Figure 1-30.—Cutaway of Barcol tester. Figure 1-30 is a cutaway drawing of the tester, showing  the  internal  parts  and  their  general  arrangement within the case. The  lower  plunger  guide  and  point  are  accurately ground so that attention need be given only to the proper position of the lower plunger guide within the frame to obtain  accurate  operation  when  a  point  is  replaced. The frame, into which the lower plunger guide and spring-tensioned plunger are screwed, holds the point in the  proper  position.  Adjustment  of  the  plunger  upper guide nut, which regulates the spring tension, is made when the instrument is calibrated at the factory. CAUTION The  position  of  this  nut  should  not  be changed.  Any  adjustment  made  to  the plunger  upper  guide  nut  will  void  the calibrated settings made at the factory. The  leg  is  set  for  testing  surfaces  that  permit  the lower plunger guide and the leg plate to be on the same plane. For testing rivets or other raised objects, a block may be placed under the leg plate to raise it to the same plane. For permanent testing of this type, the leg maybe removed and washers inserted, as shown in the drawing. The point should always be perpendicular to the surface being  tested. The design of the Barcol tester is such that operating experience is not necessary. It is only necessary to exert a light pressure against the instrument to drive the spring-loaded indenter into the material to be tested. The Table 1-5.—Typical Barcol Readings for Aluminum Alloys Alloy and temper Barcol number 1100-0 35 3003-0 42 3003  -1/2H 56 2024-0 60 5052-0 62 5052-1/2H 75 6061-T 78 2024-T 85 hardness reading is instantly indicated on the dial. Several typical reading for aluminum alloys are listed in table 1-5. The harder the material, the higher the Barcol number. To prevent damage to the point, avoid sliding or scraping when it is in contact with the material being tested. If the point should become damaged, it must be replaced with a new one. No attempt should be made to grind the point. Each tester is supplied with a test disc for checking the condition of the point. To check the condition of the point, press the instrument down on the test disc. When the downward pressure brings the end of the lower plunger  guide  against  the  surface  of  the  disc,  the indicator reading should be within the range shown on the  test  disc. To replace the point, remove the two screws that hold the halves of the case together. Lift out the frame, remove  the  spring  sleeve,  loosen  the  locknut,  and unscrew the lower plunger guide, holding the point upward so that the spring and plunger will not fall out of place. Insert the new point and replace the lower plunger guide, screwing it back into the frame, Adjust the  lower  plunger  guide  with  the  wrench  that  is furnished until the indicator reading and the test disc average number are identical. After the lower plunger guide is properly set, tighten the locknut to keep the lower plunger guide in place, This adjustment should be made only after installing anew point; any readjustment on a worn or damaged point give erroneous readings. 1-40