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Some observations on three jaw self-centering chuck accuracy.

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  • Some observations on three jaw self-centering chuck accuracy.

    I am new to lathes, but I have experience in trying to hold high tolerances in making things. I have made some interesting notes on my chucks for my new mini-lathe. No doubt most if not all of these are well known to experienced persons, but I have questions regarding the topic of accuracy.

    1. There are nine permutations of mounting and tightening sequences with a three bolt mounting. One mounting orientation and one tightening sequence emerged as most accurate with much variation (.001" to .007" deviation).

    2. The choice of which of the three chuck receptacles to use to tighten also had great effect on accuracy with as much as .006" difference from best to worst.

    3. The best I have achieved in a consistently repeatable way is .0003" run out on 3/8" diameter stock.

    My questions are:

    (a) Is .0003" run out considered very good for three jaw independent chucks?

    (b) What will most machinists accept as a deviation when using a four jaw independent chuck?

    (c) I have set up work in the four jaw to get the same .0003" and I find the time taken to achieve that is probably the same or more than what it takes me to get it by tweaking the three jaw self-centering chuck, therefore, is there some secondary reason why I should be using a four jaw for very accurate work that I am not aware of?

    I must confess a preference for the self-centering chuck. The tweaking is just as lengthy, but it is of a different nature than the four jaw independent chuck.

    Any thoughts would be appreciated.

  • #2
    I prefer the 3 jaw chuck, self centering especially, as it brings to the table most of what the 4 jaw offers but IMHO, with more precision for finer work without the hammering, shimming, etc.

    Of course, for odd shaped or eccentric work, I reach for the 4 jaw in a heartbeat.

    One factoid about the 3 jaw that always fades into the archive ... the 3 jaw chuck has ONE marked key hole which should be tightened for best accuracy. This from the horse's mouth at Rohm. I've seen this marking now on numerous good quality 3 jaw chucks.

    The scroll will ALWAYS have some clearance to the center hub of the chuck body. When the marked keyhole is used, the scroll will be forced toward one side of the chuck. Presumably, this is the same position used at the factory when the jaws were given a final grind.

    On a Bison 3 jaw, non set-tru, using the marked keyhole give around 0.001" runout while using one of the others seems to add an average of another thousandth or so. It is pretty repeateble.

    I've heard old timers advise tightening all 3 holes on keyed DRILL chucks and I haven't explored their construction enough to know if there is a good reason for this ... but I trust them.

    Den
    Last edited by nheng; 12-19-2006, 11:15 PM.

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    • #3
      Some observations on three jaw self-centering chuck accuracy.

      Hi guys;


      One thing I became aware of early on was that round stock, even stuff that looked like it had a good mill finish, is not always that concentric. I found that if I were going to have to rechuck a part it had a better chance of going back into the chuck and running true if I marked a line on the part and the #1 chuck jaw when I first put the blank stock in for the first time. Sometimes, if the stock is out of true, I can tap it lightly with a soft face hammer while the spindle is rotating. That will fix any misalignment from a small bit of swarf or grime.
      Jim (KB4IVH)

      Only fools abuse their tools.

      Comment


      • #4
        .0003"?That's getting into spindle runout range,so ya that's pretty good.

        Like said one pinion is used at the factory to lash out the scroll and jaws prior to final grind,it's called the master pinion.
        I just need one more tool,just one!

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        • #5
          A repeatable runout of 3/10's is good if fact super good. I will agree the three jaw is much easier and faster to use but when chucking odd shaped items the 4 jaw is the only way to go.

          As Nheng stated rounds are not always round.

          If you need repeatable accuracy in the 10's you need to use Collet's.

          My advise is only worth what it cost you.

          Ross
          GUNS Don't kill people
          Drivers using cell phones do.

          Comment


          • #6
            Just for the record, I erred by saying "stock". I have been using brand new high quality center drills for testing as I assumed they would be of higher tolerance than typical round stock.

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            • #7
              I just got through truing up an old 6 1/4" Bison 3-jaw for a 1 1/2"-8 threaded spindle. I'm happy with the ~.0002 runout at about 2" from the chuck. I don't do work for NASA so that's dang close enough for me. I did notice a difference if only one pinion was used to tighten the jaws. The runout jumped to .003. I'm already in the habbit of using all three pinions in the tightening sequence so I'm satisfied. (I do it with drill chucks too.)

              Keep in mind that a speck of dust can change your runout. Think what a metal chip can do.

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              • #8
                0.0003" is very good indeed for any chuck or collet. Usually those adjustable three jaw chucks hold concentricity well allowing repeat work and good contricity. They do not suffer overload and minor wrecks well so be careful when parting gummy materials.

                I don't suppoes anyone wants to hear my 4 jaw rant - again - so I'll stop here.

                Comment


                • #9
                  Originally posted by nheng
                  I prefer the 3 jaw chuck, self centering especially, as it brings to the table most of what the 4 jaw offers but IMHO, with more precision for finer work without the hammering, shimming, etc.
                  What would the "hammering and shimming" be?

                  Hammering is NOT S.O.P on any work I do with any chuck......I save that for bedding parts in the vise on the mill or shaper...... and I don't hit hard.....

                  The ONLY thing the 3 jaw offers that can beat a 4 jaw is quickness........and holding things with a number of facets divisible by 3......"more precision" is not what I associate with it..... Unless you mean a 3 jaw WIRE chuck *

                  It won't hold as tight, It can't get as accurately centered, it won't hold anything with an even number of facets well.

                  Sheesh... I think I am starting to sound like the other 4 jaw bigots...... well, too bad...... cuz they are right !

                  I did have a 4 jaw that needed to be shimmed for anything precise.... but I replaced it... it was also worn so it wouldn't grab anything under about 0.375 diameter... had a mostly stripped jaw thread.....etc, etc, etc....

                  * wire chuck = collet
                  1601

                  Keep eye on ball.
                  Hashim Khan

                  Comment


                  • #10
                    most of the inaccuracy in a 3 jaw is in the scroll. noting the tir at one spot isn't necessary indicative of the accuracy of the chuck, ie .375 might be spot on but when the scroll is turned to say .395, its a different story. if on the other hand, you've tuned your chuck to produce .0003 tir across its range, that is truly impressive and i hope to read the article soon

                    As has be discussed many times, it really only matters though if you have to keep the section held by the jaws concentric with the part being turned. so long as you are turning without removing the work, the accuracy of the scroll doesn't matter. if you do disturb it, and the work is anything but crude (ie better than 2-4 thou tolerance), use the 4 jaw or collets to do the next op. Den i disagree re the 4 jaw, it is as accurate as your indicator, far more than the three jaw and is regularly used to keep things concentric to a few tenths.

                    the question isn't what a machinist thinks is tolerable, its what the design and job at hand requires, a hitch pin for a farm wagon is different than engine parts for example. there is no standard answer, but most wouldn't expect better than a thou or two from a high quality 3 jaw, but it doesn't much matter imo, whether your three jaw is out 5 thou or 2 as most of the time 2 is just as unacceptable as 5 - neither is acceptable for accurate work. When you need to keep and OD or ID concentric to something already turned on the part, you must either use a collet or indicate with the 4 jaw. I use a tenths indicator because it lets you get easily better than 1/2 a thou, with care to 2 or 3 tenths.

                    I have never thought of the 3 jaw as being something to fiddle with beyond the inherent accuracy of the scroll, ie which pin is tightened etc. Other than experimentation, it would be a waste of time to fiddle with the 3 jaw accuracy as it lacks the direct control a 4 jaw has, ie indicator says i'm out 1 thou in this direction, tighten this jaw to correct sort of thing. the 3 jaw is a convenience we like to use frequently, but is the poor cousin the the 4 jaw both in accuracy and versatility. Use when concentric accuracy doesn't matter or more commonly when you can turn everything without disturbing the work, ie turn, face, bore, part off, is the 3 jaw's domain.
                    in Toronto Ontario - where are you?

                    Comment


                    • #11
                      "Other than experimentation, it would be a waste of time to fiddle with the 3 jaw accuracy as it lacks the direct control a 4 jaw has, ie indicator says i'm out 1 thou in this direction, tighten this jaw to correct sort of thing."

                      -----------------------------------------------------------

                      Oddly, this is at the heart of my inquiry. If I am able to develop a repeatable work mounting protocol that can net me .0003" accuracy at any dimension, does this not qualify as direct control?

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                      • #12
                        [QUOTE=Forrest Addy]

                        They do not suffer overload and minor wrecks well so be careful when parting gummy materials.

                        -------------------------------------------

                        What does this mean? Can you elaborate?

                        Comment


                        • #13
                          Originally posted by nheng
                          I've heard old timers advise tightening all 3 holes on keyed DRILL chucks and I haven't explored their construction enough to know if there is a good reason for this ... but I trust them.

                          Den
                          Nheng, this is good advise, at the very least it give a better grip without any added stress and it generally improves accuracy...

                          I mentioned this before on this site but you can make eccentrics on a three jaw and quite a variety, just put the jaws in where they dont belong or leave certain ones out one tooth and record your results, --- dont turn anything heavy or long or to fast though

                          Comment


                          • #14
                            [QUOTE=gmm22]
                            Originally posted by Forrest Addy
                            What does this mean? Can you elaborate?
                            By their nature, they are less stable than a traditional scroll 3 jaw. You have an intentional gap that the adjust-true screws use to skew the chuck on the back plate. That means instead of having a full circumference register to hold/support the chuck on extreme side loads, you now have a point load at the end of that screw. The crew can deform, the point blunt, or the chuck itself dented by the point; all to the tiniest amounts, but cumulatively you’re now out of true again. Furthermore, such “crashes” can leave it so that the chuck and spindle/backing plate are no longer even axially parallel for similar reasons. And, you still have the jaws and scroll which sustain wear and deformations as well just like a normal chuck.
                            Russ
                            Master Floor Sweeper

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                            • #15
                              What work are you doing that requires a 0.0003" tolerance and is it repeatable every time ?
                              What happens if you wind on 20 thou as a last cut, do you get the same results as winding on a 5 thou finishing cut ?

                              There is more to this than just quoting numbers, speeds, feeds and more importantly tool marks come into play at these stated tolerances.

                              I spend a lot of time doing bearing diameters, external and internal and I always aim for the best finish at a thou off what I want and then polish to size, That gets me the finished size I require and a good finish.

                              Chucks are cheaper now then they have ever been, even good ones. I'm amazed when I read adverts for someone wanting a set of jaws for a chuck that went out of production 40 years ago.
                              Chances are if they find a set they will be worn or the chuck isn't even worth it and the cost will be 75% of a new chuck with both sets of jaws.

                              If absolute accuracy is needed get a set of soft jaws and you won't look back and with soft jaws those weird jobs like disks and stepped flanges are a thing of the past.

                              I know you still have this chuck that you are loath to get rid off as it's only missing a set of jaws but believe me it isn't worth it.
                              If you are that much of a pack rat then weld it [ shudder ] to a plate and bolt it to the bench as a vise for holding round work.

                              Last place I worked we welded a big 15" 4 jaw direct to the bench and believe me that chuck could hold any shaped material safely, pipe flanges, elbows, castings.................


                              .
                              .

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



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