machine shops. The other type (Bristol) is used infrequently. Recessed-head screws usually have a hex-shaped (six-sided) recess. To remove or tighten this type of screw requires a special wrench that will fit in the recess. This wrench is called an Allen-type wrench. Allen wrenches are made from hexagonal L-shaped bars of tool steel (fig. 1-17). They generally range in size up to 3/4 inch. When using the Allen-type wrench, make sure you use the correct size to prevent rounding or spreading the head of the screw. A snug fit within the recessed head of the screw is an indication that you have the correct size. The Bristol wrench is made from round stock. It is also L-shaped, but one end is fluted to fit the flutes or little splines in the Bristol setscrew (fig. 1-17). SAFETY RULES FOR WRENCHES There are a few basic rules that you should keep in mind when using wrenches. They are as follows: Always use a wrench that fits the nut properly. Keep wrenches clean and free from oil. Otherwise they may slip, resulting in possible serious injury to you or damage to the work. Do not increase the leverage of a wrench by placing a pipe over the handle. Increased leverage may damage the wrench or the work. Provide some sort of kit or case for all wrenches. Return them to the case at the completion of each job.    This saves time and trouble and aids selection of tools for the next job. Most important, it eliminates the possibility of leaving them where they can cause injury to personnel or damage to equipment. Determine which way a nut should be turned before trying to loosen it. Most nuts are turned counterclockwise for removal. This may seem obvious, but even experienced people have been observed straining at the wrench in the tightening direction when they wanted to loosen the nut. Learn to select your wrenches to fit the type of work you are doing. If you are not familiar with these wrenches, make arrangements to visit a shop that has most of them, and get acquainted. Figure 1-17.-Allen- and Bristol-type wrenches. METAL-CUTTING TOOLS Many types of metal-cutting tools are used by skilled mechanics of all ratings. As you become better acquainted with the ABE rating, you will probably discover many tools that you use for cutting metal that are not described in this text. In this text, only the basic hand metal-cutting tools will be considered. SNIPS AND SHEARS Snips and shears are used for cutting sheet metal and steel of various thicknesses and shapes. Normally, the heavier or thicker materials are cut by shears. One of the handiest tools for cutting light (up to 1/16-inch thick) sheet metal is the hand snip (tip snips). The STRAIGHT HAND SNIPS, shown in figure 1-18, have blades that are straight and cutting edges that are sharpened to an 85-degree angle. Snips like this can be obtained in different sizes, ranging from the small, 6-inch, to the large, 14-inch, snip. Tin snips will also work on slightly heavier gauges of soft metals, such as aluminum alloys. Snips will not remove any metal when a cut is made. There is danger, though, of causing minute metal fractures along the edges of the metal during the shearing process. For this reason, it is better to cut just outside the layout line. This procedure will allow you to dress the cutting edge while keeping material within required dimensions. Cutting extremely heavy gauge metal always presents the possibility of springing the blades. Once the blades are sprung, hand snips are useless. When 1-12