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by Barry Rogers

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.


Selecting the Right Stylus

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).

Important facts when choosing a stylus

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 points.

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.

Other types of styli

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 stylus.

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 rough surfaces.

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 probing.

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.

So what is the best sensing solution for your CMMs?

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 position.

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 requirements.

To contact or not to contact?

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 solution.

Scanning or discrete-point measurement?

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 needs.

Stylus and sensor changing

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 throughput.

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.

Complete sensing solutions

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.


About the author

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 letters@qualitydigest.com.