dress mushroomed heads on chisels and points on chisels,  screwdrivers, and drills, as well as for removing excess metal  from work and smoothing metal surfaces. The bench grinder is generally equipped with one medium- grit and one fine-grit abrasive wheel. The medium-grit wheel  is usually used for rough grinding where a considerable  quantity of material is to be removed or where a smooth finish  is unimportant. The fine-grit wheel is used for sharpening  tools and grinding to close limits. It removes metal more  slowly, gives the work a smooth finish, and does not generate  enough heat to anneal the edges of cutting tools. Straight snips, or sheet metal shears, have straight  blades with cutting edges sharpened to an 85° angle.  [Figure 4-34]  Available in sizes ranging from 6 to 14 inches,  they cut aluminum up to  1 ⁄ 16 of an inch. Straight snips can be  used for straight cutting and large curves, but aviation snips  are better for cutting circles or arcs. Aviation snips are used to cut holes, curved parts, round  patches, and doublers (a piece of metal placed under a part  to make it stiffer) in sheet metal. Aviation snips have colored  handles to identify the direction of the cuts: yellow aviation  snips cut straight, green aviation snips curve right, and red  aviation snips curve left.  [Figure 4-35] Before using any type of grinder, ensure that the abrasive  wheels are firmly held on the spindles by the flange nuts.  An abrasive wheel that comes off or becomes loose could  seriously injure the operator in addition to ruining the grinder.  A loose tool rest could cause the tool or piece of work to be  “grabbed” by the abrasive wheel and cause the operator’s  hand to come in contact with the wheel, possibly resulting  in severe wounds.  The file is an important but often overlooked tool used to  shape metal by cutting and abrasion. Files have five distinct  properties: length, contour, the form in cross section, the  kind of teeth, and the fineness of the teeth. Many different  types of files are available and the sizes range from 3 to 18  inches.  [Figure 4-36] Always wear goggles when using a grinder, even if eyeshields  are attached to the grinder. Goggles should fit firmly against  the face and nose. This is the only way to protect the eyes  from the fine pieces of steel. Goggles that do not fit properly  should be exchanged for ones that do fit. Be sure to check  the abrasive wheel for cracks before using the grinder. A  cracked abrasive wheel is likely to fly apart when turning at  high speeds. Never use a grinder unless it is equipped with  wheel guards that are firmly in place. A grinding wheel is made of a bonded abrasive and provides  an efficient way to cut, shape, and finish metals. Available in  a wide variety of sizes and numerous shapes, grinding wheels  are also used to sharpen knives, drill bits, and many other  tools, or to clean and prepare surfaces for painting or plating. Grinding wheels are removable and a polishing or buffing  wheel can be substituted for the abrasive wheel. Silicon  carbide and aluminum oxide are the kinds of abrasives used  in most grinding wheels. Silicon carbide is the cutting agent  for grinding hard, brittle material, such as cast iron. It is  also used in grinding aluminum, brass, bronze, and copper.  Aluminum oxide is the cutting agent for grinding steel and  other metals of high tensile strength. Many types of hand cutting tools are available to cut light  gauge sheet metal. Four cutting tools commonly found in the  air frame repair shop are straight hand snips, aviation snips,  files, and burring tools. 4-13 Figure 4-35.  Aviation snips. Figure 4-34.  Straight snips. Straight Snips Aviation Snips Files Grinding Wheels Hand Cutting Tools