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