Using Optical Comparators In Precision Machining

The optical comparator, or shadow graph, as it is sometimes called, is one of the most useful precision measurement tools in the shop. With proper use, it can be highly accurate and save a tremendous amount of time.

This precision measurement tool is quite capable of accuracies of .0002 in./.005 mm. Time spent practicing with known gages is time well spent. There are some subtle aspects of the optical comparator that require careful attention, but not any more than any other precision tool.

A classic mistake is to have the workpiece out of square with the table of the comparator. This gives an obviously false reading, yet it happens more often than you might imagine. This is why it is necessary to spend some time practicing with gages.

Injection mold makers, tool and die makers and precision machining operations all use the comparator on a daily basis. It can be used to check angles, radii, distances between centers, intersection points, thread depths, thread pitch and many more details.

Some of these details, such as thread pitch, are actually quite difficult to inspect any other way. This is especially true in the case of tiny threads, where it is impractical to physically hold wires and a micrometer to measure.

Time proven design

The basic design has not changed since the introduction in 1920. What has changed is improved optics, user friendly interfaces, the addition of digital readouts, and now the use of computers to process information.

In the past these precision measurement tools were large and had a huge footprint. The newer generation of machines can easily fit on a table top and do everything that the huge tool of the past could do.

The cost has also come down considerably. Because the basic design and technology is unchanged, many competing companies have emerged with improvements and refinements, without increasing the cost dramatically.

Now operators can compare their results with the CAD file and download the information. This is a great improvement over the machines of the past.

Optical comparators vs. video measuring systems

In many respects, the shadow graph is being replaced by video measuring systems. This is due primarily because of the following features the video system has over the comparator:

• Fully corrected (i.e., not inverted) image

• Image processing capabilities

• Variable zoom lens, offering multiple magnification levels

• Variable illumination with a cool fiber-optic light source, plus three light sources

• Color and profile images

• Capability to capture images of work pieces or part features

• Third-axis measurement

• Capability to archive and document measurements

Nevertheless, many companies will continue to use the tried and proven comparator because people are used to it, it works and it is easy to understand.

The choice between an optical comparator and a video measuring system is not an easy one. Depending on your needs and budget, either one could do a superb job. One option is to purchase a used optical comparator.

Leading manufacturers

• Scherr-Tumico

• Mitutoyo

• OGP Optical Gaging Products

• Nikon

• Dorsey Metrology International

• Micro-Vu

• LS Starrett