# How to tighten a 3 jaw?



## RVJimD (Jan 5, 2015)

I thought I read someplace that you should always tighten a 3 jaw chuck in the hole marked 0?  Is that correct?  Can someone shed some more detail on this?

I was talking with a friend and the topic came up and we were standing In front of my lathe so I looked.  Sure enough it has a 0 mark.



Also, on the Jacobs chucks, are you supposed to tighten all three or is one good enough?

jim


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## JimDawson (Jan 5, 2015)

I have heard this discussion before and IMHO it makes no difference.  I have tried it and not noticed any difference in the way the chucks tighten.  Some recommend using all 3 holes to tighten, but I have never found a difference.  Having said that, it could make a difference if one of the pinion gears or the ring gear were fowled up a bit.

Same applies to drill chucks.


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## Bill C. (Jan 5, 2015)

RVJimD said:


> I thought I read someplace that you should always tighten a 3 jaw chuck in the hole marked 0?  Is that correct?  Can someone shed some more detail on this?
> 
> I was talking with a friend and the topic came up and we were standing In front of my lathe so I looked.  Sure enough it has a 0 mark.
> 
> ...



Since three jaw chucks are tighten at the same time by a ring gear, I have tighten using one socket.  You might get more pressure by tighting each socket.


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## chips&more (Jan 5, 2015)

I was under the impression that you might get and I said “might” have better repeatability in jaw clamping if you use the same pinion. If you have jaws and its parts that continue to have the same force and all, you should have a more constant location of your work/part in the jaws….Good Luck, Dave.


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## Ken_Shea (Jan 5, 2015)

I'm of the same opinion as others here, 
"might" make a difference, that said, I always tighten all three because it was beat into all of us during shop class by the shop treacher  so it's a habit now and no harm by doing it.


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## higgite (Jan 5, 2015)

I have a relatively inexpensive (cheap is such a harsh word) 4-inch 3-jaw chuck that came with my bench lathe. It does operate smoothly and it fits my needs, but it doesn't have a "master pinion" marked on it. So I ran my own test with various diameter drill rods that I'm confident are as nearly round as my DTI is accurate. I found one pinion that was consistently more repeatable with less TIR than the other 2. I marked that one and have been using it exclusively to tighten the chuck. So far, so good. Of course, it won't exactly center all work pieces like is possible with an independent 4-jaw, but when it's the chuck I'm using at the time, knowing I have the least runout within reason just makes me feel better.

Tom


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## Cobra (Jan 5, 2015)

I use a mark to ensure that each of the chucks goes into the spindle mount in the same orientation each time.
As far as tightening the three jaw, if I am worried about the runout, I use the four jaw.


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## shoeboxpaul (Jan 5, 2015)

For a drill chuck, I was taught to use all three holes. There is clearance in the chuck in order for it to work, and it can sorta wedge on itself giving you the feeling that the drill bit is secure. When you use all three holes, especially on an older chuck, you see the chuck turn just a little bit extra using the second and possibly third hole. This can prevent a lot of drill bits from turning in the chuck.  For a lathe chuck, I wouldn't think you would want to hold the material with that much force. Single point turning takes less power than drilling, a drill has 2 cutting surfaces usually with greater contact area.


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## Micke S (Jan 5, 2015)

It depends on the condition and wear of the scroll wheel and the support of it in the chuck. Turning the best screw only can be a major difference for low runout compared to other screws. But if everything is in order turning any or all screws does not improve.


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## dracozny (Jan 5, 2015)

Both the 3 jaw lathe chuck and 3 jaw drill chuck I have I find I need to tighten at least 2 holes, the 3rd is usually pointless. However only tightening one I find the jaws do not clamp properly. Both of them are cheap low tolerance.


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## MikeWi (Jan 5, 2015)

I tighten, or at least try to tighten all three holes on lathe and drill chucks.  I was taught that way originally, but I continue to do so because I always CAN tighten the chuck a little on each hole. Now if I found that the other key holes were already tight all the time, I wouldn't bother, but that isn't the case.


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## Mark_f (Jan 5, 2015)

Many moons ago when buffalo still roamed, I , as an apprentice was taught to tighten all three screws. I still do because I was taught that way and it is a habit. I have found no difference in run out tightening one or all three. Run out amount is determined by wear and fit in the scroll and jaws. If you set your chuck for .0005" run out at chucking on 1" diameter (this is what I did because most work is close to that size) it should repeat relatively close AT THAT SIZE and tightening one or all three screws seems to make no difference. As you chuck different diameters , the run out may ( and most likely will) change some. This is due to wear, fit , and accuracy of all the parts. My beat up Union 3 jaw on my SB 9 never shows more than .002" run out anywhere, but at the 1" or there abouts it is always best because that was where it was set to .0002" at. Tightening all three screws , I believe, will give a little more clamping pressure ( and also makes deeper marks in your part).

So after all this crap.... er ...scientific explanation.....I would say "It does not matter. Tighten one or all three, as long as it makes you feel good."

Note: the preceding was completely my own observations, opinions, and the way I was taught. )


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## Kernbigo (Jan 5, 2015)

The 0 mark is for putting the chuck back on the back plate the same, it is called a witness mark. As far as tightening i have always found one  hole that is best of the 3, called a sweat spot.


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## NightWing (Jan 5, 2015)

I always go around the three pinions, even on drill chucks.  You can always get a bit more grip by tweaking each hole.

Notice that the zero mark on the chuck has a companion zero on the backplate.  My thoughts are that those are witness marks, so if you ever take the backplate off the chuck, you can reassemble it the same way it was built.


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## darkzero (Jan 5, 2015)

I was taught to choose one pinion & stick with it to account for wear & fepeatability over the years (for new chucks). Not sure if actually makes a difference though, all my chucks started out as new, first time owner for me. In shop clas we we taught to use multiple pinions when more grip was needed & mainly because all the chucks were heavily worn. On my drill chucks I just use any hole but then again I don't use keyed chucks as often as I do keyless.


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## Bamban (Jan 5, 2015)

mark_f said:


> .....at the 1" or there abouts it is always best because that was where it was set to .0002" at. .... )



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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## xalky (Jan 5, 2015)

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.


I can dial in any of my lathe 3 jaw chucks to within .0005". Here's how I do it. I'm might catch flack for saying it, but don't knock it till you tried it!

I Chuck up the work piece and tighten all three scroll screws tight as if to hold the part for cutting. Now take a dial indicator and put it to the work piece and spin it by hand noting the run out. Now, take a brass hammer and wack the jaw that is farthest out. That's it. You might have to do it a couple of times to get it right. Works every time. This won't work if your jaw surfaces are out. I do it all the time when flipping a part around to get the concentricity as close as possible.

My speculation as to why this works, the scroll has some play in it, Particularly on older chucks. Giving it a good tap seems to move it just enough to get it right. Ironically, it doesn't move when working the part, and I usually push my speeds and feeds.

Marcel


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## RVJimD (Jan 6, 2015)

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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## hermetic (Jan 6, 2015)

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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## herbet999 (Jan 6, 2015)

I tighten all three when it matters. It always seems I can get a bit more from each. But then again, I have a cheap chuck.


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## Kernbigo (Jan 6, 2015)

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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## xalky (Jan 6, 2015)

RVJimD said:


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


 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


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## darkzero (Jan 6, 2015)

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.



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.


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## higgite (Jan 6, 2015)

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


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## Round in circles (Jan 7, 2015)

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




Nearly there  

 :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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## f350ca (Jan 7, 2015)

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


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## NightWing (Jan 7, 2015)

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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## jocat54 (Jan 7, 2015)

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.


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## atunguyd (Jan 7, 2015)

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.



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


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## Mark_f (Jan 7, 2015)

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.


 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.hew:


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## Bamban (Jan 8, 2015)

mark_f said:


> 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.:
> 
> ...




Mark,

Thank you.

It makes sense the way you laid it out, I think I can follow the procedure.  Got an Asian 3J I can try this on. It has 0.006 run out on the 1 inch test bar I picked up from PTG.


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## Mark_f (Jan 8, 2015)

atunguyd said:


> 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



My reasoning for using all three sockets is because of the "play" , very tiny amount that it is, is present in the scroll grooves, jaw grooves, and socket grooves. Let's face it, these things cant be a perfect fit ,especially being massed produced. When you tighten one screw , it locks the screw, jaw and scroll taking up all the slack and also at this point is taking all the tension to hold the part. When you tighten the other two screws, you take up any slack they had and also lock the part in the chuck with them , distributing the load to all three screws instead of one, thus also gripping the part tighter. This is MY reasoning for tightening all three sockets. It maybe hogwash , but it makes me feel better, so I do it. :nuts:


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## darkzero (Jan 8, 2015)

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.



Not necessarily. From what I've seen chucks with only 1 pinion have less max rated speed than a chuck with 3 pinions (but there are other factors too). Think about the imbalance issue at high RPMs if not dynamically balanced but if your lathe can't hit those speeds it doesn't matter much.

I agree with f350, I also hear that the marked pinion is the master that was used for tightening when the chuck was ground. And whatever pinion you use if you regrind the jaws then becomes your master.


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## Bamban (Jan 10, 2015)

mark_f said:


> 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.:
> 
> ...




Mark,

My sincerest THANK YOU for sharing your procedure. 

 I initially thought I would do the procedure on my ChiCom 3J, but changed my mind and did the procedure on Bison 6 inch 3J. I actually did the procedure a couple of days ago, but have not reported on it. Today after I got frustrated with my new to me old Buck set thru 6 inch 6J chuck, I decided to install the adjusted 3J back in the lathe and do the same test I did on the 6J, that is read the run out at the end of test bar and moved the carriage in close to the chuck and read run out. I only have the 0.001 DI, but as you can see the needle is fairly steady.

Thank you, again.

Sorry, boss, this 3J gives me the best repeatability by using a particular pinion, and I tested all of them and any combination and I keep on going back to that single pinion approach
	

		
			
		

		
	

View attachment 92007


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## Rick Leslie (Jan 10, 2015)

My lathe chuck is a quality make but very worn, so repeatability is hit or miss. I've never looked at the keys for numbers but I will now. My drill chuck is the same; very worn. I do find tightening all three key holes makes a difference in holding force with it. I would never have thought of such but read it (I believe on here) and it works. (For those of us too frugal to buy new and/or quality stuff.)


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## Round in circles (Jan 10, 2015)

There is of course the real big elephant in the workshop that most apprenticed machinists will know and follow if they are worth their salt .
It is:-
Use a three jaw to turn to some where near 
use a four jaw to turn something reasonably accurate 
Turn centre drilled items between centres using a dog & face plate if you want precision  & repeatability .

 This is why the accuracy of a three jaw & its  repeatability is not so much of a problem if you have a face plate and dogs or you can make the driving dogs.


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## gregg-k (Jan 13, 2015)

f350ca said:


> 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



I agree with Greg ... that's exactly what I learned as well ... over 50 years ago, and I've always done it that way.

My 8-1/4" Buck 6-jaw Adjust Tru chuck has one pinion socket to tighten the jaws, so there is no choice but to use that one.

.. Gregg


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## gregg-k (Jan 13, 2015)

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.



Good point, and yes, and my Buck 6-jaw chuck is made exactly that way ... it only has one pinion.

.. Gregg


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## Round in circles (Jan 13, 2015)

gregg-k said:


> Good point, and yes, and my Buck 6-jaw chuck is made exactly that way ... it only has one pinion.
> 
> .. Gregg



Is  the jaw operating device the same as a bog standard chuck back plate ?


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## hermetic (Jan 13, 2015)

Anyone having any doubts that the marked pinion is the one to use for most accuracy and repeatability should read through this catalogue of Pratt-burnerd chucks. It is known as the "nominated pinion. It is not an index mark for backplate alignment as when you buy a brand new chuck, you don't usually get a backplate with it! Doesn't stop anyone fitting a chuck to a backplate using a 0 to also align the backplate to the chuck.

Phil.


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## hermetic (Jan 13, 2015)

Whoops, forgot the link



http://www.600uk.com/data/downloads/products/PBI Workholding Catalogue.pdf


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## SonofHarold - Metal Carver (Oct 18, 2018)

OK. newbie here and I have been getting a lot of answers to my many questions just looking searching and reading here. I looked up this subject just out of couriosity based on what my old man (Harold, who was a manual machinist/ tool and die man in the aero-space industry) told me many years ago before I had any interest in learning machining. We were just doing some random thing and when using one of his electirc drill motors and I hung up the bit and slipped the chuck on it. I caught a little heck for that... " what are you doing, your ruining MY drill chuck!" Being German there was one way to do things -  his way. I was taught then to just snug it up with the first position, then tighten it up successively, around the chuck; 1,2,3 and 4. ending up going one past the first one used to initially snug things up with. He said that's the way he was doing things running his lathe at work and thats the way it should be done. So I've always done that... then more recently doing some real tho minor type stuff in the machine shop where I worked the guy helping and coaching me along told me the same, to use all three and go one more. I was a little surprised as I had heard from others  use all three, and others saying it does not matter.  Reading this there seems to be more controversy about than I would have thought but do the the Snug..1,2,3,4 method and it does seem to work well for me, now it's just a habit!
Just got the first reall cuts made on my new lathe and all is going pretty well.. and a retro thanks to all those have contributed here... there is so much good and useful information I ve not really needed to ask any serious questions as yet.


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## mikey (Oct 18, 2018)

Must have missed this thread. I agree, there seems to be some controversy about how to tighten a 3 jaw chuck. 

I actually queried Rohm once upon a time about this exact thing and was told that their chucks have a stated amount of run out that is only achieved when tightening a single pinion identified by the factory, which is marked. If you use any other pinion or multiple pinions then they do not guarantee the accuracy. 

I have 2 Rohm 3 jaw chucks, one Yuasa and one Samchully. All have a single pinion marked and that is the one I use. It is interesting that when I used any other pinion or when using multiple pinions, TIR increased. So I guess the factory is right. What I also found is that cranking a lot of torque into the pinion increases run out, and using multiple pinions to get the chuck really cranked down increased run out the most ... at least for me. So, now I use the marked pinion and tighten it firmly and the chucks have never slipped. 

I don't own cheap or worn chucks so I am not sure if the above holds in all cases. If I ever do own a chuck that isn't marked then I'll do as @higgite did and run my own tests, then mark the most accurate pinion myself.


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## WarrenP (Oct 19, 2018)

+1 on what Mikey said. My chuck has the mark and I tested it once, seems using that marked one gives the least amount of runout.


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## MrWhoopee (Oct 20, 2018)

xalky said:


> I can dial in any of my lathe 3 jaw chucks to within .0005". Here's how I do it. I'm might catch flack for saying it, but don't knock it till you tried it!
> 
> I Chuck up the work piece and tighten all three scroll screws tight as if to hold the part for cutting. Now take a dial indicator and put it to the work piece and spin it by hand noting the run out. Now, take a brass hammer and wack the jaw that is farthest out. That's it. You might have to do it a couple of times to get it right. Works every time. This won't work if your jaw surfaces are out. I do it all the time when flipping a part around to get the concentricity as close as possible.
> 
> ...



A variation on this technique was routinely used in the shop where I apprenticed and in the one I owned. First snug the part (not tight), indicate and tap the high jaw(s) with a soft hammer. Snug a little more, check and tap again. Repeat until you cannot tighten any further. Even a chuck with a badly worn or damaged scroll can be brought in quite close with this method. We rarely mounted a 4-jaw.


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