Overview
ISO 2857:1973 specifies the manufacturing tolerances on the threaded portion of ground thread taps used to produce ISO metric threads. It applies to taps for tolerance classes 4H to 8H and 4G to 6G, for both coarse and fine pitches. The standard covers short taps specified in ISO 529 as well as any other ground thread taps having the same diameters and pitches. Internal threads produced by these taps are referred to as “nut” in accordance with ISO/R 965 terminology.
Key topics
- Scope and applicability
- Tolerances for ground thread taps intended to make ISO metric internal threads (classes 4H–8H, 4G–6G).
- Valid for ISO 529 short machine and hand taps and equivalent ground taps.
- Tolerance classes for taps
- Three tap classes (1, 2, 3) recommended according to the class of nut to be produced.
- Positioning of tap pitch-diameter tolerance expressed relative to a tolerance unit t (t = TD2 pitch tolerance, grade 5 of the nut).
- Indicative offsets: class 1, class 2, class 3 (document gives specific offsets in terms of t).
- Dimensional tolerances
- Major diameter: minimum deviation Js (nominal D + Js, with Js ≥ 0.4 t); maximum left to manufacturer.
- Pitch diameter: limits d2_min and d2_max computed from basic pitch diameter using lower/upper deviations Em and Es (tables provided).
- Minor diameter: no fixed tolerance; root radius should generally lie beneath the internal profile line.
- Geometric and cumulative limits
- Tolerance on thread angle and half-angle provided by pitch-dependent table.
- Cumulative pitch error over any number of threads limited to 0.05% of the measured length (minimum 0.008 mm).
Applications
ISO 2857 is practical for:
- Tap manufacturers designing and grinding threaded portions to specified tolerances.
- Tooling engineers and machine shops specifying taps for consistent thread production.
- Quality assurance and inspection teams checking pitch diameter, major diameter, angle tolerances, and cumulative pitch error.
- Procurement and standards compliance when purchasing ground taps for metric thread production.
Using ISO 2857 helps ensure interchangeability, thread fit consistency, and predictable nut manufacturing outcomes across production batches.
Related standards
- ISO 529 - Short machine taps and hand taps (dimensional designation).
- ISO/R 965 (ISO 965) - General purpose metric screw threads; tolerances and basic data.
- ISO/R 724 - Basic thread dimensions referenced for calculations.
Keywords: ISO 2857, ground thread taps, manufacturing tolerances, ISO metric threads, pitch diameter, major diameter, tolerance class, tap design, thread inspection.
Open preview (PDF)Ground thread taps for ISO metric threads of tolerances 4H to 8H and 4G to 6G coarse and fine pitches — Manufacturing tolerances on the threaded portion