2026-04-02
Most CNC workholding is designed around rectangular stock. Vise jaws clamp parallel faces. Fixture plates bolt to flat bottoms. Zero-point systems pull down on planar surfaces. When the part cooperates — flat, compact, the right size — fixture selection is straightforward.
When it doesn't, you're guessing. A part with curved surfaces has no obvious seating face. A part with thin features might fit geometrically but produce a pocket that collides with the part itself. A part that exceeds the jaw width works in theory if you don't mind overhang, but you won't know until you've committed setup time.
This article describes how OPJAW evaluates fixture suitability for irregular parts. Three geometric metrics score each part against each fixture strategy. Then the system goes further: it actually generates the tooling and validates the result. If the tooling doesn't hold up to inspection, the strategy is ranked below one that does.
Every fixture strategy imposes geometric requirements on the part. OPJAW quantifies three of them directly from the STEP file.
Flat face ratio. The fraction of the part's surfaces that are planar and aligned with the clamping axis. OPJAW filters all faces in the model, keeps only those that are both planar and within a few degrees of the Z axis, and divides by the total face count. Fixture plates and zero-point systems need at least one large flat face for seating. A higher ratio means more surface area in contact with the fixture — better stability, less risk of the part rocking under cutting loads.
Compactness. The ratio of the part's shortest bounding-box dimension to its longest. A perfect cube scores 1.0. A long thin bar approaches zero. Compact parts fit vise jaws well because they distribute clamping force evenly and don't protrude beyond the jaw faces. Elongated parts cant, deflect, or hang off the edge.
Size fit. Whether the part's largest dimension fits within the strategy's
working envelope. A 6-inch vise has 152.4 mm of jaw width. A fixture plate has
a larger bolt-pattern footprint. Exceeding the envelope doesn't disqualify a strategy
outright — some overhang is acceptable — but it penalizes the score. The farther
past the limit, the steeper the penalty.
These three metrics produce a geometric pre-screen score for each strategy. But the pre-screen is only half the evaluation. OPJAW then runs the generation probe: it actually builds the fixture tooling — cuts the pocket, positions the clamps, validates the solid — and checks the result. Is the output a single watertight body? Does the pocket intersect the part? Are the dimensions within tolerance? A strategy that passes the generation probe scores significantly higher than one that doesn't, regardless of its geometric pre-screen. A fixture that looks compatible on paper but produces a broken pocket is ranked below one that actually works.
A standard bearing housing. The bounding box is moderate and fits within every strategy's
envelope. Compactness is 0.65 — not a perfect cube, but close enough
that vise jaws clamp it evenly without overhang.
The flat face ratio is only about 12%. That sounds low, but it's misleading. The pillow block has a large flat bottom and a flat top, which together provide ample seating area. The remaining 88% of faces are bearing bores, chamfers, and mounting flanges — curved surfaces that don't count as Z-aligned flats. What matters for fixture seating isn't that most faces are flat, but that at least one face is flat and large enough to sit on. This part has that.
All four strategies — vise jaws, multi-op vise, fixture plate, zero-point — generate valid tooling. The generation probe passes for each one. When the geometry cooperates across the board, pick whatever fixture type is already bolted to your table.
Parallel cooling fins, tightly spaced. This part has a higher flat face ratio than the
pillow block — about 32%, because every fin surface is a planar face aligned with
the Z axis. Compactness is 0.65, similar to the pillow block. The part is
small enough to fit any fixture type. On geometric pre-screen scores alone, this part
looks well-suited to every strategy.
The generation probe tells a different story. The pocket that holds this part must fit between those fins. The gap between adjacent fins is narrow, and the boolean operation that subtracts the part shape from the fixture stock is sensitive to thin-wall geometry. A pocket wall that's too thin either fails the solid-body check or produces an interference — the fixture collides with the part it's supposed to hold.
This is the case that shows why surface metrics alone can't predict fixture viability. A part can have good numbers — high flat ratio, compact proportions, well within the size envelope — and still fail the generation probe because its internal geometry creates conditions that the pocket builder can't resolve cleanly. The only way to know is to attempt the cut and validate the result.
Four cylindrical tubes meeting at right angles. This part has a flat face ratio of 1.4%
— two small planar faces out of 148 total. Nearly every surface is curved.
Compactness is 0.56, dragged down by the part extending equally in two axes
while being shorter in the third.
The fixture plate and zero-point strategies rely on a stable flat seating face. With a 1.4% flat ratio, there's effectively nowhere for this part to sit. The surface analyzer finds no Z-aligned planar face large enough to qualify, and the strategy reports an incompatibility before generation even begins.
Vise jaws take a different approach — they clamp from the sides, so a flat bottom
isn't strictly required. The compactness score is moderate, and the bounding box (67.6 x 67.6 x 37.6 mm) fits within the 152.4 mm jaw width. The vise can attempt a pocket. But the
tubes extend in four directions, and the generated pocket can't fully enclose them. The
system produces tooling with an oversized warning: the jaws hold the center of the cross,
while the tube arms protrude beyond the fixture envelope.
This is what fixture selection looks like when no standard strategy is a clean fit. Rather than discovering that through trial setups on the machine, the evaluation runs in seconds and reports the specific failure mode for each strategy: no seating face, envelope overshoot, curved profile incompatible with standard pocket methods.
Fixture selection for irregular parts is a geometric compatibility problem. The part's shape, proportions, and size determine which strategies can even attempt to hold it, and the generation probe determines which of those attempts actually produce valid tooling.
OPJAW runs this evaluation on any STEP file. Upload a part, and the system scores it against four fixture strategies. You see which strategies work, which fail, and why — flat face ratio too low for plate fixturing, part too large for the vise envelope, pocket geometry creating thin walls. No trial runs, no scrap from a bad setup.
Upload a STEP file and see how your part scores.
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