On 2009-06-28, Michael Koblic wrote:
DoN. Nichols wrote:
O.K. Did you tighten the screws holding the jaws as you
installed them? Did you press each jaw outward as you tightened the
screws? (Otherwise, they are likely to shift a bit under load.)
They were tight already.
Tight -- but perhaps not preloaded when tightening. There is
some slop in the screws fit into the soft jaws which can allow them to
shift if you don't pre-load the jaws towards the outer diameter while
you (or the factory) are tightening the screws.
[...]
Hmm ... IIRC, the washer contacts only the hard jaws below the
top (soft) jaws, not the soft jaws so there are no "nubbins" left --
unless you are turning a step to both hold and support a disk-shaped
workpiece.
No. The soft jaws cover the whole thing. There are nubbins...
O.K. Take the chuck off the lathe.
Adjust the jaws to the point where the hard "master" jaws are
level with the OD of the chuck body. The soft jaws will stick out
beyond that point.
Now -- using a small flashlight, look in through the threaded
aperture in the back which screws onto the lathe's spindle. You should
see the ends of the master jaws sticking out about 1/16" or a bit more.
Past those you will see the soft jaws closing to about the right size
to hold a 1/16" diameter workpiece (minus what you have removed in
truing the jaws).
Now -- turn something (a plug) which will just barely slip in
past the threads, and about the length of the master jaws. Close the
*master* jaws tightly onto that, and re-mount the chuck on the lathe
spindle.
Now -- use a drill bit to slightly enlarge the hole in the
center where the soft jaws almost meet -- and then use a boring tool to
reach down through there and enlarge the hole a little. If you intend
to hold something of a known diameter, drill a little undersized for
that and then bore to barely fit that. Then loosen the jaws, remove the
plug which held the jaws pre-loaded, and tighten the jaws onto your
workpiece. If you are going to hold a disk instead of a shaft which
will fit through, bore to leave about 1/2" of the jaws near the body
(which should clear the screws which mount the jaws), and to the
diameter of the disk to be held.
I was puzzled by this poor result. I could not think of an
explanation. Then I measured the washer and it turns out to be out
of round by 0.004".
Not too bad -- depending on the age and quality of the chuck. I
would expect 0.001" or better on a brand new quality chuck (like the
Austrian made chucks for my Compact-5). But those have only one-piece
jaws -- either hardened jaws with steps already made, or soft jaws
which go all the way down to the scroll plate as one piece.
This a is a US-made new Taig chuck.
I thought so, but I was writing for more than just you. Note
that I covered "quality" above as well -- The Austrian chucks for my
Compact-5 sold for more than your Taig cost complete with both chucks.
Each one comes with a certificate of accuracy -- when brand new, and my
examples were more accurate than the certificate promised.
First thing is whether there are multiple holes for the tommy
bars. If so, try with each one (using only the master jaws to clamp
with at first) and see which one gives the best concentricity. Then
*mark* that tommy-bar hole in the body so you can always use it for
the final tightening. (In the case of chucks which tighten with
keys, if there are three sockets, one should be marked with a "-0-"
or something similar by the manufacturer. Some have only one socket
so there is never any question.
There are three pairs of holes. I did play with them but it did not seem to
make any difference.
Hmm ... you don't really want to call them pairs, since
depending on the size of the workpiece, any one of the scroll plate
holes may be used with any one of the body holes. And the hole which
matters is the body hole (which mine does not have), not the scroll
plate hole. You use the scroll plate hole which is the closest upstream
from the preferred body hole as the jaws just start to touch.
But once you have bored the jaws to the proper size for the
current workpiece, you should get well under 0.001". It is only when
you move the jaws to grip a different diameter that you can get
significant change in runout -- especially if the scroll plate is not
truly concentric -- or is loose on the projection of the body so it
can shift from side to side as you tighten.
This is seems to be the recurring motif. I did not appreciate that this is a
feature of scroll chucks.
A scroll-backed (universal) chuck is mostly for quick setup
where the concentricity is not too critical -- e.g. when you are going
to turn all the features in one setup and then part off from what is
held in the chuck.
If you want *precision*, you take the time with an independent
4-jaw chuck to do the setup with a sensitive dial runout indicator.
But another use for a 3-jaw is with either reversed jaws
(assuming one-piece chuck jaws), or with soft jaws turned to make
reversed steps to hold a larger diameter disk shaped object. If you
want to handle even larger, make a set of extra-long soft jaws for the
3-jaw you have and bore them to a close fit so you can clamp the jaws
onto the workpiece with very little motion of the scroll plate. You
don't really need precision centering for this, because you will be
using it to face the workpiece not to turn the OD.
You can also reverse the jaws on your 4-jaw chuck, so the jaws
look somewhat like this (use a fixed-pitch font like Courier to avoid
*serious* distortion of the image):
+--------+ +--------+
| | | | | |
| | +---------+ +---------+ | |
| | | | | |
| | Jaw #1 +---------+ +---------+ Jaw #3 | |
| | | | | |
|____________________________| |____________________________|
+---------------------------------+ +---------------------------------+
| Chuck Body | | |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.......~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
so you can grip larger diameter stock -- though not as large as you can
with the 3-jaw with custom soft jaws. Just be careful that the custom
soft jaws are not so long that they will hit the bed.
4) What object to use for that purpose? The best I can think right
now is to get a piece of aluminium bar and turn and face it in my
4-jaw chuck and then part it off at the correct thickness.
Make it so it will fit behind the soft jaws in contact only with
the master jaws. You may have to unscrew the chuck from the spindle
to get it into place properly.
Not possible. See above.
See description above. The preload filler contacts the hard
master jaws, not the soft jaws which you are turning to true them up.
(Preload because it holds the master jaws in contact with the scroll
plate the way they will be in normal use.) Note that the advice which
you got in one of the other followups yesterday, which suggested a large
ring and backing the jaws out against that is not good for when you are
trying to true the jaws for normal gripping, as it preloads against the
wrong face of the scroll and the jaws' matching teeth. However, it is
*good* advice if you are turning the OD of the jaws, or turning steps on
the jaws to grip the ID of a workpiece, since in this case the jaws and
the scroll plate will be preloaded in the direction in which they will
be used.
Note that if you want real repeatable precision, you want
collets, and the good collets for a Taig spindle (not the standard one,
but the special one) are the WW style drawbar collets, which are size
limited -- 3/16" max for pass-through, and 1/4" for short gripped stock.
The standard collets which come with the lathe will handle
larger workpieces, but are not as precise. If you want precision for
anything larger than 1/4" with this lathe, you will have to use the
4-jaw and a good dial runout indicator.
Hmm .... I wonder whether they make an ER-16 or ER-25 collet
nosepiece for this machine?
5) If one used an object that is too thick or repeated the procedure
a few times the turned down (and hopefully true) portion of the jaws
will become smaller and smaller.
Huh? You mean the contact *length* along the faces of the jaws?
Not if you do the clamping only with the master jaws and turn the full
length of the soft jaws. For most things, you *want* the full length
of the soft jaws in contact with the workpiece.
Again, not possible.
Yes -- it *is* possible -- if you follow my suggestion at the
top of this for a plug for preloading the *master* jaws while you are
turning the soft jaws.
And you should have multiple sets of soft jaws. Either buy
more, or machine some aluminum in your small mill (which should be
large enough for this task) to make extra jaws. Make one set for
general purpose -- a stepped set for standard ID gripping, and a
reverse stepped set for larger OD griping. Depending on the
precision you need, you can often get away with turning the existing
jaws end-for-end.
Done that. Mainly on account of being sure that I would roger the first set
of jaws and would need the spare. I was not going to touch anything though
until I understood the current problem.
:-)
Remember that you can also *make* new soft jaws -- use an
unmodified set as a pattern -- and you can make them over-long to grip
larger diameter disk workpieces.
Whenever you make a set of jaws, use a number stamp set to mark
the jaws for position 1, 2, or 3, so when you put them back on, they
will be on the same master jaws.
Right.
And if there are no markings beside the jaws on the chuck body
(there are none on mine) use the same number stamps to mark the jaw
ways, starting with a randomly picked jaw as 1, and increasing as you
move in the direction that you turn the scroll plate to tighten the
jaws. Note that you will not be able to stamp the master jaws -- they
are too hard. But at least my chuck won't turn far enough to take the
master jaws out -- you need to remove the circlip on the back and
withdraw the scroll plate to do this -- and you should not need to do
so.
O.K. Now there *are* markings -- I went down to do that while
verifying the capacity of the WW series collets. I used 1/16" number
stamps which are a bit small, but are certain to fit on a part of the
side of the soft jaw which is not likely to be turned off.
What is the minimum size of the gripping
portion of the jaws to provide secure workholding?
That depends on a lot of things, including the material of the
jaws (aluminum in this case, but hardened steel for hardened top jaws
for larger chucks, or mild steel for soft top jaws for similar sized
chucks. Softer jaws, or softer workpieces need larger contact areas.
And when you are holding a long workpiece and turning some distance
from the chuck jaws, you need the full length of the soft jaws' faces
to minimize tilt in the jaws.
OK. Bigger is better.
There are other ways to hold the jaws preloaded for turning.
Do web searches and find them.
Not too sure that I understand what that means. I shall hit the books.
Instead of tightening the jaws -- or the master jaws as I have
suggested above -- onto a washer or a plug, you can do other things:
1) Put three pieces of the same thickness between the angled faces
of the jaws as you close it. I would suggest 3/16" or 1/4" HSS
tool bits, as they tend to be rather precise in dimensions.
2) Drill three holes to form an equilateral triangle around a
large center hole, tap for screws (say #10 or maybe 1/4") and
place those screws so they project into the holes for the heads
of the screws which hold the soft jaws to the master jaws.
Place these into the holes and tighten the jaws. You may want
to make it with several sets of screw holes for different jaw
positions. You bore the jaws through the center hole.
3) Three larger holes in a triangle with a center hole bored to
intersect the three so the tips of the jaws stick through for
turning while the holes' walls press on the angled faces of the
jaws while you are turning.
I hope that this helps.
DoN.
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