Method of indicating the relative roughness of a surface

#This invention relates generally to a method of indicating the relative roughness of a surface. More particularly, though not exclusively, the invention relates to a method of indicating the relative roughness of a surface that has been smoothed by machining, as by forming a plurality of substantially uniform and parallel grooves therein. The invention is particularly useful for indicating the relative roughness of the finished surfaces of projectile casings because excessive roughness can interfere with the proper operation of the projectiles.

#The standards of relative roughness, as used herein, are those of the Standard Surface Finish Blocks (S.S.F.B.) standardized by the National Bureau of Standards. The Standard Surface Finish Blocks are designated by numbers, as for example, 32, 64, 125, 250, and 500. The smaller the S.S.F.B. number is, the smoother is the surface it designates.

#It has been proposed to indicate the relative roughness of a finished surface by magnifying the surface and visually comparing it with reference surfaces, but such a method is obviously slow and subject to human error. Instruments for measuring the roughness of commercial finishes are usually of the tracer type in which a finely pointed diamond stylus is drawn across the finished surface. The motion of the stylus is highly magnified and recorded by electrical means. While satisfactory for certain materials, the aforementioned instruments may mar the very smooth finish surfaces they are measuring.

#The novel features of the invention, as well as the invention itself, both as to its organization and its method of operation, will be understood form the accompanying drawings, taken in connection with the accompanying description, in which like reference characters refer to like parts, and in which:

#FIG. 1 is a perspective drawing, partly schematic and partly fragmentary, of a block of metal in the process of having the relative roughness of a finished surface thereof indicated;

#Fig. 2 is an enlarged fragmentary cross-sectional view of the block shown in FIG. 1, taken along the line 2-2 and viewed in the direction indicated by the arrows;

#Referring now to FIG. 1 of the drawing, there is shown an object 10, such as a block of brass, for example, the relative roughness of whose surface 12 it is desired to indicate. The surface 12 has been finished (smoothed) by machining, as by a milling machine, and consists of a plurality of substantially uniform parallel grooves 14, as shown in the enlarged cross-sectional view of the surface 12 in FIG. 2. While the overall surface 12 (ignoring the grooves 14 therein) is illustrated herein as a substantially planar surface, it may also be curved (with grooves therein) in accordance with the novel method.

#The object 10 is placed on a belt 16 that is adapted to be moved, by any suitable means (not shown) in a direction indicated by arrow 18, for the purpose hereinafter appearing. The movement of the object 10 is in a direction transverse to that of the grooves 14 at a substantially constant speed. Preferably, the object 10 should be moved in a direction that is substantially perpendicular to the direction of the parallel grooves 14.

#In accordance with the novel method, a light source 20 is disposed to direct an incident beam 22 of light substantially normal (perpendicular) to the finished surface 12. The light source 20 may be either of the laser or non-laser type, but a laser source of relatively low energy output is preferred because of safety precautions. The light source 20, for example, may be a continuous wave HeNe 5mW laser source, or the like.