Roughness Comparison Specimens (Metrology)

11.11.
The roughness comparison specimens are used for qualitative assessment of the surface roughness. In this qualitative assessment of the roughness of the finished product is evaluated by visual or tactile comparison (i.e. lightly moving a finger nail across the lay of the roughness) with roughness comparison specimen of known roughness value finished by similar machining process. The roughness comparison specimens are produced in a variety of materials (steel having a hardness of about 400 HB) by direct application of the production process or by copying techniques and are available in various grades of roughness. The copying techniques consist of (a) electro-forming positive replicas of master surfaces and (b) making positive replicas of master surfaces in plastic materials by coating or otherwise and having th&feel and appearance of the natural machined metal surface. It is general practice to produce the specimen surfaces as replicas of master surfaces in a hard material like nickel alloy or metallised plastic.
The roughness comparison specimens are available either in flat form or segmented cylindrical form. The flat specimens have the size: length—25 mm, width—16 mm, thickness— 1.6 to 10 mm. The specimen is rigidly supported on a base to prevent it from being warped or distorted during use. The segmented cylindrical specimens will have either convex surface or concave surface having a surface radius of about 125 mm.
These specimens have also proved to be very good devices for training the personnel to judge the Ra value of the feel and appearance of the surfaces. While these are being used for direct comparison purposes, it is important to note that
(i) two surfaces being compared should resemble each other in material, shape and method of finish;
(ii) two surfaces must be closely alike in general character in tbe sense that the cutting technique and machine feeds used for making the specimen should resemble those used for the products;
(iii) surfaces being compared should be clean and dry,