4.4 Planar vs Datum Feature of Size

Planar vs Datum Feature of Size

Datum features come in two fundamental flavors: planar datum features (a single flat face) and datum features of size (a cylinder, pin, width, or slot — a feature defined by a size dimension). The two behave differently in how they constrain the part, what simulator they use, and whether datum shift is possible. Senior-level selection between them is driven by function and inspection practicality.

Planar Datum Features

A planar datum feature is a flat face. Its datum is a plane contacted by a surface plate (or equivalent). A planar primary datum arrests:

• 1 translation (normal to the plane)
• 2 rotations (about the two in-plane axes)

= 3 DOF. The face's orientation (perpendicularity to the next datum) is what locates the part; the face itself does not self-center because it has no size.

Planar datums are repeatable and shift-free. A surface plate contacts the high points of the face; the datum plane is the plate's plane. Because there is no size dimension, there is no MMB or LMB and no datum shift on a planar datum. RMB is effectively the only mode.

Worked Example: Planar Primary

A rectangular cover plate uses the bottom face as primary datum A, a long side as B, and a short side as C. The DRF is three mutually perpendicular planes. A hole pattern is |POS|Ø0.3|A|B|C|. The position zones are located by basic dimensions from the corner formed by A, B, and C. No datum shift is available regardless of the face's flatness — the part is locked to the planes.

Datum Feature of Size

A datum feature of size is a cylinder, pin, or width — a feature with a size dimension that produces a center (axis or center plane). Its datum is the axis or center plane derived from the simulator. The number of DOF it arrests depends on whether it is primary, secondary, or tertiary:

• Primary size datum (e.g. a bore as A): axis arrests 2 translations + 2 rotations = 4 DOF. A tertiary is needed to arrest rotation about the axis.
• Secondary size datum (e.g. a pin as B): arrests 1 translation (radial to the primary) + 1 rotation = 2 DOF, but only after the primary has arrested the others.
• Tertiary size datum: arrests the remaining rotation about the axis.

RMB vs MMB on a Size Datum

A size datum can be referenced at RMB or MMB (or LMB), and this changes behavior fundamentally:

• At RMB: the simulator expands or contracts to the actual mating size. The datum is the actual center of the feature. The part self-centers. No datum shift. This is the right choice when the feature mates on its axis and you want the part to center on its own geometry.
• At MMB: the simulator is fixed at maximum material (smallest hole / largest pin). The datum is the gage's center, not the part's center. If the part's feature departs from MMB, the part can shift on the gage — datum shift is available. This is the right choice when the part assembles to a fixed mating feature and the shift reflects functional play.

Width (Two Parallel Faces) as a Datum

A width datum is a datum feature of size defined by two parallel faces (a slot or tab). Its datum is the center plane between the faces. At RMB the simulator is two parallel plates that close to actual mating size; the center plane is the datum. At MMB the plates are fixed at MMC separation and shift is available. A width datum arrests:

• 1 translation (normal to the center plane)
• 1 rotation (about an axis in the plane)

= 2 DOF, regardless of being primary or higher (the rest of the lock-up comes from the other datums). This is one fewer than a cylinder primary because a width has no axis to arrest two rotations.

Comparison Table

Choosing Between Planar and Size Datums

The choice follows function:

• If the part locates by mating on a flat face (a cover bolted to a housing face), use a planar primary. The face sets orientation; the bolts are then located from it.
• If the part locates by mating on a bore or pin (a gear on a shaft, a pulley on a hub), use a size primary. The axis is the functional reference; a planar datum would not capture centering.
• If both a face and a bore mate (a flange with a pilot), a common pattern is planar primary (face) + size secondary (pilot bore at MMB): the face orients, the bore locates, and MMB on the bore allows the functional shift of the pilot in the bore.

Inspection Implications

Planar datums are simple: a surface plate and stops. Size datums at RMB need variable simulators (expanding mandrels, collets) that are more expensive and slower. Size datums at MMB use fixed functional gages that are faster and mirror assembly but introduce datum shift that the inspector must account for. A Senior-level drawing choice weighs: functional fidelity (MMB), repeatability (RMB), and inspection cost (planar and simple simulators).

Worked Example: Choosing Planar Primary + MMB Secondary

A pump cover bolts to a housing face (datum A, planar) and pilots on a bore (datum B, Ø20 ± 0.05, MMB = Ø19.95). The bolt pattern is |POS|Ø0.5|A|B(M)|.

• A (planar) arrests 3 DOF: Z translation, rotation about X, rotation about Y. No shift.
• B (size, MMB) arrests 2 more DOF: X and Y translation. The gage pin is Ø19.95 fixed. If the actual bore measures Ø20.02, datum shift = 0.07 diametral — the cover can shift 0.035 radially on its pilot, exactly as it will in assembly.

The combination orients the cover (face A) and centers it on the pilot (bore B) with functional play — a textbook functional DRF. Replacing B with a planar datum would lose the centering; replacing A with the bore would lose the orientation reference that the bolts need.