Precision Measurement & Metrology
Micrometers, gauge blocks, height gauges, dial indicators, tolerance interpretation, and advanced GD&T per ASME Y14.5 including feature control frames, datum reference frames, MMC boundary calculations, and complex positional tolerances.
- Select the correct measuring instrument for a given machining application
- Interpret bilateral, unilateral, and limit tolerances on engineering drawings
- Explain GD&T symbols including position, concentricity, and flatness
- Read and interpret feature control frames per ASME Y14.5
- Calculate Maximum Material Condition (MMC) bonus tolerance for hole and shaft features
- Identify primary, secondary, and tertiary datum reference frames and explain their purpose
- Apply complex positional tolerance callouts to multi-feature part inspection
Lesson 1
Tolerances & Geometric Dimensioning
Tolerance Types
A tolerance is the total allowable variation in a dimension. There are several types:
Bilateral tolerance - the dimension can vary equally in both directions from the nominal size. A dimension of 25.00 +/- 0.05 mm means the part can be anywhere from 24.95 to 25.05 mm. The +/- symbol indicates bilateral tolerance.
Unilateral tolerance - the dimension can vary in only one direction. For example, 25.00 +0.00 / -0.05 means the part can be 25.00 to 24.95 mm but never larger than 25.00.
Limit tolerance - specifies the maximum and minimum sizes directly (e.g., 25.05 / 24.95 mm).
Bilateral Tolerance
Format: 25.00 +/- 0.05
Range: 24.95 to 25.05
Varies both directions equally
Unilateral Tolerance
Format: 25.00 +0.00 / -0.05
Range: 24.95 to 25.00
Varies in one direction only
Limit Tolerance
Format: 25.05 / 24.95
Range: 24.95 to 25.05
Max and min sizes given directly
GD&T Symbols (ASME Y14.5)
Geometric Dimensioning and Tolerancing (GD&T) per ASME Y14.5 uses symbols to control the form, orientation, and location of features beyond simple size tolerances. The 2026 RSOS framework tests GD&T at a deeper level than the legacy NOA - machinists must read and apply feature control frames, not just recognize symbols.
The symbol of a circle with a cross inside represents position in GD&T. Position controls the location of a feature (typically a hole) relative to datum references.
| Symbol | Name | Controls |
|---|---|---|
| Circle with cross | Position | Location of features |
| Circle | Concentricity | Coaxiality of features |
| Two parallel lines | Flatness | Surface waviness |
| Circle with arrow | Runout | Surface variation during rotation |
| Diamond | Profile of a line | Cross-sectional shape |
Feature Control Frames
A feature control frame is the rectangular box on a drawing that specifies a GD&T requirement. It reads left to right in three sections:
Example: a position callout reading [position symbol | 0.2 (M) | A | B | C] means: the hole position must fall within a cylindrical tolerance zone of 0.2 mm diameter when the hole is at MMC, referenced to datum frame A-B-C.
Datum Reference Frames
Datums are theoretically exact points, lines, or planes established from part surfaces. A datum reference frame (DRF) uses three mutually perpendicular planes to fully constrain a part for inspection:
Primary Datum (A)
Controls the most critical direction
Contacts 3 points on the datum feature
Eliminates 3 degrees of freedom (one translation + two rotations)
Secondary Datum (B)
Contacts 2 points on the datum feature
Eliminates 2 more degrees of freedom
Perpendicular to primary datum
Tertiary Datum (C)
Contacts 1 point on the datum feature
Eliminates the final degree of freedom
Fully constrains the part for measurement
MMC and Bonus Tolerance
Maximum Material Condition (MMC) is the state where a feature contains the most material:
- For a hole: MMC is the smallest allowable hole size (more material remains around the hole)
- For a shaft: MMC is the largest allowable shaft size (most material on the shaft)
When a position tolerance includes the MMC modifier (circled M), bonus tolerance is earned as the feature departs from MMC:
Bonus tolerance = actual feature size departure from MMC
Example: a hole with a diameter of 10.0 mm minimum (MMC) and a position tolerance of 0.2 mm at MMC. If the hole is actually machined to 10.3 mm, the bonus tolerance is 0.3 mm, giving a total allowable position tolerance of 0.2 + 0.3 = 0.5 mm.
Complex Positional Tolerances
A composite positional tolerance uses two feature control frames stacked vertically for the same feature:
- The upper frame (FRTZF - Feature Relating Tolerance Zone Framework) controls location relative to datums
- The lower frame (PLTZF - Pattern Locating Tolerance Zone Framework) controls the spacing between features within the pattern
This allows a bolt-hole pattern to have a loose location tolerance relative to the part edges but a tight spacing tolerance between the holes themselves, reflecting real functional assembly requirements.
RSOS Exam Focus
The 2026 RSOS IP exam tests applied GD&T reasoning - not just symbol recognition. Expect scenario questions: given a feature control frame callout and an actual measured size, calculate the total available tolerance zone. Know MMC bonus tolerance arithmetic and be able to identify the three datum planes in a datum reference frame.
A dimension of 25.00 +/- 0.05 mm is a bilateral tolerance. In GD&T, the circle with a cross inside represents position. A feature control frame contains the geometric symbol, tolerance value with optional MMC modifier, and datum references. Bonus tolerance equals the amount the actual feature size departs from MMC. Three datums (primary, secondary, tertiary) define a complete datum reference frame.