Tightening a chuck

[tornitore45]

Mr Ron question about tightening the vise brought to mind a related topic.
Is the key sized so that the Hulk can not apply excessive force?
On a three jaws chuck the issue is not so important, there is no reason to over-tight.
Oh a four jaw chuck the final tightening has to meet two conditions: Move the part to dial centered and Assure adequate clamping.

My method reach a point at the stage where the chuck is hand tight, that the test indicator Zero coincide with the part centered.
At this point I have to tighten each jaw without affecting the part position.
So I tighten the first two jaws (perpendicular NOT opposite) to overshoot the dial zero by 1 or 2 thousand (depend what kind of operation I plan to do) then bring the dial back to zero with the opposing jaws. At this point the part runs dead on.

My question is how many thousands do you usually force a jaw to move

[whateg0]

Depends on where in the jaws the part is, to some extent. Also depends on the material being turned. Whether I am using copper pads or not. How heavy of a cut am I going to be taking? Is the stock hollow or solid? Many factors.

If, for example, an acetal part is out at the end of the jaws, I use a lot less force than if I have a solid bar of steel that goes through the spindle and I'm removing a lot of material. I put quite a bit of force on the part, but I don't use a cheater on the chuck and the tommy bar isn't bent yet.

If I'm putting almost as much as I can and it's not going to make it to center, I back off the other side and retighten until it is.

Vague enough?

Dave

[tornitore45]

I was mostly concerned about the top range of effort. Clearly there is all the range from just polishing a thin tube with 600 grit paper to tapping 5/8" but the concern is about at what point one is abusing the chuck.

My 6" is very good and dependable, once is finger tight it quickly feel seated. The 4" feels "spongy" even when fairly tight it give you the feeling you could push another couple of lines on the dial.

[ctwo]

I've never considered much the compression of the opposite jaw. Just as with torquing bolts, I feel it at the lever. But we know, real torque is measured as bolt stretch (or compression?) over some distance.

My small chucks seem compressible over a few though at most, but that is very subjective. I still go around and give them a feel good torque...

I've seen Edge Precision use an actual torque wrench...

[Richard_W]

The idea is that the chuck wrench provided is the max tightening need for a particular size chuck. While on small diameter chucks being tight is not as critical. However on larger 12 and 14" and bigger chucks if you are not tight enough, the scroll will loosen when turning on the spindle. Reason being the start surge is enough that the chuck body moves before the scroll starts to move. I have had many parts that require a light chucking come loose when the spindle is brought up to speed quickly. Lathes with a clutch where the motor runs continuously the clutch can be engaged slowly and not be a problem. That being said they are times when a short pipe is necessary as an assist. I have 2 torn rotator cuffs and use a short pipe on a 12" 3 jaw chuck as an assist. 12" chuck being necessary because the through hole in the spindle is 4+ inches.

[Harold_V]

On a three jaws chuck the issue is not so important, there is no reason to over-tight.

There are circumstances where a three jaw is called upon to be tightened beyond what one might normally expect. That very circumstance is often the cause of a sprung chuck. Visualize a part being gripped by only a very short portion of its diameter, such as a thin plate. Because little is gripped, and the pressure is applied to the extreme end of the jaws, it's relatively easy to damage the chuck due to the mechanical advantage. That condition is quite common, in fact.

In such a case, the use of soft jaws is highly recommended, as they not only grip better, but they also provide the benefit of proper registration of the part because the item to be gripped fits in a pocket of the required depth.

H

[tornitore45]

What I meant is that on a 3 jaws, you are not trying to center the part acting one one jaw. You can bear down as much as you need for the specific operation but the part will not be any better centered that the intrinsic accuracy of the chuck. On a 3 jaws case my concerns regarding the max movement (force) applied to one jaw does not apply.

[Mr Ron]

I think it was Harold who said all three jaws of a chuck should be snugged up, not just one jaw; has to do with backlash on the screws. I notice after tightening one jaw, the next jaw always has a bit more tightening to do and on to the next jaw. I have always followed this procedure. With a 4 jaw, I hand tighten each jaw until the part is as close to center as I can get. Then I start to increase pressure until the part is centered and feels secure without overtightening. I also always allow enough stock length so it is gripped fully by the jaws, not just at the tip.

This discussion and others tells me there are many who do not fully understand the most basic metal working. The questions asked appear to tell me this. I ask some basic questions and I'm glad that others are now also revealing their own concerns. Much of the questions are not found in books. It's only by addressing experienced machinists can we learn the "right" way. We are fortunate to have people on this forum who can provide good guidance.

I have found that no matter how much I think I know, I can always learn something new.

[Harold_V]

I think it was Harold who said all three jaws of a chuck should be snugged up, not just one jaw;

I did say that, but the comment was directed more towards Jacobs type chucks (drill chucks), not three jaw chucks for lathes.
That said, it can still be a valid concept, as the scroll shifts with each socket use, so it does alter how the chuck grips, and does grip tighter than simply using one socket.

There's a problem with that process, though, if you are using a chuck that is adjustable, or if you're using soft jaws. Each socket will grip the part in a slightly different attitude, due to the scroll shifting. If you are shooting for repetitive gripping, as you would if you are using soft jaws, or you have dialed in an adjustable three jaw, in order to maintain the same degree of precision, the same socket should be used at all times. Even then, if the material being gripped varies in size, or is not consistently round, the chuck won't repeat well.

H

[SteveHGraham]

My three-jaw chucks don't have three sockets. I've been wondering if that's one reason why they aren't expensive.

[whateg0]

... I also always allow enough stock length so it is gripped fully by the jaws, not just at the tip. ...

While that's nice, it's not always possible to have a part so securely gripped in the chuck regardless of chuck type. Sometimes the design of the part dictates that only a small section can be gripped. Sometimes, it's the amount of stock available for the making of said part. I'm sure there are other situations that I haven't run across.

Dave

[ctwo]

My three-jaw chucks don't have three sockets. I've been wondering if that's one reason why they aren't expensive.

Mine was inexpensive and I've been very pleased with it. It has three tightening sockets...

I get full jaw grip less often that usual. A lot of times I will have some 1/2" or thin copper shims in there.