On 2009-06-30, Michael Koblic wrote:
DoN. Nichols wrote:
[...]
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.
OK, I understand the procedure. Clearly this could have been done neither as
per the factory instruction nor using the kit provided.
Well ... it would have been possible to put the washer in there
behind the soft jaws and in contact with the master jaws.
I am still not clear about the purpose of doing it this way. I understand
the purpose of pre-loading the jaws (as per the info I have found so far) is
to prevent the cant of the jaws outwards at the tips if the workpiece is
inserted only part way.
That depend in part on the rigidity of the chuck and the jaws.
Your top jaws are aluminum, so they will give more with given force than
the steel ones will.
Let us assume that the soft jaws are loose on the screws attaching them to
the master jaws.
Then first tighten them somewhat onto a cylindrical part, and
then tighten the screws mostly. Then tighten a bit harder onto the
cylindrical part, and firmly tighten down the screws the rest of the
way. (I guess that I should suggest a torque limit, but do you have a
torque wrench which measures in inch pounds anyway?) I typically go by
the spring of the Allen key's handle as an indication of torque.
If one pre-loads the master jaws as you outlined and then
tries to turn the soft jaws they will not be supported and the cut will be
made in a way that will leave the surfaces in no relationship to what they
would be eventually when tightened on a workpiece.
Right -- which is why the first step is to make sure the jaw's
screws are tightened while the jaws are preloaded as above.
If, however, one follows
the procedure as outlined in the OP, the base of the soft jaws is tight
against the washer. This pressure will push the jaws tight against the
screws and transmit the pressure further to the master jaws which should
thus become pr-loaded in a manner identical to yours. Thus everyhting should
be nice and tight and identical to a workiece with a diameter identical to
the diameter of the washer being held in the jaws. The jaws are fixed and
turning will remove the material form the right places.
Ans as soon as you loosen the chuck to remove the washer, you
also let the jaws shift, because you did not take pains to tighten the
jaws' screws under preload first. :-)
[ ... ]
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.
I went back to this coincidentally this afternoon before reading your post
and quickly realized that this is the case.
Good!
The reason I did not get any
helpful result before is that I did not appreciate the difference between
the holes. In fact after correcting for this I could get the run out as
little as 0.001" using a specific hole on the body which is now clearly
marked. Problem solved!
Great!
[...]
If you want *precision*, you take the time with an independent
4-jaw chuck to do the setup with a sensitive dial runout indicator.
You also feel the death approaching that much more quickly...
Nope -- you are too busy setting things up to notice that. :-)
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.
Holding large objects is one thing, turning them is another. I do not care
for the noise the little fella makes when I order him to do this.
Keep the extension of everything you can as short as possible.
You can also reverse the jaws on your 4-jaw chuck,
[...]
I did that and then turned and faced a 4.5" washer. The bottom cleared the
ways by about 1/8" and getting to the side of the disk was a little tricky.
That is what the soft jaws are for. If the washer (or other
disc) is thinner than the steps of the jaws on the 4-jaw, you take a new
set of soft jaws for the 3-jaw, and turn them (after preloading) to a
step just deep enough to allow facing both sides. You can actually
make it a little deeper, and just expect to machine off a bit of the
height of the soft jaws as you do the first workpiece. Remember -- the
parts of the soft jaws are expendable. :-)
Not to mention doing it at 575 rpm...
I would swear that mine will go slower than that. How many belt
steps are on your pulleys?
And you could replace the motor with a three phase or a DC motor
and connect an appropriate controller to it to get speeds perhaps 1/6th
your current speed. That would take you down to 100 RPM or a bit
slower. My 12" Clausing will go down to 210 RPM in direct drive, and if
I engage the back gear, it will go down to 35 RPM. The corresponding
highest speeds are 1600 RPM and 270 RPM. Having a variable speed motor
would be somewhat the equivalent of having a back gear -- except that it
would not boost the torque the way a back gear does.
[...]
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.
I started up on that. Inadvertently. The first parting tool I made just did
not cut it. It spun the toolpost and shaved the jaws before I realized what
was going on.
Clamp some heavy paper between the toolpost and the table or
compound and it will increase the holding power by fitting into
But the advice on turning the OD jaws is noted. I also found some good stuff
he
http://www.ctemag.com/pdf/2002/0203-topchoices.pdf
related to what the shape of the jaws should be etc.
Of course, this is written from the perspective of someone who
is after 0.0001" runout -- and is working with a machine capable of
that. :-)
But it does have good information -- including the ring and pins
in the jaws' mounting holes which is shown in the first photograph. It
looks as though the pins are a permanent part -- under the heads of the
outer bolts holding the top (soft) jaws to the master jaws.
[...]
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.
Scratching has worked well so far. Engraving also a possibility.
O.K. Be sure to use surfaces which are not critical -- perhaps
the outer ends of the master jaws. A carbide-tipped vibrating engraver
(Burgess Vibrograver or equivalent) will probably do.
[...]
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.
No. My head just exploded.
Those are not all used at the same time. They are three
different ways to preload the jaws without getting in the way of the
boring operation. It helps to find the web pages illustrating the
techniques. I've seen (1) and (3) above illustrated in web pages.
OTOH the second (re-worked) parting tool works just fine. Once you get past
the squealing and screeching of the bigger OD down to about 3/8" :-)
Keeping the amount or projection down to a minimum helps a lot.
Making sure that the sides of the parting tool are parallel and
perpendicular to the axis of the workpiece helps. Having the parting
happening as close to the chuck's jaws as possible helps. Getting
lubricant to the bottom of the groove helps. And ideally, not having a
compound as part of the stackup will help minimize the flex of the
system.
Gotta stay positive...
Yep!
Enjoy,
DoN.
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