9.1 Hole & Slot Patterns, Multi-Segment Frames, & Composite Position Tolerancing

9.1 Hole & Slot Patterns, Multi-Segment Frames, & Composite Position Tolerancing

Quick Answer: Under ASME Y14.5-2009 Section 7.5.1, composite position tolerancing is identified by a single geometric characteristic symbol (⌖) spanning two or more horizontal segments. The upper segment establishes the Pattern-Locating Tolerance Zone Framework (PLTZF), which governs pattern location and orientation to the specified Datum Reference Frame (DRF) using larger tolerance zones. The lower segment establishes the Feature-Relating Tolerance Zone Framework (FRTZF), which governs internal feature-to-feature spacing and orientation to specified datums using smaller tolerance zones. In the FRTZF, datums constrain ORIENTATION ONLY, NEVER LOCATION. In contrast, multiple single-segment feature control frames feature separate position symbols for each segment, causing the lower segment to control both location and orientation relative to its datums.

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Feature Patterns & Simultaneous Requirements (ASME Y14.5-2009 Section 7.5)

In mechanical assemblies, mating components rarely interface through isolated features; instead, they interface through patterns of features of size such as bolt circles, rectangular mounting hole arrays, or sets of parallel alignment slots.

Defining a Pattern

A pattern is defined as two or more features of size to which a single geometric control is applied, or which are related to one another by basic dimensions. When a drawing callout specifies 4X Ø10.00 ± 0.20 followed by a position feature control frame, all four holes are bound together into a single geometric entity.

The Simultaneous Requirement Rule (ASME Y14.5-2009 Section 4.19)

A foundational principle on the ASME GDTP examination is the default Simultaneous Requirement rule:

• When multiple features or patterns of features are controlled by position or profile tolerances referencing the exact same datum reference frame in the exact same order of precedence and at the same material boundary conditions, they must be inspected simultaneously as a single composite pattern.
• The theoretical tolerance zones for all such features are locked in location and orientation to each other and to the common datum reference frame.
• Overriding the Rule: If manufacturing or inspection requires separate, independent evaluation of related patterns, the drawing must explicitly state the note SEP REQT (Separate Requirement) adjacent to each feature control frame.

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Anatomy of Composite Position Tolerancing (ASME Y14.5-2009 Section 7.5.1)

In many functional designs, the relative spacing between mating holes must be held to tight tolerances to ensure fastener assembly, while the overall location of the entire pattern relative to the part's outer edges can tolerate significantly greater variation. Specifying an unnecessarily tight general position tolerance increases manufacturing costs. Composite position tolerancing solves this dilemma by uncoupling pattern location from feature-to-feature spacing.

                    COMPOSITE POSITION FEATURE CONTROL FRAME
     ┌───┬─────────────────────────┬───┬───┬───┐
     │   │ Ø 0.8 Ⓜ                 │ A │ B │ C │  ◄── Upper Segment: PLTZF
     │ ⌖ ├─────────────────────────┼───┼───┼───┤      (Pattern Location & Orientation)
     │   │ Ø 0.2 Ⓜ                 │ A │ B │   │  ◄── Lower Segment: FRTZF
     └───┴─────────────────────────┴───┴───┴───┘      (Feature Spacing & Orientation Only)
       ▲
       └── Single Symbol Spanning Both Segments

1. Upper Segment: Pattern-Locating Tolerance Zone Framework (PLTZF)

• Pronunciation & Terminology: The upper segment defines the Pattern-Locating Tolerance Zone Framework (PLTZF) (pronounced plits-if).
• Function: The PLTZF controls the location and orientation of the pattern as an entire rigid group relative to the specified datum reference frame.
• Basic Dimensions: The PLTZF tolerance zones are located from the datum features by basic dimensions and are spaced relative to each other by basic dimensions.
• Tolerance Magnitude: The PLTZF specifies the larger tolerance value (e.g., $\varnothing 0.8\text{ mm}$ at MMC).

2. Lower Segment: Feature-Relating Tolerance Zone Framework (FRTZF)

• Pronunciation & Terminology: The lower segment defines the Feature-Relating Tolerance Zone Framework (FRTZF) (pronounced frits-if).
• Function: The FRTZF controls the internal feature-to-feature spacing within the pattern, as well as the orientation of the pattern relative to any datums repeated in the lower segment.
• Basic Dimensions: The FRTZF tolerance zones are located relative to each other by the basic dimensions that define the pattern geometry. However, they are NOT located by basic dimensions from the datum features!
• Tolerance Magnitude: The FRTZF specifies the smaller, refined tolerance value (e.g., $\varnothing 0.2\text{ mm}$ at MMC).
• Boundary Containment: To be accepted, the actual feature axes must lie simultaneously inside both the PLTZF tolerance cylinders and the FRTZF tolerance cylinders.

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The Inviolable Datum Rules for the FRTZF

Questions regarding datums in the lower segment of a composite frame represent some of the most heavily tested items on the ASME GDTP Technologist exam. Candidates must master three strict rules:

Rule 1: Orientation Only, Never Location

Datums referenced in the FRTZF constrain ORIENTATION ONLY (perpendicularity, parallelism, or angularity), NEVER location.

• Even though basic dimensions extend from Datum B or Datum C to the pattern on the drawing, the FRTZF tolerance framework is completely free to translate (float) relative to those datums.
• The FRTZF cylinders maintain their basic center-to-center distances to each other and maintain strict angular alignment (e.g., perpendicularity or parallelism) to the referenced datums, but their distance from those datums is constrained solely by the larger PLTZF cylinders.

Rule 2: Order of Precedence & Omission Rules

• Identical Precedence: Datums specified in the FRTZF must repeat the datums from the PLTZF in the exact same order of precedence.
  • If PLTZF specifies [ A | B | C ], valid FRTZF datum sequences are: [ A | B ], [ A ], or no datums [ ].
  • An FRTZF specifying [ B ], [ B | A ], or [ A | C ] is an illegal syntax error under ASME Y14.5-2009 Section 7.5.1.
• Omission of Datums: Lower priority datums may be omitted from the FRTZF:
  • No Datums ([ ⌖ | Ø0.2 Ⓜ ]): The FRTZF governs only feature-to-feature spacing. The framework is free to translate and rotate in any direction, constrained only within the PLTZF.
  • Primary Datum Only ([ ⌖ | Ø0.2 Ⓜ | A ]): The FRTZF controls feature-to-feature spacing and constrains perpendicularity/angularity to Datum A. The framework can translate freely in $X$ and $Y$ and can rotate about the axis perpendicular to Datum A.
  • Primary & Secondary Datums ([ ⌖ | Ø0.2 Ⓜ | A | B ]): The FRTZF controls feature-to-feature spacing, maintains perpendicularity to Datum A, and constrains rotational clocking (parallelism) relative to Datum B. However, it floats linearly in distance from Datum B!

Rule 3: Prohibition of New Datums

The FRTZF cannot introduce any datum feature that was not already established in the PLTZF. Specifying Datum D in the lower segment when the upper segment only referenced A, B, and C is strictly prohibited.

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Visual Mechanics & Tolerance Zone Kinematics: Floating & Orienting Cylinders

To visualize how composite position works in three dimensions, envision two distinct rigid cages or frameworks of tolerance cylinders:

                 PLTZF vs. FRTZF TOLERANCE ZONE MECHANICS

         ┌────────────────────────────────────────────────────────┐
         │                        DATUM B                         │
         └───────────────────────────┬────────────────────────────┘
                                     │ Basic Distance Y
                                     ▼
                 ┌──────────────────────────────────────┐
                 │   PLTZF Zone (Ø 0.8)                 │
                 │   Locked in location and             │
                 │   orientation to A, B, and C         │
                 │       ┌──────────────────────┐       │
                 │       │ FRTZF Zone (Ø 0.2)   │       │
                 │       │ Free to float within │       │
                 │       │ PLTZF; locked in     │       │
                 │       │ orientation to A & B │       │
                 │       │      ( • Axis )      │       │
                 │       └──────────────────────┘       │
                 └──────────────────────────────────────┘
                                     ▲
                                     │ Basic Distance X
         ┌───────────────────────────┴────────────────────────────┐
         │                        DATUM C                         │
         └────────────────────────────────────────────────────────┘

1. The PLTZF Framework: Consists of large cylinders (e.g., $\varnothing 0.8\text{ mm}$) located at exact basic dimensions from Datums A, B, and C. This framework is rigidly anchored in space.
2. The FRTZF Framework: Consists of small cylinders (e.g., $\varnothing 0.2\text{ mm}$) fixed at exact basic center-to-center distances to each other.
3. Kinematic Motion:
   • If the FRTZF references [ A | B ], this smaller framework must remain perpendicular to Datum A and parallel/perpendicular to Datum B, but it can slide up, down, left, and right (translate) within the larger PLTZF cylinders.
   • The physical feature axes of all manufactured holes must fall inside both the stationary PLTZF cylinders and the floating FRTZF cylinders simultaneously.

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Composite Position vs. Multiple Single-Segment Feature Control Frames

The distinction between a composite feature control frame and two single-segment feature control frames is one of the most critical topics on the certification exam.

     COMPOSITE POSITION FRAME           TWO SINGLE-SEGMENT FRAMES
   ┌───┬───────────────┬───┬───┐       ┌───┬───────────────┬───┬───┐
   │   │ Ø 0.8 Ⓜ       │ A │ B │       │ ⌖ │ Ø 0.8 Ⓜ       │ A │ B │
   │ ⌖ ├───────────────┼───┼───┤       ├───┼───────────────┼───┼───┤
   │   │ Ø 0.2 Ⓜ       │ A │ B │       │ ⌖ │ Ø 0.2 Ⓜ       │ A │ B │
   └───┴───────────────┴───┴───┘       └───┴───────────────┴───┴───┘
     Single Characteristic Symbol        Two Separate Characteristic Symbols

Fundamental Differences

• Visual Identifier: A composite frame has one geometric symbol spanning across multiple horizontal rows. Two single-segment frames have separate geometric symbols in each row.
• Datum Constraints in Lower Segment:
  • In a composite frame, the lower segment controls orientation only relative to Datum B. The FRTZF can float linearly relative to Datum B.
  • In two single-segment frames, each segment is a completely independent position requirement. The lower frame controls BOTH location and orientation relative to Datum B! The holes must be located within $\varnothing 0.2\text{ mm}$ of their basic coordinates from Datum B.

Master Comparison: Multi-Segment Location Controls

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Common Exam Traps: Patterns & Composite Position

• Trap 1: Believing the Lower Segment Locates from Datums: In [ ⌖ | Ø0.8 Ⓜ | A | B | C ] over [ ⌖ | Ø0.2 Ⓜ | A | B ], many examinees assume the holes must be within $0.2\text{ mm}$ of the basic dimension from Datum B. False. Datum B in the FRTZF controls orientation (rotation) only; pattern location to Datum B is governed solely by the $0.8\text{ mm}$ PLTZF.
• Trap 2: Reversing Datum Precedence in Composite Frames: Identifying [ ⌖ | Ø0.6 Ⓜ | A | B ] over [ ⌖ | Ø0.2 Ⓜ | B | A ] as valid. This is an invalid callout; datum order in composite position cannot be reversed.
• Trap 3: Confusing Composite Frames with Two Single-Segment Frames: When two separate position symbols are present, the lower frame controls location to its datums. If the lower frame reads [ ⌖ | Ø0.2 Ⓜ | A | B ], the pattern is locked in location within $0.2\text{ mm}$ from Datum B.
• Trap 4: Missing the Bonus Tolerance on Both Segments: When MMC (Ⓜ) is specified on both segments, departure from MMC provides bonus tolerance to both the PLTZF and the FRTZF independently based on actual feature size.
• Trap 5: Assuming FRTZF Can Exceed PLTZF: The lower segment tolerance value must always be smaller than the upper segment tolerance value ($t_{\text{FRTZF}} < t_{\text{PLTZF}}$).