Three versus four.

[DavidA]

A few days back I was checking the run-out on my Red lathe.
 
The spindle, with it's front bearing nipped up to a point where the drag was noticeable but not too tight, showed aprox 0.001" on the clock. Also the same on the outside edge of the three jaw chuck.
"quite good". Pats oneself on the back (spends next five minutes trying to get the muscles back in place).

But when I chucked up a piece of ground round stock, there run-out went up to around 0.0025".

I know that three jaw chucks are prone to this, and when you think of the way the scroll works, it is no wonder.
Now, the usual method of getting concentricity is to swap to a four jaw and adjust it for zero run out. But this is a bit of a drag.
I was watching Stefan Gotteswinter's channel and he seems to get by very well by having allowed a slightly oversize fit on his back plate to chuck mounting and using this to tap the chuck into line.
He just slightly slackens of the three chuck holding screws and used a copper drift to gently knock the chuck until the clock shows zero.

This seems much preferable to swapping chucks.

Does anyone here use this method ?

Dave.

[awemawson]

In short - no, but that's the principle of the Griptru chuck except the Griptru uses other screws to push the chuck about rather than a soft hammer. It also theoretically stops it moving back under load.

[Fergus OMore]

Dave,
 

Two bits of memory? The first is that Hemingwaykits made an insert to act like a Griptru and earlier still, the late Martin Cleeve elongated the holes between the back plate and the 3 jaw, knocking it to clock to- I think that he said 3 tenths of a thou. Anyway, it WAS in Model Engineer.

Hope this helps

Norman
 

[BillTodd]

Well worth the effort, to make a chuck adjustable, my old cushman is good to a thou on the first 1" or so of scroll.

wktodd.webspace.virginmedia.com/Hardinge/page2.html

[NormanV]

David, I have to disagree that knocking the chuck to achieve alignment is less trouble than changing to a 4 jaw chuck. If you use two chuck keys to simultaneously loosen one and tighten the opposite jaw of a 4 jaw chuck, whilst using a dial indicator to see progress, you will be able to true up your work with only 3 or 4 adjustments.
I find that I use my 4 jaw chuck more than the 3 jaw as the adjustment is so easy.
Norman

[bertie_bassett]

when I picked up my lathe it came with no chucks, first thing I got was an old 4 jaw.

in all honesty I don't mind not having a 3 jaw, doesn't take very long to set up, and holds things well.

imo knocking the chuck to true things up and then trying to lock it in position without it moving must take longer?

[ieezitin]

I own a 3 and 4 jaw both great quality.

The 4 jaw always resides on the spindle, it takes very little to zero a piece / job in it.  If i am going to do repetitive work that requires constant re-chucking without much care to tolerance (with-in .002 or so) the three jaw gets used. Zeroing in on a 4 jaw is a part of working with a lathe experience.

If a requirement calls for a repeat tolerance below the .002 mark obviously collets are used but that requires planning of the part to be manufactured.

Just my humble opinion.

Anthony.

[rotorhead]

Hi Lads,

Before converting my oem integrally mounted PrattB L0 3 jaw to a 'Griptru, I used the method of reducing the spigot on the backplate by at least 0.020".

I also used that much on the Griptru conversion, to allow the adjusting screws room to work.

Still use the reduced backplate spigot method on my miller diving head chuck, works very well...

[DavidA]

Norman.

One advantage of the adjustable three jaw set up is that, providing you set your dial guage vertically, all you have to do is tap down from the high point whilst rotating the chuck, watching the dial until it settles at zero. No need to loosen/tighten opposing jaws.

Dave.