1.  Always  stop  the  work  before  you  take  a measurement. DO NOT measure moving parts because the micrometer may get caught in the rotating work and be severely damaged. 2.   Always open a micrometer by holding the frame with one hand and turning the knurled sleeve with the other  hand.  Never  open  a  micrometer  by  twirling  the frame,  because  such  practice  will  put  unnecessary  strain on  the  instrument  and  cause  excessive  wear  of  the threads. 3.  Apply  only  moderate  force  to  the  knurled thimble when you take a measurement. Always use the friction slip ratchet if there is one on the instrument. Too much pressure on the knurled sleeve will not only result in an inaccurate reading, but also will cause the frame to spring and force the measuring surface out of line. 4. When a micrometer is not in use, place it where it will not drop. Dropping a micrometer will cause the micrometer frame to spring. If you drop a micrometer, check it for accuracy before you take further readings. 5.   Before  you  store  a  micrometer,  back  the  spindle away from the anvil, wipe all exterior surfaces with a clean, soft cloth, and coat the surfaces with a light oil. Do  not  reset  the  measuring  surfaces  to  close  contact because the protecting film of oil on these surfaces will be  squeezed  out. SNAP GAUGE The snap gauge compares the outside diameters of parts such as shafts and journals to a standard. It can compare diameters from zero to 8 inches at an accuracy of 0.0001 inch. Figure 2-5 shows a typical snap gauge. Most   snap   gauges   consist   of   a   frame   with   an insulated handle, a hex wrench mounted in the handle, dial  indicator  digits  calibrated  in  0.00l-inch  divisions,  a bezel   clamp,   adjustment   wheels,   locking   wheels,   a backstop, a lower anvil, an upper anvil, and a guard. Whenever you use a snap gauge, use the handle and avoid touching the gauge proper because body heat may affect  the  reading.  For  the  same  reason,  handle  the standard plugs only by their plastic end. Clean the anvils and  the  backstop  with  a  clean  cloth.  To  use  the  snap gauge,  follow  the  manufacturer’s  operating  instructions. After  you  record  the  readings  and  compare  the readings with the design specifications, clean and store the snap gauge in its appropriate storage location. Figure  2-5.—Typical  snap  gauge. BORE GAUGES The dial bore gauge is one of the most accurate tools for measuring a cylindrical bore or for checking a bore for out-of-roundness or taper. The gauge does not give a  direct  measurement.  It  identifies  the  amount  of deviation from a preset size or the amount of deviation from one part of the bore to another. A master ring gauge, outside  micrometer,  or  vernier  caliper  can  be  used  to preset the gauge. Figure 2-6 shows a typical bore gauge. Figure  2-6.—Typical  bore  gauge. 2-4