Metric (ISO) thread designations
An ISO metric thread is designated by M, the nominal diameter in millimeters, the pitch, and the tolerance class, in that order: M10×1.5-6H. ISO 965-1 sets out the designation, and ISO 261 lists which diameter and pitch combinations are standard.
- M10: the ISO metric profile, 60°, with a 10 mm nominal (major) diameter.
- ×1.5: the pitch, 1.5 mm from crest to crest. For a coarse-pitch thread it may be left out, so M10 alone means M10×1.5. A fine pitch, as in M10×1.25, is always written.
- -6H: the tolerance class. A capital letter is an internal thread (a nut or tapped hole) and a lowercase letter an external thread (a bolt), so a mating pair is written 6H/6g. The number is the tolerance grade and the letter the position of the band.
- LH: added for a left-hand thread, as in M10×1.5-6H-LH. A thread with no hand stated is right-hand.
When no class is given, ISO 965-1 takes 6H for nuts and 6g for bolts, the medium class for general use, for the common sizes. Check the drawing's notes before you assume it. A multi-start thread states its lead and pitch separately, as in M16×Ph3P1.5, which is a two-start thread advancing 3 mm per turn.
The metric thread calculator gives the basic pitch and minor diameters of any metric size from its diameter and pitch.
Unified inch thread designations
A unified thread (ASME B1.1) is designated by its size, its threads per inch, its series and its class: 1/4-20 UNC-2A.
- 1/4: the nominal size, a 1/4 in major diameter. Small sizes use numbers instead, as in #10-32.
- -20: 20 threads per inch, a pitch of 1/20 in. Inch threads are counted per inch rather than given as a pitch.
- UNC: the coarse series. UNF is fine and UNEF extra fine; UN with a fixed pitch, such as 8-UN, is a constant-pitch series.
- -2A: the class. A is external and B internal; 1, 2 and 3 run from the loosest to the tightest fit, and 2A/2B is the usual class for general fasteners.
- LH: added for a left-hand thread.
| Piece | Metric example | Unified example |
|---|---|---|
| Size | M10 (10 mm) | 1/4 (0.25 in) |
| Spacing | ×1.5 (pitch, mm) | -20 (threads per inch) |
| Series | Coarse unless a fine pitch is written | UNC, UNF, UNEF or UN |
| Class | -6H internal, -6g external | -2B internal, -2A external |
| Hand | -LH if left-hand | -LH if left-hand |
Tapped holes: thread depth and drill depth
A tapped blind hole has two depths. The thread depth is how deep the full thread must go. The drill depth is how deep the hole is drilled, which must be deeper so the tap's lead and the chips have room. A callout such as Ø5 ↧15, M6-6H ↧12 gives both. When only the thread depth is given, the drill depth is the shop's choice. When neither is given and no view shows them, both are missing.
The tap drill is a process choice, not a requirement on the part. A common starting point for metric coarse threads is a drill near the nominal diameter minus the pitch, such as 5 mm for M6×1. The tap maker's recommendation and the material decide the real size.
How threads are drawn
Threads are not drawn as helices. ISO 6410-1 draws the crests as a continuous thick line and the roots as a continuous thin line; in an end view the root is a thin arc of about three quarters of a circle. ASME Y14.6 allows detailed, schematic and simplified representations; its simplified form shows the root diameter with hidden (dashed) lines. The note carries the meaning, so a thread that is drawn but not called out is incomplete, and a callout on a hole whose drawn size disagrees with it is a conflict to raise.
A checklist for every thread callout
01 System and size
M for metric, a fraction or number with UNC, UNF or UN for unified. Read the nominal diameter.
02 Pitch
Read the pitch or threads per inch. A metric callout without one means the coarse pitch from ISO 261.
03 Class and hand
Read the tolerance class, internal or external, and LH if present. Find the drawing note that applies if no class is given.
04 Depths
For a blind tapped hole, find the thread depth and the drill depth, or the view that shows them.
05 Mating part
Check that the mating thread has the same size, pitch and hand, and a class that pairs with this one.
What TruePart reads today
In the Technical Preview, TruePart reads thread (M) callouts, and the one part family it builds has no threaded feature.
Questions
What does M6 mean without a pitch?
An ISO metric thread of 6 mm nominal diameter with the coarse pitch, 1 mm. A fine pitch, such as M6×0.75, is always written out.
What is the difference between 6H and 6g?
6H is a tolerance class for an internal thread and 6g one for an external thread. The capital or lowercase letter tells you which; they are the usual mating pair.
What does 1/4-20 UNC-2A mean?
A unified inch thread of 1/4 in nominal diameter, 20 threads per inch, from the coarse series, class 2A, which is external.
Why does a tapped hole have two depths?
The thread depth is how far the full thread must reach. The drill depth is deeper, so the tap's tapered lead and the chips have room below the last full thread.
Sources
- ISO 965-1, ISO general purpose metric screw threads, Tolerances, Part 1: Principles and basic data · ISOThe designation of metric threads, tolerance classes such as 6H and 6g, and multi-start designation.
- ISO 261, ISO general purpose metric screw threads, General plan · ISOThe standard combinations of diameter and coarse or fine pitch.
- ASME B1.1, Unified Inch Screw Threads (UN, UNR, and UNJ Thread Forms) · ASMEUnified series, threads per inch and classes 1A to 3B.
- ISO 6410-1, Technical drawings, Screw threads and threaded parts, Part 1: General conventions · ISOThe simplified representation of threads on drawings.
- ASME Y14.6, Screw Thread Representation · ASMEThread representation and callouts on ASME drawings.
What TruePart does today
- LimitIn the Technical Preview, TruePart reads thread (M) callouts, and the one part family it builds has no threaded feature.