WINTER 2021

104
THE DISTRIBUTOR’S LINK
BRUNO MARBACHER THREAD TOLERANCES ASSURE FASTENERS CAN BE ASSEMBLED EASILY from page 14
The tolerances zones are covered in ISO 965 part 3,
they are listed under the individual tolerance zones, given
for a certain size range and pitch.
The tolerance is then applied to the nominal pitch or
major diameter listed in ISO 724.
For an externally threaded fastener the typical tolerance
zone is 6g, for an internal thread the typical tolerance is
6H.
For the tolerance zone h the maximum is right at the
nominal dimensions. Applicable to both the pitch and
major diameter.
g, f, e locations (positions)
are placed farther away from the
nominal dimensions.
The maximum thread
dimensions are smaller than the
nominal dimension, thus providing
more clearance in the internal thread
(nut thread).
The ‘e location’ provides more clearance than the ‘f
location’ and the ‘f location” gives more clearance than
the ‘g location.’
The minimum major and pitch diameter with tolerance
class 8g is smaller than the one with tolerance class 6g
Calculation Examples
Based on ISO 724, the nominal pitch diameter for an
M10 thread, is 9.026mm. Please note, the pitch diameter
is responsible for the tightness of the thread fit.
On a screw with a 6g tolerance class, for the
maximum specified pitch diameter is minus 32µm (-
0.032mm) and the minimum pitch diameter is then the
nominal pitch diameter minus 164µm – (0.164mm).
¤ Max pitch diameter = 9.026 – 0.032 = 8.994mm
¤ Min pitch diameter = 9.026 – 0.164 = 8.862mm
For a screw with a 6h tolerance class, the tolerance =
-0µm (-0.000mm) = maximum diameter, for the minimum
diameter it is - 132 µm (- 0.132mm)
¤ Max pitch diameter = 9.026 – 0.000 = 9.026mm
¤ Min pitch diameter = 9.026 – 0.132 = 8.894mm
In the plated condition the pitch diameter should
not transgress the nominal pitch diameter (max of pitch
diameter with 6h tolerance)
For an M10, the difference between the maximum
diameter in the 6g tolerance class, and nominal diameter
(6h upper limit) is 32µm, Because of the thread’s
geometrical shape that value is divided by 4, which would
allow a plating or coating thickness of 8µm.
As stated above with an 8g tolerance class, the
minimum pitch/major diameter is smaller.
Minimum pitch diameter for 8g tolerance class =
9.026 - 0.244 = 8.782mm
ISO 965 part 2, as well DIN 13 part 13, list the
minimum and maximum dimensions of the critical thread
diameters. ASME B1.13M also indicates minimum and
maximum dimension of
the critical diameters.
The DIN handbook 45
contains an enormous
amount of information on
various metric threads.
Tolerance Classes of a Nut thread
On nut threads, the tolerance is applied in the reversed
direction, meaning instead of minus tolerances there are
plus tolerances.
Capital letters indicate the tolerance positions for nut
thread dimensions,
CONTINUED ON PAGE 162