WINTER 2021

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THE DISTRIBUTOR’S LINK
Bruno Marbacher
Bruno Marbacher earned his mechanical engineering degree in Switzerland, he also holds a
business degree. He started out as a tool and die maker (poly-mechanic) and over the years he
has held various management positions in quality and engineering. During his time in America
he has developed and given numerous seminars on topics related to the proper use of mechanical
fasteners and machine elements, and assists engineers in solving fastening/assembly issues. His
has groomed and directed many young engineers in fastening/assembly technology. He now
offers his 40 years of experience through writing and lecturing.
THREAD TOLERANCES ASSURE FASTENERS
CAN BE ASSEMBLED EASILY
Dear Readers, in the previous article, I covered the
international tolerance system for limits and fits.
The tolerance system for threads has some similarities
as it also works with tolerance zones. The main purpose
is to assure interchangeability on a worldwide basis.
However, the tolerance zones
are defined to meet different
purposes and conditions as
well.
DIN (German Institute for
Standardization) started
developing thread tolerances in the early 1920’s.
ISO (International Organization for Standardization)
published the first standard, which was the standard for
the metric thread, in 1947.
The International Standard ISO 965 specifies thread
tolerances.
Tolerance Classes (Zones)
As with “shafts and pins”, the tolerance symbols
consist of letters and numbers. The letters indicate
the location of the tolerance in relation to the nominal
dimensions of a threaded part, for metric threads this is
mainly the pitch diameter and the major diameter.
The numbers (called tolerance grades) indicate the
actual tolerance, the actual limits; a bigger number calls
out a bigger spread between the low limit and the high
limit.
Tolerances for internal threads are expressed with
TECHNICAL ARTICLE
capital letters; tolerances for external threads are
expressed in lower case letters.
For the thread tolerance symbol, the number goes first,
followed by the letter; that is different from the tolerance
system for limits and fits, where the letter goes first.
Thread tolerance class examples: 6g, 6h, 6H
The tolerance zones are arranged in a certain sequence
in relation to the zero line. That zero line represents the
nominal dimension of a given diameter. There are fewer
tolerance options for threads than we have for Limits and
fits.
Tolerance Classes of a Screw Thread
Depending on what tolerance zone is chosen, it
may put the maximum dimension equal to the nominal
dimension or a certain amount below. This applies to the
nominal pitch diameter and/or major diameter
Here are some of commonly used letters h, g. f, e
Nominal dimension for pitch diameter as well as major
diameter can be found in ISO 724.
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