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Turning between centers with ball bearings

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  • Turning between centers with ball bearings

    Yesterday I needed to make a tapered cone of approx 2" diameter and 12", so 4 degree taper on each side. I used the technique where you offset the tailstock with another piece of scrap metal with a divot and turn between centers, using a 1/4" ball bearing on each side.

    I had a few problems:
    1) The piece would chatter when taking a bigger cut or going to higher rpm
    2) sometimes the tailstock end would suddenly jump off center by about .01"
    3) Because the headstock end was nutating (pivoting around), sometimes the lathe dog (in this case, an indicator spindle clamp plus a piece of scrap metal c-clamped to the chuck jaw) would get pushed around from constantly moving back and forth.

    Does it really matter what type of divot you use for the ball bearing? I initially started off with a 90 degree countersink and only towards the end did I try a 1/4" ball end mill.

    What would cause the chatter? I made sure that the tailstock was pushing firmly in.

    How would I address problem #3?

    Thanks

  • #2
    Don't know ..good idea ..never used that method before

    but, how about when making the scrap metal with divot in it ..having the face chopped off at an angle less than 45 degrees ..that way you then turn the angled peice to the best position that suports the ball without it interfering (in other words, when angled, it will have more of divot in contact with the ball)..would only work for the tailstock end...but it's that end that is providing the push to keep it there, so may sort every thing out.

    as for the tailstock jumping off centre ....you must have some sideplay somewhere that is taken up by the side ways push ...barrel ...maybe worn ...or adjust your barrel lock.

    always centre your tailstock with barrel lock on ...then put the barrel lock on every time you advance the barrel.

    oh yeah divot would have to hemispherical ...that would help...hmm maybe quarter-spherical

    all the best.markj
    Last edited by aboard_epsilon; 09-09-2011, 07:55 PM.

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    • #3
      Use this method quite a lot and never had a problem with it.
      I use a boring head with a scrap piece with a centre drilled hole in it.

      .

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



      Comment


      • #4
        There must be variations on a theme possible here...how much of the ball bearing can be "buried" in the divot?
        If you use a 1/4" ball mill won't a 1/4" ball bearing be half way down in the divot? Is this what one wants?

        Are there angles past which the ball bearing will simple "refuse" to stay in place (i.e. with enough force to hold the work, the bearing pops out...) ?

        Do you use a ball bearing smaller or larger than the divot you create?

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        • #5
          Here's the method & measurements I bumped into on the web and saved. I've used it several times and it worked flawlessly.

          "This is an idea that sprung to my mind a while ago, although I'm
          sure I'm not the first one to think of it.

          When offsetting the tailstock for taper turning, or using a special
          tailstock fixture for the same purpose, the 60 degree center points don't
          fit well in the centerholes of the work being taper turned.

          This method needs custom-made lathe centers for both headstock and tailstock.
          The sharp point is turned off for a short distance, and centerdrilled just
          as is done for the work being turned.

          Hardened steel balls are captured in the centerholes between the lathe
          centers and the work, at each end.

          The correct centerhole size is important in relation to the bearing ball diameter.

          For a standard 60 degree centerdrill, the opening of the hole at the ends
          should ideally be between 88% and 90% of the diameter of the ball. (.389” for my .4325” ball, added by Milton)
          If larger, there may not be enough clearance between the lathe center
          and work to allow any offset.
          If the hole's opening is smaller than 87% of the ball's diameter, only
          the corner of the hole's opening will contact the ball and the whole
          thing may come loose under heavy cutting pressure.

          In practical experience, I've had very good results with this technique
          while turning morse taper shanks.

          For the purpose of accurately setting the tailstock setover, the effective
          length of the workpiece is measured between the centers of each ball.

          Just mike the workpiece with the balls in place, and subtract the total of
          one half the diameter of each ball.

          Be sure to use your favorite tailstock center lube on that end
          (I use white lithium grease).

          Hope this is useful,

          Ken Grunke
          West Lima, WI
          Jan. 09, 2005<<<"
          Milton

          "Accuracy is the sum total of your compensating mistakes."

          "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

          Comment


          • #6
            Originally posted by John Stevenson
            Use this method quite a lot and never had a problem with it.
            I use a boring head with a scrap piece with a centre drilled hole in it.

            Greetings John,
            While the boring head setup is a good one and must be complimented I'm more interested in the part being made. What's it for? It's a cool looking part. Especially the thread because the pitch looks so coarse for the diameter it's on.
            Eric

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            • #7
              Thanks DICKEYBIRD, part of the reason I asked is I have a center set which contains a few interchangeable nose pieces that are various sizes of cups

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              • #8
                Originally posted by John Stevenson
                Use this method quite a lot and never had a problem with it.
                I use a boring head with a scrap piece with a centre drilled hole in it.



                I'm impressed John.. That's really thinking outside the box. Hope I can remember that.

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                • #9
                  Turning Tapers

                  Sir John and Dickeybird
                  Both excellent ideas for dealing with the issue of taper turning. I have saved both together to a file so I can reference them when needed. Thanks a lot.
                  Fred Townroe

                  Comment


                  • #10
                    You can get proper center drills for drilling holes that have curved edges rather than straight 60؛ edges. Called Bell centers(or something like that). Haven't bought them in quite a while. They assure better contact than regular center drills.

                    Years ago,I made a tailstock center for tapers out of a larger than usual jeweler's lathe cross slide,a Derbyshire. Later on, made a much more substantial one. The use of the boring head is a good old stand by,too. Probably the most sturdy one,too.

                    Comment


                    • #11
                      Originally posted by etpm
                      Greetings John,
                      While the boring head setup is a good one and must be complimented I'm more interested in the part being made. What's it for? It's a cool looking part. Especially the thread because the pitch looks so coarse for the diameter it's on.
                      Eric
                      I don't know the exact title but we used to call them bobbin take off shafts.

                      They are / were fitted to lace machines. Nottingham at one time was the world leader in lace, there must have been literally 1,000's of companies all around this area making lace, many of the beautiful lace mills are still standing.

                      Anyway I digress, the large lace warps had literally 1,000 of turns of separate lace wound onto them and these went thru all the needles on the machines. because the wars always had more threads on than needed the spare threads were caught on a spinning waste shaft at the side.

                      Because as the warp runs empty the waste shaft has to run slower because the circumference is less.

                      The shaft in the lathe is the input shaft for this drive, there are two opposing shafts with the taper on and a flat belt is guided by a fork and as it slides down the shaft it changes speed.
                      The thread on the end is the worm drive of the take off gearbox.

                      Over time they wear where the belt runs and just require a quick skim up hence the setup. Used to do loads but the lace trade has all gone now, only about 10 companies left and they survive by doing exotic, high priced stuff.

                      Only customer I have left makes shawls, very high priced, about £200 a pop and if you blow your nose on one it disintegrates [ don't ask ] cool thing about his place is though he still has the wooden frame looms up in the attic dating back to the 1590's in working condition and he still demonstrates them to visitors.
                      .

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



                      Comment


                      • #12
                        This is the sort of thing they turned out besides curtains and bedding. :-



                        The Battle of Britain lace panel held at the Australian War Memorial was manufactured (between 1942 and 1946) by the lace curtain firm of Dobsons & M. Browne & Co. Ltd. It was woven to commemorate the battle and as a tribute to those who fought to save Britain.
                        During the war Dobsons & Browne had devoted most of its output to the production of mosquito and camouflage netting. As a means of retaining the skills and standards of their highly trained designers and draughting staff who were under-employed by the wartime production requirements, the firm took up the idea of making a large commemorative lace panel.
                        The design for the panel took two years and the drafting for the jacquard (pattern cards) another 15 months. The pattern required 40,000 cards, weighing a tonne altogether. Each panel took a week to produce and required 4,200 threads and the preparation of 975 bobbins for the loom. A total of 41,830 kilometres of fine Egyptian cotton went into the making of each panel, which measured 4.5 x 1.62 metres when completed.
                        The panel depicts scenes of the bombing of London, and the types of aircraft used in the battle, as well as the badges of the Allied air forces involved and the floral emblems of Great Britain and the Commonwealth. Also included are the names of the firm and the craftsmen from Dobsons & Browne who created the work. At the bottom on a scroll are Sir Winston Churchill's famous words: " Never was so much owed by so many to so few. " A cottage and a castle are also depicted, to indicate that rich and poor suffered alike. The edging of the curtain is composed of ripening ears of corn representing the season during which the Battle of Britain took place. Interwoven with these are Tudor roses, thistles, shamrocks, and oak leaves.
                        Thirty-eight panels were woven before the jacquards were destroyed. King George VI and Sir Winston Churchill were each presented with one, and others were distributed to various RAF units, and to Westminster Abbey, the City of Nottingham (where the panels were woven), the City of London, and personnel from Dobsons & Browne. As airmen from New Zealand, South Africa, Canada and Australia had been attached to various RAF units, these countries also received a panel.


                        .

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



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                        • #13
                          Thanks for the info, Dickey.

                          I've just been wondering what is the best divot profile for this task. The headstock end doesn't matter that much because it just pivots around a little bit. The tailstock end is kind of difficult because the ball needs to rotate, and it can either rotate relative to the tailstock, or to the workpiece.

                          Initially I thought that a hemispherical divot from a ball end mill would be the stiffest because it has the most contact area, but then again, once that divot starts wearing, the junction can have some wiggle.

                          I'm not sure if it is worth it to get the ball center drills that gwilson mentioned. If the work material is aluminum, won't the ball wear in kind of an annular region anway?

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                          • #14
                            Hi,
                            Regarding the Battle of Britain Commemorative lace panels.
                            Recent recalculations suggest that the figures for the lace panels are actually...
                            10,800 to 16,200 cards forming the Jacquard.
                            Roughly 14 miles of fine Egyptian cotton.
                            The loom was capable of producing 4 panels at a time in around 2 hours.
                            I have located 30 panels worldwide and welcome any further input.
                            Unfortunately the one at the AWM Canberra is not on display but there are at least 4 in Australia, 2 of which are definately displayed.

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                            • #15
                              Originally posted by beanbag
                              Thanks for the info, Dickey.
                              The tailstock end is kind of difficult because the ball needs to rotate, and it can either rotate relative to the tailstock, or to the workpiece.
                              The ball will rotate against whichever piece has the smaller seat dia.
                              If you want the bearing to rotate on the tailstock, make that seat slightly smaller than the seat on your workpiece.
                              I cut it twice, and it's still too short!
                              Scott

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