Thread: chuck troubles

I was doing a quick job on the lathe and i turned the tool post around to use the parting tool then boom i hit the chuck with the turning tool and it shattered luckily nothing was damaged although it through the chuck out of alignment. so i took the opportunity to take the chuck apart to clean it up.
anyway i took the backplate off and found that the mating register is 5 mm to small and there is alot of play and the bolt hole are 16mm and the bolt are only 8 mm so how should i go about this i was thinking a new backing plate but i am not sure.

That's a good thing. Do your bolts up a bit more than finger tight. Chuck some ground bar. Mount an indicator. Bump the chuck till the bar clocks true. Tighten bolts. You can set and forget for general 3 jaw work, but re-indicate any time concentricity is important. The way I do mine is with a DTI reading the top of the bar. I just find the highest point and bump down with a rubber dead-blow hammer. That way the indicator isn't getting shocked as much.

Andre Sounds like you definately need to make a new backing plate .. I am making one at the moment , but I messed up the internal threading , twice . Learn by your mistakes ... the register needs to be precise . MIKE

thanks for the replys the back plate has 4 screws bolted on so you can move it around on the register. i have got the accuracy to 0.04 then i tightened the bolts on the backplate up and it went to 0.12

Hi Andre,

Sounds like you were lucky that there was no other damage, I bet the adrenalin was pumping when the tool shattered..

Regards
Ray

We all make mistakes at some time or another.
Can you rotate the chuck on the back plate and drill 4 more smaller holes, say 8.5mm. The register being smaller is good as Bryan said because you can set the chuck accurately using a dial indicator. 5mm under is a bit much but it should not matter if it's 5 thou or 5mm. I would say it helped to save you this time.

I see you have tried, just tighten the bolts a little at a time and continually checking it until they are done up.

Dave

If there is enough meat left on the backing plate why not do it properly and eliminate the problem reoccering in the future.
If the need arises where you need an adjustable 3 jaw chuck it might be time to buy one,also a good time to become proficent in setting a 4 jaw.

Well I think I need to face the backing plate I finally clocked it up and the face is running out 0.2 mm. I think this combined with the loose register was causing my grief as even before the crash the chuck ran out.

Andre
I would be inclined to make a new backing plate.
Maybe it might sound a bit like a lot of work, but when you go to machine work in the chuck you dont want the thought in the back of your mind, that if I take a heavy or interupted cut then things may move.
The backing plate, I believe, should be a very neat, practically near interference fit into the register of the rear of the chuck.
These are my thoughts on the subject.
Also the other variable is the chuck itself.
Three jaw chucks need to be accurate. To keep them that way they need a bit of gentle handling. I would not grip black hot rolled or other slightly out of round stock in a 3 jaw chuck if you want it to remain accurate.
Its very easy to strain the scroll in the chuck & or the jaws, & for ever more the accuracy is hindered.
For out of round or black hot rolled work then the 4 jaw independent chuck is the way to go.
I know you like the use of the 4 jaw chuck from what I have read with your posts.
regards
Bruce

What is the problem with out of round stock in a three jaw chuck? It will grip evenly as three points of contact is self aligning. I thought that was the point of a 3 jaw. Only problem is if the variation is along the length of grip of the jaws. 3 jaw chucks are not considered accurate. If you have one that is accurate then some care is needed but how would out of round stock be a problem?

Dean

I was taught the same thing. Never use out of round stock in a three jaw chuck because it damages the scroll.

Originally Posted by Oldneweng

What is the problem with out of round stock in a three jaw chuck? It will grip evenly as three points of contact is self aligning. I thought that was the point of a 3 jaw. Only problem is if the variation is along the length of grip of the jaws. 3 jaw chucks are not considered accurate. If you have one that is accurate then some care is needed but how would out of round stock be a problem?

Dean

It's kinda the opposite of that. The jaws are self aligning so one or more of the jaws will be under strain when tightening the workpiece.

Hi welder,
As has been said you could relocate the bolt holes further around on the backing plate.
The spigot on the backing plate can be turned down a little bit further and a ring machined up to be an interference fit on the spigot and semi-finished on the outside diameter.

You can now fit the ring then machine the OD to suit your female register and machine the mount face at the same time. For a hobby lathe you could make the registration with say 0.15 to 0.25mm or so clearance to facilitate setting up the chuck as has already been said. Though I don't personally like that idea much it has merit in a non industrial situation, though I have seen it used in an industrial setting. If you go with a clearance situation then regularly check that the mounting bolts are tight, especially if doing intermittent cuts.

Good luck with you repairs

Cheers.

Thanks for the replies I think I will make or buy a new backing plate for the chuck in question. Then turn the old back plate to suit a 4 inch 3 jaw I have.

PS anyone know where to buy a 2"x8 tpi backing plate.

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.

It's kinda the opposite of that. The jaws are self aligning so one or more of the jaws will be under strain when tightening the workpiece.

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