タップの下穴の管理とは？ | オーエスジー

The pilot hole is machined with a drill that takes into account the expansion allowance from the standard dimensions of the pilot hole diameter.

In the case of a cutting tap, the pilot hole = minor diameter , so check the pilot hole with a minor diameter plug gauge.
The reason why the pilot hole is minor diameter is that the threaded valley of the tap is smaller than the pilot hole diameter, so it is not cut in the valley of the tap, and the drill diameter used in the pilot hole remains as minor diameter .

In the case of grooveless taps, after drilling the pilot holes, the internal thread raised by the grooveless taps is tested using a minor diameter plug gauge to change the pilot hole diameter until the dimensions are met.

The pilot hole diameter is different for cutting taps and non-grooved taps.