How to tighten a 3 jaw?

I disagree on the 0 marks, they are witness marks to disassemble the chuck from the backing plate, that is why they have 0 on both of them. If it was for the best adjustment it would only have one 0.
 
RVJimD said:
Marcel,

is this something you do once to correct or set the runout, or every time you chuck up a part?

jim
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I generally only do it when I flip a part around in the chuck so that the concentricity remains as close as possible. Sometimes I'll do it if i have to rework a part that's already been turned to assure it runs true. There's really no reason to do it any other time.

Marcel
 
Kernbigo said:
I disagree on the 0 marks, they are witness marks to disassemble the chuck from the backing plate, that is why they have 0 on both of them. If it was for the best adjustment it would only have one 0.
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My Bison chucks have reference marks (0) but not on the backplates. My import chucks don't have any reference marks. I made my own.
 
I guess the question is, in the OP's photo, are the witness marks aligned with one of the pinions just by coincidence? Or was the chuck marked first to indicate the favored pinion and then the backplate marked afterwards to indicate best alignment for future disassembly/reassembly?

Tom
 
hermetic said:
The 0 marked key hole on the three jaw chuck is the one marked by the manufacturers as the hole to use for best repeatable concentricity, which begs the question "so why would you want to use the other two holes ever?" and I suppose the answer is "when it really doesn't matter" (whenever that is).
Xalky, I like that idea! I will try it next time I am dialing in for accuracy!

To all, If you look at the specs of a new, good quality 3 jaw chuck, you will see that the concentricity is rarely specified as greater than .003", which is, I assume, the repeatable concentricity. You can get nearer than this with a bit of tapping and dialling in, but if you want better, "the most accurate chuck you posses is an independant 4 jaw"!
Phil
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Nearly there :anyone:

:think1:I suspect that the three holes are there to be used :rofl: ( more of this later ) .

Those chucks with the " master " mark tend to have that mark where the start of the plate screw thread starts so is it an easy or dumbed down way of putting the chuck jaws back in the right order at the starting point if you have had them out ?
jaws MY 3 jaw lathe chuck simply has 1or 2 stamped on two of the jaws & 1&2 on the body , both sets of the four jaw chuck jaws have either 1,2 or 3 on their jaws . There are no official line up marks on the body or the back plate .

Though after gently scribing matching marks on the body & back plate of my three jaw lathe chuck , the stripping & cleaning when I first received it ( it was difficult to open & close ) I found it still stiffer than I wanted so I rotated the back plate one hole and found it a perfect fit/movement .
I then dot punched align ment marks on the new favoured position of the body & back plate .

I guess it's assumed that the operator has the intelligence to work out where the unmarked jaw goes :roflmao:.

RE:-
The three holes , if you continually only use one hole to tighten the chuck it will eventually wear in one segment of those gear teeth involving the jaw spiral closing drive lips and become out of kilter so to speak. It will not run freely in the end , frequently binding up on both opening & closing of the jaws . ( A bit like starter ring wear on car engine flywheels & starter motor bendix drives jamming/ wearing ) .

There is also the possibility that by only using one hole you can end up with the work piece being gripped out of line. If you use all the holes and give them a nip up via them you'll normally discover any irregularities and rectify them before doing start up on the machine .

It's also simply good engineering practice to spread the stress load evenly when tightening things up , so it is distributed evenly over several places unless specifically stated by the manufacturer or after hundreds of years of hard won experience by many people .

I think I can truthfully say that in my 50 plus years of tightening various chuck of my own that have zero or minimal wear on the jaw faces & using all available tightening points in gentle rotation I have never ever had any drill or work piece slip .

I have however had things slip if I have not done it evenly and one of my ancient 1/4 & one of my 23 / 64 ths much sharpened twist drills have the battle scars to show it.
 
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I read somewhere that the marked socket was the one used at the factory when the jaws were ground. Its supposed to give the least runout. True or not I can't say and have no recollection where I read that.

Greg
 
Ponder this; If the chuck manufacturers wanted you to use the same pinion all the time, they would save the manufacturing cost of the other two pinions and holes and leave them out.
 
I have 2 three jaws chucks---one has three holes, the little one I like to use the most only has one hole, guess it gets worn out faster (sorta like me):)) But on the one with three holes--it only gets one tightened--whichever is easier to reach. Haven't noticed any difference so I will just keep doing one.
 
NightWing said:
Ponder this; If the chuck manufacturers wanted you to use the same pinion all the time, they would save the manufacturing cost of the other two pinions and holes and leave them out.
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I dont think it is that simple. As I have heard it explained, the scroll has a small amount of play in it due to various factors (tolerances, mass production etc... ). With this in mind it stands to reason that one of the drive sockets will use this play to give less runout than the other two.

Since this play is not designed in it cannot be determined beforehand which of the drive sockets will give the best runout. Some manufactures then test for the one with best runout and mark that one.

Sent from my GT-I9505 using Tapatalk
 
Bamban said:
Mark,

What is the trick to set a 3J scroll chuck for 0.0002 run out? I would like to explore how to do that on my 3J using a 1 inch test bar.

Thank you.
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What Marcel says actually does work for very small adjustment and wont hurt the chuck cause you ain't smacking it with a 5 pound sledge. You give it a sharp heavy tap. I agree with his method , as i have done that on occasion.

Now on to your question.:

I chuck up a 1" ground bar a few inches long ( a piece of drill rod or turn a 1" bar). I loosen the bolts holding my chuck to the back-plate. (my chuck has three dowel pins in the back-plate locating it. I think the register is bad. God only knows what this thing has been through all these years.) I get my chucked up rod indicated within as close to perfect as possible. 1 or 2 tenths at least. I then tighten the bolts and reamed and re-pinned my back-plate and chuck. Because of parts fit, wear and manufacture accuracy The chuck will generally repeat within a few tenths AT THAT DIAMETER ONLY. Because of the factors I just mentioned, when you chuck up at any other diameter there is going to be error. How much error depends on the condition , wear , and quality of your chuck. I was fortunate that this Union chuck was a very good chuck and the worst run-out I get is .0025", but if I chuck at 1" diameter, I can get it to a few tenths run-out on a turned bar.

Take note: To get the repeat-ability I have to sometimes chuck,and un-chuck a few times, and at any other diameter, I do the same till it gets as close as possible to true. If I just throw a piece in the chuck and tighten it, It may be out .002" to .003". Also you can only get accuracy on a piece that is true round. If you throw a piece of CRS or HRS steel rod in the chuck , you cant expect any accuracy till you turn it. I hope I explained this so it is easy to understand.:phew:
 
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