A place for "one-liner" hints and tips that the members have contributed

• The major diameter of a numbered thread (e.g. 6-32) is given by
  D = ( 0.013 * N ) + 0.060 ... so a #6 has a major diameter of 0.138 in.
   
• pi (3.14159) ~= 355/113 with an error of only 0.002%
  (22/7 is in error by 0.9%)
   
• Norm Wells recommends Otto's for dial indicator repair in the South Bay area.
  Otto's Dial Indicator Repair
  2614 Huntington Lane
  Redondo Beach, CA
  310-372-3618
   
• Don and Marv say "A great resource for machining work and innovative machine shop accessories."
  Fisher Machine
  11704 Inglewood Ave.
  Hawthorne, CA 90250
  310-644-8375
  John Fisher, the owner, can be reached at fishermachine@aol.com
   
• To find the tapdrill needed for ~75% depth of thread, simply subtract the pitch of the thread from the nominal diameter of the tap.
  1/4 (in) - 20 (tpi) => 1/4 (in) - 1/20 (ipt) = 0.2 in (#7 drill)
  Works for metric, too.
  6 x 1 mm => 6 - 1 = 5 mm
   
• Here's a mental arithmetic trick to (approximately) convert fractional inches to millimeters.
  Keep doubling numerator and denominator until the denominator is 256.  Then the numerator divided by ten is the equivalent in mm with an error of only 0.78%
  13/64 = 26/128 = 52/256 -> 52/10 -> 5.2 mm  (correct value = 5.159375 mm)
   
• To (approximately) convert mm to inches, multiply by 4 and divide by 100.
  30 mm * (4/100) =~ 1.2 in (correct value = 1.181 in)
   
• If you're going to check a micrometer for accuracy, it's important to use a set of gage blocks that cause the spindle to seat at different orientations so drunken thread errors will be noticed.
  The preferred set for inch micrometers is:
  0.105, 0.210, 0.315, 0.420, 0.500, 0.605, 0.710, 0.815, 0.920, 1.000.
  For metric micrometers the preferred set is:
  3.1, 6.5, 9.7, 12.5, 15.8, 19.0, 21.9, 25.0.