Measuring by Comparison
Call it an optical comparator, a profile projector, a contour projector, shadow graph or any other name attributed to this versatile instrument. But whatever its name, this measurement tool is still a much used piece of equipment for machined-parts inspection, tool and die measurement, screw thread inspection and go/no-go overlay measurement.
Since before World War II, optical comparators have been a staple in large factories and small shops, and in industries as diverse as automotive and health care. Their basic uses have re-mained the same, even as the machines have evolved to include digital readouts, mechanized travel, sophisticated software, quick change bayonet lenses or lens turets, edge detection capabilities and improved optics and lighting. Whether the operator measures small, lightweight, flat parts with a vertical projector, or heavier, round parts on a horizontal machine, the comparator still projects a magnified shadow of a part's contour onto a precision-ground glass screen for comparison.
"The optical comparator is still a workhorse in the industry," said Mike Metzgar, department manager of measuring systems at Nikon (Melville, NY). "They are so easy to use that anybody with some real basic training can walk up to a comparator and measure a part or a feature."
Right tool for right job
Whether measuring an o-ring seal on a booster rocket or cap threads on a mascara bottle, the process is pretty much the same. Measurements are taken by staging a part and moving the screen ring around the part's image to divide it and do angular measurements. By moving the part on the stage, coordinates and lengths can be measured. Even the simplest machines can measure angles, radius, diameters, width, intersection, included angle, distance, roundness and straightness.
Environmental factors have little effect on comparators and they can be placed in-line on the shop floor. This can speed measuring, which is especially important in a go/no- go scenario when there is no time to transport the part to a metrology laboratory.
"Comparators in the shop environment are especially useful for workers who are making parts on a lathe or mill," said Richard Iavelli, vice president of Deltronics (Santa Ana, CA). "They can walk up and measure a radius or the position of an angle and get an answer that can quickly alert them to production problems before they make bad parts."
One of the most traditional uses of an optical comparator is measuring a part against a detailed overlay chart. These charts, which are often made from a mylar material, are clipped to the comparator's screen and the part's image is projected onto the screen and aligned to the shape to see if it is in specification.
Throw it on "These chart gages are used to check common and standard radii angles, center lines and radius grid angles," said Donald N. Ahlegian, owner of Ahlegian Precision (Concord, OH), an overlay chart company. "People use them all the time because you can throw a part on the comparator and do verification and checks."
Construction points and theoretical intersections that do not exist on the workpiece can also be determined, said Ahlegian. "Irregular surfaces can be viewed over the entire range by comparing the actual part shadow with a maximum and minimum tolerance. So they put the part up there and compare it against the min/max border. If it falls in between the two lines, the part is good."
Another comparison technique is the use of a software overlay. The software imports a computer-aided design model that creates an electronic overlay on a computer linked to the comparator. A digital readout (DRO) sends data to that computer screen to see the entire profile of the part. This is especially helpful on non-standard geometries that have free-form curves that blend together, said Louis Todd, president of Quality Control Solutions (Temecula, CA). A car is a good example, because the curvatures of a car are not just one arc; instead it is maybe 50 arcs blended together.
"You take your existing drawing and put it in software, tell it what your tolerances are and you just start making measurements in real time," said Todd.
Optics are the heart of the comparator because the image projected is what gets measured, not the part itself. Optical comparators have a fixed-magnification lens, which can be changed with another lens depending on the required magnification. Fixed optics are important, Todd said, because the field of view can be ground extremely flat. "If you project an image on a non-flat set of optics, it will look curved, barrelled or pincushioned," said Todd.
The more a part is magnified, the better the resolution, but the field of view will be smaller. For example, in a 14-inch screen diameter with a 10X lens, the largest section of the part that can be seen at one time measures 1.4 inches.
Because of this small viewing area, parts that are longer than the field of view will have to travel on the stage along the X and Y axes. Some comparator systems have a manual worktable that is moved by handcrank. Motorized worktables, which are often run by software or joystick, are used for both repetitive and automated inspections. Helix motion, accomplished by pivoting the worktable on the Y axis, is used for thread measurement, as it allows a true profile of the thread form to be imaged.
"The more travel you can get for your dollar, the better it is for somebody who is purchasing a machine because it allows a wider range of parts to be inspected," said Iavelli. "Often, users are only interested in features that are in a small area, but someArial the same feature repeats over a longer distance. In that case, the performance of the stage is critical."
It is important to understand the comparator's sensitivity so that a company does not mistakenly exceed allowed part tolerances, said Metzgar. Optical magnification and part travel are two areas that can affect measuring accuracy. When defining the sensitivity, the magnification can vary, but the scale or encoder that tracks travel, is fixed. A comparator might have an encoder or stage scale that is accurate to within 1/10 of a micron, or it might go all the way to 2 microns, said Metzgar. "It doesn't matter what measurement is made, it is going to have that error potential. The scale or encoder tracks the movement and the system is only as accurate as the scale or rotary encoder. You have to have a high accuracy scale on the stage or a high accuracy rotary encoder to get a highly accurate measurement of that part," said Metzgar.
For automated uses, an important feature is an edge-detection system. A sensor is mounted on the screen and it recognizes contrasts between the dark images of a part and the light background of the comparator screen. As the cross hairs pass from light to dark, an audible signal is generated. This eliminates the possibility of human error in detecting the edge. (For more on uncertainty factors caused by cross hairs and other common mistakes, see Quality's Web site at http://qualitymag.com.)
"Edge sensing is binary. It recognizes light and dark," said Kenneth Parlee, vision systems manager for Mitutoyo America Corp. (Aurora, IL). "In addition to an audible signal, there is a visual acknowledgement. In a loud shop environment, the easiest way to know when it has passed is to watch the digital readout system."
The electronic digital readout is another important feature on many comparators to display measurements. DROs have capabilities ranging from simple display of X-Y coordinates to complete, programmable, geometric processing with a computer-numerical-control (CNC) worktable. The level of DRO chosen depends on the level of computational power needed.
Using a DRO requires entering X-Y coordinate points into a function, which then returns the calculated geometry and associated values. Typical measurement routines include angle, radius, diameter, intersection, and included angle. Higher-level digital systems offer form computation, such as straightness and circularity, as well as true position, concentricity and profile.
Another feature to look for is measurement software that can perform automated parts inspection, statistical-process-control analyses on groups of parts, or exportation of data to form-analysis and other software packages.
With the bells and whistles that are now available, some manufacturers may have a difficult time deciding on the appropriate comparator for their applications. Comparator manufacturers all say that to make the best decision requires determining what it will be used for--the size, weight and shape of the parts, for example--and what it may be used for in the future. In some cases, all those bells and whistles may not be needed.
"Some people want fast computers and vision equipment to make measurements, but sometimes the optical comparator with an overlay chart is the most accurate way to measure those parts," said Metzgar. "You put the parts on, and boom, it is either go or no go."