TM 9-243 USING PRECISION GAGE BLOCKS - Continued 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. 2 Shift the blocks. If the blocks are clean, they will CAUTION begin to take hold. Do not leave blocks wrung together for 3 Slide the two blocks together, using a slight pres- long periods of time since surfaces in sure and a rotary motion. contact will tend to corrode. FACTORS TO CONSIDER WHEN USING GAGE BLOCKS Ordinary changes in temperature have a sizable effect When using gage blocks consider the source of error on measurements made with precision gage blocks. resulting from temperature. Metals other than iron and The standard measuring temperature is 68°F, which is steel (such as aluminum) have a much different coeffi- just a little lower than the average temperature in most cient  of  linear  expansion  which  will  result  in  a  difference shops.  Since  the  room  temperature  affects  the  work  as between the room temperature measurement and the well as the block, the expansion in the work will be standard   measuring   temperature   measurement.   Care- matched in most cases by a similar expansion in the less handling of gage blocks may produce an error of block. The coefficient of linear expansion of several several millionths of an inch and this error increases metals and blocks is listed below: proportionally with the dimension of the block. Material Steel Iron Phosphor bronze Aluminum Copper Gage blocks Millionths  of  an  inch 5.5 to 7.2 per degree F 5.5 to 6.7 9.3 12.8 9.4 6.36 to 7.0 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 suffi- cient 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  pres- sure. 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 inspec- tion  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  clas- sification is shown on the following page. 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, caus- ing a serious error in a measurement, par- ticularly if a long stack of blocks is being handled. 15-8