It was previously stated that an essential principle is that
the actual profiles of both the nut and bolt threads must never
cross or transgress the theoretical profile. Practically, to
make a thread, tolerances must be applied to ensure that this
essential principal always applies. Tolerancing of screw threads
is complicated by the complex geometric nature of the screw
thread form. Clearances must be applied to the basic profile
of the threads in order that a bolt thread can be screwed into
a nut thread. For the thread to be made practically there must
be tolerances applied to the main thread elements.
Usually nut threads have a tolerance applied to the basic
profile so that it is theoretically possible for the nut thread
profile to be equal to the theoretical profile. Bolt threads
usually have a gap between the basic and actual thread profiles.
This gap is called the allowance with inch-based threads and
the fundamental deviation with metric threads. The tolerance
is subsequently applied to the thread. Since for coated threads
the tolerances apply to threads before
coating (unless otherwise stated), the gap is taken up by
the coating thickness. After coating, the actual thread profile
must not transgress the basic profile of the thread.
A full designation for a metric thread includes information
not only on the thread diameter and pitch but also a designation
for the thread tolerance class. For example a thread designated
as M12 x 1 - 5g6g indicates that the thread has a nominal
diameter of 12mm and a pitch of 1mm. The 5g indicates the
tolerance class for the pitch diameter and 6g is the tolerance
class for the major diameter.
A fit between threaded parts is indicated by the nut thread
tolerance designation followed by the bolt thread tolerance
designation separated by a slash. For example: M12 x 1 - 6H/5g6g
indicates a tolerance class of 6H for the nut (female) thread
and a 5g tolerance class for the pitch diameter with a 6g
tolerance class for the major diameter.
A tolerance class is made up of two parts, a tolerance grade
and a tolerance position.
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.
5 tolerance grades (grades 4 to 8) available for the minor
diameter of the nut thread.
3 tolerance grades (grades 4,6 and 8) for the major diameter
of the bolt thread.
5 tolerance grades (grades 4 to 8) 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
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
An example of such a seizure problem is illustrated below: