Flat-surface wringing and contact error variability

Abstract

Two highly finished flat surfaces brought into intimate contact by a sliding action in the presence of a trace of liquid develop an adhesive bond of considerable strength in directions both parallel and normal to the interface so formed. This phenomenon has practical importance in length metrology where gauge blocks of hardened steel (often called `slip gauges') are used as working length standards. The adhesive `wringing' bond between their flat smooth surfaces is used to build up a combination or `stack' of these blocks so as to form a standard of any required length by the addition of the individual block lengths. This technique enables the combination to be handled as a single unit without coming apart but it is important to know how consistent this addition is and to quantify the dimensional effect of the `contact error' so introduced.This paper describes a series of experimental investigations which may lead to a better understanding of the physical mechanism of the wringing bond. The contact error has been estimated anew using a surface profile fitting technique made possible by the availability of high-magnification rectilinear traces of the surfaces. From geometrical considerations the minimum value that the contact error can take for two similar surfaces lies between two and four times the centre line average roughness index. The contact error variability with different liquid media, with different gauge block materials and with repeated number of wringings has been studied. The wringing forces increase and the contact error diminishes with time elapsed after wringing. High-magnification rectilinear records of the surfaces show that the surface finish deteriorates with repeated wringing by amounts which are different for different materials but which are enough to affect the repeatability of a wrung-up combination of gauge blocks. A method of contact error comparison by interferometry has been developed and this gives results which are in good agreement with the limits set by surface texture observed when fitting the high-magnification surface profiles. A tentative qualitative theory of wringing, based on the experimental work, is advanced.