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The measurement of surface roughness is not as straightforward as most other engineering measurements, such as length or angle or hardness; and in order to get good accuracy, expensive instruments are needed. These are often unsuitable for workshop use, where roughness of machined surfaces can usually be assessed conveniently and with sufficient reliability using visual and tactile (fingernail) comparison with a standard set of surfaces machined in the same way.
Such sets of surfaces are called roughness comparison specimens. They are simple to use, inexpensive, and hence widely used in many different applications, such as:
- checking roughness of turned, milled, ground, reamed, polished or lapped parts during machining
- checking the roughness of spark-eroded surfaces as described in the VDI 3400 Guideline
- checking the roughness of steel surfaces which have been prepared for painting by blast-cleaning, as outlined in ISO 8503
- estimating surface roughness prior to accurate measurement by a stylus instrument, as recommended in BS EN ISO 4288, eg.
- checking the seriousness of damage to parts caused by nicks, dents and scratches
- checking reconditioned ship propeller blades
The range of comparators ‘100’ is for intensive use in the workshop. Each block includes patches finished or machined following the procedure indicated on the surrounding label. The value of the roughness in Ra complies with ISO 2632 and BS 2634.
Standards should be used for a visual comparison and are not intended to serve as a reference surface to calibrate roughness gauges. For this reason, it is not necessary to establish a certificate of inspection or to perform a periodic calibration.