Overview
ISO 2306:1972 - "Drills for use prior to tapping screw threads" is an ISO standard that specifies recommended drill sizes to use before tapping parallel screw threads. It covers drill diameters for ISO metric threads (coarse and fine series), ISO inch threads (UNC and UNF), and pipe threads. Drill diameter is defined approximately as the nominal thread diameter minus the thread pitch. The standard provides tables of drill sizes (Tables 1–6) and an annex summary (Table 7) to guide tap drill selection for normal lengths of engagement.
Key topics and technical requirements
- Scope: Drill sizes for pre-tapping holes for metric threads, inch (UN) threads and pipe threads; intended for threads of normal engagement length.
- Drill diameter rule: Recommended drill diameter ≈ nominal thread diameter − pitch.
- Tabulated data: Detailed tables for:
- ISO metric coarse and fine pitch series (Tables 1 & 2)
- Unified coarse (UNC) and unified fine (UNF) inch threads (Tables 3 & 4)
- Pipe threads with and without pressure-tight joints (Tables 5 & 6)
- Annex summary of drill diameters across common metric pitches (Table 7)
- Class of fit: Minor diameters and drill recommendations reference common class tolerances (e.g., 5H, 6H, 7H) so users can match drill choice to thread tolerance.
- Selection notes: The standard explains practical influences on drilled hole size - drill point geometry, work material, lubricant, alignment, feed and speed - and notes that softer materials may deform, reducing the effective tapped minor diameter.
- Standardization of sizes: Most drill sizes are selected from ISO/R 235 (jobber/stub and Morse taper drills); a small number are exceptions indicated in the document.
Applications and users
ISO 2306 is practical for:
- Machinists and CNC programmers selecting tap drill sizes for production and prototyping
- Tooling and manufacturing engineers specifying hole sizes for threaded components
- Toolroom and maintenance departments stocking drill inventory and preparing tapping operations
- Quality engineers verifying correct pre-tap hole diameters to reduce tap wear and breakage
- Technical educators and documentation authors creating manufacturing process documents or drilling/tapping charts
Benefits include reduced tap breakage, improved tapping economy, consistent thread quality, and standardized drill inventory.
Related standards
- ISO/R 235 - Parallel shank twist drills (jobber and stub series)
- ISO/R 261 - Metric screw threads - general plan
- ISO/R 263 - Inch screw threads - general plan
- ISO/R 7 and ISO/R 220 - Pipe thread specifications (pressure-tight and non-pressure-tight)
Keywords: ISO 2306, tap drill sizes, tapping drill chart, ISO metric threads, UNC, UNF, pipe threads, drill diameter, tap drill selection.
