Overview
ISO 12256:2002 - "Aerospace - Drives, internal, cruciform, ribbed - Metric series" - is an international standard that specifies the dimensions and tolerances for metric internal cruciform ribbed drives used in aerospace screws. It covers drive geometries for screws with nominal thread diameters from 3 mm to 20 mm, provides configuration drawings (Figures 1–4), and tabulates dimensional data (Tables 1–4). Dimensions and tolerances are expressed in millimetres and location tolerances follow ISO 1101.
Key topics and requirements
- Scope: Dimensioning of internal cruciform ribbed drives for metric screws (nominal thread diameter 3–20 mm).
- Drive configurations: Includes pan head, normal 100° countersunk head, and reduced 100° countersunk head configurations with corresponding drive layouts.
- Drive codes: Uses drive size codes (R1–R5) to index dimensional sets and gauge criteria.
- Dimensional tables and figures: Detailed numeric values, gauge penetration limits and geometrical parameters are provided in Figures 1–4 and Tables 1–4.
- Tolerancing: All dimensions/tolerances in mm; presentation of location tolerances conforms to ISO 1101 (Geometrical Product Specifications).
- Designation rules: The standard defines drive designation format for unambiguous specification in drawings and procurement.
- Cross-references: Drivers and gauges used with these drives are defined in ISO 14280 and ISO 14281; other normative references include ISO 3202, ISO 5856, ISO 7913 and ISO 13921.
Applications and users
ISO 12256 is directly applicable to the aerospace supply chain where consistent, interchangeably fitted fasteners and tools are critical. Typical users include:
- Aerospace fastener designers and manufacturers specifying internal cruciform ribbed drives
- Screw and bolt designers selecting drive sizes for heads (pan, countersunk)
- Tool and driver manufacturers producing mating drivers and bits
- Quality, inspection and metrology teams using gauge penetration limits and tolerance rules
- Procurement and specification engineers ensuring component conformity and interoperability
- Maintenance and assembly teams requiring correct screw/drivers for field service
Related standards
ISO 12256 is essential for ensuring dimensional interchangeability and reliable torque transfer in aerospace fasteners, supporting consistent manufacturing, inspection and tooling across supply chains.