Truing the chuck super easy

Discussion in 'Machine Modifications' started by Rudy, Oct 8, 2017.

  1. Oct 8, 2017 #1

    Rudy

    Rudy

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    I want to share my method for truing a lathe chuck. At least I haven't seen it used. There are several fixtures you can make to preload the jaws. I simply glued them with hot melt glue. Very easy to remove afterwards. Not much resistance when I tightened up the chuck to take in the play. Maybe there is not important to have normal working tension on the jaws when doing this? I managed to reduce the run out from 0,07mm to 0,01mm (aprox. 3 to 0.4 thou).

    2017-10-08 17.20.27.jpg
     
  2. Oct 8, 2017 #2

    DJP

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    I tried this for an old 3 jaw chuck and it works for the diameter that you set. I clamped the inner portion of the jaws on a round piece then trued up the jaws so it was similar to your process. For that size of round stock the chuck ran true again but for any other size it was off.

    The root problem was a worn out scroll so no amount of jaw truing was going to fix that. The only solution was a new chuck and luckily I was able to save the old backing plate and machine it to size for the new chuck.

    Unless you like to play, truing old 3 jaw chucks is a waste of time, in my opinion.
     
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  3. Oct 8, 2017 #3

    Rudy

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    BTW, I trued with the diameter seen in the picture, about 30mm. I checked the result on 10mm only. Of cause the accuracy of the scroll is important on different diameters..

    BTW2:
    Double checked the results.. I did not do several measurements in the first place. I should have known better... one measurement is not a measurement, it's a guess. After several measurements I did not manage to get repeatability, so I guess i'm only measuring the natural inaccuracy for this chuck... The truing was probably not all that helpful after all.
     
    Last edited: Oct 8, 2017
  4. Oct 8, 2017 #4

    Mechanicboy

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    Before truing the chuck jaws ---> Be sure there is not sloppy fit between chuck and spindle.
     
  5. Oct 8, 2017 #5

    Rudy

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    Jens Erik, checking the spindle was actually the only thing I did right. Tested it on all three possible positions. Loosened it and tightened it again, same result. One position was slightly better.
     
  6. Oct 8, 2017 #6

    DJP

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    Before I bought a new 3 Jaw chuck I found that I could get results out of the worn one by placing a shim under the jaw that showed the most runout. I limped along for a while this way before finally giving up.

    The good news was that machining the back plate to exactly fit the new chuck returned the old Southbend to reasonable accuracy. It's gone to a new owner now and I have a Super 7 to replace it. So far so good.
     
  7. Oct 8, 2017 #7

    goldstar31

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    All that has happened is that you have got yourself a set of 'soft jaws'

    Cheap at half the price

    Norm
     
  8. Oct 9, 2017 #8

    Charles Lamont

    Charles Lamont

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    A light grind can be useful to correct 'bell-mouthed' jaws, but the chuck really ought to be loaded. I have much improved a chuck by the method explained here:

    http://www.homemetalshopclub.org/news/aug04/aug04.html
     
  9. Oct 9, 2017 #9

    kiwi2

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    Thanks for the link Charles. I've got an old chuck which is out of true. I couldn't work out how to put the jaws under pressure and still get access to the bore. Now I know how to do it!
    Cheers,
    Alan C.
     
  10. Oct 9, 2017 #10

    goldstar31

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    I recall Tom Walshaw writing as Tubal Cain, suggesting a favourite chuck number to get consistent results
     
  11. Oct 9, 2017 #11

    Blogwitch

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    I have been following this post since the very beginning, and not one mention of how you get tapered jaws in the first place.

    No matter how old the chuck is I doubt very much if it could be attributed to wear, a chuck, even in a production environment can't wear that much.

    So you are left with one major cause, gorilla tactics, white knuckle effect, extension tubes on the chuck key etc etc. So self inflicted to get the chuck tightened up more than it should.
    It takes only one heavy application and the jaws are then physically bent from being square to being off angle (tapered) permanently.

    How to cure permanently and perfectly.

    You can't, and please bear with me, I have tried a few times to get jaws straightened again (not on my chucks BTW, I never over tighten them, but other peoples) using the techniques mentioned above, and not one gives acceptable results, no matter how famous the person is telling you to do it, you will NOT get your chuck back to it's original condition.

    Here is a a very short video of a mate of mine TRYING to rescue his chuck by using my lathe and toolpost grinder, all to no avail.

    [ame]https://www.youtube.com/watch?v=sOTDmY1RjQM[/ame]

    I am talking about self centering chucks here, no matter how many jaws, but standard independent jawed chucks can be fairly easily fixed by regrinding them back to square on the surface grinder. This also makes the jaw tips the correct shape, flat. If you are trying to grind a self centering jaw set, you will end up with jaws that have a concave tip surface that not only isn't very strong, but will dig into your workpiece ruining the surface finish.

    So what to do with a tapered jawed chuck?

    Chucks are so cheap nowadays (not second hand, don't touch them at all as they may already be tapered and you are back to where you started), so throw it away and buy a new one.

    There are ways to make your chuck last longer, get yourself a new sets of internal/ external jaws and hope that your scroll spiral isn't bent or damaged, or do as I do, when first buying a chuck, purchase a set of soft jaws. I use soft jaws for at least 60 to 70% of the lathe work I do and not only do I get more secure holding, I get great precision thrown in as well. A great shame really, as I have just bought a six jaw self centering which soft jaws are not available for, so I will have to be very careful about tightening up on that one, but they are designed to give superior grip because of the number of jaws, mainly on thin walled tubing etc, without deformation.

    Earlier on I told you to throw the bent chuck away, but don't do that, cut it up into pieces and make yourself some engines out of it.

    Take the advice or leave it.

    John
     
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  12. Oct 10, 2017 #12

    MachineTom

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    I have a jaw preload fixture, it works very good, but you need to have two piece jaws, as the tool use the holes on the jaws to attach. An issue with grinding jaws in addition to the scroll concern is the wear of the bed transfers into the grinding of the jaws,
    What I do is after I trued the jaws, next remove that chuck and install another chuck, place a hefty 1-2" spud into the chuck, with a couple inches sticking out, take a light cut on this. You now have a piece running as true as you lathe can be, now remove the backing plate from the recently ground jaws chuck, and mount the chuck backwards onto the spud you just trued. When the lathe is running, you will see the error in the chuck mounting surface, Grind or light cuts to true the back of the chuck. Before you remount that chuck, mount the backplate alone, and grind or light cuts to true up its surface as well.
    That is as good as you can get.
     
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  13. Oct 10, 2017 #13

    Blogwitch

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    I forgot to mention that the easiest way to cause tapered chuck jaws is to hold the piece parts in the first 1/3rd of the chuck jaws. Mainly machining steel and other hard materials are to blame, softer materials such as aluminium and brass will give a little, but you can still end up causing the same problem.

    Because there is so little grip on the short part, they start to turn whilst machining, certain people will tighten down the jaws, and because that pressure is at the back of the jaws, anything just held in the front part will ensure that you will bend them.

    I try not to get into that situation by machining on a longer piece of metal if possible, then parting off the bit you require, or if you have soft jaws, turn them up to hold the short part, much more secure.


    John
     
  14. Oct 10, 2017 #14

    bazmak

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    Ther are lots of ways and ideas.Have tried most with some success and sometimes not.I think in the end its down to the chuck problems
    Some worn/damaged chucks can be improved some cant
     
  15. Oct 11, 2017 #15

    goldstar31

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    Much could be avoided if people had things like faceplates and 4 jaw independent chucks.

    Just think how nice that it would be if there wasn't people with knackered chucks?

    Norm
     
  16. Oct 11, 2017 #16

    Blogwitch

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    Norm,

    Four jaw independents have their own problems when trying to hold really small items.

    I bought a brand new small four jaw, and when roughly centred the small square formed at the jaw tips couldn't hold and give me the 0.005" offset that I required on a tiny part.
    The only way around it was to take some material off the angled sides of the jaws where it comes down to the gripping tip to allow the jaws to come closer together so it would hold the part. I thought I would need to grind it off, but in fact a razor sharp 16mm multi flute carbide cutter shaved them down to perfection. It is a shame that once that job was done, I have never had to use that chuck since.

    Maybe that way could be used to get the bent jaws back to somewhere near.

    But you have forgotten to mention the very versatile Keats angle plate, one of the most useful bits in my arsenal for holding offset parts on my lathe, and it can be used on my mill and surface grinder as well, plus maybe loads of other uses as well on the surface plate.

    John
     
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  17. Oct 11, 2017 #17

    goldstar31

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    Hope things are continuing to be on the mend but I was trying to wean people away from three jaw chucks and the hapless 'I know a man, who knows a man, who knows or thinks that he knows a man who can actually mend a three jaw chuck- I think'

    Or something like that

    Kind Regards

    Norm
     
  18. Oct 11, 2017 #18

    Blogwitch

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    I know exactly what you mean Norm, it only takes one of the big names to mention something and it soon becomes unchangeable gospel.

    The way I get around it is to look at the problem logically from all angles, and you can soon and easily see that certain things just won't work.

    If that doesn't cure the problem, I go back in time to old engineering books, as there is very little that has changed since then, just people coming up with 'new' discoveries that had been forgotten years ago.

    John
     
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  19. Oct 12, 2017 #19

    Rudy

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    Since I started this thread this has been a lesson.. Both trough my own experience and trough your very informative posts. Thanks..

    I have dismantled my chuck and inspected it. Lots of shavings and dirt. Looks like they were in a hurry making this one. The scroll is probably the source to the variable accuracy. Some of these chucks are probably good, some not.
    A good thing though, the spindle seems to run very true so there is a potential. I do have an MT4 ER32 collet chuck in very good quality, but a 3-jaw self centering chuck is very convenient. Even with a lousy chuck, one can do very precise machining, like I proved to myself when building my first engine, featured here as “Another Stuart 10V”.

    This inevitably brings me to the next question. What should I look for if I want something better. Not breaking the bank. Preferably from Asia since I'm in Norway, shipping from there is next to 0. Yes I know Chinese quality and all that. I have bought nearly all my tools and equipment from there and half of it is in really good quality.
    (If I have to brake that bank, I'll do it).
     
  20. Oct 12, 2017 #20

    DJP

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    Taiwan has a good reputation for tool quality. Brands from there would be my first choice in sourcing from Asia.
     

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