Thread: chuck troubles

Hi Andre,
Is there enough material to machine the old register off and put a new one on. You only need enough register to engage the recess past the lead in chamfer. With regard to not round work in a 3 jaw chuck, I reckon Bruce is right. Imagine a slightly oval piece of bar stock and 3 jaws moving towards the stock at the same rate. This rate won't change when they hit the stock and will have uneven pressures on contact.

Phil

Originally Posted by Oldneweng

All three jaws will be under equal strain when tightening out of round or not unless you are talking about a convoluted shape. This is basic Geometry. If you take one jaw out altogether nothing will happen on an approximately round shape. Without the third jaw nothing happens, the stock falls out.

If the shape is not round at all that is different.



I would like to see a situation when only one jaw is under strain.

Maybe you are talking about micro amounts of force. This is a lathe and that is what 3 jaw chucks are designed for.

I do not understand how you come up with your reasoning and would appreciate it if you could enlighten me. What you are saying goes against all that I have learnt about it, which is not to say I am an expert in any way.

Dean

Mate that's what I was taught, take it or leave it, doesn't bother me either way.

Originally Posted by Steamwhisperer

Imagine a slightly oval piece of bar stock and 3 jaws moving towards the stock at the same rate. This rate won't change when they hit the stock and will have uneven pressures on contact.

But Phil, what stops the work from moving until the pressures equalise?

Originally Posted by Bryan

But Phil, what stops the work from moving until the pressures equalise?

Hi Bryan,
you should know better than ask me to explain something but here goes.
The work won't stop moving but either will the jaws as they keep moving at the same rate together. Perfectly round, perfect contact. Not perfectly round not perfect contact. Everyone knows that 3 legs on a tripod will make perfect contact with the ground even when the ground is uneven. That is until the point of origin, lets say the camera,(I am using a camera tripod as the example) is held in a fixed position and the legs extend out on a fixed plane. If the ground is a perfectly flat the legs will hit the ground at the same time. If the ground is uneven they wont. They will if you keep forcing the legs into the ground.
Even I know I have confused this even more.
They are called 3 jaw concentric and not independant chucks because they are just that, concentric. The idea is to centralise the work and not offset it.
I hope this helps but I kinda' know it wont

Phil

Originally Posted by slhouetteV8

Mate that's what I was taught, take it or leave it, doesn't bother me either way.

I do not understand how you come up with your reasoning and would appreciate it if you could enlighten me.

That attitude is not what this forum is about. People including me are reading this thread and others to add to their knowledge and maybe pass some knowledge on. My position is that I never ever accept anything unless I can see the reason for it. The world is full of misleading information. If you have a stand be prepared to back it up.

Dean

Originally Posted by Steamwhisperer

Hi Bryan,
you should know better than ask me to explain something but here goes.
The work won't stop moving but either will the jaws as they keep moving at the same rate together. Perfectly round, perfect contact. Not perfectly round not perfect contact. Everyone knows that 3 legs on a tripod will make perfect contact with the ground even when the ground is uneven. That is until the point of origin, lets say the camera,(I am using a camera tripod as the example) is held in a fixed position and the legs extend out on a fixed plane. If the ground is a perfectly flat the legs will hit the ground at the same time. If the ground is uneven they won’t. They will if you keep forcing the legs into the ground.
Even I know I have confused this even more.
They are called 3 jaw concentric and not independent chucks because they are just that, concentric. The idea is to centralise the work and not offset it.
I hope this helps but I kinda' know it wont[IMG]file:///C:%5CUsers%5CDEAN%7E1.GAY%5CAppData%5CLocal%5CTemp%5Cmsohtml1%5C01%5Cclip_image001.gif[/IMG]

Phil

The work piece is not held in a fixed position but is free to move until it is locked by the jaws.

I am sorry Phil but I cannot see any comparison between a tripod held by a person from above being lowered to the ground and a 3 jaw chuck controlled by a machined scroll. When I chuck a piece of round stock in the 3 jaw at the point of contact I rotate the work back and forth a little to ensure it is engaging evenly along the jaws.

By the way when the tripod hits the ground what would normally happen is that one leg would hit first. This leg would stop moving which changes the movement of the tripod. The next nearest leg would hit next, again changing the movement which would now be causing the third leg to continue down until it also hits the ground. This seems to me to be confirming what I am saying. All three points contacting the ground. I see this example as being the fact that when all three legs are on the ground they have even contact. The difference here is that if the legs are not at equal heights then they would not have equal weight on them. This is because the force is caused by gravity which is based on a vertical vector pointing towards the earth’s centre of gravity. Totally different to 3 jaws with their force vectors pointing towards the centre of the spindle line.

I really would like to understand what you are trying to say but it seems you are not sure of the "Physics" of it yourself. Three contact points will adjust themselves continuously until there is too much pressure for them to move.

From an overall point of view. If you have an accurate 3 jaw chuck I can understand being careful not to upset this accuracy, if that is what you want to do. The 3 jaw is the work horse chuck of the lathe. The vast majority of 3 jaws on lathes in the world are used as quick chucks for round or similar materials. This is what they were designed for in the first place. If you use them differently then you are not wrong to do this but I question whether this is relevant in a thread of this nature. I believe that Andre has enough to worry about already.

Dean

There must be something to it at work we never use the the 3 jaw to hold out of round hot rolled steel the same applies at Tafe I asked why and the response was that it damages the scroll.

Originally Posted by Oldneweng

That attitude is not what this forum is about. People including me are reading this thread and others to add to their knowledge and maybe pass some knowledge on. My position is that I never ever accept anything unless I can see the reason for it. The world is full of misleading information. If you have a stand be prepared to back it up.

Dean

What experince in a machining enviroment do you have.

The problem is that a number of people have said that is the case but no one can explain it and it doesn't follow my understanding of the matter. As I have said I will not accept something unless I see the reason for it. I get the same sort of thing at work all the time. This is how it is done! Why? Because we have always done it this way, because someone told me to etc etc. I have proven that some of these methods are wrong.

In 1946 (if my memory serves me) the First Aid organisation for US released its annual First Aid book with the information that eating a meal before going swimming was dangerous due to cramps. 2 years later this information was removed as it was discovered to be totally untrue. That "Old Wives Tale" exists today in the minds of many people of that generation. I was raised with that knowledge. It is false but still exists.

When I worked in engineering back in early 80's I never heard any mention of this chuck problem. My knowledge of physics does not support it. Hence I ask Why?

Dean

As asked what machining background do you have or is it book related.

Your post must have been typed at the same time as mine was. I worked for nearly 6 years in an engineering firm in Adelaide starting in about 1978. I ran centreless grinding section for a lot of that time. I also ran induction hardening for a time. Both of these sections involved some tooling work. I was trusted by the boss to do a lot of this myself. In this I was being taught by the toolmakers. I also had some work in production using manual lathes for piston turning and NC lathes.

Dean

what difference does it make pipeclay? This basically a geometry question isn't it?

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.

Originally Posted by pipeclay

As a geometric question it should be self explanetry.

Are we talking about the solar system now?