The rule  depth  gage  is  a  graduated  rule  with  a sliding head designed to bridge a hole or slot and  to hold  the  rule  perpendicular  to  the  surface  on  which the measurement is taken. This type has a measuring range   of   0   to   5   inches.   The   sliding   head   has   a clamping screw so it may be clamped in any position. The sliding head has a flat base that is perpendicular to the axis of the rule and ranges in size from 2 to 2 5/8  inches  in  width  and  from  1/8  to  1/4  inch  in thickness. The micrometer depth gage consists of a flat base attached to the barrel (sleeve) of a micrometer head. These   gages   have   a   range   from   0   to   9   inches, depending  on  the  length  of  extension  rod  used.  The hollow  micrometer  screw  (the  threads  on  which  the thimble  rotates)  itself  has  a  range  of  either  1/2  or  1 inch. Some have a ratchet stop. The flat base ranges in size from 2 to 6 inches. Several extension rods are normally supplied with this type of gage. To measure the depth of a hole or slot with reasonable accuracy, use  a  depth  gage  as  shown  in  figure  2-48,  view  A. Hold the body of the depth  gage  against  the  surface from which  the  depth  is  to  be  measured  and  extend the scale into the hole or slot. Tighten the  set-screw to  maintain  the  setting.  Withdraw  the  tool  from  the work and read the depth on the scale. To measure the depth of a hole or slot with more accuracy  than  is  possible  with  an  ordinary  depth gage,  place  a  vernier  depth  gage  over  the  slot  as shown  in  figure  2-48,  view  B.  Notice  the  clamping screws are at X and Y; the horizontal adjusting screw nut is at Z. With X and Y loose, slide the scale down into  the  slot  being  measured  until  it  is  almost  in contact. Then tighten X to make Z operative. With Z, adjust  the  scale  to  the  proper  feel  and  secure  the setting   with   Y.   By   proper   feel   we   mean   the adjustment at which you first notice contact between the end of the scale and the bottom of the slot. Then read   the   setting   as   described   under   “Reading   a Vernier Scale.” To  set  the  vernier  depth  gage  to  a  particular setting, loosen both setscrews at X and at Y and slide Figure 2-48.—Using depth gages. the   scale   through   the   gage   to   the   approximate setting. Tighten the sets crew at X, turn the knurled nut at Z until the desired setting is made, and tighten the setscrew at Y to hold the setting. To measure the depth of a hole or slot, as shown in  figure  2-48,  view  C,  with  more  accuracy  than  is possible  with  either  an  ordinary  depth  gage  or  a vernier  depth  gage,  place  a  micrometer  depth  gage over the slot and adjust the thimble until the contact of the spindle causes the ratchet stop to slip. Remove the    micrometer    from    the    work    and    read    the micrometer.  Remember,  if  extension  rods  are  used, the total depth reading will be the sum of the length of the rods plus the reading on the micrometer. DIAL INDICATOR The  dial  indicator  is  used  in  several  different ways to measure the amount of deviation (or runout) in revolving or rotating parts. Accurate shaft-to-shaft alignment    is    impossible    without    using    a    dial indicator. Dial indicators are supplied with various fittings, links,  and  adapters.  Additionally,  special  application hardware  is  available.  Figure  2-49  illustrates   the basic   hardware   of   a   dial   indicator   set   and   two different types of dial heads. The dial scale is usually graduated  in  thousandths  of  an  inch  and  has  an adjustable   bezel   around   it.   The   scale   of   a   dial indicator usually reads  plus  numbers  to  the  right  of zero and minus numbers to the left of zero. The typical setup for checking the trueness of a shaft, using a dial indicator and vee blocks (or roller blocks), is to place the shaft in the vee blocks, mount the magnetic base or clamp attachment (whichever is the  most  appropriate)  with  the  swivel  post  and  dial head attached to a solid surface. Adjust the mounting linkage  to  a  convenient  angle  that  permits  ease  in reading the dial (but does not interfere with the task being   performed).   Bring   the   sensor   button   into contact  with  the  shaft,  loosen  the  swivel  post  clamp screw, raise and lower the sensor button to determine what  the  full  travel  of  the  indicator  is.  After  the extent of travel has been determined, set the pointer at mid travel, secure the swivel post clamp screw, and then  zero  the  dial  with  the  adjustable  bezel.  Rotate the shaft slowly and observe the pointer for deviation. The combined deflection (plus and minus sides of the scale) is the total indicator reading (TIR). CARE OF PRECISION INSTRUMENTS Special   treatment    is    required    for    precision instruments   if   they   are   to   serve   their   intended purpose.  The  following  precautions  will  help  ensure their accuracy. 2-30