The present invention relates generally to an apparatus for measuring the amount of skew or sinusoidal variation in the edges of moving plates or sheet material. It is particularly intended for use in preparing steel sheet and strip for the manufacture of tin plate.
In the manufacture of tin plate from sheet and strip it is normal practice to subject a slab of steel to a hot rolling operation wherein rolling at the last finishing stand is conducted above the upper critical temperature of the metal. In such a hot rolling operation the strip is wound into a coil at temperatures ranging from 1050.degree. to 1200.degree. F., depending on the end use of the product and its desired metallurgical characteristics. The hot roll strip is then subjected to a continuous pickling operation wherein the strip first passes through cold working equipment which fragments surface scale and facilitates acid attack prior to actually passing through the pickling solutions at uniform speed to complete oxide removal, and is thereafter followed by cold water spray rinses and, if necessary, neutralizing alkaline solutions. The strip is then recoiled.
Following pickling, cold reduction of the coil takes place. The reduction in thickness of the strip may be as great as 90% and is carried out at exceedingly high rates of speed on the order of 1 mile per minute. Care must be exercised to obtain flat strip from the cold reduction mill in order to secure uniform results further on in the tinning process. Strip shape can be distorted by edge wrinkles or center buckles, most of which is caused by excessive uneven rolling pressures. Such distortion results in noncylindrical out of round coils of metal.
Prior to annealing the coils received from the coil reduction operation are uncoiled and subjected to electrolytic cleaning, rinsing and air drying and are thereafter recoiled. The strip may be annealed on a continuous line, which once again necessitates uncoiling and recoiling the strip at relatively high temperatures and line speeds.
The annealed strip may then be subjected to temper-rolling to secure the desired hardness and surface texture, to impart the required mechanical properties to the product made from the strip and to effect final flattening of the strip. Again, excessive rolling at the edge or center can cause edge or center fullness.
In the course of the foregoing operations the strip is coiled and uncoiled several times. Apart from any edge or center defects, improper recoiling can cause noncylindrical coils with resultant defects and reduced acceptance of the ultimate tin plate product.
A typical coil when treated in the foregoing operations may weigh from 30,000 to 40,000 pounds. Its length may be approximately 25,000 feet. It can be readily appreciated that handling such large units of product, with frequent coiling and uncoiling, can result in recovery of coils that are noncylindrical and undesirable.
In tin plate manufacture a coil preparation line is thus used for edge shearing and inspection of the strip at a location prior to the tin coating operation. As in prior operations, the coils of steel are placed on a motor driven mandrel and uncoiled through tension bridle rolls, side shears and then recoiled again on a motor driven mandrel while moving through the line at very high speeds such as 2400 fpm.
Noncylindrical coils when processed through the rotary side shear can result in edges on the strip which are not straight but instead have sinusodial variations. Such variations produce inferior product and can result in rejection of products and great loss to the manufacturer. Accordingly, installation of a measuring device on the coil preparation line to detect edge variations as the coil is sheared can be very advantageous and can provide information enabling corrective action to be taken before inferior products are produced. One useful arrangement is to locate the measuring system on the exit side of the side shears.
The lack of symmetry of an out of round coil and the sinusodial oscillations of the steel strip which result when it is unwound from the coil preparation line mandrel as it is fed into side trimmer knives are definite disadvantages. The amplitude of the oscillations tend to increase with the rotational speed of the mandrel and the strip speed. Such oscillations produce an alternating tension from side to side on the strip as it is fed into the shear, thus producing a sinusodial pattern of lateral weave cut into the sides of the strip by rotating shear knives. The period length of the sinusodial variation is dependent on the circumference of the payoff coil. When the edge of the strip deviates from a straight line over a given longitudinal length, that length of strip becomes unusable for prime tin plate products.
The purpose of this invention is to determine dynamically the maximum amplitude of the deviation from a straight line for a given wave length of strip, e.g. 0.025 inches in 5 feet. This is accomplished by taking three separate lateral measurements along the edge of the strip along with a measurement of wave length and thereupon calculating maximum amplitude of the lateral variations.