Thread: chuck troubles

Originally Posted by pipeclay

if by some chance two of these jaws come into contact with the workpiece before the third jaw the other two jaws will load up before the third jaw grips,this in itself will create excess load on the scrolls of the chuck causing them to apply unequall pressure to the job.

You guys have got the real problem a bit askew

" if by some chance two of these jaws come into contact with the workpiece before the third jaw " . . . . the piece will simply travel perpendicularly in the vice until it meets the other jaw. That's what a self centring means.

However, if the piece meets the third jaw at a non-normal angle then not as much radially pressure can be applied to the contact surface of the piece as the other two jaws so the third jaw does not hold as much as the other two.

It's not that one jaw is overloaded as much as one jaw being underloaded relative to the piece which leaves the piece prone to movement.

Originally Posted by pipeclay

As you are most book machinists it probably matters not.
Have you ever set a job up in a three jaw chuck using a fixed steady and had the job either walk out or walk into the chuck.This is caused in a way by the chuck gripping the material and the steady.
If the material being gripped by the chuck is not held securely by the three jaws along the full length of the material in the chuck the material will have the tendency to walk in or out as it rotates in the fixed steady,it causes a spiralling effect.
Have you ever attemted to hold black bar with scale on it in a three jaw chuck and had the material move on you,most times workshops if they do intend to hold black bar or forgings,castings etc in a three jaw chuck will have an old chuck for this purpose.
The three jaws of a conventional three jaw chuck work together,hence the term self centreing,if by some chance two of these jaws come into contact with the workpiece before the third jaw the other two jaws will load up before the third jaw grips,this in itself will create excess load on the scrolls of the chuck causing them to apply unequall pressure to the job.
As the scrolls are not independant but working from a central point the undue pressure can have a detrimental effect on the loading of the scolls causing the ones that are in contact to distort.
As a geometric question it should be self explanetry.

As you are most book machinists it probably matters not.

What does this mean? Who are book machinists?

Have you ever set a job up in a three jaw chuck using a fixed steady and had the job either walk out or walk into the chuck.This is caused in a way by the chuck gripping the material and the steady.

Actually no but I can certainly understand why it would happen. I did not say at any point that using a 3 jaw chuck in my way would provide you with a perfectly concentric work piece. That is what truing is for.

If the material being gripped by the chuck is not held securely by the three jaws along the full length of the material in the chuck the material will have the tendency to walk in or out as it rotates in the fixed steady,it causes a spiralling effect.

Yes. As above.

Have you ever attemted to hold black bar with scale on it in a three jaw chuck and had the material move on you,most times workshops if they do intend to hold black bar or forgings,castings etc in a three jaw chuck will have an old chuck for this purpose.

Again no. I would not put a scaled piece in the chuck for a critical setup. Clean it off first would be the sensible thing I would have thought. The risk of slippage is to be avoided in this case as the scale can wear the jaws due to its hardness.

The three jaws of a conventional three jaw chuck work together,hence the term self centreing,if by some chance two of these jaws come into contact with the workpiece before the third jaw the other two jaws will load up before the third jaw grips,this in itself will create excess load on the scrolls of the chuck causing them to apply unequall pressure to the job.

I always thought the term self centering referred to the stock, not the actual jaws.

As a geometric question it should be self explanetry.

I am still waiting for an answer that does explain it in geometric terms.

I should point out to all that I am not trying to oppose anybody in this matter but I am trying to get an answer that is based on genuine scientific / geometric principals.

if by some chance two of these jaws come into contact with the workpiece before the third jaw the other two jaws will load up before the third jaw grips,

If by some chance two of these jaws come into contact with the workpiece before the third jaw the other two jaws will support the weight of the workpiece until the third jaw makes contact. The jaws will slip over the workpiece until this happens. If we are talking about a work piece that is so out of round that this cannot occur then that is another situation to the one currently being discussed, as far as I was aware.

When I place stock in the chuck I do up the jaws until they are just touching and then slightly rotate the work back and forth whilst slowly tightening the chuck. This is to align the work to the length of the jaws and to find the best position. You can feel if there is a problem.

I get the impression that some are taking this personally. If you are then you have misconstrued the meaning of posts. I for one am on this forum to further my knowledge and maybe pass some knowledge on, as I have said before.

Dean

Try this thought experiment: Rotate your 3 jaw so one jaw is at 12 oclock. Place a short piece of roundish stock on the lower 2 jaws, so it can sit there on its own. Now start closing the chuck. What happens?

• The stock stays where it is and is gripped only by the lower 2 jaws?
• The stock is lifted by the lower jaws until it meets the upper one?

Maybe this will explain what is going on.

Attachment 216152

On the left the forces applied to the round piece by the jaws are ALL equal and meet at the radial centre of the piece. In the middle and right drawings the forces applied to the round piece by the jaws to the surfaces are NOT ALL equal because the surfaces are not all perpendicular to the jaws.

This in turn means the tool does not experience equal back forces as the piece goes around, which is why piece slippage can occur under these conditions.

The case on the right is the worst since the centre of the 3 jaw back pressures does not even meet at a single point and the chuck in effect creates a back torque on the piece which is probably why some pieces can walk their way out of the chuck.

BTW I have only minimal machining experience so you may not wish to pay any attention to what I am saying

As was said earlier and I agree with if the stock is not bright or peeled you shouldnt use the 3 jaw.
By all means if a book says you can go for it.
If it works for you go for.
Dont ever complain if a 3 jaw runs out more than stated after using it this way.
A book machinist is a machinist that relies on books for most of there knowledge,there appears to be quite a few on this forum with out much practical experience.

Originally Posted by pipeclay

A book machinist is a machinist that relies on books for most of there knowledge,there appears to be quite a few on this forum with out much practical experience.

That's why we come here. Some of us have jobs and kids that tend to get in the way of quality workshop time.

I have got some experience in this problem through work. At school, we trialed he use of hot rolled stocks for some time to save some coin, but it would regularly work itself loose in chucks. It most often occured when the stock was inserted all the way in the chuck, centre drilled, then extended and supported with a live centre. My thoughts on the matter were similar to the person who mentioned the fixed steady, because the centre isnt necesarily on axis, the work was forced to occilate in the chuck, causing it to move in the jaws until it had abraded enough metal away to become loose.

The problem was nearly completely eradicated when we went back to bright stocks. Replacing the chucks made it better again, the previous work done on the backing plates was pretty average and they had to be remade and trued on all of the mating surfaces.

It didnt damage the chucks though. The runout of a given chuck (of the old chucks) stayed pretty well the same for the years that I was at that school. The new chucks occasionally saw some hot rolled metal for various odds and ends and they never showed any signs of changing either. I think the damaging the scrolls or the guideways for the jaws argument is a bit of a stretch.

Bob, thank you for trying to shed light instead of heat. I don't really understand why the force lines change given that the jaws are constrained, but I'm in over my head on that one and I know you are a man of science so I'm inclined to believe you. But I thought the question was about the amount of force on each jaw? Does that also change?

Originally Posted by Bryan

Bob, thank you for trying to shed light instead of heat. I don't really understand why the force lines change given that the jaws are constrained, but I'm in over my head on that one and I know you are a man of science so I'm inclined to believe you. But I thought the question was about the amount of force on each jaw? Does that also change?

I agree the jaws are constrained but it's whats happening at the contact surface that is important since this determines the final radial force applied.

If a force X is a applied at 90º to the surface of an object then the object experiences a force X. (diagram on left)



If the force is not applied at 90º then the force experienced by the object is <X or on my drawing Xv. The non-perpendicular geometry also generates a sideways or in my drawing horizontal force (Xh). Because the chuck jaws are constrained it effectively means the chuck ends up applying, and the piece ends experiencing the sideways force. The vector sum of the sideways and vertical forces must add up to the original force X so Xv will always be less than X.

It's this sideways forces that can cause out of round pieces to turn the pieces to turn under load

I hope that helps.

Bob this certainly explains some things but what I have been discussing is slightly out of round stock. I have pointed this out in my posts.

If you were to rotate the stock in the 3rd photo as I have mentioned while tightening it would settle into a far better position than it is at the moment.

All of these situations are possible but you have to remember that the first cuts are for truing up and need care and checking of the chuck tightness.

A book machinist is a machinist that relies on books for most of there knowledge,there appears to be quite a few on this forum with out much practical experience.

What has that got to do with people using this forum to learn proceedures. I do not think that making disparaging remarks has any benefit. I myself have never implied I have a lot of experience, in fact on numerous occasions I have stated that I don't. I am still waiting for a proper explanation of this issue. I also repeat that I am trying to understand this situation in a way that makes logical sense to me. You also have to remember that I have no idea who you are or what experience you have so whether or not I take note of what you say is based purely on how well you can explain the situation. As far as I know you could be a milkman.

Maybe someone can point me to some technical reference about this.

Dean

Peter,

I imagine there are quite a few of us who are book machinists, I'm certainly one of them.

Very seldom have I been shown how to correctly perform the majority of tasks I perform these days in the shed but I think I get by. I've learnt by trial and error and there have been plenty of the latter.

This forum has been a great source of knowledge gleaned from those with far greater experience than I will ever have. The other source has been my books. Most of us participate here because we wish to learn and share ideas and experiences.

And for a lot of us this is simply a hobby. I can imagine the frustration a proficient machinist must have when he or her view some of our efforts but what can we do but heed advice and try and improve.

Bob.

Any remarks I make whether diparaging or not are based on what I read here.
Work at present is as a train guard,I blow a whistle open and close doors and answer questions.

Originally Posted by pipeclay

Any remarks I make whether diparaging or not are based on what I read here.
Work at present is as a train guard,I blow a whistle open and close doors and answer questions.

Does that make you a book machinist then pipeclay?

Or perhaps a whistleblower?

More a whistle blower,and its my own.