nervous" with chuck change. this weekend i hope to mount and check 3 jaw again. good thing about it, well there is no good thing about or a fast way to change chucks this size. it just takes time!!!!!!!! sad but true! will get to all you guys that have helped with info so far. thank you.