Bal-tec™ Home Optical Flat Made of Metal
A large, flat reference datum, is of the greatest importance in many situations. A glass optical flat can provide the needed planeitude, but it is soft and friable with practically no wear or shock resistance. In addition there is no good way to mount a glass optical flat.
A rugged “metal optical flat”, commonly referred to as an “anvil” or “gage anvil”, is through hardened, wear resistant, and has very good long-term dimensionally stability. The best body shape for such a device is cylindrical. The simple, symmetrical, cylindric4al surface is the most economical form to lap and polish, to a very good flatness and surface quality. The 2.0 inch (50.8mm) diameter anvil is 1⅛” (28.575mm) thick, or tall. This metal optical flat is made of a vacuum melted, chrome alloy steel that is through hardened to 62 HRC. This material has a relatively low coefficient of thermal expansion of 6.4 microinches, per inch, per degree Fahrenheit.
For calculating the hertzine elastic deformation, when measuring parts with a small area of contact these anvils, have a Young’s Modulus of elasticity of 30,000,000 P.S.I. and a Poisson’s ratio of 0.26. This very rugged device has a one-quarter (0.250) inch [6.35mm] diameter threaded hole with twenty threads, per inch in the center of the bottom surface. This threaded hole is used to hold the optically flat anvil, rigidly in place against the mating part. This threaded hole matches, the threaded mounting holes, in most vibration isolated, optical tables. The precision lapped rear surface, of the anvil has a shallow counter bore in the center and the remaining surface is divided into two rings. This approach is used to promote sub-microinch (nanometric) location of the optically flat anvil against poor quality mating surfaces that may only be machined, or at best, ground. The standard flatness of the top working surface of these anvils is, dead flat, to one light band (10 microinches) [0.3 micrometers] convex. The surface quality is one microinch [0.03 micrometers] maximum Ra.
Higher quality than this can be provided on special order. The top and bottom flat surfaces are lapped parallel within twenty microinches (one half micrometer) [two light bands] standard, and a higher quality can be provided on special order. There are two standard versions of the two inch (50.8mm) diameter optically flat, metal anvil, available. All features of the two versions are identical, except that the top surface of Part #AN-2-P is a plane flat surface and the Part #AN-2-S has several annular serrations that, promote microinch [sub-micrometer] seating when the matching parts are not perfectly flat. This high quality location is the result of the serrations breaking up any air or liquid film between the two flat surfaces.