Announcement

Collapse
No announcement yet.

3 Jaw Chuck Jaws Not Evenly Contacting Workpiece

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • JRouche
    replied
    Originally posted by kuksul08 View Post
    In my lathe, the jaws are two-pieces and bolt together. One part rubs against the scroll and has some contact area with the workpiece, and the outer part touches the workpiece only.


    Can someone shed some light on this?
    Not sure I can but will try.

    I turn some tubing and no matter what I always had a flat spot" with my three jaw set. I would crush the tube just to get a good bite on it to turn it. Frustrating. I was using over sized tube just to keep from deforming it.

    Then the six jaw, solved my issues. It is still not my prefered method, which is a four jaw.

    I like to dial in the part with a four jaw most of the time.JR

    Leave a comment:


  • luthor
    replied
    Originally posted by nickel-city-fab View Post

    This could actually be a great advantage if you need to hold something oddball
    just make a sacrificial top jaw for the odd job
    That's what soft jaws are for.

    Leave a comment:


  • nickel-city-fab
    replied
    Originally posted by Doozer View Post
    It is incorrect to say that workholding can not be done with the master jaws.
    Just take off the top jaws and chuck up your work. Much more positive holding force
    from the master jaws. Done all the time.

    --Doozer
    This could actually be a great advantage if you need to hold something oddball
    just make a sacrificial top jaw for the odd job

    Leave a comment:


  • Doozer
    replied
    It is incorrect to say that workholding can not be done with the master jaws.
    Just take off the top jaws and chuck up your work. Much more positive holding force
    from the master jaws. Done all the time.

    --Doozer

    Leave a comment:


  • danlb
    replied
    It seems to me that.... (and I could be wrong) The surface that was shown to be ground in situ is the tapered side of the jaw, not the gripping surface. Grinding on the side like that allows more clearance as you tighten it down on a small rod. One situation where a person would do that is to allow the chuck to close down on a smaller work piece.

    I'm surprised to see the teeth on the master jaw too. The only value that I see in that is the ability to use it without top jaws when you need all the length that you can get. It would allow you to cut all the way back to nearly the face of the chuck.

    Dan

    Leave a comment:


  • darryl
    replied
    The top jaw has a key that nests into the bottom jaw. Is there any play in that area? Perhaps the jaws were pushed toward center before they would ground to 'spec'. Then when the pressure is put on a workpiece, the play moved the other way, leaving you with the gap.

    One cure could have been to use a shim on the appropriate side of the key- but then that shim would always be required when remounting the top jaws. It would be better to take up that play in the outward direction before tightening the jaw down- then go through the process of grinding the jaws true.

    Your solution sounds like what I would have done. I can set up to grind jaws, but I have never done that- yet. My one-piece jaws are out by less than two thou in concentricity, and yes I'd like to get that down to a couple tenths ( is that laughter I hear?) but it will never be that close over the whole range- and likely not repeatable either. Apparently for a three jaw, being out by 2 thou is pretty good. In that vein, your method of sanding back the bottom jaws is perfectly acceptable. The testing is pretty easy too- and if your final sanding takes a little off the top jaws as the surfaces even out, that's ok. Ground in place would give you a slight hollow in each jaw, which I think is to be preferred, but the grip surface changes anyway as you clamp different diameters, so it's kind of just academic at this point. It's better to achieve full jaw length contact with the workpiece, and it looks like you have done that.

    Leave a comment:


  • BCRider
    replied
    The first picture with the grinding shown on the angled faces sure does look like it's a match.

    Then given that the masters are a little "lower" it might be some other issue. If the chuck is new to use but previously used perhaps someone before you loaded the jaws with a small puck of metal held back by the master jaws then lightly ground the slave jaws true? Otherwise it would appear to be a QC issue?

    You might get away with simply grinding the masters so the parts are only held in the slave jaws. You might not need to grind the whole length. But then if you're going to set up to grind the jaws anyway.... might as well do it all so you get full length support.

    Leave a comment:


  • old mart
    replied
    I would regrind those jaws in situ,after throughly cleaning the chuck parts. Then afterwards, go through the same cleaning to make sure there was no grit left to cause wear. The chucks that I have ground can be trusted to run at 0.001" tir at any diameter, I must be very lucky to get 3 chucks that good.
    With those reversable jaw tops, I would only grind the full length of the inside but not the centre when grinding the jaws when they are set outside. It would be very unlikely to be able to match the master jaws with the top parts both ways around.
    Last edited by old mart; 11-08-2021, 11:13 AM.

    Leave a comment:


  • J Tiers
    replied
    Looks clear to me, the grind marks are not the point.

    Look instead at the grip faces. Notice that they look a little wider on the top jaw. If true, that indicates that they are farther back than the narrower marks on the bottom portion of the composite jaw. The two were ground together at least on the tapered sides, but the top part either has more wear, was not ground on the grip face at the same time as the bottom one, or the manufacturer did not spark out on the whole composite jaw when grinding the grip faces.

    The grip faces on the bottom also do not even look consistent along that jaw, but that may be a trick of the reflections.

    Leave a comment:


  • Bob Engelhardt
    replied
    Originally posted by kuksul08 View Post
    Here are some photos of a jaw. I can see the grind marks are continuous, ...
    With the continuous grind marks, I have wonder how the upper & lower pieces are not aligned? How could they be ground together & not be aligned? They were probably ground separately, but on the same grinder, with the same orientation. So the uppers are original, but mismatched by the manufacturer - a QC failure.

    Leave a comment:


  • kuksul08
    replied
    Originally posted by plunger View Post
    Nice fix. Needs some pretty accurate sanding to get that right.. But if you turn the jaws around for external work wont they also be wrong. Looks like they can be slightly adjusted . Isnt the problem the key that holds the movable jaw to the main jaw.
    The key is a precision tight fit. They can be removed by hand but just barely, it's very snug. What I did should have no effect on the alignment if I flip the jaws around.

    I think the ultimate solution here would be to grind the master jaws even lower, then place a rod in there to preload against, then grind the top jaws as was recommended.

    I think what I have will work for my purposes.

    Leave a comment:


  • plunger
    replied
    Nice fix. Needs some pretty accurate sanding to get that right.. But if you turn the jaws around for external work wont they also be wrong. Looks like they can be slightly adjusted . Isnt the problem the key that holds the movable jaw to the main jaw.

    Leave a comment:


  • kuksul08
    replied
    Here are some photos of a jaw. I can see the grind marks are continuous, so I have no reason to believe these top jaws are for a different set of master jaws.

    What I did was put some fine sandpaper on a 90 degree angle plate and put that on a surface plate, then carefully removed about .002" from the master jaw only. Now there is about a .001" gap, which allows the top jaws to make full contact. Amazing how much more rigid everything is!!!

    Thanks for the recommendation guys.




    Leave a comment:


  • BCRider
    replied
    Look at how the slave jaws are shorter than the masters. If we look around at online images I see that some, not most but some chucks with two piece jaws have slave jaws that are wider than the master jaw. But ALL of the slave jaws are the same or a whisker longer than the master jaws. Yours are quite a bit shorter on the outside and apparently a few thou shorter on the inside too. Makes me think that these are not the proper slave jaws for that chuck.

    In another thread about replaceable jaw chucks someone alluded to a standard for the jaws as far as fitting to the spine and cross bar goes. If that is so it would explain why the slave jaws fit. But if there's no standard for the outer shape of the slave jaws it might explain why your jaws in are not a good fit and might not be the proper ones for this chuck in the first place.

    You might want to look into that idea of a common fitting format and if so shop for a new set which starts out longer than the master jaws.

    Leave a comment:


  • Stu
    replied
    Ditto on Danlb, they are not meant to grip the work, just remove them and take a little off the tips. They don't need to match, just need to be smaller than the top jaws

    Leave a comment:

Working...
X