Form and Contour: Measurement 101
Fundamentals of complex-surface inspection
By Jim Destefani
When you think "form and contour," chances are you think of parts with complex intersecting surfaces, such as forming dies or cavities of a plastic injection mold. But even the most mundane parts have features that fall under the form and contour category.
A prime example of a form feature is a screw thread. It can be checked using go/no-go ring or plug gages, but a thread's true characteristics are defined by distances and angles between threads, the shape of the threads, and other form parameters.
And, form and contour measurement is becoming more important. Designers using CAD tools, emphasis on reducing part counts, multiaxis machine tools, and advances in plastic injection molding are all resulting in geometrically sophisticated parts with numerous features that have critical shapes, sizes, locations, and relationships. Product performance depends on each part meeting design specifications.
What Is It? But what does the term "form and contour" mean? At its simplest, form and contour covers those metrologic part characteristics not considered size or position, according to Christopher Daly, technical support engineer, Carl Zeiss IMT Corp. (Maple Grove, MN).
"Form evaluation addresses how closely the surface profiles of the measured components match a geometric ideal," he explains. "Contour evaluation most often is linked with a requirement to evaluate small, close-tolerance radii and angles."
"Form and contour are used to describe shape attributes of a surface," explains Fred Mason of Optical Gaging Products Inc. (Rochester, NY). "They are at one end of a continuum with roughness at the other extreme and waviness in between.
"Measurement of form is measurement of the deviation from primitive geometry features including straightness, flatness, circularity, or cylindricity," he continues. "Contours are combinations of primitive geometries. In most cases, contours can be broken down into form components."
According to Mason, form and contour measurement in manufacturing should be based on definitions used in geometric dimensioning and tolerancing (GD&T). Form tolerances specified using GD&T terminology include the above-mentioned straightness, flatness, circularity, and cylindricity.
Gary Card, of N. Kingstown, RI, coordinate measuring machine supplier Brown & Sharpe Inc., also includes GD&T profile characteristics in the definition. "Profile takes into consideration the profile of a surface or of a line," he explains.
Technologies for measuring contour range from simple templates to optical comparators, video inspection machines, dedicated instruments, and coordinate measuring machines.
Gages for checking standard radii, angles, and threads are available off the shelf. For other shapes and sizes, users can fabricate a master that's used as an inspection template.
Optical comparators project a magnified image of the workpiece, allowing more accurate comparison to a template as well as dimensional evaluation of forms and contours. Modern comparators sometimes feature edge detection and other image analysis techniques borrowed from video inspection machines, which use a camera to project an image of the workpiece on a computer monitor.
Video inspection machines include software with features such as edge detection, which allows rapid and precise analysis of the form of an edge over a range as large as the stage travels of the system. An example application for such a capability is inspecting the contours of gaskets for engines, motors, and compressors.
Scanning of a part can be via contact (top) or noncontact (center) methods. Edge tracing capability allows OGP's video and multisensor machines to quickly and accurately inspect the form of part edges (bottom).
Other capabilities that facilitate form and contour inspection are autofocus, which allows a video machine to accumulate a series of focus points across a surface, and CNC stage control, which translates the part and the imaging optics as points are acquired.
Measurements taken using a CMM allow characterization of overall workpiece geometry. While video inspection machines operate by translating the workpiece on a moving table, CMMs collect detailed dimensional data by moving a sensing device called a probe along workpiece surfaces.
Other technologies for form and contour measurement include dedicated machines for checking specific features, such as roundness, as well as CMMs.
Many dedicated machines currently available were developed to check a specific form characteristic. Dedicated roundness testing machines, for example, were a response to the need to evaluate thread profiles more accurately than could be done with hard gages or standard optical comparators.
Such machines typically use a stylus or ball tip that is traversed at constant speed along a section of the workpiece surface. An advantage of this approach is very good accuracy.
Multisensor Systems. Manufacturers of both video inspection machines and CMMs are taking their measuring technology a step further by enabling users to select the type of measurement method best suited to their application. (For more information on multisensor inspection, see the article "Multisensor Systems Ramp Up" on page 61 of the June 2003 issue of Manufacturing Engineering magazine).
"Use of multiple sensors on one measurement machine can allow it to inspect surfaces with and without contact, with varying resolutions, and combine measurements from various sensors into a single analysis of the part," Mason says.
Brown & Sharpe's Card agrees. "Use of multiple sensors is becoming increasingly important," he says. "Combining contact, laser, and possibly video sensors can provide a lot of measurement versatility. For example, you may have a bore diameter with a very tight tolerance. But on the same workpiece you may have a profile with a much broader tolerance."
Contact probes allow either continuous-contact or single-point probing. Continuous-contact scanning probes follow the surface contour as it and the part move. In single-point probing, individual data points are acquired.
Contact scanning allows features such as holes, planes, and cylinders to be quickly inspected with high repeatability and accuracy while delivering a complete description of part geometry-something that's particularly important for form features, according to Card. "Acquiring a larger number of data points ensures a much better true shape of the form features," he says. "In general, the more points gathered, the greater the repeatability of the results.
Noncontact sensors emit a precisely focused laser or infrared light beam. When the beam strikes the workpiece surface, it forms the image of a spot. The diffused scattered light is then focused on a photoelectric array. Any variation on the surface distance from the sensor results in a change of the position of the spot image on the array.
Sensors that work by emitting a plane of light also are available. When this plane intersects the part, a light line forms along the contour of the part. This line is detected by the image sensor that transforms it into a measurable digital image.
Formulating a gaging strategy starts with analyzing inspection needs. "If process control is the goal, then the more data points used to describe the workpiece, the more effective the information will be in adjusting machine tools," Card says. "The amount of data that accurately defines the part feature within tolerance specifications, while maintaining acceptable inspection throughput, is the right amount of data for process control applications."
Card says selecting the probe type that's right for your application is mainly a tradeoff between inspection throughput and accuracy. "Some scanning devices let users traverse rapidly across a surface and collect up to 20,000 data points/sec," he explains. "But when accuracy greater than several thousandths of an inch is needed, you'd need to select a different type of sensor. Contact probes allow accuracies in the tenths or several micron range."
Mason also lists several other considerations when selecting the type of inspection required for a specific part. These include:
- The size of the line or area over which form or contour must be measured
- Part mounting and system travel requirements
- The part surface--pliable or fragile parts may be damaged by probe contact.
Polymer Allows Inspection of Internal Geometries
External forms can be inspected using optical, scanning, or other techniques. But what about complicated internal geometries?
One option is sectioning the workpiece so that key features can be seen visually. This technique allows accurate measurement but results in a scrapped component.
A new approach is provided by Metro-Form, an extremely stable polymer that accurately adopts the shape of internal geometries. Available from Vision Engineering Inc. (New Milford, CT), the silicone-based polymer can be injected into blind holes, cavities, and thread forms. The resulting flexible molding is easily removed without permanent distortion or damage--once free, the molding will return to the cavity's original shape to within 0.04% in any dimension--and can be measured using noncontact gaging equipment.
Available in a kit containing an applicator gun, a range of nozzle sizes and shapes, and putty to block off open channels in the part, the system consists of three silicone compounds of varying softness to handle a variety of cavity forms. The entire molding process takes less than 30 minutes.
This article was first published in the January 2004 edition of Manufacturing Engineering magazine.