GB646760A - Improvements relating to the optical measurement and/or calibration of tubular surfaces - Google Patents

Abstract

646,760. Optical gauging-apparatus. BURCH, C. R., WILLCOCKS, M. J., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. July 30, 1948, No. 20366. [Class 106 (ii)] A Michelson interferometer consisting essentially of a light source 1 with a collimating lens 2 to provide a beam of light of narrow angle which is split into two portions by halfsilvered mirror 3 placed at an angle to the beam, with a reference surface 5 and a specimen surface 4 and an eye-piece 6, is modified or adapted for the comparison of cylindrical, elliptical or polygonal forms by the addition of an auxiliary reflecting surface which is so shaped and arranged that the image of the specimen surface as seen by the instrument has nominally the same figure as that of a standard reference surface arranged on the other arm of the interferometer, whereby irregularities of the specimen surface are made visible as interference fringes. As shown in Fig. 2, a gap gauge having faces 7, 8, is positioned with the faces parallel to the optical axis of one arm of the instrument and an auxiliary reflecting surface in the form of a 90 degree prism 9 with reflecting faces 10, 11, is placed between the faces 7, 8, as shown. Light from the source 1 after reflection from the surfaces of the prism 9 strikes the surfaces 7, 8, normally, and is reflected back along the optical axis to the halfsilvered mirror. The combination of the surfaces 7 and 8 of the gauge and the reflecting surfaces 10, 11 of the prism 9 forms an apparently flat image giving rise to interference effects in known manner, the apparent position of the image being the point where the surfaces 10 and 11 of the prism, if extended, would intersect the surfaces 7 and 8 of the gauge. The arrangement shown in Fig. 3 indicates the method of testing a right cylindrical bore, for example a ring gauge. In this case the bore 13 is arranged with its axis in line with the instrument axis and a 90 degree cone of speculum metal 14 or any metal with a reflecting layer 15 is positioned as shown, the combination of the surface of the bore of the cylinder and the cone forming an apparently flat image as before for production of interference effects with the reference surface. In Fig. 4 the converse arrangement is shown with a rod 16 arranged coaxially with an auxiliary reflecting surface 18 in the form of hollow cone 17 with a central aperture to clear the rod 16. Polygonal bores or rods may be similarly dealt with and in some cases a cone of parabolic form may be employed for bores of large diameter. With the apparatus it is possible to inspect a bore and detect any departure from a truly circular shape by observation of the interference fringes. Furthermore, by substituting for the specimen surface a test-piece of known diameter it is possible to measure accurately the bore under test.