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A profile on optical comparators.

Optical comparators have been used in the machine tool industry for more than 60 years. Commonly referred to as "shadow graphs" by early users, optical comparators have also come to be known as "optical projectors" or "contour projectors" by some manufacturers and users. In operation, comparators are used to project the enlarged shadow of an object's profile (or contour) onto a ground-glass screen where it can be compared to a template, or gaged either directly or indirectly.

Some comparators offer surface illumination through direct or oblique lighting, or through the use of fiber optics. On comparators so equipped, a detailed image of the part is projected onto the screen.

Today, optical comparators are used for checking simple and complex forms, contours, and shapes. The comparator provides a simple means for measuring angles, radii, concentricity, and parallelism, as well as linear dimensions. A particular advantage of the optical comparator is the ease with which a multiplicity of dimensions can be checked simultaneously, making it an excellent choice for special measurement applications that would be difficult to assess by any other means.

The part under inspection is usually placed in a staging or holding fixture, which is then placed onto the comparator's specimen table. The table is then adjusted in its X and Y axes until the part's shadow is contained by a template on the comparator's viewing screen. Some comparators offer a table that can also be moved in the Z axis, rotated, or tilted-all of which assist the user in moving the workpiece into position so that its silhouette will take the desired position on the screen. Tables may be either manually adjusted or motorized. Some motorized tables can be positioned through CNC programs.

Comparators use a variety of screen charts depending on the type of inspection being done. Grid and radius charts are common, as are charts for comparing fillets, thread forms, chamfers, and gear tooth patterns. Custom charts which replicate the profile of a part being compared are also available. With the wide use of CAD/CAM programs, many users produce their own comparison templates on acetate.

Today, many comparators are equipped with image-edge sensors that are either attached to the viewing screen or integral with the screen and provide a high degree of accuracy and repeatability in measurement applications. When an edge is detected, the edge-sensing system provides an audible beep and display on a digital readout system.

Tabletop comparators

The Leitz TP 301 optical comparator offers high stability, mechanical and optical precision, with ease-of-use to provide fast, accurate measurements on a tabletop. It is adaptable to a wide range of component geometries and measuring tasks, profile testing, surface measurement, and production inspection.

The unit's most commonly used magnification lenses (10X, 20X, and 50X) are permanently mounted on a revolving nosepiece for quick changes. The 5X and 100X lenses can be changed separately. A wide variety of XY coordinate systems are available with up to an 8" x 4" measuring range.

Several types of illumination are provided to accommodate different types of inspection pieces. Transmitted light illumination, continuously variable, is used for measurement and inspection with the silhouette method, whereas brightfield incident light illumination produces a perfect image of the surfaces to be measured on the screen. Darkfield incident light illumination is advantageous for showing up irregularities as well as poorly reflecting rough surfaces and plastics.

Leitz Ultra-Precision Group, Brown & Sharpe Manufacturing Co, North Kingstown, RI, circle 210.

Heavy-duty projectors

The LS Starrett Co's HC 800 optical measuring projector is said to provide the largest standard workstage (20" horizontal and 10" vertical measuring travel) in the industry. The floor-standing unit has a large, 32" (800 mm) screen. Tungsten-halogen profile illumination (24 V, 100 W) provides bright, clear, contrasting images. On-axis surface illumination (24 V, 500 W) comes directly through the lens, eliminating the need for oblique lighting or fiber optics. It's digital rotary screen allows for direct angle display. Incremental or absolute screen position can be selected by the operator and the display can be switched between decimal, degrees, and degree-minutes, with resolution of 0.01 deg or 1 min of arc.

The HC 800 can be used with the full range of Starrett Quadra-Chek electronic readout systems. The capabilities of these systems range from basic XY displays to full geometric computations and fully automated systems that include edge detection capabilities.

Starrett Precision Optical Div, Farmington Hills, MI, circle 214.

Auto-zeroing comparators

The Trans-O-Limit B-4 electronic comparator from Edmunds Gages includes an auto-zeroing feature that eliminates time-consuming setting of meters. The universal B-4 makes comparative ID, OD, and length measurement, providing internal/external measurement with a resolution of 0.000 001". LCD digital display permits simple, at-a-glance operation.

Edmunds Gages, Farmington, CT, circle 235.

Half-micron precision comparators

Nikon Inc's Precision 12 vertical comparator enables users to measure precisely to a half micron. Engineered for specimens with dimensions to 6" x 2" x 3", the comparator can be used to measure X and Y axes of any specimen. When outfitted with a digital or dial micrometer, Zaxis measurement can also be taken. The system is optimally designed for examining specimens that can be placed on a flat surface, and its vertical orientation allows simpler fixturing for most specimens.

Some of the features offered by the system include a pre-centered, pre-aligned lamp socket, which enables users to measure or inspect both contours and surfaces without having to hand-adjust the bulb's alignment. Also, the pre-aligned lamp housing makes bulb changes fast and error-proof. The focus knob is conveniently located on the front of the instrument, and an easy-to-use, two-position condenser replaces the focusing or replaceable lens that some equipment offers.

A three-lens turret allows the use of 5X, 10X, 20X, 25X, 50X, 100X, and 200X lenses. All lenses come with built-in half-reflecting mirrors for surface illumination, so users will never have to replace broken mirrors.

Nikon Inc, Melville, NY, circle 221.

Profile projectors

Mitutoyo's vertical-beam profile projectors offer versatile measurement and inspection capabilities for the calibration department as well as the production floor. The PV series provide a clear, distortion-free image over the entire viewing screen. Once the image is focused on the screen, subsequent measurements require no focus adjustment.

The floor standing units are available with 5X, 10X, 20X, 50X, and 100X projection lenses and 14", 20", and 24" plain or protractor screens.

For precise measurement of the projected image, Mitutoyo offers high-linearity, cross roller bearing XY cross travel stages, and high accuracy micrometer heads with digital or LCD display. Other options include an image edge sensor to improve measuring accuracy and repeatability, and 2D data processors to facilitate trigonometric calculations.

Mitutoyo Mfg Co, MTI Corp, Paramus, NJ, circle 234.

Multi-plane and motorized comparators

Deltronic's 14" profile projector features a programmable, motorized, focus-axis stage movement option that enables accurate automatic measurement of multi-planed parts, which previously would have required refocusing before subsequent measurements could be taken. The programming is done by manually focusing the first piece. Thereafter, focusing is done automatically for electronic edge-sensing measurement.

The profile projector also offers a manual remote-control joystick, focus-axis and X-axis motors, digital readout, and surface illumination.

Deltronic, Santa Ana, CA, circle 212.

Budget-priced comparators

A new, American made optical comparator with high-quality construction and an economical price has been introduced by Swiss Precision Instruments Inc. The full-featured comparator carries a price of $2995, which the company states is thousands less than competitors' models.

The 14" comparator features a single long-life tungsten-halogen light source; surface illumination is provided through Duplex Fiber. A three-position rotary switch selects profile, surface, or combination surface and profile modes. A heavy-duty, welded, steel frame and ball-way construction is rated for loads to 100 1b. The unit is less than 33" tall and 22.5" wide.

The high-capacity 4" x 8" travel stage features a 4.5" x 19" work surface and a quick release for rapid table movement. The rotary, frosted-glass screen has 90-deg crosslines and a 360-deg protractor rim. Vernier readings are accurate to 5 min of arc.

A 10X lens is standard with 20X and 50X lenses available separately.

Swiss Precision Instruments Inc, Los Angeles, CA, circle 226.

Electronic edge sensing comparators

Gage Master Corp's 24" optical comparator offers an 8" x 13" coordinate table and CNC table drive. Electronic edge sensing is accomplished with internally mounted sensors and the full CNC driver stage offers joystick controls.

Other options for Series Ninety are: fiberoptic surface illumination, choice of geometric computing digital displays and workholding fixtures such as matched centers, riser blocks, heavy-duty vise stage, and glass workstage.

Gage Master Corp, Rancho California, CA, circle 223.

Small comparators

The MP6B projector provides quick inspection and accurate measurement of opaque objects such as small metal parts and assemblies, electronic components, and similar materials. Two built-in adjustable fiberoptic light guides produce high-intensity illumination that produces bright images on the circular 6" crossline viewing screen.

To eliminate the clutter of any external components from the inspection area, the fan cooled, 100 W, halogen light source and transformer are built into the projector. The 5X, 10X, 20X, 50X, and 100X projection lenses provide a wide range of magnifications, working distances, and fields of view.

Unitron Inc, Bohemia, NY, circle 233.

Toolmaker's microscope

Olympus Corp, Precision Instrument Div's STM5-UM microscope uses built-in linear optical scales and precision stages for measurements of X, Y, and Z dimensions down to 10 [Mu] (or 0.1 [Mu]m).

A wide range of magnifications, from 50X to 800X, and numerous stage and illumination options adapt measuring conditions to workpiece requirements.

A digital readout, either 10 or 20 micro-inch and English or metric display options, is mounted at eye level for quick confirmation of measuring results.

Stages from 2" x 2" to 2" x 6" are available and adaptation to varying workpiece heights is facilitated by the generously dimensioned rack and pinion coarse focus.

Olympus Corp, Precision Instrument Div, Lake Success, NY, circle 236.
COPYRIGHT 1992 Nelson Publishing
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:Jul 1, 1992
Words:1647
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