Document dqx1J4zD1kN3mgN18n34bR7q

PRINCIPLES OF GUARDING where a moving part crosses the fixed part of a machine is illustrated by the tables of a planing mill, the side of a shaper, spot welders, and clamping fixtures. In at least two types of machines, the re ciprocating mobon is especially dangerous --the guillotine and the shear--where a knife moves up and down past a fixed knife (Fig. 22-3). Some mechanisms, a cam gear for in stance, use a combination sliding and turn ing movement Others use an oscillating movement as u a pendulum These mecha nisms have their special dangers, and often the compound motions are more dangerous than the simple ones from which they were developed Squeezing action is found in toggle presses, press brakes, and hydraulic presses Types of guards To eliminate the dangers involved in ma chine operation, either guards may be built and installed over the hazardous areas or the equipment may be redesigned to have no exposed dangerous parts. The modem lathe is a good example of machinery made safe through improved de sign Its motor drive and gear box are en closed so that line shafts, pulleys, and belts are dispensed with. The modern power press, in which all the working parts with the exception of the slide are enclosed, is another good example Types of guards which are used to make machinery safe include the fixed guard, the interlocking guard, and the automatic guard. The fixed guard is considered preferable to all other types and should be used in ev ery case unless it has been definitely deter mined that this type is not at all practicable. The fixed guard at all tunes prevents ac cess to the dangerous parts of the machine. Fixed guards may be adjustable to ac commodate different sets of tools or vari ous lands of work Once adjusted, they should remain "fixed," and there should be Fig. 22-3. Protection ogainst all variations of these common cutting and shearing mechanisms should be provided. 22-4