Tools and machines by Barnard Charles 1838-1920

Author:Barnard, Charles, 1838-1920
Language: eng
Format: epub
Tags: Tools, Machine-tools
Publisher: New York, Boston [etc.] Silver, Burdett and company

ting of hay and cornstalks in the lever feed-cutter. From the invention of the pivoted knife it was only a short step to the invention of the double-bladed pivoted knives.

Lay a visiting-card on the bench and with your knife try to cut it into two pieces by pressing the knife-blade down upon it. It is not easy to cut it in this way, because the whole of the cutting-edge of the blade is used at once. There is another and better tool for such work, called a pair of scissors. Hold the card in the left hand and the scissors in the right. We see that we now have a double acting pivoted cutting-tool, and we observe, in using it, that the blades do not press upon the card through their whole length. The card is separated with a moving or progressive cut. We say the pivoted blades give a shearing cut. This progressive or shearing cut enables us to do work that would not be possible with a single-bladed cutting-tool, and we shall find the principle of the shearing cut applied to a great number of cutting-machines.

The ancient men who first used a clam shell or a flake of obsidian as a knife had no idea of a double-bladed knife. They could not imagine how a pair of scissors would work. Double cutting-tools or shears were not invented until long after the men who used stone knives had disappeared. The earliest shears of which we have any trace were simply two iron knives joined together at one end by a spring formed of a strip of elastic metal. Such a tool was held in the hand and the blades pinched together. They were

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