3 years ago

91430 SPS cover edited - Electronic Fasteners Inc

91430 SPS cover edited - Electronic Fasteners Inc


THREADS IN BOTH SYSTEMSThread forms and designations have been the subject ofmany long and arduous battles through the years.Standardization in the inch series has come throughmany channels, but the present unified thread form couldbe considered to be the standard for many threadedproducts, particularly high strength ones such as sockethead cap screws, etc. In common usage in U.S.A.,Canada and United Kingdom are the Unified NationalRadius Coarse series, designated UNRC, Unified NationalRadius Fine series, designated UNRF, and several specialseries of various types, designated UNS. This thread,UNRC or UNRF, is designated by specifying the diameterand threads per inch along with the suffix indicating thethread series, such as 1/4 - 28 UNRF. For threads in Metricunits, a similar approach is used, but with some slightvariations. A diameter and pitch are used to designatethe series, as in the Inch system, with modifications asfollows: For coarse threads, only the prefix M and thediameter are necessary, but for fine threads, the pitch isshown as a suffix. For example, M16 is a coarse threaddesignation representing a diameter of 16 mm with apitch of 2 mm understood. A similar fine thread partwould be M16 x 1.5 or 16 mm diameter with a pitch of1.5 mm.For someone who has been using the Inch system, thereare a couple of differences that can be a little confusing.In the Inch series, while we refer to threads per inch aspitch; actually the number of threads is 1/pitch. Finethreads are referenced by a larger number than coarsethreads because they “fit” more threads per inch.In Metric series, the diameters are in millimeters, but thepitch is really the pitch. Consequently the coarse threadhas the large number. The most common metric threadis the coarse thread and falls generally between the inchcoarse and fine series for a comparable diameter.Also to be considered in defining threads is the toleranceand class of fit to which they are made. The InternationalStandards Organization (ISO) metric system providesfor this designation by adding letters and numbers in acertain sequence to the callout. For instance, a threaddesignated as M5 x 0.8 4g6g would define a thread of5 mm diameter, 0.8 mm pitch, with a pitch diametertolerance grade 6 and allowance “g”. These tolerancesand fields are defined as shown below, similar to theFederal Standard H28 handbook, which defines all of thedimensions and tolerances for a thread in the inch series.The callout above is similar to a designation class 3A fit,and has a like connotation.COMPLETE DESIGNATIONSMetric Thread DesignationNominal SizePitchTolerance Class DesignationM5 X 0.8 – 4g6gTolerance Position(Allowance)Tolerance GradeTolerance Position(Allowance)Tolerance Grade))))))Crest Diameter Tolerance SymbolPitch Diameter Tolerance Symbol80

METRIC THREADSExample of thread tolerance positions and magnitudes.Comparision 5/16 UNC and M8. Medium tolerance grades – Pitch diameter.µm+200+150+100+500–50–100–160–200µm5/16 UNC2BNUT THREAD6H2A 6g 6h5/16 UNCAllowanceM8PlainBOLT THREADAfterplatingAllowance = 0DEVIATIONSexternal internal basic clearanceh H noneg G smallelargeNOTE:Lower case letters = external threadsCapital letters = internal threads81

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