Using
Extended Chuck Jaws
Recently
we discussed the safe use of chuck jaws in this column.
We reviewed the forces involved in using hard and soft jaws
and tips for safe operations.
Another type of jaw common in job shops is the extended jaws,
which allow you to hold longer parts in the machine.
Back
in my job-shop days I would purchase extended chuck jaws when they
were needed for a specific operation.
When placing the order for the jaws, the sales staff never
said, “Be careful!
These chuck jaws can be dangerous.”
I guess they thought we know what we were doing!
As
foolish as it seems now, safety was not always a main concern.
There were times when we would use a 12-inch chuck to make
parts that should have been made with a 15-inch chuck.
The
12-inch counter centrifugal chuck was rated for 4000 RPM and the
part extended out a long way from the face.
The 4000 RPM rating was for standard jaws; with extended jaws
we had to drastically limit the G50 maximum RPM setting.
The
RPM rating is just one concern when using extended jaws.
You must also be aware of the weight of the jaws themselves.
The weight and the mounting distance from the spindle
centerline are part of the WR (weight/radius) factor.
The greater the distance from the centerline the greater is
the gravitational pull caused by the spindle rotation.
Remember
the early days of the space race when scientists used a centrifuge
to measure gravitational stresses?
Astronauts were spun in a large centrifuge machine where
scientists measured the high stresses inflicted on the human body by
large gravitation forces.
Well
the lathe chuck is our very own centrifuge; although it is tough to
measure the centrifugal force.
There is a special gauge that can be purchased with a rotary
coupling that allows the chucking force to be measured as the chuck
is spinning. The chuck
force will be reduced when a part is placed into the chuck due to
the weight of the part.
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These
forces can be great, so we must apply strict safety considerations
when using extended chuck jaws. The first step is to use a G50 / G92
preset command so the machine cannot reach a dangerous speed. The
definition of dangerous depends on your point of view. Remember, not
only are we using extended jaws, we also have a facing tool that is
effectively trying to push the part out of the chuck. Any time we face
to the center of the part in the G96 constant surface speed mode your
machine hits the G50 / G92 maximum preset.
Another
safety step is to cut away some of the jaw to reduce the weight.
Take a look at the jaws before mounting them to the chuck.
It may be possible to cut the outside corners, reducing the
weight but not affecting the grip of the part.
Lastly,
if we are going to bore these jaws to suit a specific part diameter,
we should bore a back taper (larger in the back) in the jaws to ensure
that we are gripping the part with the front of the jaws. The work
holding arrangement deflects due to the clearance / slop in the chuck
itself.
A 0.001 clearance at the chuck face will magnify into a larger
clearance at the front of the jaws.
Use
your judgment to determine how much taper is required. A simple test
is to set a piece of paper at the front of the jaws and close the
chuck to grip the part. If the paper comes loose there is not have
enough back taper.
Following
these basic safety tips we should be ready to use our extended chuck
jaws.
Be sure to keep your door closed and your procedures safe, if
not the part will be chasing you down the shop.
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