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  1. #1
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    Default Tightening a scroll , self centring chuck

    I had my 3 jaw scroll chuck apart to day to clean it out which brought to mind something that I've read about a lot.
    That is that "a scroll chuck should be tightened using all of the available chuck key holes" (pinions).

    So tonight I did a bit of searching and there are several threads on various forums, going back several years.

    Lots of different opinions expressed as to how to "correctly" tighten a scroll chuck.

    Lots of people in favour of using all the pinions (because that's how they were taught) plus plenty of others saying use the same pinion every time or it doesn't matter.
    Others believe that the chuck is marked to show the best pinion to use. ( which is useless if the chuck has ever been apart and re assembled with the pinions in different holes)

    Reading some of the posts I cant help but wonder if some of the people who made them actually know how a scroll chuck works.

    To me it shouldn't matter which pinion is used (but I'm always open to an opinion change) as the pinion only turns the scroll and its the scroll that actually tightens the jaws.
    Anyway I thought Id put it out here for discussion.

    peter

  2. #2
    Join Date
    Sep 2012
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    Default

    Hi Peter, Guys,

    I personally don't bother going round the pinions or using just a specific one to tighten the jaws on the lathe three jaw chuck. I understand the arguments that the scroll isn't a precision fit in the chuck body and can move slightly when force is applied from different pinions. But if you have to tighten a chuck to that extent to get a firm grip on your work then the jaws are probably bell mouthed and quite worn.

    On the other hand, I do occasionally go round the holes on the drill chuck, particularly if the drill is large and is likely to slip under pressure. Large reduced shank blacksmiths drills spring to mind here along with hole saws.

    Happy New Year All.
    Best Regards:
    Baron J.

  3. #3
    Join Date
    May 2011
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    Murray Bridge S Aust.
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    Default

    Some good food for thought, will be intersting to see what comments come back from the machinists and amatuers. Thanks for bringing this up.
    Kryn
    To grow old is mandatory, growing up is optional.

  4. #4
    Join Date
    Aug 2011
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    Perth, Western Australia
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    Default

    I have always run a four jaw (non scroll) on my lathe but every now and then I stick a 100mm scroll four jaw in the big one for little jobs. As I was taught for a scroll chuck I tighten all the pinions.
    Drill chucks now are manly keyless but if using a key chuck, as I was taught, I do go round and tighten on all holes.

    Tony

  5. #5
    Join Date
    Dec 2011
    Location
    Sydney
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    Default

    I have an Elliott Forte 3 jaw 5 inch steel body chuck with one pinion marked "0", which presumably is (or was when new) the one best pinion to use.
    While I never actually measured the concentricity of grip with different pinions, the Elliott has always seemed a good accurate chuck regardless which pinion is used,
    and from memory more consistent than my Pratt Burnerd iron body chucks were they were new.
    Bill

  6. #6
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    Default

    On a scroll chuck in reasonable condition, I can't see it would matter which pinion is used to tighten it.
    I usually only use one (randomly selected) to tighten. However, I occasionally check all three and get a little bit out of 'the others'. I can only surmise that friction within the chuck allows the scroll to hang slightly on the pinion.

    Michael

  7. #7
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    Mar 2014
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    South of Adelaide
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    Default

    I normally only use one pinion, but if i need something super tight in the chuck i will use all the pinions. On cheaper chucks you can tighten the pinion before the high point to tweak the run out.

  8. #8
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    Interesting answers .
    I'm not disputing any of them but I still don't get how using 1 or all three pinions can change how the chuck tightens. Unless there is a large amount of wear (or poor manufacturing quality control) in the chuck between the scroll plate and the chuck body. Maybe using different pinions might effect it then but I still cant see how. You could even assemble the chuck with out the pinions installed and still tighten it by sticking a screwdriver or similar into the hole and levering on the gear teeth.
    Am I missing something? I'm just a dumb fitter at the end of the day.

    peter

  9. #9
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    I suppose some of the things we are taught just stay with us.
    On the other hand I was taught to come over the spinning chuck if filing a job in the lathe. I cannot do it, I have visions of getting my arm caught.

    Tony

  10. #10
    Join Date
    Sep 2012
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    York, North Yorkshire UK
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    Hi Peter, Guys,

    Quote Originally Posted by bollie7 View Post
    Interesting answers .
    I'm not disputing any of them but I still don't get how using 1 or all three pinions can change how the chuck tightens. Unless there is a large amount of wear (or poor manufacturing quality control) in the chuck between the scroll plate and the chuck body. Maybe using different pinions might effect it then but I still cant see how. You could even assemble the chuck with out the pinions installed and still tighten it by sticking a screwdriver or similar into the hole and levering on the gear teeth.
    Am I missing something? I'm just a dumb fitter at the end of the day.

    peter
    If you can visualise the scroll wheel not being a perfect fit inside the chuck body or a perfect fit on the inside bore, there will always be some slight play, maybe only a few thou, but its there. The pinions will push the scroll wheel to take up this play. The next pinion will push the scroll to take up any further play, and so on.

    Now imagine the scroll unwound and layed out. It would look like a straight sloping line. Any play in the scroll wheel will cause the line of the scroll to vary its position, either up or down. Further imagine the three jaws in fixed positions along that line. Each jaw will have its own amount of play which will vary depending upon the diameter of the work in the jaws and the scroll position at that diameter.

    In short there will always be one pinion that will be tighter than any other, conversely there will always be one pinion that is more loose. The same applies to the jaws, each jaw will have a different force applied to it, simply due to machining tolerances. This is the basic reason that a three jaw scroll chuck will never hold a work piece with zero run out and the same reason that it is very hard to get the chuck to repeat at any setting once you remove the work piece.

    Not an easy thing to explain, but tolerances and wear and tear play a big part. In my opinion people over tighten three jaw lathe chucks needlessly. I've seen a couple of chuck scroll plates that have had distorted scrolls !
    Best Regards:
    Baron J.

  11. #11
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    BaronJ.
    Thanks for that explanation. Its actually the best I've seen yet.
    I don't use my 3 jaw all that much and when I do its not for jobs that need repeat-ability. I'm not on the clock at home so if it take an extra few minutes to set up in the 4 jaw then so what.

    thanks
    Peter

  12. #12
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    Quote Originally Posted by bts View Post
    I suppose some of the things we are taught just stay with us.
    On the other hand I was taught to come over the spinning chuck if filing a job in the lathe. I cannot do it, I have visions of getting my arm caught.

    Tony
    Tony, funnily enough, when I did my apprenticeship we were taught to be lefties when filing. Some of the other blokes had trouble doing that but I was lucky as I'm fairly ambidextrous. The thought of leaning over the chuck to file scares me as well.
    Peter

  13. #13
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    Jun 2007
    Location
    Victoria, Australia
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    When it matters I use the indicator with the 3 jaw and achieve adjustments from 0.07 to 0.02mm by adjusting tension on the scrolls. Sometime when the moons are aligned I am able to get 0.00m But if it really matters and work piece has to removed and swapped around or worked elsewhere if the workshop, the 4 jaw is mandatory.

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