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  1. #1
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    Default Centering stock in 3 jaw chuck

    Hey Guys,
    Firstly, I apologise for this basic question, but I am having some trouble.
    I have some alum. round stock (30mm) , I have cut a short section to work (90mm) to work on, I have mounted it in the 3 jaw chuck and I cannot get it centered.
    I am taking measurements with my dial indicator, but no matter what I do, I cant get it true.

    My spindle bore is only 20mm so it wont fit through there.
    I thought the 3 jaws are pretty much self centering. I tried for about 1 hour and gave up. No matter what I do I cant get it line up.
    I am thinking of fitting the 4 jaw chuck and using the 2 key method to get the work centered.

    Is there something plainly obvious that I am doing wrong. I can post pics if required.

  2. #2
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    Default

    Quote Originally Posted by oohsam View Post
    I thought the 3 jaws are pretty much self centering.
    They are pretty much.
    How far out is it?
    A few 0.1mm then I'd say its as good as you can do with that chuck.
    A few mm and you may have a jaw or jaws in wrong.
    I assume if you're putting a DTI on is must be pretty close.

    Whats wrong with using the 4 jaw?

    Stuart

  3. #3
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    Default

    Nothing is wrong with the 4 jaw, I just have not used it yet.

    Here is a picture and video of my issue.
    I've tried minor adjustments and got it close but not as close as I've gotten it with other stock. I'm sure this stock is not warped.

    IMG20140614235453.jpgIMG20140614235440.jpgIMG20140614235501.jpg





  4. #4
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    Default

    Hello,
    You are not doing anything wrong. However you may well be expecting more positioning accuracy than the system can achieve.
    Three jaw chucks are self-centering, meaning that the workpiece is drawn towards the centre of the spindle axis without special effort on the part of the operator, but the precision of the result is variable depending on several factors.

    First, the workpiece may not be circular in section. To be pernickerty, it will never be exactly circular, but the error may be acceptable to you. Often however "round" stock turns out not to be very round.

    Then there are the errors within the chuck, originating in manufacturing, wear (typically in the scroll or jaws), or mounting of the chuck.

    Last, and hopefully not relevant in your case, bad spindle bearings can upset measurements of round workpieces.

    Even if the lathe and 3-jaw chuck are well made and in good condition, and the workpiece truly circular, it would be rare to put the work into a 3-jaw chuck and find no deviation on the dial indicator, so the 3-jaw is generally considered to be a convenient work holder, not as good as you can get.

    To do better, get out a 4-jaw (independent) chuck and see how good you can get it. Shouldn't take an hour!

    Cheers,
    Bill

    Edit - typing too slow!

  5. #5
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    hahaha dont worry Bill, the slow typing is no issue for me!

    I'll put the 4 jaw in and see how I go...

    I thought I could get away with using the 3 jaw. You are right, nothing is truly 'round' but I thought I'd get much much closer than this. I can't seem to get it to an 'acceptable' level of workability.

    Based on that video, Im sure none of you would accept that piece as workable in that state.

  6. #6
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    May 2012
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    Default # Jaw centring.

    Hello Oohsam,
    Couple of questions arise. Is it the chuck, or the stock, or both? Find a piece you have already machined, preferably round the 30mm dia. and mount it in your 3 jaw. Now indicate it. How close to centred is it? This is the basic accuracy of your chuck and/or its mounting at around 30mm dia. Now try machined pieces at larger and smaller sizes, (indicating as close to the chuck as you can). This will tell whether the scroll in the chuck is accurate over its range, or whether perhaps it has been damaged or strained.
    Put your piece of stock back in the chuck and see if it has just one high point and one low. If so, you can assume it's near round, but not centred. If readings are all over, then the stock was never round. Some of it is extruded, not rolled, and does not even feel round.
    Run your indicator on the outer body of your chuck and see if it runs true. Any variations suggest that it is not fitting truly on the spindle nose, or on its backing plate. Check for any grit ir swarf on the mounting surfaces and be sure they are VERY clean. Remount the chuck and check runout again.
    Find a foot or so of straight machined shaft and mount it. Indicate it round 200mm from the chuck. Note the variation with reference to the chuck and rotate it half a turn in the chuck. If the variation is constant with reference to the chuck then the error is in the chuck or its mounting,. If the variation has moved round 180 degrees then the bar is not straight.
    These are a few of the things I have had to figure out for myself in a remote area over the years. I am very sure there are people here who can describe far more methodical ways of checking out and calibrating a machine for accuracy. Regards,
    Combustor.
    Old iron in the Outback, Kimberley WA.

  7. #7
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    Default

    Excellent info.

    I'll run out to the garage and check that stuff.
    be right back.

  8. #8
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    Quote Originally Posted by oohsam View Post
    Here is a picture and video of my issue.
    The workpiece is only gripped over a very short length, so it is likely the work axis is at an angle to the spindle axis, adding to the dial gauge variation as you rotate the spindle. You could try tapping the outer end of the workpiece sideways to minmise the error (maybe you have tried that).

    Cheers,
    Bill

  9. #9
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    Default A bit slow.

    Hi again,
    I am a bit slow and inclined to rabbit on. Others have come in with good info while I was writing the above, so listen to those who have been there and done that. Combustor.
    Old iron in the Outback, Kimberley WA.

  10. #10
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    How far out of round are you talking about? Although Stuart said "They are pretty much." there is a lot of variation and generally the chucks on cheaper Chinese lathes are likely to be less than perfect. I checked an accurate round shaft in the 3 jaw chuck on my new (to me) lathe and found it was out by about 0.002". I thought this was a very good result. I would not expect you to get that sort of result from your lathe, but I have no experience with that sort of lathe. I am just going on hearsay.

    You cannot change the concentricity of work in a 3 jaw chuck to any great degree unless you use shims to pack out the jaws. Often you can improve it by turning the material around in the chuck, and maybe a bit of a tap will help slightly, but in general you should try to cut the shaft to make it concentric and leave it in position until all operations are done on the diameter. Facing as well, but that is not quite as important. A 3 jaw chuck is not generally considered accurate enough to refit work and have it concentric. It you do need to do this, either use the 4 jaw or get a collet system which will give the best accuracy and will be accurate straight off without adjustments. They just cost money, that's all.

    As to this being a basic question, you need to learn this sort of stuff somehow! You could try watching some videos from UTube for beginners or reading books, or you could ask on the forum. That is what the forum is for after all.

    Keep trying, and learning. There is a lot to learn and it is so much fun.

    Dean

  11. #11
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    Ueee is offline Blacksmith, Cabinetmaker, Machinist, Messmaker
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    That is a lot of run out for a new chuck. I would try it with the normal jaws not the reverse jaws and see if it makes a difference. Personally i don't like how little you are holding in the chuck and i see the wider part of the jaws as potential problems if you are trying to turn right up to the jaw.
    Most of my 3 jaws hold better than .1mm TIR, but i do have one that is more like .5 TIR, but it has seen a pretty hard life. I also have a cheap new one that is pretty poor.

    Ew
    1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.

  12. #12
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    Default

    Thanks so much for all the help guys. Really appreciate the support.
    I have read a great deal about machining, and downloaded a lot of instructional videos, however, the videos and books make things sound fairly easy, but when you apply this to real life, little things are a lot harder!

    I checked the runout on the 3 jaw chuck itself, and it was 0.001mm I think thats pretty good.
    I mounted the 4 jaw chuck, which is not really a full chuck (i'm not sure what the proper name is) but it looks like this. Its more of a plate.
    I'm sure these are not recommended, and a proper good chuck should be purchased.

    8872481357854.jpg 8872481390622.jpg

    anyway, the runout on the 4 jay was .001mm .
    It took me a while to mount the piece in the 4 jaw and get it right, but I got it within .001mm runout. I'll get better with this over time but I
    I think the issue was that all the stock I was mounting before was going into the spindle bore, so it was helping to hold it true, however when I had a piece of stock that didn't fit the bore, the 3 jaws just were not able to hold it well enough, or maybe it was me. The 4 jaw is definitely more stable and holds the work piece very nicely.
    I'll probably go and buy a proper 4 jaw chuck this week sometime because this was quite hard to adjust with 2 keys..

    This machining thing isn't cheap, but I really am enjoying it.

    IMG20140615034402.jpg


    This is the method I (tried) to use, took a while but I worked it out.


  13. #13
    jatt's Avatar
    jatt is offline Always within 10 paces from nearest stubby holder
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    This machining thing isn't cheap, but I really am enjoying it.




    oohsam wait till you discover milling!!!!!!!
    Frisky wife, happy life. ​Then I woke up. Oh well it was fun while it lasted.
    From an early age my father taught me to wear welding gloves . "Its not to protect your hands son, its to put out the fire when u set yourself alight".

  14. #14
    jatt's Avatar
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    oohsam wait till you discover milling!!!!!!!
    Looks like I will have to take that one back...... a quick search of your previous in archives. It would seem you already have one.

    Would be nice if I just could buy a couple of chucks and grind up some HSS to cover the lions share of my milling jobs!!
    Frisky wife, happy life. ​Then I woke up. Oh well it was fun while it lasted.
    From an early age my father taught me to wear welding gloves . "Its not to protect your hands son, its to put out the fire when u set yourself alight".

  15. #15
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    This machining thing isn't cheap, but I really am enjoying it.

    When I first got my gear I was told I'd end up with a small fortune, what I wasn't told was that I had to start with a big one.

    It's certainly not a cheap hobby but what hobby now days is?
    Kryn

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