Patent GB-549987-A (GB)
Title: Improvements in or relating to the measurement of surface roughness
IPC: G01B7/34
Filed: 1941-03-15
Abstract: 549,987. Electric measuring apparatus. KAPELLA, Ltd., and REASON, R. E. March 15, 1941, Nos. 3586/41 and 4898/42. [Class 40 (i)] [Also in Group XXXIX] In electric responsive apparatus for measuring small movements such as a gauge or a surface roughness indictor, and comprising a pair of electro-magnetic devices having magnetic cores linked with windings differentially responsive to motion of an armature controlled by a stylus, feeler or the like, the armature is controlled by one or more springs arranged and dimensioned so that the resultant of the mechanical and magnetic forces acting thereon is substantially constant throughout the working range, thus compensating the varying magnetic force in the armature and providing uniform working pressure on the stylus. The output of the apparatus is indicated or recorded as described in Specifications 539,271, 539,272, 539,273 and 539,274. The electromagnetic devices may be transformers, auto-transformers or chokes fed with A.C. or pulsating D.C. which is modulated by movements of the armature. In one arrangement, Fig. 1, for measuring surface roughness, the armature B carries at one end a stylus B2 for movement over the surface and is pivoted on a ligament spring B1 so that its other end moves towards and away from the poles of a pair of E- shaped cores A, A<SP>1</SP> carrying primary windings D, D<SP>1</SP> connected to parallel or series and secondary windings E, E<SP>1</SP> connected in series opposition. The cores A, A<SP>1</SP> may be separately adjustable to vary the gaps or as shown, both are carried by a frame H adjustable in the casing F by a screw F2. The balancing springs G, G<SP>1</SP> engaging the armature are also adjustable. The stylus B2 projects through a presser foot C<SP>1</SP> and the adjustment of the cores adjusts the datum line of the measurement whereby the full scale of the indicating or recording instrument or a part thereof may be caused to correspond with a part of the total possible range of movement of the stylus. The casing of the instrument is of high permeability material for shielding purposes. The ligament spring B<SP>1</SP> may constitute the spring for counterbalancing variations of force or may assist springs G, G<SP>1</SP>. In a modification, Figs. 3 to 5 (not shown), the end of the armature is secured to the centre of a transverse leaf spring anchored at its end to the frame H. Another modification, Figs. 6 and 7 (not shown), comprises substantially the gauge head shown in Fig. 11 of Specification 541,628 with an adjustable leaf spring for compensating the variations of magnetic force on the armature (movable core). In another gauge head shown in Fig. 8, the armature T is pivoted at one end on a ligament spring T<SP>1</SP> and carries at its other end an adjustable stud T<SP>2</SP> bearing against the centre of a transverse leaf spring X which compensates the magnetic force variations and is supported against four studs X<SP>2</SP> on a fixed support. The armture operates between a pair of electromagnetic devices V<SP>2</SP>, V<SP>3</SP> adjustably mounted in the casing U. The gauging member W sliding in tube U<SP>1</SP> is held against the armature by spring W<SP>3</SP>, the armature being provided with an adjustable wedge W<SP>2</SP> to engage the gauging member and being held down against that member by a spring W<SP>5</SP>.