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3 Jaw Chuck Issues

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  • 3 Jaw Chuck Issues

    My five year old Chinese lathe, a "CX701" from busy Bee tools in Canada came with a 6" three jaw chuck with a 1.5" thru hole in the chuck and spindle. When new I tested the run-out on the spindle and on a test piece held in the chuck, and got .003" total indicted runout. This seems more or less what is normal for a lathe like this, and it never caused me any problem. However, gradually over the last year, the chuck has started holding anything clamped in it a bit crooked. Runout on the spindle, as checked without the chuck in place is still +/- .003 total indicated runout. I have a bearing on a square shanked tool, and with that in place I can still persuade the piece in the chuck to run straight. This is an added step and a great pain in the arse. I priced a new 6" chuck this morning from Busy bee, and they cost around $260 Canadian. I have unbolted the chuck from the spindle and cleaned all of the mating surfaces, but this doesn't fix the problem. The lathe gets used almost every day. I numbered the jaws and the slots they fit in when I got the lathe new, so it isn't the jaws being in the wrong slots. I'm at a bit of a loss here. I don't want to have to buy a new chuck, but I want the chuck I have to hold pieces true without having to resort to secondary methods to make the piece run true. Any suggestions?
    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

  • #2
    Jaws may be bell-mouthed, letting the work shift around. You can grind them to be straight again, and the work will not shift around.

    That will not act to make it "centered" perfectly everywhere, but it WILL fix the issue of the work shifting, unless the chuck is really fouled up. As you know, the 3 jaw is not reliably centered anyhow, so that's OK.

    BTW, did you mean 0.0003" runout on the spindle? You typed "+/-0.003" , which is a ton of spindle runout. I assume you meant the 0.0003"

    Last edited by J Tiers; 11-07-2020, 11:54 AM.
    CNC machines only go through the motions


    • #3

      Have you disassembled the chuck and cleaned the dirt and chips out of it? That's the first thing I would do before doing anything else. It's cheap and easy and often solves the issue with runout on a scroll chuck.
      If you think you understand what is going on, you haven't been paying attention.


      • #4
        No Mike, I meant what I typed. The runout is about 0.003" tir. That is only 0.0015" out of center.
        Brian Rupnow
        Design engineer
        Barrie, Ontario, Canada


        • #5
          There is a lot that can go wrong with a three jaw chuck! Take it off, strip it and clean it. Whilst it is stripped, check to see if the jaws are loose in their slots, check the scroll for damage to the spiral caused by overtightening, check for lateral play between the chuck body and the scroll, and check the teeth on the jaws for damage and embedded swarf. Swarf can be removed, but for any of the others, the chuck is worn out. If you use the lathe daily this would not be surprising on a chinese chuck. Reassemble the chuck, refiti it to the lathe, get a peice of good round stock, and making sure it fits over the entire length of the jaws, grip it fairly lightly in the chuck, and as you tighten it, look carefully for the front of the jaws moving towards the tail end of the lathe, indicating wear in the slots or jaws.With a small torch, see if you can see light getting through between the stock and the front of the jaws. If you use stock that does not cover all the depth of the jaw on a regular basis, it could be that the jaws are belmouthed on an otherwise good chuck, in which case closing the jaws onto a ring, or three peices of stock cut to fit between the jaws, and then grinding the jaws will cure the bellmouthing, however, it is very likely on a chuck of this quality that there will be all of the faults I have listed above, and if this is the case, you will only improve the accuracy when gripping stock of the diameter set when you grind. All of this is immaterial if you always make sure that the stock goes right through the chuck jaws, and turn all the diameters. If however you use the stock diameter as a finished size, what you have turned will be out of concentricity with the stock diameter. 3 to 4 thou runout on a 3 jaw chuck when new is acceptable tolerance, three jaw chucks are not perfectly accurate ever. If you want to use the outer diameter of the stock as a finished diameter you should use a collet chuck The most accurate chuck you have is a 4 jaw independant chuck!
          Man who say it cannot be done should not disturb man doing it!


          • #6
            That runout on the spindle really needs addressing. Are the bearings adjusted ok? Is the end of the spindle a large flange with holes in to the mount three and four jaw chucks?


            • #7
              Brian with as much as you use your lathe I think cleaning it might help unless you have already and it didn't help. Also, as Jerry mentioned the jaws may be bell mouthed and as much as you use the lathe I wouldn't be surprised. I too agree with JT 0.003" TIR is a bit much at the spindle nose.

              I'm not sure if you get Harbor Freight service where you are but I bought a long nose die grinder a while back just for grinding chuck jaws. It was pretty reasonable about $40 USD and the bearings in it are quite good. I mention this in case you don't have anything to grind your jaws. I use BXA tool holders so I bored out a 1" boring bar holder to fit the nose of the grinder and I was in business.



              • #8
                I concur on the spindle run out, address that first, Do you have a 4 Jaw Chuck?
                Just grab a rod no dialing needed,put a dial on the spindle or chuck and grab the rod . Should be no movement or very little.
                A good quality chuck of that size is approaching $1000 , not saying that the cheap stuff cant be used but sometimes putting lipstick on a pig is all you will get even after tweaking.
                And if there is no movement then your spindle wasnt machined properly.
                Last edited by redlee; 11-07-2020, 02:02 PM.
                Beaver County Alberta Canada


                • #9
                  If the spindle bearings are ok, then the only way forward would be to check the register for concentricity and also the face of the spindle. You may have to take the smallest skim off of one or both to true things up. Don't worry about the register not being tight on the three jaw chuck, I have about 8 chucks for the museums lathe and every one has an intentially loose register. This is to allow fine tuning of the runout if necessary. The bolts holding the chucks on the backplate never move when tightened.
                  As you have numbered the jaws and chuck body, which the manufacturers should have already done, there are no dangers of getting the chuck fully dismantled and not being able to reassemble it correctly. Any large parts can have a small centre punch dot on them for realignment.
                  If there are three keys in the body for tightening, you can try each in turn to find the one which gives the best accuracy. I check with a MT bush for a turret/ capstan lathe.


                  • #10
                    Here's another vote for disassembling and thoroughly cleaning the chuck, if you haven't already.

                    It can't hurt.


                    • #11
                      A few things to check.
                      Take a 6 " piece of ground round stock about .75" to 1 " in diameter ( O-1 or silver steel) and chamfer the edges very slightly to eliminate cutoff burrs
                      Roll it on a surface plate and if it rolls very smoothly ,it is not warped and usable
                      Number the sockets 1 to 3
                      Chuck the piece using socket # 1 and tighten to your Normal amount.
                      Measure run-out and mark the chuck where the the high spot occurs with a magic marker and the amount as well and match mark the round stock to a jaw to keep orientation the same
                      Repeat , only using Socket # 2
                      Repeat and do Socket # 3
                      Note the variation. This is scroll plate wear and also tells you WHICH socket to use for tightening to have minimum TIR .
                      You may wish to repeat the above to confirm your recordings

                      Using the best socket ( Now called the "Master" (paint it white !)) rechuck the round stock, but wrap a piece of paper -one wrap !- about 1/2" " wide and 1 inch from the chuck end first.
                      This paper shim will only touch the jaws near the end and will point out bell-mouthing issues as you are loading the jaws more at the tips.
                      If you get better readings, then you have a Bell-Mouth problem.
                      If not, then you have a jaw regrind to do.
                      If bell mouthed, ONLY regrinding will solve the problem

                      Last edited by Rich Carlstedt; 11-07-2020, 08:17 PM.
                      Green Bay, WI


                      • #12
                        Thanks Rich.--Thanks to the rest of you fellows as well. I will try the take it apart and clean everything up approach. If that fixes it, then great. I don't have equipment to regrind the jaws so if the problem remains I will probably buy another chuck.---Brian
                        Brian Rupnow
                        Design engineer
                        Barrie, Ontario, Canada


                        • #13

                          No Dremel tool and a grinding point?

                          That is really all it actually takes as far as grinding equipment. The rest is just a means to hold the jaws in loaded condition without blocking the grinding wheel/point.

                          For that, Rich Carlstedt's trick of drilling holes in the end of the jaws (small masonry bit, like 1/8") and use of a ring is very good.

                          Like this:

                          Last edited by J Tiers; 11-07-2020, 10:02 PM.
                          CNC machines only go through the motions


                          • #14
                            To go along with Jerry's picture of the chuck is a simple "Jury Rigged " grinder for a quick jaw grind.
                            This is just a Sears Craftsman Chain Saw Grinder that was clamped to a tool holder.
                            Yes, it's nice to have a Dumore Tool Post Grinder to do Lathe work, but this is not fancy stuff as far as requirements are concerned


                            Click image for larger version

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                            Green Bay, WI


                            • #15
                              The root problem here is the +/- .003 runout at the spindle without a chuck ! That is extremely high for a lathe spindle, sub-thou being the norm. Any runout at the spindle itself will only be amplified by a chuck.

                              All the suggestions on checking the chuck and cleaning it are good practice but in this case there is a problem with the spindle itself, you need to start there. Problem could be bearing wear or as simple as preload needing adjustment.

                              Lathes of the type Brian has typically use tapered roller bearings, preload is very important. Other possibilities are worn out bearings or bearing/race that has spun on the shaft/housing. Don't know if Brian's lathe is a oil filled head or what it uses for lubrication but that is obviously important to bearing life.

                              Of course its possible that the spindle had .003 spindle runout when it was brand new, in which case it a quality issue that should have been addressed when it was first bought. That ship sailed long ago.

                              Bottom line: Fix the spindle problem first, then move on to any chuck issues.
                              Last edited by Sparky_NY; 11-08-2020, 06:41 AM.