5.1 Inclined and Translated Datum Features

Inclined and Non-Orthogonal Datum Features

ASME Y14.5-2009 explicitly permits datum features that are not mutually perpendicular to one another. A part may have a primary planar datum A, a secondary datum B that is a planar surface angled to A, and a tertiary datum C that completes the DRF. The standard's rule is that the datum feature simulators are oriented to each other by the basic angle (a theoretically exact angle, framed on the drawing) between the features, and the simulators then contact the actual datum features at the highest-points / least-material condition logic appropriate to the modifier.

The key concept is that the Datum Reference Frame (DRF) is a theoretical, mutually perpendicular coordinate system, but it is established from datum features that need not themselves be perpendicular. The simulator for an inclined secondary datum is held at the basic angle to the primary datum simulator; the intersection of the contact constraints removes the remaining rotational and translational degrees of freedom. The result is a DRF whose axes are perpendicular to each other but whose origin/orientation is offset by the inclined feature.

Why this matters at Senior level

The Y14.5-2009 Senior exam weights Datum Referencing at 30%, and inclined datums are a frequent discriminator between Technologist and Senior candidates. The trap is to mentally "square up" the part and evaluate position/profile in the drawing's orthographic coordinate system. The standard requires evaluation in the DRF, which is rotated relative to the drawing by the basic angle.

The Datum Translation Modifier

The datum translation modifier is the triangle symbol (the "dynamic" or "tangent" note) placed in the feature control frame after the datum letter. Its function: it allows the datum feature simulator to translate along the datum it establishes, so the simulator seeks the actual mating size of the feature and the contact progresses geometrically rather than being pinned at a single theoretical tangent point.

• Without the modifier (RMB default on a planar feature): the simulator contacts the high points of the surface; for a feature of size at RMB, the simulator expands/contracts to actual mating size but does not translate along its own axis after contact.
• With the translation modifier: the simulator is allowed to translate, which means a secondary or tertiary datum of size can shift along its datum to maintain contact with the primary while the DRF is finalized. This is critical for non-orthogonal datum schemes because translation lets the angled simulator slide along its angled datum to keep the DRF consistent.

When it is used

Use the translation modifier when:

1. A secondary or tertiary datum is a feature of size (typically a hole or pin) referenced at RMB.
2. The geometry of the part is such that the simulator must translate to find a stable, repeatable DRF—commonly with inclined datums or when the primary is a complex/curved surface.
3. You want the datum shift behavior of the simulator to follow the actual mating envelope as it translates, rather than locking the contact.

Worked Example: Inclined Secondary Datum

Consider a cast bracket with:

• Datum A = the bottom planar mounting face (primary).
• Datum B = a machined planar face angled at a basic 30° to A (secondary).
• Datum C = a datum target hole (tertiary).

The feature control frame for a bore reads: ⌖|Position|Ø0.2(M)|A|B|C.

Step 1 — Establish the simulators

The A simulator is a surface plate contacting the high points of A. The B simulator is a planar bar held at exactly 30° to the plate; it is brought into contact with the high points of B. Because the angle is basic, there is no tolerance on the 30°—the simulator is at the theoretical angle. The C simulator is an expanding pin centered in the datum target hole.

Step 2 — Orient the DRF

The DRF is a right-handed coordinate system where one plane is A, the second plane is the B-simulator plane rotated 30° about the line of intersection with A, and the third is perpendicular to both. The origin is at the intersection of A, the B-simulator, and the C center.

Step 3 — Evaluate the controlled feature

The bore's position tolerance zone Ø0.2 at MMC is centered at its basic location measured in the DRF, not in the drawing view. If the drawing dimension shows the bore 50 mm "up" from A and 40 mm "along" from B, those 50 and 40 are projected onto the inclined DRF axes. A Senior candidate must recompute the basic coordinates in the rotated frame (or, equivalently, the CMM is aligned to the DRF and measures directly).

Trap: Squaring the part

If the inspector squares the part to A and the drawing's vertical edge, the bore will appear to be off-location by the tangent of 30° times the 40 mm dimension—a false rejection. The fix is to align the CMM to the DRF established by A|B|C, which is the same DRF the drawing's basic dimensions are defined in.

Trap: Modifier mismatch

If B is referenced at MMB instead of RMB and the actual B feature departs from its maximum material condition, the resulting datum shift must be added to the position tolerance in the direction the shift allows. The translation modifier interacts with this: with translation, the shift can be consumed anywhere along the B simulator's travel; without it, the shift is constrained to the contact geometry. Mixing these up is a top-drawer Senior error.