How to Select the Right Comparator
- Horizontal light path instruments have a beam of light traveling horizontal across a stage. This type of machine is ideal for large heavy parts and shafts to be held on fixtures such as V-Blocks or between Centers. Applications include machined components, castings, transmission shafts, and thread form measurement.
- Vertical light path instruments have a beam of light that travels vertically. Parts that are being inspected and/or measured are placed on a plate of glass. The glass plate is on the systems XY stage, which the beam travels though. Vertical systems are ideal for flat parts such as stamped parts, gaskets, electronics, and O-rings.
Step 2 - What screen size and stage size best suits your application?
Typical screen sizes range from 14" to 32". Before choosing a screen size you should determine the exact features of your application that need to be measured at one time. It may not be possible or necessary to view the entire part when measuring. Calculations can be made by dividing the screen diameter by the lens magnification. For Example, using a 10X lens on a 16" optical comparator would enable viewing 1.6" of the part on the screen (16"/10 = 1.6"). It is considered good practice by many to keep within one inch of the screen margin when viewing an image with an overlay. Ensure the stage size, travel, and weight capacity will accommodate all of the parts that are intended to be measured or inspected. Generally comparators with screen sizes of 16" or smaller are bench-top units that have weight capacities of up to 150 pounds.
Step 3 - What lens/lenses does your application require?
A basic rule of thumb is that a typical attentive operator can repeatedly discriminate .004" on a comparator screen. Dividing the "discernible resolution" by the lens magnification determines the minimum resolution attainable for each lens. The chart below will help you determine the proper lens needed with regards to the tolerances required.
|LENS MAGNIFICATION||DISCERNIBLE RESOLUTION|
|5X||.0008" (0.020 mm)|
|10X||.0004" (0.010 mm)|
|20X||.0002" (0.005 mm)|
|25X||.00016" (0.004 mm)|
|31.25X||.0001" (0.003 mm)|
|50X||.00008" (0.002 mm)|
Step 4 - What type of readout and software should you invest in or will you use overlays?
If you are planning to use overlays it is good practice to keep within one inch of the screen margin when viewing an image. If basic measurements like positions and distances are required, select a basic XY digital readout. If your application demands the measurement of circles, angles, and parametric distances then a readout with geometric capability is required. CNC capable readouts should be considered if your inspection and measurement are repetitive. Another worthy consideration is optical edge detection. Edge detection eliminates operation subjectivity and improves the overall repeatability and accuracy in which measurements are made.
Step 5 - What options or tooling will be required?
In order to ensure measurements are repeatable and accurate your work-piece should be held securely to the comparator table. Be sure to explore the variety of fixtures available to find one that presents the features of your part according to the light path that best suits you application.