Study guide
9.3 Projected Tolerance Zones
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
9.3 Projected Tolerance Zones
Key Takeaways Explain This Topic AI
A projected tolerance zone extends the position zone above (or below) the surface by a specified height, controlling the axis where the mating part actually engages rather than where the feature originates.
Projected zones are required for threaded holes, press-fit pins, and welded/bolted studs because the engaged length of the fastener — not the thread depth or the hole bottom — controls whether the mating part assembles.
The projected height is specified in the feature control frame as the value after the projected-zone symbol (a circle with a vertical line through it), and equals the maximum engagement length of the mating part.
Inspection uses a gauge pin or threaded plug that projects the specified height above the surface, and the position is verified at that projected height, not at the surface.
Using a non-projected zone is a common Senior trap: a threaded hole can pass a conventional position check yet allow a tilted stud to bind the mating part above the surface.
Last updated: August 2026
9.3 Projected Tolerance Zones
Quick Answer: A projected tolerance zone is a position zone that is projected above the surface by a specified height, so it controls the fastener's axis where the mating part engages, not where the thread starts. Use it for threaded holes, press-fit pins, and studs. The symbol is a circle with a vertical line through it, placed in the feature control frame with the projected height.
Why a projected zone is needed
Consider an M8 threaded hole in a base plate. A mating plate 15 mm thick will bolt to it. The position tolerance controls the threaded hole's axis — but which axis? The thread is cut several millimeters deep into the plate, and the axis can tilt within the thread. If the position tolerance is checked only at the threaded portion (a non-projected zone), a tilted thread can still lie within the Ø zone inside the hole while the stud's projected axis — where the mating plate actually engages — is far off-center.
The engaged length of the fastener, not the thread depth, controls assembly. The mating plate sits on the surface and engages the stud for the 15 mm of its thickness. If the stud is tilted, the top of the stud is displaced by tilt × engagement length , and the mating plate's clearance hole may not fit over it. A conventional position zone that ends at the surface cannot see this.
The projected tolerance zone solves this by projecting the cylindrical position zone above the surface by a specified height (the engagement length). The entire projected axis of the threaded hole — and therefore of the engaged fastener — must lie within the zone.
Symbol and notation (Y14.5-2009)
The projected tolerance zone symbol is a circle with a vertical line extending above it (modified by a small upward tick in some renderings). In the feature control frame it is placed after the tolerance value , followed by the projected height : | ⊕ | Ø0.2 Ⓟ 15 | A | B | C |
Reading: position, diameter 0.2, projected 15 mm , datums A, B, C. The zone is a Ø0.2 cylinder that begins at the surface and extends 15 mm above it (in the direction of fastener engagement). The height value is the minimum projected height and equals the maximum engagement length of the mating part — typically the mating plate thickness or the stud height above the surface, whichever controls.
For a blind threaded hole the projected zone extends away from the hole opening, above the surface. For a through hole with a press-fit pin, the projection is in the direction the pin protrudes.
How to specify the projected height
The projected height should equal the maximum engagement length of the mating feature:
- Feature Projected height =
- Threaded hole + bolted cover plate Cover plate thickness (max engagement)
- Threaded hole + stud + nut Stud height above the surface (or nut engagement length, whichever governs)
- Press-fit dowel pin locating a mating plate Dowel projection height above the surface
- Threaded hole, deep engagement Full engagement length, not thread depth
Do not use the thread depth as the projected height — the thread depth only governs pull-out strength, not the geometric engagement that controls assembly.
Worked example — stud and cover plate
An M8 × 1.25 threaded hole is tapped into a cast aluminum housing. A steel cover plate 15 mm thick is bolted to the housing with an M8 cap screw that engages the full 15 mm. The drawing specifies:
4 × Ø8.5 clearance hole in cover; position Ø0.25 at MMC, A, B, C.
4 × M8 × 1.25 threaded hole in housing; position Ø0.2 Ⓟ 15 , A, B, C.
Analysis:
The cover clearance holes get a conventional (non-projected) zone; the cover sits flat on the surface, so the hole position at the surface is what matters.
The housing threaded holes get a projected zone 15 mm above the housing face. The Ø0.2 cylinder starts at the face and extends 15 mm upward.
A threaded plug gauge 15 mm long is screwed into each threaded hole. The gauge pin represents the stud axis. The CMM (or a functional receiver gage) verifies that the pin's axis lies within Ø0.2 over the full 15 mm projected height.
If the thread is tilted 0.1 mm over its 20 mm thread depth, the thread-bottom axis might be within a non-projected Ø0.2 zone. But the projected axis at 15 mm above the surface would be displaced by approximately (0.1 / 20) × (15 + depth_to_surface) — easily exceeding Ø0.2 and binding the cover. The projected zone catches this; a non-projected zone would not.
How it is inspected
Screw a threaded plug gauge of the projected height into the threaded hole (or insert a gauge pin into a press-fit pin hole). The gauge pin represents the projected fastener axis.
With a CMM, measure the pin's axis at two heights within the projected length and confirm the axis lies within the Ø zone over the full 15 mm.
Alternatively, a functional receiver gage with a pin of the projected height and a gage hole at the virtual condition is placed over the projected pin; if it seats, the projected zone is satisfied.
The inspection deliberately does not measure the thread itself at the surface; it measures the projected representation of the engaged fastener.
Interaction with MMC and bonus
Projected zones can carry an MMC modifier . When the threaded hole is at MMC (the condition of maximum material — for an internal thread this corresponds to the minimum effective diameter, i.e., the tightest thread), bonus tolerance grows as the actual thread departs from MMC. The virtual condition is then built at MMC and the functional gage is sized accordingly. In practice, gaging internal threads at MMC is complex because the thread's MMC is defined by the maximum-material virtual condition of the pitch diameter; many drawings specify projected zones at RFS for simplicity and rely on CMM measurement.
Senior-level traps
Specifying a non-projected zone for a threaded hole that engages a mating part. The thread may pass inspection at the surface while the projected axis is far out of tolerance. The Senior exam will show a tilted stud binding a cover and ask which control was missing — the answer is a projected tolerance zone.
Using thread depth as the projected height. Thread depth controls pull-out strength, not engagement. The projected height is the engagement length of the mating part.
Projecting in the wrong direction. The projection must be in the direction the mating part engages — above the surface for a blind hole that receives a bolt from above, below for a through hole engaging from underneath. The drawing must make the direction unambiguous (a note or a direction-indicating symbol where needed).
Confusing the projected-zone symbol with the depth symbol. The depth symbol (↧) specifies how deep the hole is cut; the projected-zone symbol (Ⓟ) specifies how high the position zone extends above the surface. They are unrelated controls and are not interchangeable.
Forgetting that the projected zone replaces, not adds to, the surface zone. The position tolerance applies within the projected height ; it is not an additional zone on top of a conventional zone. The drawing specifies either a projected zone or a conventional zone, not both, for the same feature.