2026-04-04
The surface the CMM touches to inspect the part should be the same surface the fixture touches to hold it. When they are not, every tolerance on the drawing is referenced to a datum the fixture does not control.
Three contact points define the primary plane — constraining Z translation, X rotation, and Y rotation (3 DOF). Two contact points define the secondary line — constraining Y translation and Z rotation (2 DOF). One contact point defines the tertiary position — constraining X translation (1 DOF). Total: six degrees of freedom, fully constrained.
Every fixture implements some version of this, even if the operator does not think about it that way. A vise jaw is a secondary datum. A hard stop is a tertiary datum. The bottom of the vise or fixture plate is the primary datum. The language is GD&T. The physics is older than GD&T.
The primary datum is the seating face — the largest Z-aligned flat face on the part.
It must be large enough to resist cutting torque without the part rocking. In OPJAW's
surface analyzer, a face qualifies as "flat" if its normal is within 5 degrees of the
approach axis. The alignment threshold is cos(5°) ≈ 0.9962.
The function
get_flat_faces(shape, approach_axis=Axis.Z, angle_tolerance=5.0) collects all
planar faces, filters by alignment, and returns them sorted by area descending. The largest
qualifying face becomes the seating surface. orient_to_seating_face(part) takes
this face, flips the part if the face normal points up, and translates so the seating face
sits at Z=0, centered on the XY origin.
If no face passes the alignment filter, there is no primary datum. The function raises
ToolingCompatibilityError. The part cannot be fixtured with a planar seating
scheme — it needs a conforming fixture, V-block, or collet.
When multiple faces are equally large, the choice is ambiguous. The surface analyzer picks the first by sort order, but two faces with near-identical area and different orientations can produce completely different fixture setups. This is where operator judgment — or a more sophisticated heuristic — must intervene.
For vise jaws: the clamping axis is the secondary datum. The jaw faces constrain the part's position perpendicular to the clamping direction. The part seats on the primary datum (vise bottom or parallels), and the jaw faces establish the secondary reference.
For fixture plates: a side stop or dowel pin pair constrains rotation around the seating face normal and translation in one horizontal axis. Two pins on one edge give you two contact points — the minimum for a secondary datum.
A hard stop, locating pin, or jaw shoulder sets the part's position along the clamping axis. Without this, the part can slide between the jaws.
On a fixture plate, this is typically a single dowel pin or an end stop perpendicular to the secondary stops. One contact point — the minimum for a tertiary datum.
The tertiary datum is the one most often neglected. Operators push the part against the jaw and call it located. For single-operation roughing, this is usually fine. For multi-operation work where you need to return to the same position after flipping, it is not. See Multi-Op Fixturing: Datum Transfer Between Operations for the full picture.
No flat face. The pipe cross (04_pipe_cross.step in the stress
test set) has a flat ratio of 1.4% — effectively no seating surface. The ratio is
computed as z_aligned_flat_count / total_face_count. The surface analyzer finds
no qualifying face and raises ToolingCompatibilityError. The auto-selector
scores this part low on every strategy that requires a seating plane.
Ambiguous primary datum. Parts with multiple equally-large faces create ambiguous primary datums. A cube has six faces of identical area. The seating face depends on which orientation the operator chooses — or which face the sort order happens to pick.
Curved primary surfaces. Spheres, cylinders, and other non-planar surfaces cannot define a plane with three points. They need conforming fixtures: cradles, V-blocks, or custom-shaped nests that make contact along a curve rather than at discrete points.
For a deeper look at how OPJAW handles parts that defeat the standard strategies, see Fixture Selection for Irregular Parts.
Collet holding. A collet constrains four degrees of freedom simultaneously — radial position in X and Y, plus tilt around both horizontal axes. The collet grip replaces both the primary plane and the secondary line. Only axial position and rotation around the collet axis remain unconstrained (set by a stop and an orientation key, respectively).
Adhesive fixturing. The adhesive bond constrains the seating plane, but secondary and tertiary location depend on part placement accuracy. There is no mechanical datum — the operator places the part by eye or against a reference edge.
Magnetic chucks. The magnetic field pulls the part flat against the chuck surface (primary plane), but does not constrain XY position. Lateral location depends on stops, fences, or the operator. This is a one-datum fixture.
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