The Punching Process
Punching is performed by shearing, the result of placing a metal sheet between an upper tool (punch) and a lower tool (die). The upper tool plunges into the sheet producing a punching slug, which is pushed through the lower tool. Aside from punching openings, this machine can be used to produce other work processes, the most important being nibbling and creating formed areas. The latter is a process in which a work piece is shaped, such as extrusions, beads, and louvers.
Before processing the operator places the sheet into the clamps of the coordinate guide. A tool-punch, stripper, and die are placed into the tool adapter. The positioning of the metal sheet is CNC controlled and punch strokes and positioning are synchronized. Once all the internals have been input the machine nibbles the parts out of the sheet through a chute if the parts are small enough. Sometimes the parts are tabbed into the sheet, which acts to hold the parts to the skeleton by micro-joints, which allow for easy removal as an extra operation.
Tools can be manually located in a turret or on a rail and they are either mechanically or hydraulically clamped. The ram, as the upper tool adapter, holds the punch with the stripper with the lower tool adapter holding the die. These tool adapters rotate or index to bring up the next tool for production. Some of the tool stations have a rotational axis, which allows the tools to be rotated.
Work piece clamps hold the sheet metal and guide it as it is processed. The area of the sheet upon which the clamps are located remains unprocessed, because the clamps would otherwise collide with the punching head. If the collision area is also to be processed the sheet has to be repositioned. The sheet is clamped with the stripper and the coordinate guide moves to the new position with the clamps open. Once in the new position, the sheet is clamped and the stripper retracts. Repositioning is also necessary for sheet lengths greater than the working area. Retractable clamps also allow parts to be nested under the clamps, which move automatically out of the collision area when necessary.
When processing, punching tools create holes and openings of any shape but most often are round, square, rectangular, and obround tools. Outer part contours can be nibbled or processed using slit tools, which are narrow rectangular tools with a corner radius which in combination with the tool rotation can quickly cut out pieces.
Tool life is generally between 400,000 and 600,000 strokes. To attain this tool life, care must be taken to ensure a stable tool guide, material-specific tool lubrication, a sharp cutting edge, and the exact central position of the punch and die.
Formed areas with limited height may be produced on the punching machine using specific tools. Louver cuts, extrusions, and beads are prime examples. The cost-of-operation for the hydraulic turret press includes electricity and tooling. For the servo-electric turret punch, however, energy costs are considerably lower. Maintenance costs, on the other hand, on mechanical drives and wear items are higher.
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