Detector of defective coating

#During the conventional coating of sheet metal small surface areas are sometimes inadvertently allowed to remain uncoated. That is, the coating material, such as enamel, fails to cover the entire sheet leaving small areas of the metal'surface exposed to the atmosphere. Left undetected, such bare spots cause serious problems as rust and other undesirable chemical reactions will result. The canning industry is an example of an activity that cannot afford to make its product from defectively coated sheet metal. Sheets which have bare spots must be detected and removed before fabrication.

#Efforts have been made to detect such bare spots. The results have been disappointing. One attempt has been to use electrically charged free-end terminals which contact the coated sheet metal as it moves under the terminals. Proximate terminals carry opposite electrical charges. A current does not normally flow. However, when a bare spot exists between the terminals it causes the sheet metal itself to become a conductor between the terminals. The complete circuit activates means to stop the fabrication machinery so the defective sheet may be removed.

#Unfortunately, with protracted use the terminals become bent out of shape and no longer contact the sheet metal. Also, they can only be used on one side of sheets which are often coated on both sides. Until my invention it has been necessary to check each coated side, one after another, rather than both simultaneously.

#FIG. 1 is a top plan diagram showing the alignment of my invention between the supply stack of sheets and the fabrication equipment.

#FIG. 2 is a cross sectional side elevation showing a sheet between the double form of my invention.

#FIG. 3 is a fragmentary sectional view taken along lines 3--3 of FIG. 2.

#Referring to the accompanying drawing wherein like numerals represent like parts throughout, the numeral 10 is a novel sensor member. It is made of metal for conducting an electrical current. For best results I use rigid strips of arcuate configuration in their central portion with flat end portion for appropriate connections. See FIG. 7. A plurality of such strips are aligned parallel to each other and are spaced sufficiently close to each other, preferably about one-fourth of an inch, to detect an exposed spot 38' coming between them. A panel or set of such sensor members support the weight of light sheets moving over it, eliminating the necessity of "dead plates" over which sheets usually slide on their way from a stack to fabricating machinery. For heavier sheets, additional support means may be used.

#Each end 16 of each strip is planar to facilitate its remaining in position at all times. A boss 14 surrounded by a spiral spring 12 is mounted at each end to complete the sensor assembly. The sensors "float" on the springs.

#There is a mounting plate 32 with a groove 34 in which the sensor assembly is freely positioned. Also support strip 36 is secured adjacent thereto to support ends 16 which rest thereon when a sheet 38 is not present.