"Truing" chuck jaws. Your opinions?
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    Default "Truing" chuck jaws. Your opinions?

    So I've been seeing Youtube videos showing people truing chuck jaws with a grinder. They all show that the jaws are bellmouthed by chucking up a pin and showing it wobble, or by showing clearance at the top of the jaws. It's my opinion that this method does not indicate bellmouthing. My thought is that a chuck that shows this indication probably has wear on the jaw raceways, and that the jaws are lifting the same way many mill vices lift. I don't see anybody suggesting grinding the jaws of their mill vise to make them parallel under load. If I suspected bellmouthing of the jaws, I would take them out and put them on my granite plate to check if the face is perpendicular to the raceway. It just seems to me that grinding the jaws without really knowing where the problem lies is a bad approach. Kind of like grinding the ways of a lathe to correct for twist when the lathe hasn't been leveled. What's your opinion?

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    It's pretty hard to regrind the scroll on a lathe chuck so most people grind the jaws to be rid of as much bellmouth and runout as possible. If you have a worn chuck this is a bandaid fix, the chuck will eventually need to be replaced or you can live with a bad chuck. This is very natural with a three jaw chuck, which in my opinion is the single largest stumbling block when it comes to an accurate set up.

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    Default no suprise

    Quote Originally Posted by Forestgnome View Post
    So I've been seeing Youtube videos showing people truing chuck jaws with a grinder. They all show that the jaws are bellmouthed by chucking up a pin and showing it wobble, or by showing clearance at the top of the jaws. It's my opinion that this method does not indicate bellmouthing. My thought is that a chuck that shows this indication probably has wear on the jaw raceways, and that the jaws are lifting the same way many mill vices lift. I don't see anybody suggesting grinding the jaws of their mill vise to make them parallel under load. If I suspected bellmouthing of the jaws, I would take them out and put them on my granite plate to check if the face is perpendicular to the raceway. It just seems to me that grinding the jaws without really knowing where the problem lies is a bad approach. Kind of like grinding the ways of a lathe to correct for twist when the lathe hasn't been leveled. What's your opinion?

    Most of medicine treats the symptoms. No reason not to do the same with machine tools.

    (crowding the jaws of a vice to close first at the top is not unheard of. common practice for wood butchers!)

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    Just load them up at about half-travel and hard-turn them. Won't make the jaw "right" at all diameters, but will improve things. Soft jaws makes things "right" at all diameters :-).

    Remember to load them up in the appropriate way -- grip an OD to true them for OD, and an ID for ID. Mixing these up doubles the bellmouth error!

    Regards.

    Mike

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    Quote Originally Posted by Forestgnome View Post
    So I've been seeing Youtube videos showing people truing chuck jaws with a grinder. They all show that the jaws are bellmouthed by chucking up a pin and showing it wobble, or by showing clearance at the top of the jaws. It's my opinion that this method does not indicate bellmouthing. My thought is that a chuck that shows this indication probably has wear on the jaw raceways, and that the jaws are lifting the same way many mill vices lift. I don't see anybody suggesting grinding the jaws of their mill vise to make them parallel under load. If I suspected bellmouthing of the jaws, I would take them out and put them on my granite plate to check if the face is perpendicular to the raceway. It just seems to me that grinding the jaws without really knowing where the problem lies is a bad approach. Kind of like grinding the ways of a lathe to correct for twist when the lathe hasn't been leveled. What's your opinion?
    I think some of them grind to try to 'true' the self centering jaws a bit because maybe its not holding parts in a concentric manner. I saw some videos. What some are not doing properly is actually putting say a thin washer of some sort right at the back of the chuck face where it clears the actual jaws and tightening the chuck so that it pushes the jaws back. The biggest problem with self centering chucks is the backlash of the jaws. So just grinding them in the open position will not true them at all because of the backlash, even if they are spinning it while grinding and the centrifugal forces will push the jaws back. They should tighten the jaws with the back washer and then true them. Not to mention, the cheap hand grinders they secure on to grind their chuck, likely has really poor runout. So the end result is probably not the most precise truing job. Like the other guy said, hard turning them instead of grinding might be better but grinding puts less cutting forces and probably more accurate. Or unless there was a purpose built low runout grinder used and not the stuff picked up from home depot.

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    Quote Originally Posted by Forestgnome View Post
    If I suspected bellmouthing of the jaws, I would take them out and put them on my granite plate to check if the face is perpendicular to the raceway.
    ...and then what would you do? Cant regrind the slots, jaw slides, or scroll. So what are you going to do, moglice or turcite the jaws?

    Whatever the source of the error, if you properly load the jaws and grind them using the lathe ways, they will be as accurate as you can get with the jaws you have.
    The idea is to make a used chuck grip better and more consistently. Your other option is a new chuck.

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    Always buy an adjustable three jaw.......Problem solved.

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    The scroll is never worn evenly, grinding the jaws will correct the runout at that diameter but may make them worse at other diameters.

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    Quote Originally Posted by split tenth View Post
    Always buy an adjustable three jaw.......Problem solved.
    Uh, no. Wear of the sliding surfaces can't be fixed by adjusting for zero TIR. My older adjustable 3-jaw wouldn't grip worth a s__t using the hard jaws. The jaws just sprung open when you tightened, and the stock was only held at the back of the jaws. You could see daylight at the front of the jaws even when torqued super tight. I hard-turned them, and got the hard jaws working better, but it's still now only a soft-jaws chuck.

    Regards.

    Mike

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    Quote Originally Posted by split tenth View Post
    Always buy an adjustable three jaw.......Problem solved.
    Doesn't help an old chuck that you already own.
    The purpose of grinding chuck jaws is to make the chuck you have more usable, in lieu of replacing it.

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    Since the sources of errors in a three jaw chuck are numerous, this method, grinding the jaws, is of limited value. In fact, it may make the problem worse as it only "trues" them in one position in the slots and on the scroll. Try it if you want to, but do not be surprised if it does not help much.

    And yes, I do have first hand experience with this. I have plans to turn that chuck into a collet chuck.

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    I did it to an older Bison that suffered a very rough life. It looked like a lot of stuff torqued out of the jaws but the OD of the jaws was the same (not belled). Chucked up a 1" ball bearing on the scroll jaws and used a Dremel with a 80 grit sanding drum hose clamped to a tool holder to clean up the ID of the removable jaws. Went from .015" out to less than .001" after about 30 passes at slowest feed and slowest RPM. I was pleasantly surprised with the results and didn't notice any uneven wear in the scrolls.

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    Default There are two issues here, grip and concentricity

    Quote Originally Posted by Ranchero50 View Post
    I did it to an older Bison that suffered a very rough life. It looked like a lot of stuff torqued out of the jaws but the OD of the jaws was the same (not belled). Chucked up a 1" ball bearing on the scroll jaws and used a Dremel with a 80 grit sanding drum hose clamped to a tool holder to clean up the ID of the removable jaws. Went from .015" out to less than .001" after about 30 passes at slowest feed and slowest RPM. I was pleasantly surprised with the results and didn't notice any uneven wear in the scrolls.
    Ranchero50 Might not see the same improvement at diameters other than that ground for.

    But

    Any correction in the jaw bell mouth will translate across any stock size.

    Me?

    I'm happy to improve the GRIP of a scroll chuck. shims or "adjust-tru" will take care of the run out.

    With that, the last time I NEEDED the runout of a three jaw scroll chuck to be spot on was.... well,....... I just don't use the chucks for that requirement....

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    Ah, heck. I've DONE that grinding. Both with less-than-great grinders and with a decent TP grinder. It works, and I wasn't the least bit concerned with concentricity.... I wanted the jaws to grip right, they were known to be bell-mouthed in all cases.

    IMO there is exactly ONE good method to do it, and the credit goes to Rich Carlstedt.

    Drill a hole in the top of each jaw. Put a pin in each hole. Using the pins as if they were the jaws, grip a ring with them and snug it up.

    Now bring in the grinder, and go until you have hit all jaws for their entire distance axially along the bed. Spark it out thoroughly.

    It is best if it is a good grinder. But if you are religious about sparking it out thoroughly, even a not-so-great grinder will work OK.

    I've recovered usability on chucks that had screwed up jaws. I have never had a bit of trouble with the error varying at different parts of the scroll. Mostly that is because I fully expect any 3 jaw chuck to be "out" several thou, so all I ever ask of one is to hold the work tightly and cause no problems. I just don't give a rat's ass if it's "more out" here than there.

    If I didn't indicate it "in", then it's "out".

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    Quote Originally Posted by JST View Post
    Ah, heck. I've DONE that grinding. Both with less-than-great grinders and with a decent TP grinder. It works, and I wasn't the least bit concerned with concentricity.... I wanted the jaws to grip right, they were known to be bell-mouthed in all cases.

    IMO there is exactly ONE good method to do it, and the credit goes to Rich Carlstedt.

    Drill a hole in the top of each jaw. Put a pin in each hole. Using the pins as if they were the jaws, grip a ring with them and snug it up.

    Now bring in the grinder, and go until you have hit all jaws for their entire distance axially along the bed. Spark it out thoroughly.

    It is best if it is a good grinder. But if you are religious about sparking it out thoroughly, even a not-so-great grinder will work OK.

    I've recovered usability on chucks that had screwed up jaws. I have never had a bit of trouble with the error varying at different parts of the scroll. Mostly that is because I fully expect any 3 jaw chuck to be "out" several thou, so all I ever ask of one is to hold the work tightly and cause no problems. I just don't give a rat's ass if it's "more out" here than there.

    If I didn't indicate it "in", then it's "out".
    How did you protect the bed of your lathe or any other machinery component from the abrasive dust flying everywhere?

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    Quote Originally Posted by moonlight machine View Post
    The scroll is never worn evenly, grinding the jaws will correct the runout at that diameter but may make them worse at other diameters.
    This.

    Thats why actually you should use soft jaws with self centering chucks and grind or bore out the jaws close to the diameter of the part you're trying to grip to do precision work. Otherwise the concentricity of the part will always be an issue. But as others say, they are grinding to eliminate bell mouthing which is also a legitimate reason to regrind.

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    Quote Originally Posted by split tenth View Post
    Always buy an adjustable three jaw.......Problem solved.
    The problem with independent jaw chucks are they are a PITA to set up every time you change diameters, and they are often still not really that true, even if setup by the best of them. A self centering chuck, with jaws bored out near the diameter of part grip (jaws pushed back while boring) will be more true than independent jaw chuck done really well. In theory you can get a perfect setup also with independent jaws but often ppl quit after you get within accepted runout tolerances.. maybe 0.005mm or 0.01mm.

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    If you have 2 part jaws, this isn't even an issue. You already bore them for the job as needed, and they are consumable. It's really only for one-piece jaws.

    Quote Originally Posted by cncdumm View Post
    How did you protect the bed of your lathe or any other machinery component from the abrasive dust flying everywhere?
    In the usual way. Cover it up.

    You could also make a stub arbor or other holding means, and take the thing to a cylindrical grinder if you wanted.

    For this, however, that may be an over-emphasised point. If you are not heavy-handed about it, you won't generate a lot of abrasive dust, and reasonable precautions will be fine. If the chuck is worth messing with, you will be taking off very little material, and only from the gripping area of the jaws.

    And, in the end, the machine, while deserving of reasonable protection, will accumulate abrasive dust from other sources also. Overnight, for instance, as "the dust settles", the usual ambient rock dust will settle on all machines. Got concrete floors? You have rock type dust.

    Machines get used, and eventually worn. OK, you gonna die too, so no point in going crazy with wrapping and sealing. Just make a reasonable effort and get on with making parts.

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    the problem is not the dust on the ways, but the gritt in the chuck. therefore hardturn. if you grip on a ring in the back of the jaws, use the compound adjusted to make a slight taper. i had it 0.01 mm (on the radius) tighter in front and it worked out just great. runnout is usually less than 0.02 mm on any diameter.

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    Quote Originally Posted by Forestgnome View Post
    So I've been seeing Youtube videos showing people truing chuck jaws with a grinder. They all show that the jaws are bellmouthed by chucking up a pin and showing it wobble, or by showing clearance at the top of the jaws. It's my opinion that this method does not indicate bellmouthing. My thought is that a chuck that shows this indication probably has wear on the jaw raceways, and that the jaws are lifting the same way many mill vices lift. I don't see anybody suggesting grinding the jaws of their mill vise to make them parallel under load. If I suspected bellmouthing of the jaws, I would take them out and put them on my granite plate to check if the face is perpendicular to the raceway. It just seems to me that grinding the jaws without really knowing where the problem lies is a bad approach. Kind of like grinding the ways of a lathe to correct for twist when the lathe hasn't been leveled. What's your opinion?
    .
    i have reground chuck jaws when the chuck mount surface was clean and ding free and of course scroll
    was cleaned too and still part in chuck was wobbling too much like over .002"
    .
    every one i ground was normally much better after grinding jaws and usually within .0005"..... what i notice
    is a worn scroll can give variations at different diameters. it is hard to get concentric at all diameters and
    of course if scroll is dirty eccentricity will increase.
    .
    some people grind a extra .0005" off back part of jaws so more pressure at the 1" section near jaw ends.
    not sure it matters too much as jaws sink into part more than .001 usually


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