4-jaw chuck question

[kvom]

The lathe I just bought came with an 8" 4-jaw chuck (tag says Cushman). I cleaned most of the surface rust off of it, and retracted the jaws halfway.

Two of the jaws have a bit of "give" when not clamped. That it, the jaw and its screw can move back and forward slightly in their channels. However, when I clamp a piece of metal they are not loose.

I am wondering if this is how they are designed.

 

[carbide_burner]

Highly unlikely that a chuck will be designed with two loose laws! Take all the jaws out and compare the screws in the chuck. Something loose/missing, because with only two jaws having play, it also can't be wear.

 

[DickDastardly40]

When you say forward and back,do you mean radially in the direction of the slot and screw or axially headstock to tailstock.

Compare all the jaws to see if they seem the same, 2 may have been replaced or swapped in the past for some reason

You could try moving the jaws around from slot to slot to see if the slop is caused by the jaw being damaged or the slot slightly oversize in one place. The jaws in a 4 jaw do not need to be placed in a specific position, a la a 3 Jaw, so you could tinker around to get best fit.

Have a look at the teeth on the back of the offending jaws and the screws for damage. The screws can probably be removed and possibly replaced.

That said, if there is no movement when you have a job in the chuck and all jaws nipped tight, then probably little to worry about.

Hope this is of help

Al

 

[Bogstandard]

Kvom,

If it is a second hand chuck, the previous owner just might have tightened it up a little too much, trying to get a part into position, and strained the jaws/thread union. That is a fairly common occurence.
This does not happen normally with a scroll chuck, as all jaws come into contact at the same time, and usually the load is spread evenly between the jaws and scroll.

If they all tighten down OK, as Al said, nothing really to worry about.

John

 

[kvom]

Thanks for the replies.

Jaws are loose axially (towards center). The movement involves the screw as well as the jaw, so it's not a case of a damaged screw. The two that are loose are at 90 degrees to one another, so I had thought perhaps it was a design to allow them to adjust easier.

I haven't been able to remove the jaws since the chuck wrench was missing, and one I borrowed from a friend allows me to withdraw them only halfway (it's a larger wrench whose tip was ground down, so the barrel diameter is too big once the jaws are out of the body of the chuck).

 

[DickDastardly40]

If a square drive chuck key is what you're missing, a square drive socket extension in either 3/8 or 1/2" AF will often suffice, if hex then an allen key might be the ticket.

Sounds like, as Bogs suggested, a previous user wound up a bit extra on the two looser jaws to get the job into a closer TIR.

Al

 

[kf2qd]

This is normal behaviour for 4 jaw chucks. Since you will be tightening with opposite jaws you won't see the slop once you tighten up on something. Chucks where the jaws have zero slop are difficult to use and a real bother under actual shop conditions. Your 4 jaw chuck is just fine and will probably give you years of great service. The bigger the 4 jaw the more the jaws will move in and out when not under load. I repeat - you will get years of reliable service out of that chuck. (the tight jaws probably need removing and cleaning...) i have even used 4 jaw chucks that would rattle as you slowly rotated the chuck and they are still in use.

 

[rake60]

I bought a used 4 jaw chuck once.

It didn't have a chuck key either.
I made one from a short length of CR steel.
Milled the 4 flats on the end with the mini mill and drilled a
hole in the opposite end for another, smaller piece of CR steel to make
the T handle.

4140 would have been a better choice of stock, but if your putting enough
pressure on it to twist the square on cold roll, your probably overtightening
the chuck anyway.

100PSI is enough pressure to hold a piece in a chuck for most turning operations.
For grooving you might want a little more.
With just a 4" T handle it's very easy to up to 300PSI on the chuck screws.
The redneck operators will tell you, "That ain't gonna go nowhere" LOL

Over time the screws and jaw threads will wear if that's your normal practice.

Rick

 

[Kludge]

However, when I clamp a piece of metal they are not loose.

As has already been said, this is all that counts. Sitting idle, it can be as wiggly as it wants to be (within some sort of limits, I suppose); it's only when it is actually working that counts.

Look at the SR-71 as sort of an extreme example. They leaked fuel like a sieve on the ground but were tight doing what they were designed to do.

When you finally get a key, take it apart, clean it all up, oil it, put it back together and enjoy the daylights out of it.

BEst regards,

Kludge

 

[kvom]

The chuck takes a 5/16 square, so neither the 1/4 or the 3/8 ratchet extension works. I borrowed one from school that was originally larger and had been ground down to size. However the size of the main barrel hits the jaws once they exit the body of the chuck. I considered making on once I get my mill running, but since they cost less than $10 at ENCO I'll likely just buy one.

Covered it with WD40 and rubbed it up with Scotchbrite; got most of the bad surface rust off. Looks pretty good.

 

[Mcgyver]

if it moves, any mating part on any machine, has to have some give or clearance. In what direction and my how much does it move is the pertinent question. Cushman is a quality chuck, but any chuck be be damaged or worn out.

I'm not so quick to agree that if they are stationary when clamped all is ok - that will still be the case with worn or damaged chuck. its whether they are holding the work in line work in line with the the lathe's axis and evenly in contact with the work over its length (assuming the work is reasonably straight to begin with) that counts.....it's probably fine, but if there is a enough slop there for you to bother posting about it, check it out; chuck something up that is straight and put an indicator on it to make sure its not at a angle to the lathe axis and with your smallest feeler gauges make sure that each jaw is fully in contact .

 

[Kludge]

its whether they are holding the work in line work in line with the the lathe's axis and evenly in contact with the work over its length (assuming the work is reasonably straight to begin with) that counts.

Okay, I think this is where my gross inexperience with big boy toys is showing. In the itty bitties it takes a lot to cause a chuck to go bad, mostly because the loads and all are a lot lighter. In my experience (and that's all I can offer), as long as they're maintained even halfway decently, they will work properly until they just give up, and that's more often due to something external like driving a cross slide into it. (Chucks don't like that, especially when it's hard enough to stall the spindle. Ask me how I know. :-[)

(Oh, yeah. The spindle wasn't too crazy about it either. )

Anyway, I think I'll just sit back and learn when the discussion's about big machines and get all happified when it gets back to me size "toys".

Best regards,

Kludge