Chuck jaws - making them parallel...?

[AdeV]

So, I sling some bar stock in the 4-jaw chuck, line it up by eye, then pop an indicator on it - say 4" out from the chuck. After a bit of fiddling, I can usually get it indicating less than 0.002" runout. If I'm really trying, I can get it under 0.001", but that needs patience...

Anyway, slide the DI down to  just by the jaws, and suddenly it's reading "several" thou of runout (for a long - say 4" - piece, maybe as much as 0.010"). I can dial that out of course, but then I've got runout at the far end.

I can only assume, therefore, that my chuck jaws aren't parallel any more. Is there a simple fix which I can do with the lathe itself, or do I need to investigate replacing the chuck?

[Bogstandard]

Ade,

What you are experiencing is one of the failures of smallish independent jaw chucks (or sometimes big ones, depends how white knuckle you can tighten things).

With a self centring chuck, when they are tightened, all jaws exert the same pressure, so they all wear or deform the same.

With an independent, pressure is applied to each individual jaw, so they don't wear or deform equally. This is usually caused by people applying pressure to get the piece into position rather than slackening the opposite jaw.

If you know someone with a surface grinder, it is only a few minutes job to bring them back to like new by grinding them all square, together as a block.


Bogs

[DMIOM]

...... Is there a simple fix which I can do with the lathe itself, or do I need to investigate replacing the chuck?

Ade,

Have you whipped the jaws out and had a good look in case there's a ding or a burr?

How is your chuck mounted? is there a chance that the mounting of the chuck body isn't square to the spindle, especially if its on a back-plate?

(also assume you've tried this with several bits of stock in case the first one was just the type of over-bent banana/cucumber the European Union was trying to stamp-out)

Dave

[Stilldrillin]

Ade,

I would grip onto a thin disc, held at the very back of the jaws. Then take a very light cut with a tipped boring tool. Same as soft jaws! 

David D.



P.S.

Nice to see you here again John!

[No1_sonuk]

(also assume you've tried this with several bits of stock in case the first one was just the type of over-bent banana/cucumber the European Union was trying to stamp-out)

This was my first thought.

One way to check is to mark the high spot on the stock and jaws with tape, then rotate the stock 90 degrees.  Do the clocking again, and see if the high spot has moved round with the stock.  If the high spot moves, it's more likely the stock.

[Bogstandard]

David,

Thanks, nice to be with old friends again.

Unfortunately, your suggestion is a might dangerous. Maybe OK with a self centring, but with a four jaw independent, the jaws could be a fair way off exact centre to begin with, unless you used a largish washer and clocked up first on it's bore. Also some jaws are harder than others, that is why I recommend a surface grind method, tipped tools have a tendency to shatter when subjected to tough intermittent cuts.


Ade,

Being as you are on the same little island as myself, if it is your jaws that are out of square, get them to me and I will true them up for you on my surface grinder. A little extra 15 minute job like that I can easily cope with.


John

[AdeV]

Chaps,

Firstly, thanks for the suggestions & offers. Some more info for you:

The chuck is a 10" 4-jaw independent. I just measured around 0.005" runout in the chuck body, but zero taper across as much of the length as I could read without changing the dial gauge setup, around 3" worth. This is the chuck that was on the lathe when I bought it, so I have no idea what abuse it has suffered in the past, only what abuse it has suffered since I had it (and that's some - this chuck has seen me from never touching a lathe, to now - nearly a year later).

The 2" steel bar I dialed in last night shows 0.002" TIR at the far end, and about 0.018" TIR at the chuck end. I know the bar is straight because it rolls quite cheerfully on my surface plate with no appreciable light gap. I also get the same effect whichever stock I put in the chuck - I have some 2" ali which is the same, some 5/8" tool steel (same) & 5/8" regular steel (same)... I can also sometimes knock it square with the plastic hammer (provided it's not in the jaws too tightly) - inevitably, when one tightens the jaws, you can feel when the stock moves off parallel again.

Dave - it is true I've had the chuck off a few times (it's bolted to a back plate type affair). I did once trap some swarf between chuck & back plate; that was instantly noticable (you could see the chuck wobbling), so I know it's clean back there at the moment. I've not actually removed the jaws to give them a really careful look-see, but I'm pretty sure it's not compacted swarf that's the problem. That said, I'll give 'em a good wire brushing to see if that fixes it.

David D - I had the same thought as John, the jaws are clearly hardened (they mark everything that comes into contact with them, they've even (slightly) marred the ER32 chuck I have which has a hardened body. I guess a carbide tip might touch them, but I'd be worried about chippage, not to mention getting the things accurately set in the first place.


John - thank you for the offer of the surface grind, if the wire brushing fails to have any effect, then I will be in touch for sure. I'm only about an hour's drive from you - if that - and I may well be passing Crewe in the next few days on another errand... I will PM you this afternoon if I may.

[Bogstandard]

No problems Ade, for that sort of job, it is a wait while it is done type.

John

[Stilldrillin]

Ok!

It's the way I've done it, over many years. Jaws clocked true, and a light duster pass, with a carbide boring tool......

David D

[AdeV]

Ho-kay... I took each jaw off in turn (only 1 at a time so I didn't put them back in the wrong place  ), and whilst I don't have the facilities to measure taper, I did put each one on my surface plate; and not one of them sat dead steady; it was possible to rock all of them: one had a noticable end-to-end rock; two of the others a sort of diagonal-to-diagonal rock, the 4th was pretty close, with just a tiny bit of side-to-side action.

I took a bunch of photos which I'd hoped would show the gaps, but they're not terribly distinct.

Looks like I'll be taking up your offer of a surface grind, John... I'll PM you later on about that.

At least once the jaws are true, I'll know for sure how much spindle mis-alignment I've got....  <-- That's a slightly manic laugh...

[No1_sonuk]

Also some jaws are harder than others

I found out last night that the fronts of Chester DB7 jaws are softer than tungsen carbide...
I got WAY too close with an indexable tool, and now have a shallow groove on the front of the jaws.