reference axis at right angles to these planes. This action may  be illustrated by a rod fixed solidly at one end and twisted  by a weight placed on a lever arm at the other, producing the  equivalent of two equal and opposite forces acting on the rod  at some distance from each other. A shearing action is set up  all along the rod, with the center line of the rod representing  the neutral axis.  Bending (or beam stress) is a combination of compression  and tension. The rod in  Figure 4-1E  has been shortened  (compressed) on the inside of the bend and stretched on  the outside of the bend. Note that the bending stress causes  a tensile stress to act on the upper half of the beam and a  compressive stress on the lower half. These stresses act in  opposition on the two sides of the center line of the member,  which is called the neutral axis. Since these forces acting in  opposite directions are next to each other at the neutral axis,  the greatest shear stress occurs along this line, and none exists  at the extreme upper or lower surfaces of the beam. Without modern metalworking tools and machines, the  job of the airframe technician would be more difficult and  tiresome, and the time required to finish a task would be  much greater. These specialized tools and machines help the  airframe technician construct or repair sheet metal in a faster,  simpler, and better manner than possible in the past. Powered  by human muscle, electricity, or compressed air, these tools  are used to lay out, mark, cut, sand, or drill sheet metal.  Before fitting repair parts into an aircraft structure, the new  sections must be measured and marked, or laid out to the  dimensions needed to make the repair part. Tools utilized  for this process are discussed in this section.  measure any angle between the head and the blade, and a  center head that uses one side of the blade as the bisector of  a 90° angle. The center of a shaft can be found by using the  center head. Place the end of the shaft in the V of the head  and scribe a line along the edge of the scale. Rotate the head  about 90° and scribe another line along the edge of the scale.  The two lines will cross at the center of the shaft.  [Figure 4-4] Dividers are used to transfer a measurement from a device  to a scale to determine its value. Place the sharp points at  the locations from which the measurement is to be taken.  Then, place the points on a steel machinist’s scale, but put  one of the points on the 1-inch mark and measure from there.  [Figure 4-5] Scales are available in various lengths, with the 6-inch and  12-inch scales being the most common and affordable. A  scale with fractions on one side and decimals on the other side  is very useful. To obtain an accurate measurement, measure  with the scale held on edge from the 1-inch mark instead of  the end. Use the graduation marks on the side to set a divider  or compass.  [Figure 4-3] A combination square consists of a steel scale with three  heads that can be moved to any position on the scale and  locked in place. The three heads are a stock head that  measures 90° and 45° angles, a protractor head that can A rivet spacer is used to make a quick and accurate rivet  pattern layout on a sheet. On the rivet spacer, there are  alignment marks for  1 ⁄ 2-inch,  3 ⁄ 4-inch, 1-inch and 2-inch rivet  spacing.  [Figure 4-6] Fiber-tipped pens are the preferred method of marking  lines and hole locations directly on aluminum, because the  graphite in a No. 2 pencil can cause corrosion when used on  Bending  Tools for Sheet Metal Construction and  Repair Layout Tools Dividers Scales Combination Square Rivet Spacers Pens Marking Tools 1 2 3 4 5 6 7 8 9 1 2 3 4 6 7 8 9 8 16 24 32 40 48 56 4 8 12 16 20 24 28 28 4 56 8 1 9 1 2 3 4 5 6 7 8 9 4 8 12 16 20 24 28 4 8 12 16 20 24 28 4 8 12 16 20 24 28 8 16 24 32 40 48 56 8 16 24 32 40 48 56 8 16 24 32 40 48 56 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 6 7 8 9 5 1 2 3 4 6 7 8 9 5 1 2 3 4 6 7 8 9 5 9 1 5 10 THS 100 THS 32 MOS 64 THS 3 3 2 1 2 5 5 4 1 1 2 3 4 5 4 8 THS 16 THS Figure 4-3.  Scales.  4-4