On 2009-06-27, Michael Koblic wrote:
I just got a 3-jaw chuck fo my Taig lathe. It comes with two Tommy (?) bars,
a hex key, a washer and a set of instructions. The jaws are aluminium and
unfinished, the instructions tell you how to turn the jaws true.
Before I started I measured the runout using a 1/2" drill rod. It came to
0.006".
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.)
The two-piece jaws for larger chucks have both a groove along
the length to keep the top jaws parallel to the master jaws, and a
projection from the master at right angles to the jaws and a matching
cross groove on the underside of the top jaws so they always have the
same length projection -- no depending on the fit of the screws as the
Taig chuck does. (For this reason, on the Taig, I mount the jaws, turn
them to dimension and use them without ever removing them.)
I did as the instructions told me. I used the washer provided, chucked it up
at the back of the jaws, made sure it was nice and flat and carefully turned
0.002" off the jaws. The I filed down the little nubbins at the back of the
jaws where the washer was being held during the procedure.
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.
I re-measured the
runout: This time it was 0.004". I inspected the jaws: There was evidence of
"clean-up" on all three, the filing seemed satisfactory (I touched up one of
the jaws just to make sure).
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.
I have a number of questions:
1) Is 0.004" TIR satisfactory for a small 3-jaw chuck? I suspect that far
from it but I do not want to be unreasonable.
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.
Mine (an older one) only has tommy bar holes on the scroll
plate, none on the body, so I usually put a Crescent wrench around the
jaws close to the body to hold it from turning when I use the Tommy bar
on the scroll pate.
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.
2) Is the washer the most likely culprit?
It could have been a contributing factor -- as could shifting of
the jaws between the truing and the clamping of the test workpiece.
3) How to rescue the situation? The obvious solution (assuming the washer is
the culprit) is to find something tthat is perfectly round, chuck it up
again and repeat the procedure. Presumably the object will have to be
thicker than the filed-down portion of the jaws so it is held by the turned
down portion but I may be wrong.
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.
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.
The exception is when you are making soft jaws to hold a disc,
so you turn it true only for a short distance (a little less than the
intended final thickness of the workpiece, so you can face both sides in
the chuck.
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.
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.
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.
There are other ways to hold the jaws preloaded for turning.
Do web searches and find them.
Good Luck,
DoN.
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