Mechanics of a shear cutting process

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Title: Mechanics of a shear cutting process
Author: Meissner, Stephen
Abstract: A unique laboratory device, the Laboratory Shear, has been designed and built to simulate and examine the shear angle cutting process, which is the predominant method used in many cutting operations. The device is capable of force and velocity measurements of the shear angle cutting process with the abilities to vary cutting speed, knife preload, and blade geometry. The dimensions of the device are such that mobility to and from conditioning rooms in the laboratory environment are not limited. Assembly and disassembly ease as well as automatic versus manual operation options were incorporated for greater experimental flexibility. The device has the ability to evaluate products at a shear velocity of 200 inches per second, which equates to nearly 1000 feet per minute on a conventional slitter with 10 shear angle. The force measurement capability is 10 pounds, which is sufficient to handle the majority of Kodak films. Knife preload can be adjusted to provide preloads from 2 lbs to 12 lbs, in increments of 2 lbs. Debug of the Lab Shear Device was conducted and included knife sharpness measurement as well as calibration of the load cell and velocity transducer. The upper (moving) knife was found to have a cutting tip radius of approximately 0.0002 inch and the lower (stationary) knife was found to have a cutting tip radius of approximately 0.0001 inch. Both knives could be considered as "medium dull" based on the Kodak standard for slitting knife sharpness (X < 0.0001 inch = sharp, X > 0.0005 inch = dull). Edges for both knives appeared smooth and defect free as viewed with SEM. Load cell and velocity transducer calibration resulted in less than 10% standard deviation, which was within acceptable limits. Variability of the system was assessed through several experiments using polycarbonate film. Overall system variability was determined to be within 10%, which was acceptable for the device's intended use. A linear relationship between the average cutting force and the average sample thickness was observed, similar to the linear relationship between tearing force and film thickness reported during the trouser-tearing of films. Given this observed linear relationship, tearing of polycarbonate seems to be the dominant failure process despite that the shear angle cutting is a mixed mode fracture process. Fracture morphology examination was performed and proved to be beneficial, showing the ability of the device to generate consistent fractures associated with a specific speed-preload combination. For the polycarbonate tested, fracture morphologies observed tend to be independent of speed and dependent on preload. Finite Element Analysis (FEA) was applied to gain a better understanding of the cutting system mechanics and frictional mechanisms of the Lab Shear Device. Analysis provided explanation for an observed force shift during cutting in addition to providing a model for further examination.
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Date: 1997-05

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