By: Matthew Ong

Date: October 26, 2021

<aside> ❗ This document is to introduce GD&T practices in engineering drawings

</aside>

Definitions

• Datum: Theoretical reference location for a dimension (point, centre line/line, plane) [1]
• Feature: A physical portion of a part
• Datum Feature: Physical reference location (surface, holes, faces, slots) where a theoretical datum exists [1]
• Maximum Material Condition (MMC): The condition using corresponding size with minimum material reduction (largest size of an external feature or smallest size of an interal feature) [1]
• Least/Minimum Material Condition (LMC): The condition using corresponding size with maximum material reduction (smallest size of an external feature or largest size of an interal feature) [1]
• Regardless of Feature Size (RFC): The default condition that dictates a cotrol not being defined by feature size; overriden by MMC and/or LMC modifiers [2]
• Size Tolerance: Numeric values/bounds that define the size of the feature
• Geometric Tolerance: Numeric values/bounds that define the allowable form deviation of the part
• +- Tolerance: Upper and lower bounds on size (lenghts, widths, diameters) and position deviations
• Features of Size (FOS): Features that can be measured across opposing points (holes, shafts)
• Tolerance Zone (TZ): The space of allowable deviation defined by GD&T controls

Basic Conventions

1. Rule #1 of GD&T (Envelope Principle) → Size tolerances (+- values) can indirectly control geometric and size variations [2]

   → A shaft with a nominal diameter of 10mm and a tolerance of +-0.5 has an allowable size variation of 1mm and a circularity tolerance of 1mm (must fit inside a 10.5mm diameter circle or at MMC)