Author Topic: 3 jaw vs. 4 jaw question  (Read 6645 times)

Offline Tennessee Whiskey

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3 jaw vs. 4 jaw question
« on: November 06, 2016, 07:38:25 PM »
I have been watching a few of the YouTube machinists and it seems that the 4 jaw chuck is becoming all the rage.  Now,  I understand the need for one when offsetting or working with square stock,  however,  if your 3 jaw is accurate,  what's the advantage of the 4 jaw on round stock.  I just tested my 3 jaw with a. 750" gage pin and it shows less than a thou TIR.  I chucked up a piece of 1" round bar and took one. 050" cut and then a .010" cut and checked the runout and still less than  .001" . To be honest I doubt that if I had indicated it in the 4 jaw I would get much better results.  This is just one of those "hmmmm" thoughts and questions and just wanted to get you guys take on it.

Cletus

Offline philjoe5

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Re: 3 jaw vs. 4 jaw question
« Reply #1 on: November 06, 2016, 07:44:58 PM »
Cletus,
One advantage of the four jaw is that it will hold a piece of stock more securely.  Often, that isn't a problem but when max holding power is needed the 4 jaw rules

Cheers,
Phil
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Offline Jim Nic

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Re: 3 jaw vs. 4 jaw question
« Reply #2 on: November 06, 2016, 07:51:05 PM »
I think that the important part of the query is " if your 3 jaw is accurate,".  My own Chinese 3 jaw chuck, supplied with my lathe which was bought on price, is not particularly accurate so if I intend to remove a part from it and later need to work that part again then the 4 jaw is the way to go.
Having said that, I have also noticed the fondness for the 4 jaw on videos shot using industrial standard machinery and I guess it's a bit like bell bottom trousers - fashionable for a while until sense returns.
Jim
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Offline Bluechip

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Re: 3 jaw vs. 4 jaw question
« Reply #3 on: November 06, 2016, 07:53:55 PM »
Only if the stock is perfectly round. Otherwise a 3-jaw is better on the grounds that 3 points always define a circle but 4 points may not.

Or so I was told all those years ago ...  :old:

Geometrically it holds true but whether it does in practice I don't really know.

My TOS SC 4-jaw is the chuck usually on my Myford BTW ...  :headscratch:

Dave

PS Are we referring to SC chucks or a 4-jaw Independant ???

« Last Edit: November 06, 2016, 08:00:38 PM by Bluechip »

Offline 10KPete

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Re: 3 jaw vs. 4 jaw question
« Reply #4 on: November 06, 2016, 08:04:22 PM »
I have been watching a few of the YouTube machinists and it seems that the 4 jaw chuck is becoming all the rage.  Now,  I understand the need for one when offsetting or working with square stock,  however,  if your 3 jaw is accurate,  what's the advantage of the 4 jaw on round stock.  I just tested my 3 jaw with a. 750" gage pin and it shows less than a thou TIR.  I chucked up a piece of 1" round bar and took one. 050" cut and then a .010" cut and checked the runout and still less than  .001" . To be honest I doubt that if I had indicated it in the 4 jaw I would get much better results.  This is just one of those "hmmmm" thoughts and questions and just wanted to get you guys take on it.

Cletus

You are very fortunate to have a three jaw that will center that well over a range of diameters. If you really want to check it do about 6 different diameters over the range of the chuck. If they all come in that close, then set that chuck aside in a special, protected place, and use it only for those jobs where you need that accuracy. Then go buy another chuck for normal work...

That's a rare one! Unless you are willing to incur the national debt....

Pete
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Offline Gas_mantle

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Re: 3 jaw vs. 4 jaw question
« Reply #5 on: November 06, 2016, 08:15:43 PM »
I find that forgetting about round stock for which either chuck will do, the question comes am I more likely to want to turn hexagonal bars and need the 3 jaw or more likely to want to turn irregular / square bars and need the 4 jaw ?

I find I need to turn square and irregular stock far more than hexagonal so the 4 jaw stays on my machine unless I need the 3 jaw for a specific job.

Offline Maryak

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Re: 3 jaw vs. 4 jaw question
« Reply #6 on: November 06, 2016, 08:33:23 PM »
I have been watching a few of the YouTube machinists and it seems that the 4 jaw chuck is becoming all the rage.  Now,  I understand the need for one when offsetting or working with square stock,  however,  if your 3 jaw is accurate,  what's the advantage of the 4 jaw on round stock.  I just tested my 3 jaw with a. 750" gage pin and it shows less than a thou TIR.  I chucked up a piece of 1" round bar and took one. 050" cut and then a .010" cut and checked the runout and still less than  .001" . To be honest I doubt that if I had indicated it in the 4 jaw I would get much better results.  This is just one of those "hmmmm" thoughts and questions and just wanted to get you guys take on it.

Cletus

You are very fortunate to have a three jaw that will center that well over a range of diameters. If you really want to check it do about 6 different diameters over the range of the chuck. If they all come in that close, then set that chuck aside in a special, protected place, and use it only for those jobs where you need that accuracy. Then go buy another chuck for normal work...

Pete

Lucky you!

To keep it that way only machine brightstock, never black iron. When I was an apprentice, we were punished if caught with black bar in our 3 jaw chucks, at best extra factory, at worse 14 days stoppage of leave and pay. Helps you remember when you have no money for fags for 2 weeks!

A simple test for accuracy is take a short piece of brightstock, machine about half of it down by about 0.050".....Reverse it in the chuck and machine the other half to the same diameter. Usually there is a step where the 2 diameters cross. If you can feel the ridge at the crossover you chuck is a least 0.002" out.

If I understood your test correctly it is no test for accuracy of the chuck, but a test of the lathes overall alignment.

HTH

Regards
Bob
Если вы у Тетушки были яйца, она была бы Дядюшкой

Offline BillTodd

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Re: 3 jaw vs. 4 jaw question
« Reply #7 on: November 06, 2016, 10:17:57 PM »
 
Quote
I chucked up a piece of 1" round bar and took one. 050" cut and then a .010" cut and checked the runout and still less than  .001"

You can have chuck up a piece of square and it won't run out after a cut !   

A quality new 3jc is good for a few thou" TIR over a range of diameters. They are rarely better due to their construction (bits have to be able to move).

A four jaw has to be adjusted each time, so can be as accurate as you have time to be.  If you are re-chucking a part (matching concentricity) you can only be accurate in a 4jaw (or an adjustable 3jc - grip-tru or similar)

One type is not better than the other, different chucks have different uses ,  3JC are quick and hold consistently, 4 JCs are adjustable, 6 JCs are expensive and only really of use on thin wall tubular pieces (where a pot chuck cannot be used) .

Bill - Whose current chuck count is 3 x 4JCs , 3 x 3JCs for the hardinge (the hardinge taper chuck mount makes changing the chuck easy, so sometimes its handy to have more than one job chucked) , 4 misc 3JCs for the rotab ,  3&4 JCs for the cadet and a stack of jaws that don't seem to fit anything  :embarassed:

Offline PStechPaul

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Re: 3 jaw vs. 4 jaw question
« Reply #8 on: November 06, 2016, 10:44:46 PM »
It is important to note that essentially all 3-jaw chucks are self-centering (to the extent of the accuracy of the scroll and other components), while most 4-jaw chucks have independently adjustable jaws. A long time ago I bought a new 8" 4-jaw scroll chuck, but when I more recently decided to try it on my 9x20 lathe, I found that (1) it was too large for my lathe, (2) I did not have the proper adapter for my threaded spindle, and (3) it was useless for anything but perfectly round or perfectly square stock. I traded it for a well-used 6" Cushman 4-jaw independent chuck that needed an adapter and also new adjusting screws (which I made), and it is now my most used chuck. Here is a video of my adventures:

<a href="https://www.youtube.com/watch?v=fO_WpIo5gzs" target="_blank">http://www.youtube.com/watch?v=fO_WpIo5gzs</a>

A three-jaw scroll chuck will usually provide equal gripping force on all three jaws, but it is a good idea to snug it up and then lightly tap on the work while tightening it fully. This will also provide the best centering the chuck is capable of. A four-jaw scroll chuck will usually apply most of the gripping force on two opposite jaws, while the other two will exert less. There is some spring to the components, and some compression of the work, so it will usually hold reasonably well if it is accurately machined.

Six-jaw chucks are best for soft or thin tubular material that flexes enough for all the jaws to provide relatively equal grip. There are also "grip true" or similar chucks that have the ability of fine centering adjustment, but it is probably just as much work as dialing in a four-jaw. For really accurate and repeatable work of relatively small diameter round, collets are a good choice.

Offline steamer

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Re: 3 jaw vs. 4 jaw question
« Reply #9 on: November 06, 2016, 10:59:45 PM »
If I only have 1 chuck....it's a 4 jaw.   Very versatile, and can be made to do a lot of things.

.001"  run out shouldn't be difficult with a 4 jaw....  .0002" shouldn't be either......below that....well....if your good.

Dave
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Offline gbritnell

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Re: 3 jaw vs. 4 jaw question
« Reply #10 on: November 06, 2016, 11:23:27 PM »
I'm with Dave on this one. If I had to pick between a 3 or 4 jaw chuck it would definitely be the 4. It will do everything the 3 will and more. Yes it takes a minute or two to zero everything out but after practice it isn't a problem at all. Heck I have a 5C collet chuck that is only as good as the collets I put into it. Sometimes it's .0005 and sometimes it's .0015. With the 4 jaw I can make it zero all the time.
gbritnell
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Offline Tennessee Whiskey

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Re: 3 jaw vs. 4 jaw question
« Reply #11 on: November 07, 2016, 12:10:54 AM »
What a great bunch of replies and a good testament to the quality of discussions that can be had on this forum.  I guess I should have worded the original ponder differently,  but,  as worded,  it received good information.  Phil mentioned extra holding power and I can see that.  I also fully agree with BillTodd,  even if a piece of round stock is clocked a bit off center it should run true once turned past the point of being eccentric.  Now,  will  the old three jaw do that once you flip ends with the stock as Bob mentioned,  or through a range of sizes as Pete said.  I think everyone agrees, that like most of the rest of our tools,  each has a specific range of duties they are suited for, but, as Dave and George said: if you could only have one, the 4 jaw would be it. As Jim mentioned,  4 jaw use is being shown more often. I watch guys clock in  a 2" rod to turn it to 1.843 for 2.456" and then part it off  :shrug:. That's what I should have ask,  why?

Cletus

Offline steamer

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Re: 3 jaw vs. 4 jaw question
« Reply #12 on: November 07, 2016, 12:18:33 AM »
Clocking it in just to rough it off isn't required , but it can be convenient if you need to flip the part.   Now having said that, if you're parting it off, it really doesn't matter.

Other than,   a 4 jaw is far more ridged than a 3 jaw for parting operations.

Any chuck will work with turn and part off operations....as long as it doesn't move once you start.    If you have a worn out scroll chuck, it will move around under heavy cuts and drive you crazy.

Dave
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Damned ijjit!

Offline kvom

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Re: 3 jaw vs. 4 jaw question
« Reply #13 on: November 07, 2016, 01:00:03 AM »
I have an entire collection of chucks for my lathe, but the ones I use the most are the 6-jaw self-centering and the 5C collect chuck.  Assuming that the runout of the spindle is good,  then a 3 or 6 jaw will produce round parts as long as the stock radius is greater than the target radius by more than the total runout of the chuck.

Even with a somewhat sloppy 3-jaw, if it has two-piece jaws you make soft jaws that will be extremely accurate and repeatable at the size there are bored. Similarly, turning between centers is very accurate as long as the center in the chuck is not removed.

Offline Zephyrin

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Re: 3 jaw vs. 4 jaw question
« Reply #14 on: November 07, 2016, 09:12:13 AM »
A chuck with 4 concentric jaws is a great help if you do engines from bar stock...
and with it, a few calibrated shims give accurate eccentric, much easier than with a 4 independent jaw chuck.

 

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