4-jaw question

[Sshire]

I was thinking about 4-jaw independent*chucks the other day. I get that a better quality 3-jaw self-centering chuck will (or should) have less runout and hence, be more accurate. Since the part in a 4-jaw is being indicated to (ideally) zero runout, what is the advantage to a good (i.e. Pratt-Burnerd, Rohm, etc) 4-jaw as opposed to the "free" one that was included with my Grizzly G0602?

 

[Walltoddj]

If it's like the chuck I got on eBay made in Korea I'm finding I have to break the chuck down and deburr all parts. The jaws dig into the part due to sharp corners and edges. I had to do the same to my 3 jaws it would bind in some places after deburring iit feels a lot smoother and I'm not getting jaws marks on my parts.

Todd

 

[goldstar31]

Stan,
This question has been posed many times- in various ways and so on.
The stock answer( well, mine) is that NO 3 jaw chucks are accurate whereas ALL 4 jaw independent ones can be. You CAN get more accuracy as you go up the price range but you ( apart from things like Tru-grips) will you never be 'spot' on. The only holding self centring affairs are collets.

So really, if you want a lowish level of accuracy in holding round materials, use what you have and then get the best out of being able to hold irregular stuff in the 4 jaw.

Sadly, there is a even sadder world that flogs only a 3 jaw on the end of a lathe spindle for those who are starting out in model engineering. In proper days, the lathe was bought with centres at each end. The lathe came with a catch plate so that the work could go- between centres. If that was impractical, you bought a face plate and held your work onto it. It's a big topic and it lacks being explained to newcomers. It is a wonderful way to machine stuff. Then came some bright spark who modified the faceplate with slots and separate jaws and finally came the 3 jaw as a very much overrated but easy to use workholding.

So my advice- for what it is worth- is stick with the existing 3 jaw and buy an independent 4 jaw- and if you are able- a faceplate as well.

Apologies for the long ramble- but that is the logical order of events.

Have fun- enjoy

Norman

 

[Sshire]

Thanks for the comments.
90% of my lathe work is done with a Bison 5C set Tru collet chuck.
I think what my question should have been is "How does a Pratt-Burned or Rohm-quality 4-jaw chuck differ from a cheap Chinese import?"
Sorry if I wasn't clear.

 

[goldstar31]

Sorry, Stan! Probably the differences are in quality rather than actual holding.
If you are aligning something in a 4 chuck, it is academic if it is held firmly for the actual machining. It may be easier to align in a more expensive chuck but I have a Myford Pratt-Burnerd 4 jaw but it relies on ME to determine the accuracy. If accuracy of a high standard is required, one moves to better than average dial mikes to assist. Mine, when I need it is a Boxford tool room affair with an accuracy of tenths of a thous- but with only 2 thous range all together.

Clearly, this is not the answer expected but it is how precision is achieved. Now this isn't me, this is how people like Martin Cleeve set up his lathe. Last night, I was just reading his old article in Model Engineer on exactly that subject of setting a cheap lathe to tenths.

Cleeve was the guy who couldn't afford a whole lathe and made the remainder of the bits out of lumps of steel- but bought the top flight measuring kit- and then made his own micrometers!
Cheers

Norman

 

[kvom]

A three jaw is accurate after a turning op as long as the work remains in the chuck. For accuracy between chucking using soft jaws gives the best results.

 

[cheepo45]

I think the difference between the cheap and expensive 4 jaw chucks is probably parallelism of the jaws and smoothness of the jaw movement. I have successfully used both the cheap ones and the expensive ones.
I prefer 4 jaws because of the extra holding power and the ease of getting the part to run true, especially thin wall parts that are easily distorted.
Once you get the hang of adjusting them (adjust opposite jaws only) it doesn't take long to get the part running true.
They are also useful on out of round parts-adjust so the indicator reading is the same at each jaw.
cheepo45

 

[purpleknif]

For the utmost accuracy you would use truing rings and bore or turn the jaws for that particular piece or run of pieces but this requires replaceable jaws. Not really practical for most home machinists.

 

[kvom]

For the utmost accuracy you would use truing rings and bore or turn the jaws for that particular piece or run of pieces but this requires replaceable jaws. Not really practical for most home machinists.

The only thing it requires is a chuck with 2-piece jaws. Mill blanks from aluminum to mount to the lower jaws. I use this a lot.

 

[Hopefuldave]

I'll weigh in with Cheepo, the difference in quality shows first in smoothness of operation, then actual fit (particularly on camlock chucks), then accuracy in the sense of squareness / parallelism of the jaws- an out-of-square 4-jaw will let you be as accurate as you can centring up at the.jaws, but by 6" from the chuck could be way off centre. I have a choice of 4-jaws and if I'm after real accuracy (and assuming the part's big enough to hold in it) usually go to the10" Pratt-Burnerd that came with it from the factory in 1955, still accurate, still smooth, and steel so I can run it at high speeds...
I don't remember last time I used a 3-jaw for anything important... And that's a Pratt-Burnerd that has the machine serial number stamped on the back, too!