2.5 Screw Threads, Gears, and Splines

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:

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

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

1. Assuming gears and splines default to pitch diameter. They do not default at all.
2. Putting the qualifier inside the feature control frame. It goes beneath the frame or beneath the datum feature symbol.
3. 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.
4. Applying MMC bonus at the wrong diameter. Bonus for a threaded feature is computed from the size of the diameter the callout actually invokes.
5. 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.