Thread: chuck troubles

Well that's dead interesting Bob. So the amount of force changes because the direction changes. Physics huh? Thanks again for taking the time to explain and draw it. But what it means in practical terms is another question I guess. I mean even ground stock has a roundness tolerance. How fussy is too fussy? (That's a rhetorical question.)

I made that comment and have no regretts with it.

What becomes the bigger concern and not necessarlly with this particular post but a lot of posts in general is that people will read Theroy books and Practical books believing that they are the be all and end all,when really one is just theroy and the other practical mainly based on each authors machining experiences.

Trade manuals and journals I believe are different.

Where a lot of problem arises from the practical books is that I believe that a certain amount of prior knowledge is exspected of the reader.

At times you just have to read some of the questions being asked and the refferences being quoted to see that they dont explain everything for the begginer,but as I have been informed before you shouldnt dumb things down for someone with no knowledge.

Originally Posted by Bryan

Well that's dead interesting Bob. So the amount of force changes because the direction changes. Physics huh? Thanks again for taking the time to explain and draw it. But what it means in practical terms is another question I guess. I mean even ground stock has a roundness tolerance. How fussy is too fussy? (That's a rhetorical question.)

The force changes are directly proportional to the cosine of the applied angle of the force.

For example; if the out of round leads to a 1º change in the vertical application of the force the % loss in the perpendicular force lost is 0.02%. Even a 5º deviation only leads to a 0.4% loss in vertical force. I think the real devil is not the loss in the vertical force but the generation of those side ways forces since they are what contribute to the piece walking out of the chuck.

I am really at a loss knowing where the book learning came into this discussion except of course for the following quote from Pipeclay.

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

Pipeclay it appears to me that you have a preconceived idea about my knowledge. The vast majority of my reading about machining was done on this forum where most people are willing to help others learn. Did you totally ignore my answer to your post by the way?

Bob your post #40 is exactly what I was wanting when I asked for an explanation. When you say

I think the real devil is not the loss in the vertical force but the generation of those side ways forces since they are what contribute to the piece walking out of the chuck.

and

Because the chuck jaws are constrained it effectively means the chuck ends up applying, and the piece ends experiencing the sideways force.

I would have thought that the t-slots in the body of the chuck would constrain the sideways force and so only allow vertical forces to be applied. Up to a point of course. We are at last getting somewhere with this.

Your post showing the vectors of vertical and sideways force must have passed mine on the web. I missed it until I was checking back just now

By the way Bob you were concerned about the height of your lathe which led to numerous creative suggestions to remedy. I have just finished moving my new lathe from the trailer onto a block and timber stack in the implement shed which is 700mm high. I have done this so I can just roll it straight back on to the trailer when I have the slab ready for it.

Dean

So these deflected forces. Will they also occur in a 4 jaw independent chuck on less than perfectly round stock?

And are those forces of such a magnitude to conceivably damage a chuck? That may be a 'piece of string' question but it seems worth considering, since that was the claim that started this debate.

Originally Posted by Bryan

So these deflected forces. Will they also occur in a 4 jaw independent chuck on less than perfectly round stock?

I would say so. However having an extra jaw helps apply more force all round and the independence of the 4 jaws means more radial force can probably be applied. I would have to think about it some more and draw a few diagrams.

And are those forces of such a magnitude to conceivably damage a chuck? That may be a 'piece of string' question but it seems worth considering, since that was the claim that started this debate.

On their own, unlikely. However even though we cannot see any movement, any sideways flexing will lead to wear and reduce the performance of a chuck.

Originally Posted by Oldneweng

I would have thought that the t-slots in the body of the chuck would constrain the sideways force and so only allow vertical forces to be applied. Up to a point of course. We are at last getting somewhere with this.

The chuck jaws being constrains always applies a radial or vertical forces but if the surface is not absolutely perpendicular to these forces the surface will experience sideways force. This will in turn apply a back force in a sideways direction to the jaws. Bearing in mind what I said about wear above the sideways forces on the jaws are not an immediate problem, it's the sideways forces on the pieces that may lead to the piece turning or walking out of a chuck.

Up until this thread I had not thought about this issue and it was only after I started drawing some diagrams and thinking things through that convinced myself of this possibility and now that I have convinced myself of it I will be more careful with my 3 jaw chucks.

So Andre. Sorry about the distraction. Maybe should have split that off.

Anyway what's the verdict with the backplate, skim it? I would. As long as it's mating perfectly to the spindle.

I would too as it seems much easier than making a new backplate or cheaper than buying one, that is provided there is enough material available.

Dean

I have an apology to make Bob. I was talking about the wrong Bob.

Too many Bob's.

Regarding your post #49

For example; if the out of round leads to a 1º change in the vertical application of the force the % loss in the perpendicular force lost is 0.02%. Even a 5º deviation only leads to a 0.4% loss in vertical force. I think the real devil is not the loss in the vertical force but the generation of those side ways forces since they are what contribute to the piece walking out of the chuck.

And

Up until this thread I had not thought about this issue and it was only after I started drawing some diagrams and thinking things through that convinced myself of this possibility and now that I have convinced myself of it I will be more careful with my 3 jaw chucks.

My gut feeling is that if the vertical loss is only 0.4% then the sideways force caused by this would be very small. This would be offset in many cases by the force from the other 2 jaws which would be huge in comparison. The total force on each jaw has to be the same overall. By turning the stock while tightening as I have mentioned, it is possible that a high spot under a jaw could be avoided and indeed this is one reason for this action. I would have thought that stock walking out of the chuck would be more likely caused by uneven force along the length of the jaws causing the stock to occilate.

I can certainly understand you being concerned but from my point of view, as my chucks are large and solid and nowhere near new I would have to have more concrete evidence before I seriously changed my methods. I have researched this issue on the net and have not found any reference to it. I am now trying to think of some more terms to search for. If this is such an issue surely somebody could point to some online evidence. The only thing I have found of any interest is a suggestion to rotate the stock while tightening it.

Dean

I had a look there's not enough meat left to drill more holes so I faced it to suit my 4 inch 3 jaw I will have a go at making one soon.

What would this do to the chuck

Attachment 216481

Some serious side forces there I would think.

Dean

Hi Dean,
I reckon the only forces involved there are the ones compelling me to buy a 4 jaw chuck lol.

Yours in jest,
Phil

Originally Posted by Oldneweng

What would this do to the chuck

Attachment 216481

Some serious side forces there I would think.

Dean

No really, what on earth is that for?
I would skip the four jaw and go straight to a faceplate.....

How many lathes are sold without a 4 jaw? I am guessing mostly (all) would be hobby type?

Definitely would be a compelling force in this case LOL.

Dean