On 2008-09-30, Michael Koblic wrote:
[ ... ]
***I was thinking along the same lines. I have actually found a real Jacobs
chuck in my possession. I think it is this one:
http://www.jacobschuck.com/images/pr...No%20Crops.pdf
Cost less than $20, but the specs are about the same I am getting now. And
it is, of course, smaller. It was just an interesting exercise.
I note that those are chucks with threaded mounts, not Jacobs
Taper mounts. I don't expect as good a runout on the threaded back
chucks.
BTW what TIR would you expect on a, say, 3-inch 3-jaw chuck in a lathe?
That depends on who made the chuck, how old it is, and how it
has been cared for.
For the 3-jaw for my Emco-Maier Compact-5/CNC (which I bought
new) I would expect less than 0.001" runout at the present stage in its
life -- assuming the jaws were clean before clamping on the workpiece.
This was made in Austria, and mounts to a flat spindle nose with through
bolts clamping it to the spindle. The mounting surface is a surface
ground plate inside the back of the chuck.
For a cheap Chinese one, still new, perhaps 0.003" (more for a
larger chuck).
What
sort of improvement do you expect going to 4-jaw chuck?
What kind of patience do you have? How much resolution on the
indicator being used to center the workpiece? How good a finish on the
workpiece? Assuming a ground finish on a dowel pin, and a tenths
reading indicator I would expect perhaps to be able to tune to 0.0001"
-- if I had the patience at that particular time.
With a rougher finish on the workpiece, it would get in the way
of measuring, and result in poorer concentricity.
When you are
mounting a chuck, are most of the mounts self-centering (The Advanced
Machine Work says they just screw on to the spindle but that is surely not
the case in every instance)?
The lathes which I have which currently have a threaded spindle
nose are the Unimat SL-1000, the Taig, and the Atlas-Clausing 6x18"
which I no longer use.
I've described the Emco-Maier above -- with the exception of
forgetting to mention that the OD of the spindle nose is cylindrical,
and slides into a pocket on the back of the chuck. This gives quite
repeatable concentricity.
The 12x24" Clausing came with a 2-1/4x8 threaded spindle nose,
but I fairly soon replaced it with a L-00 (long taper) spindle nose
which is quite self centering.
Can you center the chuck by using the indicator
on the periphery of the chuck
You can't depend on the chuck's body to be concentric with its
grip. On a good one, it will be fairly close, but what really matters
is the faces of the jaws, not the OD of the chuck. There is not even a
certainty that the register turning inside the back of the chuck is
truly concentric with the OD of the chuck -- but it *should* be
concentric with the grip of the jaws.
or do you need to chuck something demonstrably
round and center on that?
Yes -- and if you are checking the chuck, you need to check
multiple diameters of workpiece, because the centering of the jaws will
be controlled by imperfections in the thread of the scroll plate.
In a chuck with three sockets for the chuck key, one of them
*should* be (but is not always) be marked with a stamped "0" or
something similar to indicate the one which gives the best centering.
(Yes, it does make a difference which key socket you use to tighten it.
Those others are convenient for releasing the workpiece when it is
finished, but should not be used for the final stages of tightening.
Note that there are chucks available with a feature called
"set-tru" by one manufacturer, and lots of other (trademarked) names by
others. The chuck body is mounted to the backplate by screws which are
accessible from the front, and there are four radial screws near the
back which bear on a projection from the backplate. These screws can be
used like a 4-jaw chuck to adjust so a single diameter of workpiece runs
true in the jaws. Other diameters are a different thing, and if you
care about them you'll have to re-adjust at the different diameter.
Also -- you can use two-piece jaws in your larger chucks, and
bolt soft jaws onto the master jaws and turn them to perfectly fit your
workpiece for precise centering of that one diameter -- and while about
it to provide a step to set how deep into the chuck the workpiece sits
for predictable facing to length. (Actually, the little Taig comes with
two-piece jaws, with the soft jaws being made of aluminum.) I have soft
jaws for many (but not all) of my lathe chucks.
Enjoy,
DoN.
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