Implant Manufacturer Maintains Quality with Renishaw
System
Renishaw’s QC10 ballbar system
The human body is an intricate
association of complex mechanisms and synergistic collaboration.
When a joint fails--especially if it’s a knee or hip--quality
and precision are paramount to manufacturing a replacement.
Smith & Nephew Orthopaedics in Memphis,
Tennessee, has always emphasized accuracy and quality in
its products, so change was imminent when the company found
itself mired in a quality control bog in early 1998. The
quality crackdown commenced when Richard Grimes was placed
in charge of the company’s CNC testing, repair and
validation program. Grimes, a calibration specialist, was
faced with the task of calibrating and maintaining the 325
machines that create Smith & Nephew implants--a daunting
task considering the implants are produced from titanium,
stainless steel and other metals in myriad shapes and sizes.
Smith & Nephew had been creating medical devices,
lubricants and implants since 1931. Although advancing technology
has brought unparalleled progress, it has also increased
the opportunity for error. By February 1998, performance
of the plant’s most critical machines had reached
unacceptable levels. Based on the Six Sigma scale, thread
whirling machines were operating at only 3 sigma. That meant
that 7 percent of the time, those machines failed to perform
their tasks within a specified tolerance. Vertical mills
operated at 2.25 sigma (greater than a 20-percent failure
rate) and horizontals operated at less than 1 sigma (a 69-percent
failure rate). Instead of smooth, efficient production,
Smith & Nephew was becoming increasingly involved in
rework and scrap--to the detriment of the company’s
bottom line. That was before Grimes implemented ballbar
diagnosis with Renishaw’s QC10 ballbar.
The QC10 ballbar system reduces scrap and unexpected machine
downtime by checking machine performance before component
manufacture. This concept--recognizing problems before they
adversely affect production--is a benchmark of the Six Sigma
strategy. The QC10 ballbar calculates machine circularity
error, servo gain mismatch, vibration, stick slip errors,
backlash, machine geometry, and repeatability and scale
mismatch. All data are analyzed to ISO 230-4, ANSI B5.54
or B5.57 performance standards, and machine errors are ranked
according to their significance to overall machine accuracy.
The wide range of machines at Smith & Nephew would usually
pose a problem for a system like the QC10, but adaptors
are available, allowing the QC10 to be used on different
machines.
The QC10 ballbar measures multi-axis errors that are apparent
only when the machine is in motion. The QC10 is portable
and plugs directly into the RS-232 port of any Windows-based
personal computer, including notebook models. Renishaw’s
diagnostic software calculates machine errors, weighs the
feedback based upon the measured values, and converts the
ballbar data into a polar plot of the machine’s true
movement.
Grimes uses the ballbar to check 13 performance parameters.
The company’s quality department determined that those
13 items combined must be within ±0.003 in. “When
new equipment comes in, I first test the machine with the
ballbar,” explains Grimes. “If a machine is
moved, crashes or loses a spindle, I run a ballbar test.
Regardless, each machine gets an annual test and will not
run until it meets our established specification.”
Grimes notes that the ballbar tests typically take about
15 minutes. The standard 100 mm ballbar attaches magnetically
between the machining center’s spindle and table,
tracking machine movement with a sensor accuracy of ±0.5
µm (0.000020 in.). Grimes uses the 50 mm ballbar attachment
for small machines and has implemented Renishaw’s
Windows-based Ballbar 5 software. “The Windows environment
is much easier to use and makes tests much quicker to set
up,” notes Grimes. The latest release includes a “history”
feature with an NC part program generator and integrated
simulator. The latter allows maintenance personnel to carry
out “what if?” scenarios on test data without
actually adjusting the machine.
Smith & Nephew has experienced a quality revival since
Grimes implemented the QC10 ballbar. The plant’s 325
CNC machines are currently operating at Six Sigma. “Cost
savings haven’t been figured, but the result on our
bottom line can be imagined,” says Grimes. “We
now make better products and have less downtime because
we can catch problems before they happen.” Smith &
Nephew’s commitment to strict compliance and thorough
machine evaluation ensures that implants fit precisely and
function flawlessly. This dedication to quality appeals
to not only business professionals but also to implant recipients
eager to reclaim their active lifestyles.
Renishaw’s QC10 ballbar system
- Portable device provides rapid assessment of machine
accuracy and performance.
- Analysis in accordance with ISO 230-4, ANSI B5.54,
B5.57 and JIS B6194
- Ballbar5 HPS software maintains detailed history of
machine performance over time.
www.renishaw.com
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