74
Class 1 Tap
This is closest to basic, having little oversize allowance, and is
normally specified for "close" fit threads, eg. Unified 3B, Metric 4H,
5H.
Class 2 Tap
This is normally specified for "medium" fit threads, eg. Unified 2B,
Metric 6H, 4G, 5G.
Class 3 Tap
This is futhermost above basic size and used for "free" fit threads,
eg. Unified 1B, Metric 7H, 8H, 6G.
Under favourable working conditions, the following thread tolerances
should be produced by the new class taps.
All Somta HSS taps are supplied to Class 2, 6H unless otherwise
specified.
RE-SHARPENING
Maximum productivity and tap life can only be obtained from a tap
that is kept in good condition and handled with care.
When re-sharpening becomes necessary, regrinding by hand is not
recommended, though it is probably better than using chipped or
worn taps. The recommended method is to use special tap grinding
attachments or machines, and to follow the original form of the tap.
Metric
Unified
Whitworth Form
BA
Class 1
4H, 5H
3B
Close Class
Close Class
Class 2
6H, 4G, 5G
2B
MediumClass
MediumClass
Class 3
7H, 8H, 6G
1B
Free Class
Free Class
