Quality Magazine

Quality 101: Optical Comparator Basics

June 1, 2005
By magnifying the image, optical comparators prove to be a powerful inspection tool

Because of the lens and mirror combination, most images projected on the screen are the image with right hand left and left hand right. Profile projectors that can project the corrected image have a pentagon prism to correct the image into the right orientation. This is a costly proposition, making projectors with the corrected image difficult to find.

Common to most projectors, when the workpiece is moved to the left by the stage, the image will move in the opposite direction on the screen. Some operators are unaware of this but quickly learn to reverse stage controls.

Source: Mitutoyo America Corp.

Determining the exact edge has always been the key source of error in the past with optical comparators, but the automatic edge sensor has reduced that ambiguity in the measurement. Regardless of the moving direction of the image, an edge sensor can detect the edge as long as it produces a contrast on the screen. With this device attached, there is no parallax error or personal bias from one operator to another. Source: Mitutoyo America Corp.

The profile projector, or optical comparator, has been a powerful tool for inspectors since it was first produced by Baush & Lomb in Rochester, NY. What it did was simple: It magnified the image. It was a simple "shadow graph," a name still used by many machinists.

Of the lenses available, 10X and 20X lenses most often are used because of their brightness and details projected on the screen. The 31.25X and 62.5X lenses are for inch systems only so the image can be measured using a steel rule with a fraction scale of 16th and 32nd. The reason is because 1⁄16 = 0.0625 inch, or 62.5X and 1⁄32 = 0.03125 inch, or 31.25X. This practice is limited to the United States and limited to old overlays.

To measure the angle, bring the staggered or hairline—depending on personal preference—on one edge and move the protractor until the line matches up with the other. Then read the angle scale. Source: Mitutoyo America Corp.

How to measure angles

Among the methods known for measuring angles, the easiest and most often-used method to use is a profile projector. By virtue of magnification, it becomes easier to measure the angles projected on the screen to an accuracy of few minutes of one degree. 10X is the most is the most widely used magnification. When a 20X or 50X lens is mounted and, as the magnification increases, the screen brightness decreases proportionally. For a 50X or 100X lens, the projector may need to be covered to eliminate light reflection for a clear view. Under higher magnification, the edge may be harder to define. For higher magnifications-200 or 1,000X or more-a Toolmakers' microscope or vision system with a CCD camera might be the answer.

2-D software for profile projector

The image projected on the screen remains two-dimensional; the geometric software for profile projectors is much simpler for this reason than the 3-D counterpart. Software for profile projectors can be packed into one panel with icons. Individual points need to be registered on the memory chip.

After a point is memorized, the resident X-Y coordinate system takes over. A line can be drawn by entering two points, and another line can be entered by the same method. Having two lines memorized in the system, the included angle can be calculated instantly. It may be difficult to determine the correct entry point. It may not be easy to define angles at times because the edges projected no longer are straight under high magnifications.

Circles entered by multiple points are calculated on the basis of "best-fit" or least square circle. Out-of roundness may be found in the variation of radii.

This ready-made overlay measures 60-degree threads ±5 degrees. Straight horizontal lines are the pitch lines. Pitch diameters can be measured by checking one side with the overlay and then flipping it over to the other side to measure the distance between the two opposing lines. Source: Mitutoyo America Corp.

Power of the overlay chart

To quickly find out if a part is round enough, the profile projector with a radius chart clipped on the screen surface and 10X magnification will provide an immediate answer. The operator will read the chart or move the stage forward to read the gap.

A radius chart consists of concentric circles with dimensions for 5X, 10X, 20X and 50X indicated on the chart. The chart is self-calibrating. Place a certified gage block on the stage and move it. Do this in two directions. The correct way to calibrate the screen with a master glass scale on the stage, magnify it on the screen and measure the image projected by another glass reading scale. If an overlay is not used or magnified, the image on the screen is never measured. In this case, the magnified accuracy should not be a problem. The X-Y stage movements must be checked periodically.

A contour is shown under 10X magnification. A common application for optical comparators is to measure radii. Source: Mitutoyo America Corp.

How to measure radii

A typical application for the profile projectors is to measure radii and angles as long as they are external features. Magnifying the feature with a 10X or 20X power is the best choice. If larger power is needed, the next option is a 50X lens. Larger images projected on the screen reveal more details but with some rough edges. The image gets darker as the magnification increases.

A combination chart has radius, protractor-or angle-and grid patterns in a single overlay chart.

When radii are found on the outside, the measurement is simple and straightforward. When they are placed on the inside, they may pose a problem.

Suggestions are limited for checking internal radii, but solutions include dissecting the part in half and measuring the cross-section using a profile projector, or using a contour checking machine with its outstretched stylus for this type of application. The instrument generally is known as a contracer for contour tracer. It can magnify internal contours up to 200 times.

Nobuo Suga is the education advisor at Mitutoyo America Corp. (Aurora, IL). He can be reached at (630) 723-3621 or nobuo.suga@mitutoyo.com.