Coordinate measuring machines offer one of the most effective ways to establish and maintain manufacturing process control. Two qualities distinguish CMMs from other measuring systems&emdash;the ability to quickly and accurately capture and evaluate dimensional data, and providing the operator with meaningful information concerning the condition of the manufacturing process.
Choosing the right CMM for any application requires considering several factors. For example, the size, weight and shape of the parts to be measured is important in choosing a CMM with the appropriate configuration, i.e., fixed or moving bridge, horizontal arm, cantilever, gantry or portable. Because direct computer-controlled CMM's movements are controlled automatically through motorized axes drives, it is better suited than a manual machine to applications requiring high accuracy and repeatability. Does the CMM's software interface permit operators of all skill levels to use the equipment with equal success? Can the dimensional data be interfaced easily with CAD/CAM systems to create an integrated measuring/inspection/process control program?
Just as important as any of these considerations is the choice of the device that the machine uses to gather data. CMMs measure work piece dimensions by moving a sensing device, called a probe, in the machine's 3-D measuring envelope, i.e., the X, Y and Z axes, until the probe contacts with the work piece feature to be measured. The precise position of the contact is recorded as an XYZ coordinate and is stored by the measuring system. Using these data points, the CMM's software calculates the desired distance measurements (dimensions), geometric shapes (features) and the relative position of geometric shapes (feature relationships).
The probe is literally at the center of the CMM operation. A variety of probe designs are available today, and although most probes are compatible with most CMMs, users should understand what each type of probe does and which probes best meet application-specific requirements.
Probes fall into two general categories: contact, or tactile, probes and noncontact probes. As the name suggests, a contact probe gathers data by physically touching the work piece. Contact probes are classified as hard, or fixed, probes, touch trigger probes and analog scanning probes, which maintain contact with the work piece surface during data collection.
To use a hard probe, the CMM operator manually brings the probe into contact with the work piece, allows the machine to settle and manually signals the CMM to record the probe's position. The CMM's software automatically adjusts the readings to compensate for the diameter of the probe tip. For instance, when reading a bore diameter, the software compensates for the diameter of the CMM probe tip so that the actual diameter of the bore is shown.
Hard probes are available in a variety of configurations and continue to have a broad application in coordinate metrology when used in conjunction with manual CMMs. They are most frequently used to measure curved surfaces, distances between work piece features, angles and the diameter and centerline location of bores in applications that require low to medium accuracy. Hard probes are simple to use and rugged, but their repeatability depends upon operator touch. Because every operator has a different touch when moving and bringing the probe into contact with the work piece, inconsistencies in measurement results can occur from one operator to the next.
Touch trigger probes
The touch trigger probe is the most common type of probe used on CMMs today. Touch trigger probes are precision-built, touch-sensitive devices that generate an electronic signal each time the probe contacts a point on the work piece. Contact with the part is usually indicated by an LED and an audible "beep." The probe head itself is mounted at the end of one of the CMM's moving axes. It can be rotated manually or automatically, and can accommodate many different stylus tips and attachments. These features make the trigger probe a versatile and flexible data-gathering device.
Touch trigger probes eliminate the influence of operator touch on measuring results and can be fitted on direct computer control and manual CMMs.
An improvement on the basic touch trigger probe design incorporates piezo-based sensors. These sensors translate the deflection of the probe into a constant digital acoustic signal that is recorded by the CMM. This design improves the accuracy of touch trigger probe measurement results because it eliminates the effect of stylus bending (caused by force variations when the touch trigger probe contacts the work piece) and inaccuracies caused by the probe's internal electromechanical parts.
A further improvement is the use of strain gage technology. This principle of operation effectively triggers the probe at a constant force no matter what the probe's contact angle with the work piece. The design eliminates direction sensitivity common to other touch trigger probes. Submicron accuracy is possible, even with long stylus combinations.
Analog scanning probes
Analog scanning probes are a type of contact probe used to measure contoured surfaces such as sheet metal assemblies. The analog scanning probe remains in contact with the work piece surface as it moves and produces analog readings rather than digital measurements.
Continuous analog scanning is a relatively new technology. It adds versatility to CMMs by offering dramatically increased levels of data acquisition, which speeds and improves the accuracy of measurement and inspection operations.
CAS technology is based on continuous rather than point-to-point acquisition of data with specialized probes and software. It is particularly useful for gaging and surface-mapping complex, contoured shapes, including crankshafts and cams, turbine engine blades and prosthetic devices. It is also suitable for inspecting the form of large sheet metal assemblies, such as automobile bodies.
Continuous analog scanning systems can acquire 10 to 50 times more data than traditional touch trigger probes in a given amount of time. The added data provides users with more confidence in the measuring and inspection process. More confidence may be needed if there are large gaps between data points using point-to-point probing techniques.
CAS allows users to scan irregular shapes. This is particularly valuable for measuring work piece features that change continuously, such as the arc on a turbine blade. The ability to acquire data in this manner also allows CAS systems to be used in reverse engineering applications where a new part has to be built to match or fit an existing part.
Form and shape measurement require a different approach than prismatic parts measurement. CMMs used in form measurement applications must be capable of collecting large amounts of data quickly, and measurement software must be capable of processing this data accurately. Because of this, special scanning routines not found in other CMM software packages are required.
For example, scanning software must have a filtering ability to distinguish between subtle changes in the surface direction and variability in the work piece surface finish, such as the rough area on a turbine blade. Filters can also eliminate the effects of vibration caused when the probe tip moves along the work piece surface.
Nonscanning CMMs use probe center coordinates for measurement in that the data generated by the machine is the location of the ball's center point. In scanning applications, the data must be shifted by the radius of the probe, using the parallel curve function, to represent the real surface of the work piece. During analysis, spline functions are used to remove the mismatch between the scanned points and the nominal points. This way, deviations from the nominal to the actual can be calculated.
Two types of CAS systems are used in measuring and inspection applications: closed-loop and open-loop.
In a closed-loop system, the probe automatically detects changes in surface direction of the part and adjusts itself to maintain contact with the part surface. Closed-loop scanning is particularly useful for digitizing unknown complex shapes. In the past, closed-loop scanning was performed at a relatively low speed, although improvements in controller technology have helped increase closed-loop scanning speeds markedly in the last five years. Five years ago, closed-loop scanning could be performed at only 10 mm/sec. New systems can perform closed-loop scanning at 50 mm/sec., with extremely high accuracy.
Open-loop scanning is a high-speed data-gathering technique used with continuous analog probes on parts whose geometry is well defined by surface points and vectors, or CAD data. The CMM drives the probe along a path using dimensional information from a data file. An example would be a sheet metal assembly such as an automobile hood. The probe head is driven along a vector perpendicular to the nominal surface and records the magnitude of the error between the actual surface and the nominal. CMMs now available can perform open-loop scanning at up to 150 mm/sec.
For large, bulky parts and those having complex geometries, measurement using a contact probe system is the most efficient and effective method of gathering accurate dimensional data. For less complex, smaller and higher precision parts, there is a growing trend toward the use of noncontact probes on CMMs. Noncontact inspection systems are also used to measure flexible parts whose soft material and geometry might be distorted with a contact probe.
Noncontact trigger probes are used in the same manner as touch trigger probes. However, with noncontact probes, a beam of light operating as an optical switch contacts the work piece. The noncontact probe is permanently set to a specific stand-off distance at which the light beam is triggered and measurements are taken. Because the probe never comes into contact with the work piece surface, damage is eliminated, and measurement speed is greatly improved.
Laser probes project a laser beam onto the surface of the part, the position of which is then read by triangulation through a lens in the probe receptor. The technique is much like that used by surveyors to find a position or location with bearings from two fixed points that are a known distance apart. Laser probe triangulation provides the actual position of the feature on the work piece being measured.
Vision probes are another form of noncontact sensing and are especially useful where very high-speed inspection or measurement is required, particularly on very small 2-D parts. The part is not measured directly. Essentially, a "picture" of the part is electronically digitized, creating accurate dimensions of work piece features that are measured and evaluated.
The camera used in vision systems generates a multitude of measurement points within a single video frame. The features of the image of the work piece are measured in comparison with various electronic models of expected results by counting the pixels of the electronic image. From this comparison, the true nature of the work piece being inspected can be inferred. Vision probing systems are fast and accurate, and are particularly useful in inspection operations that require frequent work piece changeovers. Unlike traditional probes that must be recalibrated for each work piece, the vision system's lens need only be calibrated once.
Adding flexibility and versatility
The measurement routine of a given part may call for the use of different probes or sensors to measure particular features. For example, a deep bore may require the use of a probe with an elongated tip, while other features may require different touch trigger, analog or vision probes. Probe changers store alternative and/or backup probes, allowing the automatic exchange or replacement of various probes. The type of probe to be used for the measurement of every part feature is written into the software program that controls the CMM operation. By using automatic probe changers, the probe changing operation can be carried out without stopping the CMM and without operator intervention.
A motorized probe head allows inspection of complex components with minimum operator involvement. The motorized head orients the probe in two axes in angular, indexable steps under manual or program control. In effect, the motorized head changes a three-axis CMM into a five-axis machine. Motorized heads make it possible to qualify all positions of a single tip probe at one time, avoiding time-consuming requalification.
Selecting the right probe
Selecting the right probe is like selecting the right CMM for a specific application. The application determines the probe system best suited for the measuring and inspection operation. Some CMMs available today come equipped with both contact and noncontact probe systems to give users virtually unlimited flexibility in data-gathering capability.
Also important, particularly when comparing probe systems, is measuring uncertainty, repeatability, and the gaging force specifications. These specifications should be matched to the requirements of your measuring and inspection operation. Your CMM supplier is best qualified to give you advice on which type of system is best suited to your needs.
Probe system selection is critical to achieving the maximum benefits from coordinate metrology. The proper selection will add value to your CMM choice.
About the author
For the past 20 years, David H. Genest has been involved in product engineering, development and marketing at Brown & Sharpe Manufacturing Co., North Kingstown, Rhode Island. He is currently director of marketing and corporate communications for the company. His background in metrology system design and development includes the introduction of Brown & Sharpe's Process Control Robot and other systems for shop floor measuring and inspection applications.