2.5 Screw Threads, Gears, and Splines

ASME GDTP Senior Level Exam

Track your progress as you read

Finished sections get a check mark.

1 Introduction & Exam Overview

1.1 Current Exam Facts 1.2 Senior Level vs Technologist and Y14.5 Revisions 1.3 How to Use This Guide and Study Strategy 1.4 The Y14.5-2018 Senior Exam and What Changed from 2009

2 Dimensioning and Symbology

2.1 Y14.5 Dimensioning Rules, Limits, and Fits 2.2 Basic Dimensions and Their Role 2.3 Feature Control Frame Structure and Reading 2.4 GD&T Symbols, Modifiers, and Communication 2.5 Screw Threads, Gears, and Splines 2.6 Angular Surfaces, Tapers, Radii, and Special Tolerance Cases

3 Datum Referencing I: Selection and Precedence

3.1 Functional Datum Feature Selection 3.2 Datum Precedence and Degrees of Freedom 3.3 Datum Targets 3.4 Datum Feature Simulators

4 Datum Referencing II: Modifiers and Shift

4.1 Multiple Datum Features and Datum Reference Frames 4.2 RMB, MMB, and LMB on Datum Features 4.3 Datum Shift Behavior 4.4 Planar vs Datum Feature of Size

5 Datum Referencing III: Setup and Calculations

5.1 Inclined and Translated Datum Features 5.2 Inspection Setup and Datum Establishment 5.3 Datum Calculation Walkthrough 5.4 Customized Datum Reference Frames and Rotational Constraint 5.5 Free State, Restrained Condition, and Nonrigid Parts

6 Form Controls

6.1 Flatness and Straightness 6.2 Circularity and Cylindricity 6.3 Rule #1, Form, and Size

7 Orientation Controls

7.1 Parallelism, Perpendicularity, and Angularity 7.2 Orientation Zones and Material Modifiers 7.3 Avoiding Unnecessary Orientation and Over-Control

8 Location I: Position and Boundaries

8.1 Position Tolerancing and True Position 8.2 Virtual and Resultant Condition 8.3 Bonus Tolerance at MMC and LMC

9 Location II: Application and Modifiers

9.1 Selecting MMC, LMC, and RFS by Function 9.2 Fixed and Floating Fastener Formulas 9.3 Projected Tolerance Zones 9.4 Coordinate Conversion and Appendix B Position Calculations

10 Location III: Patterns and Allocation

10.1 Composite Position Tolerancing 10.2 Patterns, Simultaneous Requirements, and Multiple Segments 10.3 Slots, Tabs, and Tolerance Allocation 10.4 Coaxial Feature Controls, Concentricity, and Symmetry

11 Profile I: Fundamentals and Strategy

11.1 Profile of Line and Surface 11.2 Defining the True Profile 11.3 Datum-Referenced Profile Strategy 11.4 Equal and Unequal Disposition

12 Profile II: Scope and Verification

12.1 All-Around, All-Over, and Limited Scope 12.2 Composite Profile and Coplanarity 12.3 Castings, Weldments, Sheet Metal, and CMM Evaluation 12.4 Combined Controls and Profile as a Refinement

13 Runout Controls

13.1 Circular and Total Runout 13.2 Datum-Axis Simulation and Rotating Parts 13.3 Runout vs Position, Profile, and Form

14 Exam Strategy and Calculations

14.1 Virtual/Resultant Condition and Bonus Formulas 14.2 Datum Shift and Worst-Case Boundary Calculations 14.3 Functional Gaging and Verification 14.4 Test-Day Tactics, Common Traps, and Review

2.5 Screw Threads, Gears, and Splines

Key Takeaways Explain This Topic AI

Each tolerance of orientation or position and each datum reference specified for a screw thread applies to the pitch cylinder by default.

To override the screw-thread default you place a note such as MAJOR DIA or MINOR DIA beneath the feature control frame or beneath the datum feature symbol.

Gears and splines have NO default: the drawing must always state MAJOR DIA, PITCH DIA, or MINOR DIA for every geometric tolerance and every datum reference on them.

Omitting the qualifier on a gear or spline is a drawing error, not a defaulted specification, and Senior questions test exactly that distinction.

A threaded hole that locates a fixed fastener is normally toleranced with position and a projected tolerance zone, because the pitch cylinder controls the direction the stud will lean.

Last updated: August 2026

Screw Threads, Gears, and Splines

Quick Answer: Geometric tolerances and datum references on a screw thread default to the pitch cylinder ; override with a note such as MAJOR DIA or MINOR DIA placed beneath the feature control frame or datum feature symbol. Gears and splines have no default — the qualifying note is mandatory on every callout.

A threaded hole, a gear tooth flank, and a spline all look like features of size on a drawing, but each of them has three candidate diameters: major, pitch, and minor. A geometric tolerance is meaningless until the drawing says which one it applies to. ASME Y14.5 handles the ambiguity in two different ways, and the difference between them is exactly the kind of detail the Senior exam uses to separate candidates who have read the standard from candidates who have only used GD&T.

Screw threads: pitch cylinder by default

For screw threads, the rule is a default with an override :

Each tolerance of orientation or position and datum reference specified for a screw thread applies to the axis of the thread derived from the pitch cylinder .

So a feature control frame applied to a tapped hole with no additional note controls the pitch cylinder axis . Likewise, a datum feature symbol on a thread establishes a datum axis from the pitch cylinder.

To override, place a note beneath the feature control frame or beneath the datum feature symbol:

Note Meaning
MAJOR DIA The tolerance or datum applies to the major-diameter cylinder
MINOR DIA The tolerance or datum applies to the minor-diameter cylinder
(no note) Pitch cylinder — the default

The placement matters. The note goes below the frame or symbol, not inside it, and it is not a modifier in the tolerance compartment.

When to override

The pitch cylinder is the functional default because that is the surface that transmits load and controls how a screw actually seats. Override when a different diameter does the real work:

MAJOR DIA on an external thread when the thread's major diameter pilots into a counterbore — the major diameter, not the pitch cylinder, is the locating surface.

MINOR DIA on an internal thread when a dowel or shoulder pilots in the tapped hole's minor diameter, or when the tap drill is the feature actually being inspected.

MINOR DIA when the design must guarantee wall thickness from the tapped hole to an adjacent surface — the minor diameter is the material-side boundary that matters.

If you cannot articulate a functional reason, leave the default. A gratuitous MAJOR DIA note on a tapped hole adds inspection cost and usually contradicts how the joint works.

Gears and splines: no default at all

Gears and splines are treated differently, and this is the trap:

Each tolerance of orientation or position and datum reference specified for gears and splines must designate the feature to which it applies: MAJOR DIA, PITCH DIA, or MINOR DIA.

There is no default . A geometric tolerance on a gear or spline without a qualifying note is an incomplete drawing specification — not a pitch-diameter callout by implication. The standard shifts the burden to the designer because gear and spline geometry varies far too much for a single default to be functionally sensible: an involute spline may pilot on its major diameter, its minor diameter, or its pitch diameter depending on the fit class, and the standard refuses to guess.

Senior questions frequently present a spline with a bare perpendicularity or position callout and ask what the drawing means. The correct answer is that the specification is incomplete and must be qualified , not that it defaults to the pitch diameter.

Summary table

Feature Default Override method Bare callout is...

Screw thread Pitch cylinder MAJOR DIA or MINOR DIA note under the frame/symbol Valid — means pitch cylinder
Gear None MAJOR DIA, PITCH DIA, or MINOR DIA note is required Incomplete drawing
Spline None MAJOR DIA, PITCH DIA, or MINOR DIA note is required Incomplete drawing
Threaded holes and projected tolerance zones

A tapped hole almost never wants a plain position tolerance. Because the fastener is fixed by the thread, the pitch cylinder's orientation determines which way the installed stud leans, and the stud's misalignment grows with the length of the mating part it must pass through. That is precisely the case a projected tolerance zone exists for: the zone is projected out of the threaded hole to the height of the mating part, so the tolerance is applied where the interference will actually occur. Section 9.3 develops the calculation; the point here is that the projected zone acts on the pitch cylinder axis unless the drawing says otherwise.

Verification consequences

The choice of diameter changes the gage. A functional gage for a tapped hole controlled at the pitch cylinder uses a threaded gage pin whose thread engages the part's thread and whose plain projecting shank represents the projected tolerance zone. A gage for the same hole qualified MINOR DIA uses a plain cylindrical pin at the minor diameter's virtual condition. Those are different tools with different costs, so the qualifier is not a paperwork detail — it determines what the inspection department has to buy.

Traps to rehearse

Assuming gears and splines default to pitch diameter. They do not default at all.

Putting the qualifier inside the feature control frame. It goes beneath the frame or beneath the datum feature symbol.

Forgetting that the rule covers datum references too. A datum feature symbol on a thread establishes the datum from the pitch cylinder, subject to the same override.

Applying MMC bonus at the wrong diameter. Bonus for a threaded feature is computed from the size of the diameter the callout actually invokes.

Specifying a thread as a primary datum feature when the part is located by a shoulder. A thread is a poor datum feature when a machined face or pilot diameter is doing the locating.