Optical comparators come in an often bewildering variety of shapes and sizes. Benchtop, floor model, in-line, side-screen, side-table. How do you know what type of system to get, and which options to specify? As is the case with most capital equipment purchases, the answer is to know what your needs are today and to plan for changing needs tomorrow.
We have arranged this selection guide to help you through this decision making process. If you have any questions, feel free to email us for assistance.
How large of a viewing screen should you get? Depending on manufacturer, you will find systems with screen diameters of 10″, 12″, 14″, 16″, 20″, 24″, 30″, and even larger. How big is big enough?
The answer is based on the type of measurements you plan to make.
Measurement by comparison (with overlay charts or screen templates) can use the entire viewing screen. A larger screen means you can see more of the part at any one time, perhaps measuring many features at once with the same chart. Measurement by motion (with visual alignment or edge detection) uses the movement of the worktable assembly and screen rotation to measure the part. How large of a part you can measure is based solely on the overall travel of the worktable.
If you need to use overlay charts, for thread or form measurement for example, then a larger screen may be needed depending on the size of the parts. The table below shows the field-of-view (how much can be seen at one time) for some common screen sizes and magnification levels. For other combinations, simply divide the screen size by the lens magnification.
Field Of View
Benchtop or Floor Model?
Whether you choose a benchtop or floor model comparator may ultimately depend on the size and weight of the parts to be measured. Other factors to consider include space (floor models typically take up more space than benchtop systems), required screen size (benchtop systems rarely have screens larger than 14″ or 16″), and cost (floor models are generally more expensive).
What kind of parts are being measured?
Benchtop models are smaller, lighter, and have lower load carrying capacity and measuring range than most floor model systems. Consequently, one usually finds benchtop comparators employed for measurement of small, lightweight parts such as plastic moldings, stampings, lathe work, etc.
Floor models, on the other hand, offer substantial load capacity and large travel, and are often used to measure large machined castings, dies, and heavy tooling. The following table gives some idea of the relative capacities of CCP benchtop and floor model comparators.
|Load||45 lb||45 lb||45 lb||60 lb||550 lb||550 lb||550 lb|
|X Travel||12″||12″ / 15″||12″||15″ / 18″||15″ / 18″||15″ / 18″||15″ / 18″|
How much room do you have?
The amount of floor space needed is an often overlooked part of the selection process. Although benchtop systems are fairly compact, some floor model machines are quite large. CCP optical comparators, regardless of configuration, use the minimum space possible – comparisons to competitive systems can be quite enlightening. The following table shows the floor space requirement for CCP systems.
|Size L x W||36″ x 30″||40″ x 32″||36″ x 30″||38″ x 32″||67″ x 38″||71″ x 42″||88″ x 72″|
|Area (sq. ft.)||7.5||8.9||7.5||8.4||17.7||20.7||44|
The optics are the heart of any comparator. When you think about it, the lenses in a comparator are really gages. Why? Because it is the image they form that is measured, not the part itself. And to make sure the image is accurate, optical quality (in design and manufacture) is critical.
You will find several different types of optics offered on optical comparators. Regardless of the type, however, the optics must be free of distortion (magnification change across the image), produce a flat field (image over the entire screen is in sharp focus), and have excellent resolution (sharpness of the image). Let us take a look at each type and discuss the relative merits of each.
Simple And Corrected Optics
The least expensive systems often employ simple optics. In this case, a projection lens magnifies the object and the image is directed to the screen by one or two mirrors. If one mirror is used, the image is inverted (upside down) and reversed (left to right). With two mirrors, the image is correct top to bottom, but still reversed. This second system is often referred to as corrected optics.
In a simple lens system, changing magnification involves removing a lens from the front of the machine and replacing it with another. Some systems make this more convenient by providing a multiple lens slide or turret. In most cases, however, working clearance (the space between the part undergoing inspection and the optics) decreases as the magnification is increased. For three dimensional parts, this means that inspection at higher magnifications (50x – 100x) is generally limited to flat parts, prismatic parts with little thickness, or turned parts of small diameter.
All CCP comparators provide a constant working distance at all available magnifications.
Relay Lens And Fully Corrected Optics
Relay lens systems use additional optics to form an intermediate image (usually at 1x magnification) which is in turn magnified by the projection lens. The final image is fully corrected, or in other words, erect and unreversed.
Another important benefit afforded by fully corrected optics is a constant working clearance, regardless of magnification. There is no limitation on part size or thickness due to the magnification selected.
Due to the extra optics in relay lens systems, the magnification lenses are usually arranged in an internal turret, often motorized for convenience.
CCP’s CC-20, CC-30, and CC-30S feature relay lenses and fully corrected images.
This mouthful of a word may be one of the least understood terms of optical inspection. We will leave an explanation of the details behind this type of optical design to the physicists, and talk about the benefits it provides.
Everyone knows that the closer an object is to you, the larger it appears. This is one of the fundamental principles of optics. The same is true for any optical system, whether your eye, a camera, or an optical comparator. But this can cause an error when measuring a three dimensional part, or if the image is simply a little bit out of focus. Why? Because adjusting the focus on a non-telecentric, simple optics system changes the distance between the part and the optics, and that changes the magnification. The effect is not that large (typically several thousandths of an inch measured at the part), but in today’s world of ever shrinking tolerances, you need to be as precise as you can.
Telecentric relay lens optics were developed by OGP and Eastman Kodak in 1945 to alleviate this problem. Here is a good example that shows the difference between non-telecentric and telecentric optics:
A simple part, with a step and two through holes machined into it, will appear “in perspective” to a non-telecentric system, as shown below (the effect is exaggerated for clarity). The machined step on the top (which is farther away from the optics) looks smaller. A telecentric system will create an image free from this effect, with no measurable difference in magnification.
Telecentric Lens Benefit
This type of optical system benefits the optical comparator user in the following ways:
- Magnification accuracy is not affected by part geometry or configuration.
- Magnification stays the same regardless of different operators focusing the instrument in slightly different places.
- When coupled with relay lenses, coaxial or through-the-lens surface illumination is possible.
- Coaxial surface illumination provides brighter and more even illumination.
- Depth of field (the distance you can move the part along the focus axis before the image appears to go out of focus) is increased.
All CCP optical comparators feature true telecentric optical systems. Some manufacturers claim “telecentric illumination” but you owe it to yourself to verify the benefits of such a system. Only an optical system designed from the start as telecentric delivers the advantages stated above.
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