CMM technology has evolved
during the past 30 years to meet the increasingly tighter
tolerances demanded by today's manufacturing and design
engineers. These accuracy demands, combined with the perpetual
drive for increased inspection efficiency and throughput,
have led to diversified sensing approaches on CMMs. This
can lead to confusion about which sensing system is best
for your shop's applications. Because a CMM represents a
significant investment in capital equipment for large manufacturers
and small job shops alike, it must be tailored to handle
your specific needs while providing flexibility for growth
as inspection demands change.
CMM data collection starts at the stylus tip, the
part of the measuring system that makes contact with
the piece being measured. The type and size of stylus
used is dictated by the feature to be inspected. In
all cases, however, maximum rigidity of the stylus
and perfect sphericity of the tip are vital.
A variety of stylus shaft materials are available,
including ceramic, steel, tungsten carbide and carbon
fiber. Styli made from carbon fiber-reinforced material
provide maximum stiffness, low mass, thermal stability
and high-impact fracture resistance.
Ruby, the industry standard, is the optimum stylus
ball material for the vast majority of measurement
applications and is one of the hardest known materials.
Machined into a highly spherical form, ruby balls
are exceptionally smooth on the surface, have great
compressive strength and provide a high resistance
to mechanical corrosion. In addition to ruby, other
ultra-hard stylus ball materials are available, including
silicon nitride and zirconia, for matching the optimum
stylus material to a specific application, such as
material being measured or measurement type (i.e.,
scanning or discrete-point measurement).
It's critical to keep the stylus short and rigid for
most probing applications. Minimal stylus length for
the application is suggested, and a one-piece stylus
is recommended. Every time you join styli and extensions
together, you introduce potential bending and deflection
It's also important to keep the stylus ball as large
as possible to ensure maximum ball-stem clearance,
reducing the chances of a false measurement. Choosing
the largest ball possible gives the added benefit
of maximum stylus stiffness, due to the larger stylus
stem diameter, and improves the measurement performance.
Using a larger ball will also reduce the effect that
component surface finish may have on your measurement.
Star--Multitip star styli can be used to inspect extreme
points of internal features, such as slides or grooves
in a bore, minimizing probe movement. Each tip on
a star stylus requires datuming similar to a single-ball
Pointer--Although not appropriate for conventional
X-Y probing, these styli are ideal for probing threaded
forms, specific points and scribed lines. Radius-end
pointer styli can be used to inspect the location
of very small holes.
Ceramic hollow ball--These large styli are ideal for
probing deep features and bores in X, Y and Z directions,
and require datuming of only one ball. Probing with
a large-diameter ball averages out the effect of very
Disc--A disc stylus comprises a slice through a sphere
with a spherical outside diameter. This gives the
benefit of a large diameter for planar measurement
without the mass of a sphere. These styli are ideal
for probing undercuts and grooves. A simple disc requires
datuming on only one diameter (usually using a ring
gage) but limits effective probing to only X and Y
directions. Adding a radius-end roller allows Z-direction
Cylinder--These are ideal for probing holes in thin
sheet material and threaded features, and locating
the center of tapped holes. Ball-ended cylinder styli
allow full datuming and probing in X, Y and Z directions.
Custom design styli--Custom styli are also available
for providing tailor-made product solutions for specific
customer application requirements.
The following five factors will help determine what type
of inspection system will deliver the biggest benefits to
your specific application.
The part print of the components to be measured--The part
print determines the design intent and identifies the dimensional
and geometric tolerances required. Features that form functional
fits with other parts are best measured by scanning, whereas
discrete-point measurement is best suited for the measurement
of size and positional features.
The type of measurement required--The type of measurement
required, combined with the part print, will determine whether
a bridge, gantry or horizontal-arm CMM is best for the measurement
task. The type of CMM required often dictates which sensing
system is best. For example, the measurement of gap and
flush of a body-in-white stipulates different probing requirements
than those optimized for prismatic or powertrain applications.
Machining process capability--The performance of your machining
process relative to the required tolerance will also affect
your choice of process control method. If your machining
processes reliably produce good features with consistent
form, you'll need to focus on controlling feature size and
Discrete-point measurement is ideal for this. By contrast,
if your machining processes produce features with form that
varies by a significant proportion of the tolerance, you
need to monitor and control the form. Scanning is the best
process for this task.
Required factory throughput--High accuracy, high speed and
low cost of ownership is the mantra of today's manufacturing
world. Required factory throughput--or cycle time--may also
be an important determination in selecting the right probe
or right measurement system for the job.
Adaptability to capacity and function requirement changes--Because
a new machine, or even a CMM retrofit, can represent significant
expenditure, it's vital that it meets current inspection
needs and has the flexibility to adapt with changes to measurement
Today both contact and noncontact sensors are available,
allowing CMMs to scan the form of a component or to take
discrete-point measurements. The part print and type of
measurement will largely specify whether contact or noncontact
is the best method:
Contact measurement is currently the most accurate method
of sensing for most features and components.
Noncontact is the best solution for soft, malleable materials.
When throughput is the highest priority and high-accuracy
measurements aren't required (such as for checking gap and
flush on a body-in-white), a noncontact sensor is the best
Typically, contact scanning is useful for determining
the shape and form of a feature. Collecting hundreds of
data points is very useful when looking at the form. However,
the majority of manufactured features--such as small threaded
holes--don't require this detail, nor do location or clearance
features, such as holes for roll pins. For these features,
position is the critical factor, not form. Discrete-point
measurement, which involves taking a critical number of
data points and fitting a constructed feature to them, is
best suited for verifying these features.
Traditionally with scanning, the faster the machine travels,
the less accurate the data it collects will be. This "dynamic
effect" is due to inertia, or the weight of the machine
and sensors constantly changing directions while accelerating
and decelerating during the scanning cycle. The dynamic
change in the machine structure itself also has a direct
effect on the accuracy of the measurement.
However, the dynamic effects placed on CMMs when scanning
can now be dynamically compensated. For example, Renscan
DC is a new development available on Renishaw's UCC1 control
platform; this process first scans the part feature slowly
and then remeasures the feature at a higher velocity and
teaches itself the errors introduced by the greater speeds.
The CMM is then able to measure at a higher speed with
accuracy more in line with the lower speed measurement.
Even with these latest developments, the combination of
scanning and discrete-point measurement provides the most
accurate and efficient way to measure the majority of components.
Scanning sensors are probably the most flexible sensors
you can fit to your CMM, as they can also be used to acquire
discrete points. However, touch trigger probes measure discrete
points faster because scanning probes need to settle at
a target deflection before taking the reading. In each case,
the dynamic errors are minimized with discrete point measurement.
The machine is either stationary (if a scanning probe is
used) or moving at constant velocity (touch trigger probes)
when the point is measured.
Noncontact sensors are often the best solution for more
specialized tasks such as measuring soft materials. Therefore,
one sensor may not be suitable for all your measurement
Unless you're measuring a simple component, you'll need
to change your stylus configuration to suit different measurement
tasks. This has traditionally been done manually using a
threaded connection. However, probe systems are now available
with a repeatable automated means to switch styli.
This greatly increases system flexibility by allowing
you to quickly switch to long or complex styli, as well
as use different tips (e.g., sphere, disc or cylinder),
needed for different surface configurations. Automated stylus
changing reduces operator intervention and increases measurement
Stylus changing also provides the added bonus of robustness
via crash protection. The break-out force required to uncouple
the stylus and the probe is lower than that between the
probe and probe head to enable automated changing to occur.
In the case of a collision, this ensures that the intrinsically
robust stylus disconnects from its mounting before any damage
is done to the more valuable probe or probe heads.
Many manufacturers find that they need the flexibility
of stylus changing and sensor changing. The combination
means that you'll always be using the right sensor and stylus
for the given task, increasing your measurement accuracy
while minimizing measurement cycle times. Renishaw's patented
Autojoint has recently been adopted by the Optical Sensors
Interface Standards Committee as the industry standard coupling
for probe changing and is compatible with most Renishaw
and third-party probes.
You'll also need a means to store those sensors that are
not in use on the machine and allow automated changing within
inspection cycles. Renishaw's ACR1 and ACR3 autochange rack
systems are designed for this purpose and are compatible
with all probes using Renishaw's patented Autojoint.
An ideal sensing system needs to deliver the benefits
of speed, accuracy and robustness while providing the best
probe and stylus configuration for each measurement. Additionally
it must be flexible in configuration and easily upgraded
if it's to meet the growing demands of off-line inspection.
Renishaw's new SP25M probe system, when coupled with the
industry standard PH10M motorized head, provides a solution
to match these requirements. The SP25M has been designed
for measurement and sensor flexibility, with a modular design
providing the ability to swap probes, probe modules and
styli. The SP25M can also carry the TP20 range of touch-probing
modules, providing a single source that can be optimized
for scanning and discrete-point measurement. Because the
PH10M can also carry noncontact probes, the SP25M can be
used alongside noncontact sensors to provide a complete
scanning, touch trigger and noncontact solution.
However, when submicron accuracy with a stylus is required,
Renishaw's SP80 is the ideal solution. The SP80 can carry
and automatically change styli measuring up to 20 inches
long and weighing up to 1.1 lb. The probe is mounted to
the CMM with the same simple mechanism as Renishaw's PH10
series, making it easy to either change probes or to mount
a motorized head to the CMM if necessary.
All Renishaw scanning sensors feature lightweight, passive
mechanisms for simplified design and robust operation. Technologies
such as isolated optical metrology, in which precision readheads
directly measure the deflection of the probe, ensure that
excellent performance is achievable at higher speeds when
combined with Renscan DC.
If discrete-point measurement will satisfy current requirements,
touch trigger probes are an excellent cost-effective solution.
Their small size and great versatility have provided huge
benefits to the inspection process over the last three decades.
The latest touch trigger-probing solutions, such as the
TP20 and ultra-high-accuracy TP200, are now scaleable systems
that adapt as the requirements on them change. If scanning
is required at a later date, upgrading your UCC1-equipped
CMM from Renishaw touch probing to Renishaw scanning is
a simple task.
The ultimate in flexibility is provided via the UCC1's
ability to talk to different front-end software packages,
allowing you to use your preferred package on all CMMs installed
with a UCC. This will allow the usual benefits of standardization,
such as a reduction in training costs and staff, and capacity
flexibility, by enabling easy transfer of part programs
between CMMs as required.
Prior to his current position as national sales and marketing
manager with Renishaw, Barry Rogers was general manager
for LK Inc.'s Detroit Technical Center, a skilled trade
supervisor for John Deere Harvester, and a journeyman tool
and die maker for MicroSwitch Honeywell. Letters to the
editor regarding this article can be sent to email@example.com.