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  • question about centering in the 4-jaw

    Hey Guys, I'm curious here so would like to hear from "the pro's" on this one (or anyone else with an opinion...it is the internet right?).

    Since both of my 3 jaw chucks have more or less packed it up...i've been nursing them along for years, finally sick of 'em and on the hunt for another one I can afford. I've been using my trusty 4-jaw lately.

    I've been, of course, centering things up as near as I can by eye, and using the ref rings on the chuck face, then...here's what I do: stick my Starrett last word indicator...think its 0-15-0 on its holder in the tool post. Bring that up to the work and gently feed it in 'till I get about half the "dials worth" registering on the gage. Zero it. I try to have the gage contact the work inline (roughly...i just eyeball this) with one of the jaws. Rotate the chuck by hand, checking the gage and trying to slack one jaw and snug the opposite one 'till I get it reading around .003 or so, then go to work! I thought I'd be getting quicker at this but occasionally it takes a good deal of time and basic "screwing around". My question: Is this the right way to go about this? Should you try and center it in sort of an x and then y manner? (i've also tried, if I've noticed any eccentricity to move it diagonally, seems to just throw everything "out"), also...even though I know everybody TRIES for zero. And occasionally you can get it on the dial gauge...one bit of finger pressure on the end of the workpiece will register on that same gauge. WHAT is an acceptable level of accuracy? I know it depends on what you're doing....but for typical day to day type stuff...including maybe threading? Guess what I want to know is whats the typical accuracy of a 3-jaw (i've never measured it on mine) and how long does it take an "actual machinist" to center-up a workpiece using an indicator?

    thanks
    C.

  • #2
    I'd still say get a decent new 3 jaw for quick & easy (dirty?) work.
    4 jaw is great for offsets & funny shapes.
    Get a collet chuck & set of collets (ER type) for repeated accurate work.
    Don't tighten your 4-jaw then adjust, adjust while finger tight & tighten while checking & tightening in the right places to correct your readings,
    Regards,
    Nick

    Comment


    • #3
      centering in the four jaw

      I'm not a pro, but I do like my four jaw. If you are using a dial test indicator, that may be overkill for initial centering. I use a dial indicator, like 1" overall motion, and can usually get within .001" pretty quickly. John Stevenson had a good explanation (check the archives). He suggested thinking of it as a pair of two jaw chucks. Center jaws 1 and 3, then 2 and 4, check again and your done.
      Unless you are re-centering something, if you're under .001" ought to be close enough.

      Comment


      • #4
        It has been mentioned previously to think of the four jaw chuck as two two jaw chucks.

        Roughly center the part, and then indicate. The indicator should be as close as possible to the centerline of the work.

        Start with jaws 1&3 Set the indicator to zero on jaw 1, turn to jaw 3 and note the reading. Adjust the part to 1/2 the difference. Repeat, and it should be zero zero.

        Now do the same procedure on jaws 2&4. You will probably have to go back over jaws 1&3, but the part should be well centered at this point. It should take only a minute or two to accomplish.

        Look for some of the posts by Forrest Addy on the subject. The four jaw chuck is definitely not just for non-round or odd shapes.
        Jim H.

        Comment


        • #5
          I like using my height gage for the job when the workpiece is square:



          Otherwise, I use an indicator just as everyone has said.

          Best,

          BW
          ---------------------------------------------------

          http://www.cnccookbook.com/index.htm
          Try G-Wizard Machinist's Calculator for free:
          http://www.cnccookbook.com/CCGWizard.html

          Comment


          • #6
            Hi,

            I've never thought of using a height gauge for square parts in a 4-jaw. May have to try that sometime. I use a level placed on the jaw to "index" the indicator back to the same spot on the part. A square-head works well for this, but I've got 4 small magnetic levels that work even better.

            dalee
            If you think you understand what is going on, you haven't been paying attention.

            Comment


            • #7
              One more wrinkle. You mentioned that you've got a 0-15-0 indicator, which means if you moved the toolholder to about half travel on the indicator and zeroed it, you might not have enough travel when you rotate the chuck 180 degrees. That is, if the part is off center by more than .015, you don't know exactly how much to move.

              Check high/low on two jaws, turn the high side to the front move the indicator into the work until it registers and zero it, THEN set the crosslide dial to zero. You can now rotate the chuck move the crosslide until the indicator reads zero and read off the crosslide the double error. For example, if you have to move the crosslide in .080 to get the indicator to zero again, back the crosslide off and set it to .040. Now slacken the front jaw and tighten the back jaw until the indicator zeroes again at your .040 mark. Now look at the other two jaws, again select the low side to the front, slack and tighten to again zero the indicator and you should be quite close. Recheck, and tweak if you need to be very close, but this sequence will minimize the turn/move/check/turn business.

              JM
              .
              "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

              Comment


              • #8
                Forrest is going to like this thread
                I use my four jaw for most everything. I hate my 3 jaw. It's accurate enough but it packs with chips far too easy and is a PITA. Always trying to clean it etc.
                I rough in the four jaw with chalk, then finish off with a test dial.
                I always try for "zero" or awful close.
                Takes maybe a couple minutes...except for that "once in awhile" when I'm all thumbs and can't feel the wrench right. Get to fighting it and it can take awhile. Just a dumm thing but it happens.
                If I keep this lathe I would like to buy a good 3 jaw chuck for it so I can start using soft jaws.
                Russ
                I have tools I don't even know I own...

                Comment


                • #9
                  chkz, take a look at this thread.

                  http://bbs.homeshopmachinist.net/showthread.php?t=20974

                  Rgds
                  Michael

                  Australia

                  Comment


                  • #10
                    Setting a Four-jaw Chuck in Two Revolutions

                    Hi C.

                    Here’s the most efficient method I know for quickly and accurately aligning work in four-jaw chuck. I learned this technique from the late Robert Bastow of r.c.m. fame who used it for centering huge steelmill rolls in a 108-inch swing lathe with a top jogging speed of 2 minutes (yes, Minutes!) per revolution.

                    1. After rough aligning to the chuck rings, using a dial indicator on the work-piece, rotate the spindle through one complete revolution noting the highest and lowest indicator readings;

                    2. Continue rotating the spindle and halt at exactly Midway between the above two readings, then zero the indicator bezel to the needle;

                    3. Rotate the spindle to bring jaw #1 ‘on plunger’ and adjust jaws #1 and #3 to re-zero the indicator; finally

                    4. Rotate the spindle 90 degrees and adjust jaws #2 and #4 to zero the indicator once again.

                    That’s it – you’re all done! -- and accurately set to whatever tolerance your indicator will allow.

                    Hope this little trick helps you as much as it has helped me.

                    Kind regards,

                    Jack

                    Comment


                    • #11


                      Keep it up. 4 jaw chucks are more accurate, versitile, robust and grip better than three jaw chucks. Three jaw chucks provide significnt savings in time for repeat jobs partucularly when soft jaws machined for the job are employed. That said, both are only spindle tooling.

                      The controversy arises when three jaw chucks are used as training wheels. Use them at first, if you must but graduate to a four jaw as soon as possible.

                      Comment


                      • #12
                        Originally posted by Forrest Addy


                        The controversy arises when three jaw chucks are used as training wheels. Use them at first, if you must but graduate to a four jaw as soon as possible.
                        Wow, donning asbestos suit, fire retardant gloves and ball protector as I'm going to have to disagree with Forrest.

                        Get a decent 3 jaw. By decent I mean any of the home grown jobbies or east European like the TOS or Bison.
                        There are some decent chucks coming out of China but at the moment it's a toss up from which factory.

                        A decent 3J will repeat to 1 to 2 thou every time and then the trick is to organise your work to suit.
                        If you are making a bush then inners and outer diameters should be done at one setting as even on a crap chuck with 10 thou run out your bush is still accurate.
                        If you are making a shaft allow a bit to hold on and turn all diameter in one go, don't turn half and turn around, turn it all and cut the holding piece off.

                        To save changing chucks over for more accurate work keep a few short stubby pieces that have a collar on them and a centre turned on the end, mild steel will do as these are not rubbing centres.

                        When you need to do an accurate shaft, cut to length and face and centre both ends. Then fit one of the centre stubs in the 3J and take a light skim on the centre part to ensure it runs true.
                        Then fit your shaft between centres with a lathe dog driving against a chuck jaw.
                        If you need to spin the shaft round you can do without loosing accuracy or concentricity.
                        This is exactly the same as running between centres but you don't have to disturb your chuck.

                        If you have to keep swapping from 3J to 4J for more accurate work you are wasting time, you might as well swap to a collet chuck but with a decent chuck and planning most jobs can be turned out accurately on a 3J for normal work.

                        If you have a 3J that's so bad it won't repeat to a couple of thou then dump the bugger, stick it on the welding bench or drill press for holding round bits. Given current prices for decent 3J chucks there is no reason to accept crap.

                        In the past I have pissed about grinding jaws out where they have bellmouthed, spent a whole half day once only to find out the scroll register had worn and because of this it tighten up where it wanted to on different diameters. Sorry life's too short to mess with something that now cost a quarter of what they used to cost given earned income and quality has risen to boot given modern manufacturing techniques.

                        Many will disagree with what I have written but read it again and it's only common sense, the chuck is a prime part of a lathe, would you turn with blunt tools ?
                        Most people want the best lathe they can afford and the spindle, bearings and chuck are the prime units to achieve this no matter what it looks like.

                        Bring on the flames ...............................................


                        .
                        .

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



                        Comment


                        • #13
                          I use a 4 jaw all the time doing barrel work. I have a descent 3 jaw, a Buck set-true, but I need the ability to indicate to ZERO runout. Sure, I can do it with the set-true, but that is only a 4 jaw in disquise. If you are making something out of stock material, a 3 jaw is easy to use. But, when you HAVE to work to an existing diameter, you gotta be able to dial it in.

                          Comment


                          • #14
                            You got it John.

                            Here are some tricks I use in lathe work.

                            1) If the job runs out a couple thou in the chuck, put a .001 feeler under the high jaw to bring it in.

                            2) Rough it ALL out before you start to finish.

                            3) Make all finish cuts in the same setup (chucking).

                            4) For multipal parts, do the job in operations leaving a finish true up cut. Mark the stock, and one jaw, to reset the parts in the chuck the same way.

                            5) I turn a chucking diameter to hold on to that will prevent the stock pushing back in the chuck. This is the piece John cuts off and discards.

                            6) On thin wall parts, make a plug to support the jaws, and/or chuck very gingerly!

                            7) When making the finish cuts, turn the machine on before you move that dial. Even a fine lathe will creep a couple tenths from cross feed screw pressure from the starting vibration.

                            Those four jaw chucks are too heavy to fool with unless you absolutely positively have to. I am perfactly happy to leave mine on the floor!

                            If you need an adjust true chuck, I can provide one.

                            Kap

                            Comment


                            • #15
                              If you ever work in a job shop where few jobs are the same and most jobs are one off you will seldom use a 3 jaw. Once you understand and learn how to center work in a 4 jaw you will seldom use the 3 jaw. The only time we ever used a 3 jaw was for fast work that didn't need to be accurate. For instance, thread the end of this shaft Carl. If a lathe had a 3 jaw on it already, I would use that one, other wise I would use the 4 jaw. The lathe I called mine had 3 and 4 jaw chucks but I very seldom used the 3 jaw. The problem with 3 jaw chucks is trash getting in the scroll and then the jaws are not centered. Then you have to pull the jaws and blow the scroll and jaws off.

                              At home I keep the 3 jaw on my lathe. Why? I have time to remove the jaws and clean the scroll and jaws and most the work at home involves using a big chunk of metal and turning a part out of it. Also, it's a 13" lathe and I can change chucks by hand. If I want accuracy I put the 4 jaw on. The 4 jaw is KING.
                              It's only ink and paper

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