Choosing the best probing system for your CMM

December 2002 – CMM technology has evolved over 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 as to the best sensing system for your shop's applications. Because a CMM represents a significant investment in capital equipment for large manufacturer and small job shop, alike, it must be tailored to handle your shop's specific needs, while providing flexibility for growth as inspection demands change.

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 discreet-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 will be best for the measurement task. The type of CMM required will often dictate which sensing system is best. For example, the measurement of gap and flush or body-in-white will stipulate 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, then you will 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, then you will 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 – that 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 – As a new machine, or even CMM retrofit, can represent significant expenditure, it is vital that it meets current inspection needs and has the flexibility to adapt with changes to measurement requirements.

Contact or non-contact sensing?
Today both contact and non-contact sensors are available, allowing CMMs to scan for component form or to take discrete point measurements. The part print and type of measurement will largely specify whether contact or non-contact is the best method.
· Contact measurement is currently the most accurate method of sensing for most features and components.
· Non-contact is the best solution for soft, malleable materials.
· When throughput is the highest priority and high-accuracy measurements are not required (such as for checking gap and flush or body-in white) a non-contact sensor can provide the best solution.

Typically, contact scanning is useful when 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, do not require this detail, nor do location or clearance features such as holes for roll pins. For these features, it is position that 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. 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 has a direct effect on the accuracy of the measurement.

However the dynamic effects placed on CMMs when scanning can now be dynamically compensated with Renscan DCTM, a new development available on Renishaw's UCC1 control platform. This process first scans the part feature slowly, then re-measures the feature at a higher velocity and teaches itself the errors introduced by the greater accelerations. The CMM is then able to measure at the higher speed with the accuracy of 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, since scanning probes need to settle at a target deflection before taking the reading. In each case, the dynamic effect is non-existent 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.

Non-contact 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 are measuring a simple component, you will need to change your stylus configuration to suit different measurement tasks. This has 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 (sphere, disc, cylinder, etc.) 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, so as to allow 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 OSIS (optical sensors interface standards) committee as the industry standard coupling for probe changing and is compatible with most Renishaw and third party probes.

You will also need a means to store those sensors that are not in use on the machine. Renishaw's ACR1 and ACR3 autochange rack systems are designed for this purpose and are compatible with all autojointed probes.

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 upgradable if it is to meet the growing demands on 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 modular design providing the ability to swap probes, probe modules and styli. The SP25M can also carry the range of TP20 touch probing modules, providing the best of both worlds. As the PH10M can also carry non-contact probes, the SP25M can also be used alongside non-contact sensors to provide a complete scanning, touch-trigger and non-contact solution.

Renishaw's SP80 is ideal where sub-micron accuracy with a long styli is required. The SP80 can carry and automatically change styli measuring up to 20 inches long and weighing 1.1 lb. Also, the probe is mounted to the CMM with the same simple mechanism as a PH10M, 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, where precision readheads directly measure the deflection of the stylus, ensure that excellent performance is achievable at higher speeds when combined with Renscan DC TM.

If discrete point measurement will satisfy current requirements, touch trigger probes are an excellent low-cost 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 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 more staff and capacity flexibility, by enabling easy transfer of part programs between CMMs as required.