Thursday, December 3, 2015
The Importance of Proper Cutting Clearances
By: Peter Ulintz, PMA technical director
An important requirement of any die operation is the proper alignment between all the working components. In stamping operations, accurate alignment is necessary to maintain proper clearances between punch and die steels.
Cutting, punching and trimming operations require cutting clearances that are held within close limits. Because many stamping features are not symmetrical or totally round, cutting clearance usually is measured at one side of the cutting profile and specified as a “per-side” clearance. The amount of clearance applied and the sharpness of the cutting steels have a direct effect on the quality of the sheared edges.
When the cutting clearances are small, press and die alignment becomes critical. If this alignment is not maintained properly, the punch and die details may contact each other and the cutting edges may be damaged. Clearances that are too tight will produce an edge defect known as secondary shear - sometimes referred to as a “double-break.” Small cutting clearances also require increased punching and stripping forces to extract the punch point as the pierced material grips firmly around it.
Larger cutting clearances make press and die alignment less critical and also require less cutting and stripping forces. But when the cutting clearance becomes too great, extreme rollover can occur and undesirable burrs may develop. In extreme cases, the metal may actually tear or crack in the rollover zone if the surface is stretched beyond its ultimate tensile strength.
The best way to protect a die from damage is to make sure that nothing is physically out of place during a press cycle. This involves mounting sensors in the tooling and equipping the press with a controller to interpret the signals from these devices. Sensors are proven to reduce the potential for die damage by detecting speed, accuracy, part orientation, feature positions, part presence and part ejection.
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Proper sensor selection, placement and function are critical. Sensors often are placed in progressive dies at multiple locations for critical point-of-operation detection, such as bending and punching, short feed and long feed detection, and monitoring slugs and missed hits. In transfer dies, sensors also are incorporated into the grippers to detect that parts are in place before they are transferred to the next station.
Sensors in stamping dies help stampers reduce downtime and lost production, as well as associated maintenance costs and inadvertent shipping of bad parts.
Interested in learning more about die sensors, controls and punching technology? Register for PMA’s Punch & Die Technology Seminar and Sensor & Control Systems Seminar .