9.3 Projected Tolerance Zones

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:

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

1. 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.
2. 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.
3. 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

1. 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.
2. 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.
3. 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).
4. 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.
5. 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.