Why use surface finish charts? Reason six

Different types of comparators

Just as different metal cutting processes have different ranges of average surface finish, there are also different comparator plates for different processes.

This is because, for example, edM cutting parts produce tiny "pits" on the endcut surface, while the abrasion method produces very thin, slightly arcing lines on the endcut surface. Even the exact same Ra value can look very different, depending on the cutting process used.

Therefore, it is important to make sure that you are looking at the comparator board to get the process right.

Limitations of comparators

In addition, the comparator board does not help when you achieve lower (smoother) surface finish requirements, such as 2 μin or 1 μin Ra. This is because unless they are at high magnification, the difference between 2 μin and 1 μin Ra (or less) will not be significant.

If your particular application has a critical range (for example, each part must be between 10 μin Ra and 25 μin Ra), a comparator board may also not be the best choice. In these cases, mechanical testing of the parts may be required to ensure that the surface finish is within specified limits.

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