GlossaryKind of term: Standard and conventionThread tolerance class (6H, 6g)
What is a thread tolerance class?
A thread tolerance class is the grade and position written after a metric thread size, such as 6H for a tapped hole or 6g for a bolt, defined in ISO 965-1.
A thread tolerance class is the tolerance designation of an ISO metric thread: a number for the tolerance grade, which sets the width of the zone, and a letter for the tolerance position relative to the basic profile. Capitals apply to internal threads, in practice G or H; lower case to external threads, usually e, f, g or h. With H and h the zone touches the basic profile; G, e, f and g leave an allowance. The class follows the size, as in M8-6H or M10x1-6g.
What does the standard say about the thread tolerance class?
ISO 965-1 sets out the system for ISO metric threads, with the limits of size in the later parts of the series. It defines fine, medium and coarse tolerance qualities, each with recommended classes per length of thread engagement: short (S), normal (N) or long (L). At medium quality and normal engagement this is 6H for internal and 6g for external threads, and in the usual reading that class may be left out of the designation; the smallest sizes have other default classes. For coated threads the limits generally apply before coating unless the drawing says otherwise. Where two classes appear, such as 5H6H, the first applies to the pitch diameter and the second to the crest diameter: the minor diameter of a nut thread, the major diameter of a bolt.
How does the thread tolerance class work in practice?
The class carries through to tooling and inspection. Taps come in tolerance classes under ISO 2857: ISO2 is intended for 6H, ISO1 for finer classes such as 4H and 5H, ISO3 for looser ones such as 7H. Whether the hole actually lands in class also depends on material, lubrication, tap wear and the tapping method. Thread milling sets the size through the cutter compensation, so one cutter can produce several classes. It is inspected with a go and a no-go gauge. For internal threads in position H the go gauge is the same for every grade and the no-go gauge belongs to the grade. A finer or unusual class can therefore mean a different tap and another gauge.
What is the thread tolerance class often confused with?
A thread tolerance class is not an ISO 286 fit. The notation looks similar, and in both systems H means the zone starts at the nominal size, but 6H concerns the pitch and crest diameters of a thread and H7 a plain bore, with tables of their own. Nor is the class the pitch: in M8x1-6g, x1 gives the fine pitch and 6g the tolerance.
How do you see the thread tolerance class in Blake?
In Blake the thread, with its class as the drawing gives it, sits with the part as a proposal. Whether you confirm the class or change it, the record shows who did so and when, with a note if you add one. Drilling and tapping are separate lines in the estimate, and their figures are marked as estimates. More on requests.
Frequently asked questions
Which class applies when the drawing only says M8?
If the drawing refers to ISO metric threads without a class, the usual reading of ISO 965-1 is the medium class: 6H for internal and 6g for external threads. Where a company standard applies or the part is functionally critical, ask.
What happens to the thread class after plating?
The coating takes up room on the flanks. Position g leaves a small allowance that can absorb a thin coating; for thicker coatings the designer can choose e or f. Agree whether the class applies before or after coating.
How do I check a 6H thread?
With a 6H thread plug gauge. The go end must screw in over the full length, the no-go end may enter only a turn or two; the gauging standard has the exact rule. Check frequency goes in the control plan.