We operate with the following thread sizes:
Coarse – used for manufacturing difficult threads as deep holes.
Average – used for regular threads.
Fine – used for precision threads.
Thread engagement is categorized:
L - long.
N - normal.
S - short.
In the ISO Thread Tolerance System, there are 11 tolerance grades from 01 to 9 for bolts and from 02 to 8 for nuts.
Letters G and H show the positioning of the nuts.
Letters a, b, c, d, e, f, g and h show the positioning of the bolts. If the thread cut needs surface treatment, letters G, a to g show the positioning. After the surface treatment, the nominal value cannot be exceeded.
Møtrikgevind = nut thread. Skruegevind = screw thread
Positioning of the tolerances relative to the nominal profile.
There is no need to use finer tolerances than needed to secure the thread function and to transfer the force from one part to the other. This becomes easier in longer threads than shorter and can therefore be manufactured with greater tolerances.
The highlighted tolerances are to be preferred.
Kvalitet – quality, fin – fine, middle – average, grov – coarse, skruegevind – screw thread, møtrikgevind – nut thread, tolerancebeliggenhed – tolerance positioning, indskruningslængde - screw length, Tabel over ISO gevindtolerancer – Chart of ISO thread tolerances, Valg – choice, bør undgås – should be avoided.
ISO Inch Screw Threads.
There are three tolerance classes for screws 1A to 3A and three classes for nuts 1B to 3B.
Møtrik – nut, skrue - screw
The figure illustrates the mutual dimensions and positioning in relation to the nominal measurements.
The most common combination is the 2A screw and the 2B nut.
A 2A screw can also be used with a 1B and a 3B nut.
The tolerance of the pitch diameter can be written:
5/16-18-UNC-3A PD 0.2764-0.2792
PD (pitch diameter) indicates minimum and maximum pitch diameter in inches.