chuck problems

[BrokenBiker]

so, after actually making parts with the lthe now im noticing some issues with the 3 jaw chucks.

the burnerd has an issue with the jaws, where when tightened down there apears to be a very minor taper, where the back edge clamps first and the front edge has a very small, but noticable gap, meaning the work piece can be moved...not ideal. i stripped it and there is nothing stuck in the scoll or the jaws, no play in them etc so im wondering if its just damage. could i just open them into a bit of pipe to get some force on them then use the die grinder in the tool post (and covered ways) to grind them flat and true?

the pratt 3 jaw has a different issue though. it holds work just fine, has very little run out and is reasonably repeatable, but dear got the scroll is stiff to turn. now, i stripped this chuck last night and as soon as you take the pressure off the 3 allen bolts that hold the back plate/scroll housing, everything gets nice and free to turn. stripped everything, checked for damage and wear and all i could find is a very slight burr on the scroll gear opposite jaw 3 so took that off with a hone, reassembled and rebuilt...with the 3 allen bolts 'snug' it runs fairly freely, but anouther half turn to actually tighten them and it basically seizes up...im wondering if i can just mount the backing plate, skim the spigot that presses into the chuck and therefore take a bit of pressure off the internals but still let the bolts fully tighten? that said, i used this chuck yesterday to alter the wood lathe tool rest posts and while i had to use a bit of pipe on the chuck handle, it worked wonderfully as far as concentricity and work holding goes so being able to free it up would make this a very useable chuck.

its a shame as the burnerd has 3 sets of jaws, and the pratt has 2 so they will be usefull chucks if i can get them running right.

the 4 jaw that came with it works just fine, i cleaned the scrolls out as they were full of aluminum swarf and its sopt on, but sadly only has the 1 set of jaws which means clamping externally on a large diameter isnt going to happen with it so could really do with either of the 3 jaws working properly.

 

[Pete.]

The burnerd chuck is bell-mouthed and whilst is might just conceivably be the jaws it's much more likely to be the chuck body that's deformed. People jump to blaming the jaws (mostly because grinding the jaws "fixes" the problem but let's face it you have either a small solid piece of hardened steel and a large flat thin (around the jaw slots) piece of bendy cast iron. If the chuck is abused then it's the thin bendy iron that's going to deform not a block of hardened steel.
I had a Pratt chuck that was doing the same thing you could waggle the workpiece about and hear it knocking even when clamped up. I took the jaws out and put them in another Pratt chuck and the held perfectly. Still in that chuck to this day.
My reasoning is further borne out by the common assertion that if you grind the jaws to fix the error it will only hold repeatably at the diameter it's been ground. This is because the face of the chuck/jaw guide slots is now bent, as in bent curved, so sure if you move the jaws in or out from where they were ground you would expect them to now act bell-mouthed because they are on a different spot on the curve.

 

[Brad93]

We've got a monkey at work that puts a pipe on the chuck handle everytime even with soft jaws bored for a specific job.

Thats how chucks get abused

 

[E T]

the 4 jaw that came with it works just fine, i cleaned the scrolls out as they were full of aluminum swarf and its sopt on, but sadly only has the 1 set of jaws which means clamping externally on a large diameter isnt going to happen with it so could really do with either of the 3 jaws working properly.

An independent 4 jaw usually comes with 1 set of jaws. You can flip the jaws around for use as inside jaws or outside jaws.

 

[BrokenBiker]

The burnerd chuck is bell-mouthed and whilst is might just conceivably be the jaws it's much more likely to be the chuck body that's deformed. People jump to blaming the jaws (mostly because grinding the jaws "fixes" the problem but let's face it you have either a small solid piece of hardened steel and a large flat thin (around the jaw slots) piece of bendy cast iron. If the chuck is abused then it's the thin bendy iron that's going to deform not a block of hardened steel.
I had a Pratt chuck that was doing the same thing you could waggle the workpiece about and hear it knocking even when clamped up. I took the jaws out and put them in another Pratt chuck and the held perfectly. Still in that chuck to this day.
My reasoning is further borne out by the common assertion that if you grind the jaws to fix the error it will only hold repeatably at the diameter it's been ground. This is because the face of the chuck/jaw guide slots is now bent, as in bent curved, so sure if you move the jaws in or out from where they were ground you would expect them to now act bell-mouthed because they are on a different spot on the curve.

As it happens I can hear that chuck knocking as it rotates...

So the chuck it goosed then? Not a huge issue, I have figured out the problem with the really stiff one. It just needs a shim between the back plate and the chuck as the housing is pressing on the scroll wheels/gears so I'll make one from aluminium to test the theory then make a decent one later so will have a repeatable 3 jaw of good quality either way.

I may save for a week or 2 and buy some new chucks. I fancy some larger ones anyway and would be fun to make backing plates to fit them to the spindle. 1-3/4" 8tpi thread is well within the range of the lathe

 

[Pete.]

I have an excess of chucks her eI'll see if I got anything that suits.

 

[BrokenBiker]

An independent 4 jaw usually comes with 1 set of jaws. You can flip the jaws around for use as inside jaws or outside jaws.

Didn't realise that, just checked and you are correct. I assumed they would be the same as 3 jaw chucks and need different jaws. The 4 jaw on the mini lathe was naff and the jaws never came out so never had the chance to try on that

 

[BrokenBiker]

I have an excess of chucks her eI'll see if I got anything that suits.

That's a very kind of you. I think it's one of those spi does that was very specific as I can't find much in the way of second hand chucks for that lathe...

You have just reminded me though. I have a 6" 3 jaw with no branding on it that would only need a backing plate, and I have 4 spare face plates that could be used

 

[Agroshield]

It just needs a shim between the back plate and the chuck as the housing is pressing on the scroll wheels/gears so I'll make one from aluminium...

The shim is good as in 20 years time when the chuck has worn, you can remove it. Even if you sweep the floor you would not find the material you grind off, let alone reassemble it in the same order.

Tinfoil is thin. Turkey foil is thicker.

Three feeler gauges adjacent the bolts would allow you to work out the thickness you need with the bolts tight.

 

[not done it yet]

Repeatability - as in remounting a workpiece in the chuck - is not expected and rarely achieved with a 3 jaw chuck. They are not designed for that - every concentric cut should be carried out before removal of the workpiece from the chuck. Concentricity can only be guaranteed by machining that way in a 3 jaw chuck.

4 jaw chucks are used when remounting parts, because the workpiece can be centralised to a high degree of accuracy, before further machining.

 

[Seadog]

What a pity, there's a fantastic 3-jaw, at a bargain price on FB.
Unfortunately, it's 6tpi...

 

[Pete.]

What a pity, there's a fantastic 3-jaw, at a bargain price on FB.
Unfortunately, it's 6tpi...
View attachment 496736

That's the guy I bought my TOS mill off. He's a nice guy he's probably selling it on behalf of the preservation society same as my mill.

 

[dave h 2]

What a pity, there's a fantastic 3-jaw, at a bargain price on FB.
Unfortunately, it's 6tpi...
View attachment 496736

And that photo leads to the one thing that's bothering me about the proposed solution above.
The chuck body is in two parts which are not normally visible with the back plate on, they should be done up tight. If the centre of the backing plate is pushed too far into the chuck it's bearing on the inner half, which is already tightened up anyway.
However if the mounting bolts are a touch too long they might interfere with the back of the scroll so just try some washers.

 

[BrokenBiker]

Repeatability - as in remounting a workpiece in the chuck - is not expected and rarely achieved with a 3 jaw chuck. They are not designed for that - every concentric cut should be carried out before removal of the workpiece from the chuck. Concentricity can only be guaranteed by machining that way in a 3 jaw chuck.

4 jaw chucks are used when remounting parts, because the workpiece can be centralised to a high degree of accuracy, before further machining.

fully aware, however i was surprised at how well this 3 jaw will hold concentricity when taking the part in and out...its all well and good saying a 4 jaw should be used to mount parts but its always nice to have a good repeatable 3 jaw for jobs that arent that critical...like most of the parts i will be making