plead for 3 jaw chuck knowledge

casca92

casca92

MURPHY:Was an optimist ,when compared to me
H-M Supporter Gold Member
Joined
Jan 22, 2017
Messages
85
Greetings wise ones
hopefully soon I will have this JET BD 920 N lathe out of your hair.
my confusion today is the chuck jaws do not square up. lathe was pre-loved probably more than once when I purchased it.
jaws are marked with what I believe is the correct location by stamped number.

I have juggled the jaws into different jaws and still same effect.

2 jaws will center up nicely with one always short of target.
not sure if it is always the same jaw not squared or if it is one slot out of wack..

jaws appear to be in good condition, and has been oiled and fairly clean. it mounts to spindle square and runs square.

OR
Am I not being a realist and my expectations are to high. would like to hold off on a new chuck for a little while until I return to work from medical leave.

I a wait your wisdom and guidance.
casca
 
Often the jaws are marked 1.2.3 or something, It is a pain getting them in right. First, you need them in the right slot, and then you need the first one (hopefully marked 1), to engage the spiral first, then turn the spiral to the next slot, and put the second jaw in, and so on.

If they are not marked, mark them with tape, and also mark the slots. then try your luck. remembering that they all need to follow the spiral. Whats it? 27 possible ways to assemble them?

If you have 2 jaws fit nicely, it might just be that the third jaw needs to be put in its slot before the other two.

Also, if you flip the jaws over, you can see that one has the first thread closer to the tip, then one is a bit further back, and the other is back more. I think that is your 1, 2, 3. or 3,2,1.
 
ahh a light is starting to come on , not real bright but that is some guidance I was hoping for.
 
Superburban said:
Often the jaws are marked 1.2.3 or something, It is a pain getting them in right. First, you need them in the right slot, and then you need the first one (hopefully marked 1), to engage the spiral first, then turn the spiral to the next slot, and put the second jaw in, and so on.
Click to expand...
I'll beg to differ just a bit here. The order in which you insert the jaws is important. But which slot the #1 jaw goes into is a lot less so. If the spiral inside the chuck, the teeth on the jaws, and the angular locations of the three slots are made correctly, it should make no difference at all. Sometimes it will make a small difference in how well the jaws center the work.

None of the chucks I've bought have had the slots numbered. All of them have had the jaws numbered. Per a suggestion I'm pretty sure I read about in a HM post, I tried marking the slots in one of my 3-jaw chucks A, B and C. I then tried all three positions of the #1 jaw and measured the runout of a test part. IIRC the difference between the best result and the worst was something like .002" runout. I've since done this with my other chucks, and recorded the "sweet spots" for all of them. Of course, all of this was at just one test diameter. It might well vary for other diameters, depending on the various various factors mentioned above.
 
John, That certainly does make sense, and in a perfect world should work. I have a small 5" 3 jaw that I got off E-bag, To fit in the flat rate box, the seller took the jaws off. Neither the jaws, or the slots are marked. Took me awhile to get them in right. Once I got the right order, they were close, but they all still had to move over one slot to close right. Now you have me wondering if the jaws are cut right. I have used it on a rotary table, so would not notice if they are centered. It was obvious they were re-cut.

Your explanation, along with Mikeys make the 3 jaw sound so simple. Now I'm going to have to go out, and check that 3 jaw.
 
I buy high quality chucks because the chuck is the heart of the lathe. All of my chucks have numbered jaws and slots. If it has more than one key hole, one key hole will also be marked. And all of my 3-jaw chucks are ajust-tru type. I would not buy anything else. If you buy a cheap chuck or a worn out one you will probably get frustrated with your project results. I would not regrind the jaws either. That whole picture means something is really wrong and you will end up with an iffy chuck. Yes, I know a good chuck can cost as much as the lathe. But, the results of your effort will be money well spent…Dave
 
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