Pandey's Fasteners

ISO SCREW THREADS TOLERANCE AND POSITIONS AND GRADES

A number of tolerance grades have been established for the pitch and crest diameters (the crest diameter is the minor diameter in the case of a nut thread and the major diameter in the case of a bolt thread. Tolerance grades are represented by numbers, the lower the number the smaller the tolerance. Grade 6 is used for a medium tolerance quality and a normal length of thread engagement. Grades lower than 6 are intended for fine tolerance quality and/or short lengths of thread engagement. Grades higher than 6 are intended for coarse tolerance quality and/or long lengths of thread engagement.
There are:
5 tolerance grades (grades 4 to  available for the minor diameter of the nut thread.
3 tolerance grades (grades 4,6 and  for the major diameter of the bolt thread.
5 tolerance grades (grades 4 to  for the pitch diameter tolerance of the nut thread.
7 tolerance grades (grades 3 to 9) for the pitch diameter tolerance of the bolt thread.

Tolerance positions are indicated by letters, upper case letters for nut threads and lower case letters for bolt threads. The tolerance position is the distance of the tolerance from the basic size of the thread profile.
For nut threads there are two tolerance positions, H with a zero fundamental deviation (distance of the tolerance position from the basic size) and G with a positive fundamental deviation.
For bolt threads there are four tolerance positions, h has a zero fundamental deviation and e, f, and g negative fundamental deviations. (A positive fundamental deviation indicates that the size for the thread element will be larger than the basic size. A negative fundamental deviation indicates that the size for the thread element will be smaller than the basic size.
One practical problem that is often encountered is what thread tolerance to apply to a tapped hole. The standard tolerance classes of 6g for the bolt thread and 6H for the nut thread are typically included on a drawing as default. A problem that sometimes occurs is that on long thread engagements (that are frequently used for tapped holes in soft materials) there can be an interference between the nut thread and the screw thread as the screw is rotated into the tapped hole. There can be a slight mis-match in the thread pitch between the internal thread and the external thread necessitating a wrench to rotate the fastener to the bottom of the thread i.e. it can't be freely rotated. The standard tolerance classes apply strictly only when a relatively short length of engagement is used (such as with a nut which is typically 0.8d where d is the thread size). The pitch diameter tolerance must be able to accomodate pitch and flank angle errors which can sometimes only be done by changing the tolerance position say from a H to a G for the internal thread (since standard screws - 6g - are wished to be used). Failure to change the tolerance position can result in thread seizure and damage especially if high speed tools are being used for the tightening process.