Tapping hole tolerances
Tap tolerance vs. tolerance on internal thread (nut)
|Tolerance class, tap||Tolerance, internal thread (nut)||Application|
|ISO 1||4 H||3 B||4 H||5 H||||||||Fit without allowance|
|ISO 2||6H||2B||4 G||5 G||6 H||||||Normal fit|
|ISO 3||6 G||1B||||||6 G||7 H||8 H||Fit with large allowance|
|-||7 G||-||||||||7 G||8 G||Loose fit for following|
treatment or coating
Normal tap tolerance is ISO 2 (6H), which generates an average quality fit between screw and nut. Lower tolerance (ISO 1) generates a fine fit without a gap on the flanks between screw and nut. Higher tolerance (ISO 3) generates a rough fit with a large gap. This is used if the nut is coated, or if a loose fit is preferred. Between tolerances 6H (ISO2) and 6G (ISO3), and between 6G and 7G, there are also taps with tolerance 6HX and 6GX. “X” means the tolerance is outside the standard and is used for taps working with high strength or abrasive materials such as cast iron. These materials do not cause oversize problems so higher tolerance can be used in order to increase tool life. The tolerance width is equal between 6H and 6HX. Forming taps are usually produced with a 6HX or 6GX tolerance.
Pipe threads refer to the following standards:
- G threads to ISO 228-1. One class for internal thread (tap)
- Rc and Rp threads to ISO 7-1
- NPT and NPSM to ANSI B1.20.1
- NPTF and NPSF to ANSI B1.20.3
The tolerance width on a tap designed for a specific tolerance is much smaller than the tolerance width on the finished thread. The tap tolerance is positioned so that the tap cuts a correct thread from the start. When the tap is used, it wears gradually and will eventually be too small to cut a thread correctly, having become smaller than the lower tolerance of the GO gauge. The optimum situation would be to position the tap tolerance at the upper part of the internal thread area, but then there is a risk that the tap cuts an oversized thread, above the upper limit of the NO-GO gauge. Taps with an “X” tolerance, used for materials that do not cause oversize problems, are positioned higher. The result is a longer tool life because more wear can occur before the tap cuts a too small thread.