GS GstarCAD Academy
Manufacturing Standards · Mechanical CAD & Quality Control

True Position Tolerance for Hole Patterns

A GD&T control defining a cylindrical tolerance zone within which the center axis of a manufactured hole must lie relative to specified datums.

1. Definition & Engineering Significance

Traditional ±0.1mm linear coordinate tolerances create a square tolerance zone, artificially rejecting good parts whose diagonal deviation is functionally acceptable. True Position defines a round cylindrical zone, providing 57% more usable tolerance area without compromising fastener clearance.

2. Key Technical Characteristics & Rules

  • Cylindrical Tolerance Zone (⌀ symbol): Encompasses all 360 degrees equally around the theoretically exact basic dimension.
  • Fastener Clearance Formula: Minimum Clearance Hole = Maximum Fastener Body + Positional Tolerance Zone.
  • Composite Positional Control: Top segment controls pattern location to datums; bottom segment tightens hole-to-hole spacing within the pattern.

3. Cylindrical Tolerance Zones & True Position Math

True Position per ASME Y14.5 replaces legacy rectangular plus-minus coordinate tolerance zones with cylindrical tolerance fields:

  • 57% Extra Usable Area: A cylindrical tolerance zone (∅ 0.28mm) circumscribes a square ±0.10mm box, providing 57% more allowable manufacturing zone without reducing functional clearance.
  • True Position Formula: Position Deviation = 2 * √((X_actual - X_basic)² + (Y_actual - Y_basic)²).
  • Pattern Composite Callouts: Dual-tier feature control frames control pattern location relative to external datums in the top tier, while tightly controlling hole-to-hole spacing in the lower tier.

4. Functional Assembly Verification

When inspecting multi-hole flange patterns, coordinate deviation must be evaluated simultaneously across all holes when Maximum Material Condition is invoked. Inspecting holes individually on calipers masks collective pattern rotation errors that cause assembly bolt jamming.

Cross-Disciplinary Resources

Browse Related Tutorials Skill Certification Quiz CADGuide Toolbox (560+) ↗

Enterprise Production Implementation & GstarCAD 2027 Workflow

  • Database Serialization & Binary Fidelity: Ensure all entities and symbol table records associated with True Position Tolerance for Hole Patterns are authored on standardized ISO 13567 layers rather than Layer 0 or Defpoints, and verify lossless round-trip persistence in native DWG (AC1032) format.
Implementation Stage GstarCAD / AutoCAD Execution Quality Assurance (QA/QC) Gate
1. Environment Setup Load corporate .dwt template; verify INSUNITS and LUNITS. Confirm 1:1 metric (mm) or imperial unit consistency across all linked Xrefs.
2. Geometry & Metadata Authoring Apply associative constraints, block attributes, or GD&T tolerance frames. Verify zero disassociated annotations via ANNOMONITOR = 2.
3. Deliverable Exchange Publish via Sheet Set Manager (SSM) or export to DWG / IFC / STEP. Inspect output in DWG FastView or FastView 3D before client release.

Enterprise CAD Governance, Interoperability & Quality Assurance Reference

Published and maintained by the GstarCAD Academy Engineering Editorial Board under Suzhou Gstarsoft Co., Ltd. (SSE: 688657), this documentation resource adheres to international technical product documentation and geometric modeling standards including ISO 128 (General principles of presentation), ISO 13567 (CAD layer organization), ASME Y14.5-2018 (Dimensioning and Tolerancing), and ISO 19650 (Building Information Modeling information management). Whether evaluating enterprise DWG software, executing a zero-downtime migration from AutoCAD to GstarCAD 2027 Professional, or training multi-disciplinary engineering teams, organizations must establish verifiable technical governance across four operational pillars:

Verification Dimension Engineering Standard / Command Enterprise Production Acceptance Gate
Drawing Seed & Units INSUNITS, LUNITS, MEASUREMENT Enforce corporate .dwt seed template; verify 1:1 metric (mm) or imperial consistency across Xrefs.
Layer & Annotation Hygiene LAYTRANS, DIMASSOC=2, ANNOMONITOR Zero entities on Defpoints; 100% associative dimensions and multi-scale annotative text styles.
Transmittal & Field Review ETRANSMIT, PUBLISH, DWG FastView Package all dependent SHX/TTF fonts, CTB tables, and relative-path Xrefs; verify multi-page PDF/DWG output.