As already suggested, the market divides broadly into two parts, contact and noncontact point capture devices. Some argue that there is a third category, a hybrid category, where, for example, noncontact scanning devices can be at- tached to the head of a coordinate measuring machine, a device more normally associated with some form of contact probe. Perhaps it is better to think in terms of the probing device, the work-holding device and the probe-positioning device. To compare probes, the major attributes to be considered are accuracy, speed of data gathering, and repeatability of the measurements. Figure 4.3 offers a taxonomy of measuring systems.
4.4.1 Contact Devices–Hard or Manual Probe
The oldest and slowest form of contact probe is usually known as a “hard probe”. It requires that the user manually position the probe on the surface of the object to be measured, allow the machine to settle, and then manually tell the coordinate measuring machine (CMM) to record a measurement. The coordi- nates of the point are automatically calculated by the CMM’s software, making
due allowance for the diameter of the stylus tip. This form of probe is the slow- est, and its accuracy and repeatability depend on the operator’s “feel”. In the hands of a skilled operator, the accuracy and repeatability can be good, but where multiple operators are used, there is a risk that each will “feel” the probe touching with slightly differing amounts of force and thus get very slightly dif- ferent readings. However, systems based on this sort of probe are at the cheaper end of the spectrum.
4.4.2 Touch-trigger Probe
The touch-trigger probe overcomes some of the limitations of the hard probe in that the device itself triggers the taking of a measurement when the probe con- tacts the surface of the object. Typically, an LED flashes or a beep is heard to
Figure 4.3. A taxonomy of measuring systems
82 4 Selecting a Reverse Engineering System
indicate that a point has been recorded. This sort of probe is typically mounted on a manual or computer-controlled coordinate measuring machine and is the staple of much engineering metrology. The advantage over the hard probe is that it eliminates the element of operator feel, and when used in conjunction with a computer-controlled measuring machine, can be a reasonably rapid and very repeatable way to obtain data. Further enhancements to look out for in- clude piezoelectric sensors that allow the software to compensate for deflection of the stylus caused by variable forces when the probes touches the workpiece, and strain gauge technology which ensures that the force applied by the probe is constant when a measurement is taken. Though they add cost, these features also improve accuracy.
4.4.3 Continuous Analogue Scanning Probe
The final class of probe is the drag probe or continuous analogue scanning probe. This type of probe is literally “dragged” across the surface of the object to be measured and continuously records the location of the probe. This dramati- cally increases the speed of data acquisition, but speed can suffer from variabil- ity in the surface finish and from vibration, as the probe is dragged across the surface. Some systems provide software that can filter out these effects. “Closed- loop” systems adjust the position of the stylus relative to the surface based on trends in the measurements being taken. This allows measuring a workpiece of unknown topology easily. However, where the shape is already known, from a CAD model, for example, the stylus can be driven over the surface by a prewritten “part program”. This allows comparing the actual part with the CAD model, and reports of variance can be easily generated.
4.4.4 Other Facets of Probe Selection
Two additional facets of selecting a probing system are deciding whether or not to buy a motorized probing head and which styli to buy. When taking measure- ments, it may be necessary to adjust the angle of the stylus to maintain optimal contact with the surface or to access difficult to reach areas. Manual heads re- quire operator intervention; motorized heads can be changed directly by the CMM controller. Styli are available in a bewildering array. The stylus is the part that actually contacts the surface of the workpiece. Typically, styli have spherical tips that are available in a range of diameters and materials. Larger diameters are less affected by surface finish but make access to small features problematic and vice versa. Styli are also available with cylindrical, hemispherical, star, and disc configurations. Styli also come in a variety of lengths. The shorter the sty- lus, the less it will deflect. However, access to some feature may require long styli. Some vendors provide optical systems to measure the deflection of the
stylus and hence compensate for stylus deflection. Stylus tips are available in a variety of materials; each material has accuracy and wear implications. To in- crease stylus configurational flexibility further, stylus extension bars are also available, but each additional joint provides the possibility of additional joint deflection during measuring. Again, the stem of the stylus can be made from a variety of materials, each has differing deflection properties and coefficients of thermal expansion. Using the right stylus for a particular feature is essential to maximize the accuracy of measurements. For some jobs, more than one stylus may be required. Some vendors offer automated stylus changing without the need to requalify the new stylus.
4.4.5 Noncontact Devices
Noncontact devices are typically based on some form of light measuring system, although for medical applications, sound, X-rays and magnetic resonance are all used regularly. Noncontact devices are now available which can be mounted on the head of CMMs and thus provide further flexibility for measuring objects which might deflect if a contact probing approach were used. However noncon- tact devices can also be free-standing. There are so many variations of device in this category that it is quite difficult to produce a convincing classification.
However, there are some common themes.