Optical comparators measure more than simple dimensionsby Tad A. Davis. The old adage "seeing is believing". Because these measurement tools display a magnified image of a part, a tremendous amount of information about that part can be gathered in a.
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Optical comparators, for those unfamiliar with them, are inspection machines that project magnified images of parts onto a glass screen using. D measurements. Dating back as far as calipers and micrometers, optical comparators have been used for more. Originating from static overhead projectors. Comparator advantages Optical comparators. Length and width measurements of the part shown above, for example, can be quickly obtained from two separate measurements by using a. These superficial measurements, however, might not reveal burrs, scratches, indentations or undesirable chamfers.
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Such imperfections are best detected on a comparator. In addition, a. comparator's screen can be simultaneously viewed by more than one person and provide a medium for discussion, just as a white board might facilitate a conference.
Another advantage of. D space. Unlike micrometers and calipers, which measure one dimension at a time, comparators measure length and width simultaneously. To do this, the. operator lines up the lower left- hand corner of the image with the screen centerline to establish a zero point, as illustrated above, and then checks the upper right- hand corner to get a. The straight- line distance from corner to corner can be obtained with a single keystroke.
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Measuring Length and Width. In addition, constructed points, gage points or gage lines that appear on part drawings can be established quickly on optical comparators, making relative measurements from.
This technique is illustrated below. The right side of the part is aligned with the vertical centerline, and a zero position is set.
The part. is moved to the right by the nominal depth given on the drawing. A diameter is then measured at this depth by moving the part vertically and measuring points on each angled surface."Points in Space" Measurement. Optical comparators are among the easiest measurement instruments to use. In less. than two hours, users with only a minimal amount of gaging experience can make accurate measurements using these devices. Because a comparator displays a part's.
D image on- screen, the image can be easily associated with the part's 2- D CAD drawing. This simplifies the process of developing measurement procedures for parts. For example, using a sine plate and a height gage to measure an angle might require a. Optical comparators are noncontact gages, another key advantage. Nothing but light touches a part during measurement, which means that delicate parts won't change. Noncontact gages also eliminate the "feel factor" and resulting human error of hand gages.
Contact measuring methods. Also, the size and location. A tiny radius of 0. When. magnified on a comparator with a 1. X lens, however, this radius would be 1 in.
Cost savings Optical comparators save time. Ease- of- use factors and ergonomic designs reduce the. And because comparators generally allow parts to be held in one orientation for each. D view, costs associated with parts handling and setup times are significantly reduced. Custom hard gages are subject to wear and need frequent recertification, which takes. Additionally, high- volume production can require multiple sets of fixed gages, increasing costs further.
The inflexibility of these fixed gages means that. Optical comparators, on the other hand, are general- purpose. Dimensioning techniques designed to give more leeway to parts in relation to their true functional purpose, such as profile tolerancing and true- position tolerancing with. For example, an arc by itself might be out of. The optical comparator is ideal for such dimensioning.
Types of measurements. Measurement by comparison. At first, the only way to measure with an optical comparator was by comparison- -hence its name. Part images could be compared to. For example, a measurement of 1/3. X lens. Eventually, these measuring tools were incorporated into precise glass- overlay screens, commonly called "chart gages." The most common chart gage is the toolroom chart. It. can measure angles, radii, lengths and widths.
One of the most effective ways to measure small radii- -an inherently difficult measurement to make on any inspection. Albeit a subjective measurement, this technique can provide meaningful and reproducible results in a short time. Toolroom Chart Standard chart gages, such as toolroom charts or custom chart gages with minimum and maximum tolerance zones, can be used as go/no- go gages for quick inspection, as illustrated on. In fact, using chart gages this way on an optical comparator is still one of the fastest and most cost- effective methods of measuring profile dimensions.
Nevertheless, dedicated charts are being replaced by electronically generated chart gages. During the past few years, these have evolved to the point that part features are automatically. In addition, a software package capable of. CAD file by rotating and translating the data set can be used offline for analysis.
The advantages of this "soft". And because the "charts" are stored on a computer's hard disk rather than in a cabinet, there's no risk of wear or breakage.
In addition to accurate chart gages, measuring by comparison requires accurate, distortion- free images across the full screen. High- quality optics and illumination. Using Chart Gages for Go/No- Go Gaging Measurement by screen rotation. Another early measurement technique still. Similar to measuring with chart gages, measuring angles is still quick, simple and highly effective.
Vernier scales provide angle measurements when rotating the screen ring. In recent years, digital rotary encoders have eliminated the need for vernier scales, making angle. Angles are digitally displayed in 0. The screen ring is rotated (as shown on page 3. The angle between the two edges is digitally displayed in the LCD window.
Screen- ring calibrations can minimize thermal effects and ensure accurate angle readings. These calibrations can be performed in seconds by rotating the screen 3. Measurement by Screen Rotation Measurement by motion. Before moving worktables were incorporated into optical.
Eventually, micrometer heads. Comparators now use glass scales integrated into a geometric processor with a digital readout for making measurements. There are two main advantages to measuring by motion. First, because part features are measured at the screen centerline, the need for a large screen and corresponding. The more compact systems used today take up less floor space. Second, comparators can operate in CNC mode when worktables are combined.
This significantly increases productivity and reduces operator subjectivity. In order to measure accurately using worktable motion, optical stability and mechanical accuracy are paramount. In addition, when measuring in CNC mode, edge. There are many tools and techniques used to calibrate the mechanical components of optical comparators to ensure accurate measurements when. The frequency of recalibrations required is a function of the system's mechanical stability and the working environment in which the machine is. High- quality systems are typically recalibrated every six months.
Types of optical systems. The simplest optical design used in comparators is appropriately called the "simple optics" design. It incorporates a light source, a magnification lens, a main reflecting.
Machines with this design display images that are both upside- down and reversed. A second setup is called the "corrected optics" design. This system uses two internal mirrors to flip the image so that it's displayed right- side- up, but it's reversed on the. On systems with simple- optics and corrected- optics designs, working distance (i. More sophisticated machines incorporate a relay- lens system, which provides a constant working distance at all magnifications. There is no tradeoff.
A third type of optical design is the "fully corrected system," which displays images. The convenience of seeing parts on- screen in the same orientation as they are seen on the worktable makes the machine easier to use. A useful addition to these optical designs found in many modern comparators is. A small opening, similar to a camera's shutter opening and called the "telecentric stop," is placed inside the optical path to block light rays that aren't parallel. This design increases depth of field, which in turn ensures magnification consistency across a range of focal settings. Thus, the size of the image. This reduces measurement variability from one inspector to another.
Telecentric systems provide a flat field and a. D parts in relation to their 2- D drawings. Types of illumination.
Illumination used to project a part's shadow is referred to as "direct projection" or "profile" illumination. Illumination sources include filament types, such as. Mercury- arc sources typically are brighter and last longer than filament lamps. Illumination systems incorporate collimator lenses that collect light from the source and project it onto the part. By projecting parallel light rays across the part, collimator.
An illustration on page 3. Collimated- vs. Diverging- Light Sources. Surface illumination is used to inspect dimensions that can't be viewed in profile. There are two types of.
Square- on illumination typically works better for measuring flat surfaces or blind holes, but oblique illumination is better for measuring angled. Fully corrected telecentric systems usually use square- on sources, while simple- optics and corrected- optics systems use oblique lighting. Types of edge detection Edge detection on optical comparators reduces operator subjectivity of measurements. External types of edge detection use a wand with an imbedded photo sensor positioned directly. Higher- end systems incorporate internal edge- detection hardware with multiple sensors.