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PLUGS AND HOLES

The nominal, or "name" size of this hole is 1/2 - inch.  If the draftsman wanted to give the impression of great precision, he could name it .500000-inch.  But that, too, is only a name.  The nature of matter is such that no hole made with human tools can be precisely .500000-inch across all diameters measured at various angles and at various depths.  When measurements are made in millionths of an inch, there is no such thing as perfectly round, perfectly straight or perfectly parallel.  Further, no two holes will be precisely the same.  A little matter has worn off the drill during the first pass and now is slightly smaller for the second pass.

Given this state of affairs, the design engineer must decide how close to .500-inch this hole needs to be.  This will depend, of course, on the performance expected of it when the parts are assembled.  If it is simply a clearance hole, it can be .500" more or less.  If it must freely accept a half-inch shaft ( running fit ), it can be no smaller than .500".  If it is to make a rigid joint with a half-inch shaft ( force fit), it must be no larger than .500".  Unnecessary precision is expensive.  The more leeway the design engineer can allow the machinist who makes this hole, the cheaper the part will be.  So he draws upon his experience and decides, let us say, that a variation of .004-inch would be tolerable.  This is the tolerance on the dimension.  His instructions on each of the three holes mentioned above would be revised as follows:

Type of hole                                    Drawing call-out

 No smaller than .500"                       .500"/.504" or .500" + .004"

No larger than .500"                         .496"/.500" or .500" - .004"

    .500", more or less                            .498"/.502" or .500" +/- .002"

Instructions of this type have a clear meaning to machinists and inspectors.  The hole may not be perfectly round and it may be slightly tapered along its length.  But the diameter may not be less than the low limit or greater than the high limit.  A machine operator would obviously have to make several instrument readings on each piece to be sure these conditions existed.  This would require more of his time than he had spent making the hole.  So he uses a cylindrical plug gage, which has been made for the sole purpose of checking this particular hole.  It's quick, it's easy and it does not require any special skill.