Lathe
Workholding
Back
to the Basics
Modern
CNC lathes are high performance machines designed to quickly rip
material off a part. While
the part is spinning at multiple thousand RPM and the tools are
removing material, how well are we holding that part in the machine?
Workholding
is a fundamental part of machining and is a core safety issue in the
use of CNC lathes.
Hard
jaws
Most
machinists agree that hard serrated jaws should be used in first
operation turning. The
maximum pressure should be employed to hold the part securely.
Sometimes
the pressure is reduced when turning a thin wall part.
In this case you must reset the pressure during the next
set-up. Be sure to check
the hydraulic pressure gauge that controls the grip force applied to
the part. Inadequate
pressure in the chuck can result in a part flying out causing
considerable damage to the machine and more importantly, the
operator.
Remember,
machine tools can be dangerous and flying parts can kill you.
People
talk about using 500 PSI to hold a part when in fact the chucking
cylinder may magnify this value to over 10,000 PSI gripping force.
The pressure is applied to a chucking cylinder that dramatically
magnifies the actual grip force.
The ratio of increase in force is a function of the cross
section area of the input pressure acting on the cross section area
of the chucking cylinder.
A
machine with an 18” diameter chuck may have grip forces in excess
of 29,000 PSI. Sounds
like a lot, but consider all
of the forces involved. If
you have a 50-pound part spinning round with a heavy duty insert
programmed to remove 0.375” of material per side you had better
make sure that the part is held securely.
Jaw
placement in the chuck depends on a number of factors.
Most jaws are supplied with 1/16” or 1.5 mm serrations.
The jaws can be placed anywhere along the serrations.
When
I load a new set of jaws I mount them at the top of the chuck’s
stroke. This provides two advantages.
With hard, serrated jaws the part is gripped securely. even
if the rough outside diameter of the part varies. When using soft
jaws, we have room to re-skim the jaws before relocating on a new
serration position.
|
Many
newer machines have a switch mounted on the chucking cylinder that
alarms if the jaw is mounted towards the bottom of the stroke. This
alarm helps to eliminate the problems and dangers of setting up the
machine and running out of grip stroke.
Again, with inadequate grip force the part can fly out of the
chuck.
To
ensure that I have sufficient safety stroke, in set-up, I measure the
stroke with the jaws gripping the part and then remove the part and
measure the total stroke available. This is an easy check to ensure
that we can safely grip the part.
Soft
Jaws
Soft
jaws are made from a soft steel (normally 1018 steel), bored to grip a
part for second operation machining.
Soft
jaws must grip the part
safely and with good concentricity.
To ensure a high degree of concentricity a boring ring should
be used to remove the slack in the chuck.
The boring ring, shown in this sketch, is gripped in the jaws
as the diameter is bored.
Many
machinists bore jaws by hand. They
use the manual pulse generator or the axis jog buttons to move the
slides and bore the jaws. This method depends on the skill of the
machinist to produce quality jaws.
I
recommend that a program be written to bore the jaws.
This method may be time consuming, but investing time in a
program now is ultimately more efficient.
I write and save the program for each individual job.
With a program on file I can allow an operator to bore the jaws
and do not have to rely on a fully skilled machinist.
The
program can be easily altered to add features such as an undercut
adjacent to the jaw location face, which ensures a good seating
surface. We can program a corner break on the front of the jaws to
eliminate the sharp edge. Lastly, we keep this program and have the
depth of the jaws and grip diameter on file for the next time we run
the job.
Next
month we’ll talk about the variety of chuck jaw types available. If
you have any comments or suggestions, we’d be pleased to hear from
you.
|
