This article first appeared in Practical Engineering 1940 Vol1 No2. The information contained within the article is therefore correct as of 1940. The article updates the reader with information regarding new measurement techniques that are applicable in Manufacturing and Engineering.
The "Oehmichen" caliper is a high-precision measuring device designed for the continuous automatic measurement of one dimension (thickness, width or diameter) of any drawn material (strip, film or wire) which is moving in the direction of its length. The special features of this instrument open up a wide field of industrial applications. These are probably best illustrated by a brief description of its application to the cold rolling of steel strip.
It is generally recognised that if production is to take place under the best and most economical condition, it is indispensable to know at any moment with a high degree of precision the thickness of the strip as it leaves the rollers. Up to the present, the apparatus employed for this purpose is either rudimentary and insufficiently accurate for practical requirements, or, on the other hand, it is too complicated, delicate and expensive for general workshop use.
The "Oehmichen" caliper functions with extreme accuracy during the manufacture or processing of the material, which may pass between the measuring members slowly, or at speeds ranging up to 500 feet per minute. It is mechanically operated, robust in construction, remarkably simple, unaffected by vibration and reasonable in cost.
The new method of measuring employed by this instrument consists in determining—by means of a micrometer screw which automatically makes several measurements per second—the distance separating two heavy articulated metallic jaws. Fixed to these jaws are two rotating members through which the band to be measured is passed. The two metallic jaws, one fixed and the other movable, are placed horizontally and are constrained by a spring, which tends to bring, the upper and lower rotating members into contact. The micrometer screw, perfectly centred, traverses a thread carried by the upper (movable) jaw. This screw is caused to descend in the thread by a spring which tends to give it a fraction of a turn. It is also lifted periodically, i.e., turned in the opposite sense, by a cam which is operated by an electric motor mounted on the upper jaw.
When the micrometer screw descends it meets a hard steel stop. The height of this stop in relation to the fixed jaw is regulated by the graduated control which may be seen on the top right-hand of the instrument. The position of the stop is so adjusted that the spherical end of the screw in its downward movement meets the steel stop before it has quite completed its descent. The position in relation to the instrument at which this contact takes place is registered by the apparatus and indicated on the dial in the following manner. The movements of the micrometer screw are transmitted to a shaft, carrying at its end a dog or projection. This projection is constrained by a spring, to maintain contact with a pin, which is held in position during the greater part of the cycle by means of a cam-operated brake.
When the screw has completed its descending movement, the brake is released and the pin, under spring tension, moves until it meets the projection. The brake operates immediately afterwards, fixing the pin in the position just taken up. This pin is secured to a plate, which is keyed on a shaft, which passes through the hollow shaft, and carries the indicating needle, moving in front of a graduated dial. So long as the thickness of the strip remains constant the pin remains motionless, since the final position of the projection, is unchanged.
If the thickness of the material increases or diminishes, the projection stops in front of or behind its previous position, and the pin is accordingly displaced towards the right or left. This movement is transmitted through the shaft to the dial needle.
If it is required to manufacture a metal sheet of pre-determined thickness, the jaws are separated and a shim of the thickness required is inserted between the rotating members. The jaws are then closed together until these rollers pinch the shim between them. The apparatus is started and the screw is actuated to displace the stop until the indicating needle rests opposite zero on the graduated dial.
The jaws are then separated and the metal sheet, as it comes from the rolling mill, is introduced between the rotating members, and the rolling mill is started. Although the successive measuring operations are separated by intervals of only a fraction of a second, the indications of the needle are entirely aperiodic, resembling those of an electro-magnetic apparatus. The slightest variations in the thickness of the strip even of a thousandth of a millimetre (.00004in.)—are immediately revealed and rendered clearly visible at a distance of several yards.
Although the "Oehmichen" universal caliper is an instrument of high precision, owing to its extremely strong construction and foolproof design it is capable of being put into almost any hands. Its operation requires no special knowledge.
The indicating device is well protected and requires no lubrication or other maintenance. The motor, reducing gear and the main shaft require a few drops of oil every fortnight. Their replacement cost is small. They require no further lubrication after leaving the laboratory. A recording device can be supplied at an extra cost.