TM 9-243 USING PRECISION GAGE BLOCKS - Continued 2. Shift  the  blocks.    If  the  blocks  are  clean,  they will begin to take hold. 3. Slide   the   two   blocks   together,   using   a   slight pressure and a rotary motion. 4. Shift gage blocks so that their sides are in line. Any combination of gage blocks may be stacked together  in  this  manner.    The  combination  will be as solid as a single block. NOTE The    adhesive    force    that    binds    two    gage blocks together is a combination of molecular attraction  and  the  suction  cup  action  due  to the  film  of  oil  or  moisture  on  the  surfaces wrung   together.      Separate   gage   blocks   by sliding them apart, using the same movement as when wringing them together. CAUTION Do  not  leave  blocks  wrung  together  for  long periods of time since surfaces in contact will tend to corrode. FACTORS TO CONSIDER WHEN USING GAGE BLOCKS Ordinary  changes  in  temperature  have  a  sizable  effect on measurements made with precision gage blocks. The  standard  measuring  temperature  is  680F,  which  is just a little lower than the average temperature in most shops.  Since the room temperature affects the work as well  as  the  block,  the  expansion  in  the  work  will  be matched  in  most  cases  by  a  similar  expansion  in  the block,   The   coefficient   of   linear   expansion   of   several metals and blocks is listed below: Material Millionths of an inch Steel 5.5 to 7.2 per degree F Iron 5.5 to 6.7 Phosphor bronze 9.3 Aluminum 12.8 Copper 9.4 Gage blocks 6.36 to 7.0 Handle blocks only when they must be moved and hold them between the tips of your fingers so that the area of contact  is  small.    Hold  them  for  short  periods  of  time only. NOTE Avoid conducting body heat into the block   by   careless   handling.      Body heat may raise the temperature of the block,  causing  a  serious  error  in  a measurement,   particularly   if   a   long stack of blocks is being handled. When  using  gage  blocks  consider  the  source  of  error resulting from temperature.  Metals other than iron and steel    (such    as    aluminum)    have    a    much    different coefficient   of   linear   expansion   which   will   result   in   a difference between the room temperature measurement and the standard measuring temperature measurement. Careless handling of gage blocks may produce an error of several millionths of an inch and this error increases proportionally with the dimension of the block. The  temperature  of  the  work  may  be  either  lower  or higher   than   the   room   temperature   as   a   result   of   a machining     operation     and     this     difference     may     be sufficient to cause a sizable error. Theoretically,  the  measuring  pressure  should  increase proportionally  with  the  area  of  contact.    For  practical purposes,   it   is   better   to   use   a   standard   measuring pressure.  The most commonly used pressure is 1/2 to 2 pounds. Gage blocks are used in the layout and in checking the accuracy of tools, dies, and fixtures.  They are also used in  machine  setups  and  in  checking  parts  in  process  of manufacture and finished parts. Gage  blocks  are  commonly  used  in  setting  adjustable instruments and indicating gages and verifying inspection  gages.    Gage  blocks  are  used  to  verify  the accuracy  and  wear  of  ring  and  snap  gages  and  many other    special-purpose    gages.        The    classification    of blocks    depends    largely    on    the    accuracy    required. Typical classification is shown on the following page. 15-8