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3 Jaw Chuck Issues

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  • old mart
    replied
    That is not bad for that type of chuck, you might be able to improve it with grinding of the jaws, but possibly only at the diameter that the jaws were at when they were ground. For precision, a four jaw independent chuck and a little more time spent getting the runout minimised is the way to go.

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  • brian Rupnow
    replied
    Today, with the chuck back on the lathe and all of the nuts tightened down, I'm reading .0025 total indicated run out measured 1" out from the jaws with a 1" hardened and polished shaft held in the chuck. That is as good as it is going to get. I can live with that.---Brian

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  • brian Rupnow
    replied
    The three original 10 mm studs come out without much of a fight. I then stoned that face of the chuck with a 10" medium grit stone. I measured how far above the surface the studs stuck up. I cut the heads from the three bolts and after making certain that the nuts would unscrew or screw on from the cut ends, I coated the ends which would screw into the chuck with Loctite 262 and screwed them into the chuck until the correct measurement was reached. The three original studs with damaged threads are laying on top of the chuck. I will wait 24 hours now before reassembling the chuck to the lathe.

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  • brian Rupnow
    replied
    The three studs which came with the chuck are made from some kind of very soft Chinesium, and as a consequence I have messed up the threads somewhat. That may well account for the reason I can slide a 0.0015" shim between the chuck and the spindle mounting flange on one side. I went down to my nut and bolt store today, and although they didn't have studs, they had grade 5 bolts with a long enough thread to become studs. I also picked up a bottle of Loctite 262 thread locker. Next order of business will be to see if I am able to get the original studs out of the chuck. If so, I will cut the heads off the three bolts and loctite them into place. to replace the original studs.

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  • J Tiers
    replied
    Originally posted by brian Rupnow View Post
    I have found something!!! I can slide a 0.0015" feeler gauge in between the chuck flange and the spindle flange on almost 125 degrees around the spindle circumference, with all the bolts which hold the chuck in place bolted down. That indicates that the centerline of the chuck is not in plane with the centerline of the spindle. I will now disassemble things one more time and dress the mating face of the chuck.
    This is significant.

    Can you check to see if the face of the spindle where the chuck mounts is flat as well as not having runout? If that surface is slightly convex, you will just never get the chuck to mount consistently no matter how you tighten.

    If it is dished slightly, you will not have a problem. Since you do have a problem, either there is a convexity, or a poor fit where the chuck fits against the "inside corner" where the spigot meets the flat face of the spindle, or a bit of swarf has got in.

    Since the chuck is tight, where it was not originally, and wear does not generally add material, odds are that there is swarf in there.
    .
    .
    .

    With regard to scroll wear, that is often cited as a major issue. Does not seem to be the issue here.

    But WHAT scroll wear do people mean? There are two kinds, one of which is fixable, and one not so much. And there is jaw wear also.

    The one probably meant by most who mention it is wear of the actual scroll, the portion which engages the jaws and moves them. That may not be the biggest issue. That type wear would be an issue if the chuck is always used at a particular size of workpiece. That would tend to wear a particular area, but is probably more typical of a production situation (and why would they be using a scroll chuck?). If, as with most hobby folks or general repair, a fairly wide variety of sizes of work are used, then wear will be distributed over much of the scroll. There is no reasonably practical way of fixing this type wear, although it would be "possible" to do.

    The other form of wear is wear of the central pivot of the scroll disk. This affects any size part, and is the prime contributor to inconsistent centering. It is the reason behind always using the same pinion to tighten the chuck, since that will tend to force the slop in the same direction every time.

    For that, it is possible to shim the bearing area with a thin shim wrapped around the pivot. I have done this and while it is not elegant (everyone hates shims, right?) it does work effectively.

    The final type of wear that affects jaw position is wear of the "gear teeth" on the back of the jaws. The area of the scroll used may vary, but the jaw teeth are used every time in the same location. If they do not wear evenly, perhaps due to variations in the steel, etc, then jaws will drift out of synch as they wear.

    Last edited by J Tiers; 11-24-2020, 11:43 AM.

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  • redgrouse
    replied
    Well I am late to the party ! Not read all the posts but had a look to see what a CX 701 lathe was, reading the information on Busy Bee see here https://www.busybeetools.com/content...uals/CX701.pdf I could not believe what I read on page 20 regarding chuck runout and their “fix”
    It seems there is little faith in the product so Brain may be fighting a loosing battle?
    However as has already been said 3 thou TIR on a 3 jaw, particularly a Far East one is not too bad but the main spindle runout is a bigger problem and I would look to resolve this then make a new chuck backplate to suit the inevitably reduced spindle diameter thus removing one problem then look at other considerations after assessing the results.
    John

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  • Bruno Mueller
    replied
    I found some videos how to grind out the jaws of the lathe chuck.

    https://www.youtube.com/watch?v=o3NK5BPaTz0
    https://www.youtube.com/watch?v=zofS-ewP4Zs
    https://www.youtube.com/watch?v=mh7pRLZZV2k

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  • dian
    replied
    somehow i dont believe this. 3 thou tir? i have never seen such a chuck, that would be absolute junk. my 250 mm chuck has 0.02 mm 200 mm out and even the 100 mm chuck on the chinese 3-in-1 has 0.02 mm 100 mm out.

    of course you have to tap it concentric on the register and find the procedure of thightening that yields best results. e.g. on the big chuck i snug up #2 and fully tighten #0.

    btw, "angular disparity" based tir can decrease as you move away from the chuck also.

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  • danlb
    replied
    Originally posted by brian Rupnow View Post
    David--You're not wrong. Be that as it may, I have built 40 working model engines using primarily my 3 jaw chucks. Sometimes I have to use a four jaw chuck to make an eccentric or something which requires an offset. It seems that model engines are tolerant of .003" tir. Occasionally I think of using collets, but then I look at the price of a quality collet set, and that quickly makes me more tolerant of .003" tir also.
    I was wondering why you were looking for better than .003 on a scroll chuck. Yes, the spindle needs to be under .001 to allow chucks and collets to be precise. Unfortunately the wear on a scroll of a chuck almost guarantees that the chuck jaws will eventually be off just a little. Even so, you can (and do) make perfectly usable precision parts simply by doing all operations on a part in one setup.

    There may be a reason that many blueprints call for a .005 tolerance.

    Dan

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  • Bob Engelhardt
    replied
    Originally posted by brian Rupnow View Post
    ... and still have angular disparity between the centerline of the spindle and the centerline of the chuck. ...
    An angular disparity will give you runout, but that runout will increase as you move away from the chuck. Easy check.

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  • J Tiers
    replied
    The parallel fit of the chuck over the "spigot" MUST have clearance or you cannot assemble the two. The question is not "whether", but "how much" clearance. And that is what ideally determines the concentricity of the mounting as well as the repeatability of that mounting.

    There are other dimensions of the spindle nose that will affect the concentricity and alignment of the chuck. Those may be in-play here.

    The resulting concentricity of the workholding is a separate issue that has to do with the device in question. But it is clear that errors in concentricity of mounting will not likely enhance concentricity of the workholding, regardless of the goodness or badness of the workholding device.

    The dealer has stated that the spindle is OK, without further comment that we are aware of. But what is the actual runout of the spigot? And what is the actual runout of the face to which the chuck is mounted? Those numbers are important to determining the nature of the issue, and are pretty easy to find out..

    Beside runout, what is the clearance between the "spigot" and the chuck recess? That is another number that is relevant to the situation. That spigot must have a standard size, is it at that size? I believe that is a DIN standard. In any case, if the spindle is undersize, then every spindle fixture mounted will have slop. If oversize, then the chuck may not mount correctly. If too long, the chuck may not seat on the correct surface. It is unlikely to be too short to register correctly, but I suppose that is at least possible.

    So, is it known that the current chuck mounting is OK from the standpoint of size, fit, and repeatable concentric and parallel mounting? If yes, then the problems are in the chuck itself.

    The stated issue of 3 thou eccentricity seems to me to be a pretty good 3 jaw chuck, but we do not know if the chuck is the cause.






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  • brian Rupnow
    replied
    David--You're not wrong. Be that as it may, I have built 40 working model engines using primarily my 3 jaw chucks. Sometimes I have to use a four jaw chuck to make an eccentric or something which requires an offset. It seems that model engines are tolerant of .003" tir. Occasionally I think of using collets, but then I look at the price of a quality collet set, and that quickly makes me more tolerant of .003" tir also.

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  • David Powell
    replied
    Three jaw self centering chucks are not suitable for reliably holding material or parts concentrically.
    Sometimes they may do so but that is a matter of luck
    Certainly some are better than others, some when new.may hold workpieces within a couple of thous or even less.
    They possess the advantages of speed in set up and versatility to hold varying sizes simply by a few turns of the key.
    Every good textbook I have ever read , and all the experienced lathe hands I have worked with have agreed that if you want to hold material or machined parts truly you need either chucks where the jaws can be adjusted individually or collets.
    (I know of the set true 3 jaws. but they usually set true only at the diameter they are set for)
    I once had a 3 jaw independent, that was a nightmare to use and it quickly went away!
    Chasing rainbows can be fun, but if you want quality parts all the time a three jaw self centreing chuck is NOT the tool for the job.
    Regards David Powell.

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  • brian Rupnow
    replied
    Parallel fits work quite well to give concentricity IF the mating flange surfaces are perfectly flat. However, if a "burr" has been raised around one of the studs where it screws into the chuck, you can crank all three nuts down tight, and still have angular disparity between the centerline of the spindle and the centerline of the chuck. Since these studs are not hardened, I have messed them up somewhat switching between 3 jaw and 4 jaw chucks. I have ordered 3 new studs, and after I remove the three existing studs I will "stone" the mating side of the chuck before putting the new studs in.

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  • J Tiers
    replied
    Parallel fits by nature cannot be exact, or they would be more nearly press fits. There is always some radial play, and the issue is whether that is excessive and capable of causing the problem. The stud/bolt clearance has all the same issues with parallel fit, and my understanding is that the holes are normally made with a generous allowance of room.

    So it may be a good idea to take a very good look at the fit of the chuck onto the spindle to see if there is any radial play in the system that might explain the issues found. The problem is of course what to actually do about it if some looseness is found. But at least the nature of the problem would be known.

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