Conforming Workholding in
a Non-Conforming World
Virtually distortion-free chucking of fragile workpieces is possible with the development of
the Soft-Touch chucking system. Since its inception in the late 1980’s, tens of millions of parts
have been machined to roundness quality levels never before achieved with
standard workholding devices.
We focus on thin walled and fragile workpiece applications and/or those
parts, which are distorted by conventional workholding methods. Soft-Touch chucks are applicable
where light roughing to finishing type operations are being performed.
PROBLEM:
 Standard sliding jaw
chucks will distort thin walled workpieces as described in diagram #1
. If a surface is turned and/or bored round, it will change shape and go out-of-round when
released by the chuck. Similarly, if faces are machined flat while chucked, they will
change shape and show signs of run-out when released.
Likewise, the opposite holds true. If a part is out-of-round and is chucked with a jaw chuck having wrap around
jaws, or with a diaphragm or collet chuck having wrap around part contact,
the part will be made round when clamped. Surfaces that are machined round
and flat will then spring back to the parts natural shape when
unclamped.
Floating jaws and compensating six jaw chucks with multiple contact points help,
but their massive size along with huge actuator elements do not possess
the finesse required to chuck these fragile parts. Attempting to reduce clamping
pressure may be dangerous, if the risk of throwing a part exists.
SOLUTION:
 Soft-touch Chucks
feature dozens of light-weight, compensating fingers which come into
conforming contact with the part. Chucking stresses and distortion are
nearly eliminated. Diagram #2
charts the relationship between distortion and the
number of jaw contact points.
The secret to our success is the “Vector-Lock” principle of clamping. An
inflatable bladder (see fig.#1
) within the chuck body expands dozens of
fingers that contact the part, conforming to the workpiece
contour. Serrated grippers are mounted at the tip of each finger that contact the workpiece to
lightly penetrate the surface. This creates a footprint for a high
coefficient of driving friction.
 This "Footprint” is only a scratch and generally is
not detrimental to the finished part’s surface quality. It provides high
torque driveability at a low, non-distorting force. The workpiece is
rigidly clamped, with this Vector-Lock principle, through high density
placement of gripping points. Force vectors array around the part (see diagram #3
) for a firm but delicate
grasp. The fingers flex with a slight downward arcing motion to firmly rest the part against a work
stop. Lock-up against a work stop resists deflection from cutting tool forces.
Dampening & drive characteristics derived at the clamping diameter result in better part finishes and often
allow higher cutting speeds and increased cutting tool penetration
rates.
An example of Soft-Touch’s capabilities is
illustrated in diagram #4
. The part, a torque converter impeller assembly
is made from a steel stamping. The component is stamped from .210” sheet
and formed into the shape of a bowl having a 12.82” O.D. Turbine vanes are
brazed to the inside of the bowl, causing stresses that induce an
out-of-round condition having an oval characteristic. Typically, these
parts range from .005” to .040” ovality. We chuck on the major O.D. and
bore a 12.47” diameter. This critical feature is used as a piloting
diameter effecting assembly. The data shown is from a randomly
picked production part produced in 1999 from a chuck that has been in
production for 5 years.
Soft-Touch concentrates on the tough jobs and challenges other workholding
methods by either:
- making better initial quality parts.
- making them repeatedly more economically
through less scrap and rework.
- providing more uptime and/or increased speeds & feeds to improve
overall production rates for added economy.
If you're interested in increasing your level of quality or reducing
the cost of quality, please see our contact
page or request information.
Diagram #4a
–
Configuration of part O.D. at chucking point showing .01652”
out-of-roundness. 100x magnification with .001” scale resolution per line
increment.
Diagram #4b – Configuration of 12.47” bored I.D adjacent to chucking point showing
.00201” out-of-roundness. 100x magnification with .001” scale resolution per line
increment.
Diagram #4c - Configuration of
12.47” bored I.D same as shown in diagram #4b showing .002007”
out-of-round. 1000x magnification with .0001” scale resolution per line
increment. This magnification depicts residual stresses inherent
from the stamping process and showing that orientation is basically
unchanged.
Figure #1 - A typical installation includes a Soft-Touch chuck that is hydraulically fed
via two pipe tubes through the center of the machine spindle. At the back, a dual passage rotary
coupling is attached to the spindle drive with an adapter. The coupling is fed by a hydraulic
power supply with sequenced valve capabilities and individual
pressure regulation control.
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