Oversize the Cutter: Reliable Boolean and SDF Modeling in Rig Cad

A through-hole has an exact finished depth, but the temporary solid used to create it should rarely end on the same faces as the part. This focused guide builds on Constructive Solid Geometry in Rig Cad and Transforms and Reference Frames in Rig Cad.

Suppose a panel is 50 units thick. Subtracting a cutter that is also exactly 50 units long makes its entry and exit faces coincident with the panel’s outer faces. Those surfaces are neither clearly crossing nor clearly separated, so the construction can become fragile after rotation, scaling, mirroring, repetition, or regeneration.

The dependable pattern is simple: make a through-cutter pass completely through the material it removes. The intended opening stays exact; only the disposable construction geometry receives a margin.

For a cutter that begins at one face and travels through a part, extend it beyond both sides:

$$d_{cutter}=d_{part}+2m$$ $$o_{cutter}=-m$$

where $m$ is the cutter overshoot measured along the local cutting direction. For a 50-unit panel and a 1-unit margin:

The removed region inside the panel is unchanged. The additional cutter volume lies outside the finished solid, where it cannot enlarge the intended through-opening.

If the cutter primitive is centered, increase its length while keeping its center fixed. If it grows from an end face, lengthen it and move its origin so the extra distance is distributed across the entry and exit sides. The important condition is not a particular origin convention; it is that the transformed cutter clearly crosses both final boundaries.

An exact-size cutter creates coincident or tangent relationships:

• an entry face lies in the same plane as the part's outer face;
• an exit face shares the opposite plane;
• an edge touches another edge without crossing it; or
• two surfaces overlap over an entire region.

Boolean algorithms must classify geometric regions as inside, outside, or on a boundary. Coincident faces concentrate the work in the ambiguous on case. Small coordinate and tolerance differences can then produce different classifications after a transform or regeneration.

This is a general constructive-solid-geometry problem rather than a peculiarity of one model. The OpenSCAD manual recommends giving subtractive tools clear overlap and extending them fully outside the removed volume. The Open CASCADE Boolean documentation likewise treats touching, overlapping, and near-coincident entities as special interference cases and documents additional tolerance handling for difficult inputs.

Common symptoms include:

A useful rule follows: do not ask a cutter to stop exactly on the surface it is cutting.

Name the overshoot explicitly: cutterOvershoot, booleanMargin, or cutExtension. Keep it separate from hole diameter, fit allowance, printer compensation, and every other dimension that defines the physical result.

A clearance changes the finished relationship between parts. A cutter overshoot should not. For a through-hole, increasing the cutter's axial length outside the host solid leaves the hole diameter and useful depth unchanged.

There is no universal margin. Choose a value that is small relative to the model but comfortably larger than irrelevant coordinate noise and accumulated transform error. Once the surplus remains entirely outside the retained material, making it vanishingly small offers little benefit. Making it enormous can slow previews, obscure debugging, or bring the tool into unrelated geometry, so the goal is a deliberate margin—not an infinite cutter.

Define the cutter in a convenient local coordinate system, extend it along its own cutting axis, and then apply the shared placement or rotation.

This ordering matters. Enlarging the world-axis bounding box of an already rotated cutter does not necessarily lengthen the cutter along the direction in which it passes through the part. The bounds may grow while the cutting solid still stops at the target surface.

For a transformed feature:

1. Build the host and cutter in feature-local coordinates.
2. Derive the cutter length from the host thickness plus the two margins.
3. Position the additional length across the entry and exit sides.
4. Group the feature when the host relationship should move as one assembly.
5. Apply rotation, mirror, scale, or pattern placement.
6. Subtract only after the final relationship contains clear volume overlap.

Nested transforms deserve special attention. The cutter does not merely need to be oversized in its original sketch; it must still cross the final transformed host. Nonuniform scaling, independently transformed branches, and repeated cutters can invalidate an assumption that was correct before placement.

The center finder separates product dimensions from construction margin. The plate body uses plateT; its centered window and pencil-hole cutters use plateT + 2. Their parent transforms place them at plateT / 2, so they extend one unit beyond each face and remain reliable when the plate thickness changes.

Inspect these nodes in the embedded editor:

• Plate → Windows → Window: depth plateT + 2
• Plate → Hole: height plateT + 2
• Plate → Sink: height sinkH

The sink is intentionally depth-controlled because its endpoint defines the countersink. Change plateT, stockW, or holeD: the through-cutters continue past the plate, while the sink preserves its designed depth.

Signed-distance-field subtraction needs the same through-cutter margin for an additional reason: smooth subtraction evaluates both fields across a transition band. If a cutter end cap enters that band near the host surface, the result can pinch, ridge, or lose its blend at the exit.

Use this practical starting rule:

$$m_{SDF}\geq r_{smooth}+m_{mesh}$$

Here $r_{smooth}$ is the requested transition distance and $m_{mesh}$ is a small sampling margin. This is a construction heuristic, not a universal constant; the required value depends on model scale, field quality, and extraction settings.

Extend the cutter only along its local through direction. Its width, diameter, or profile still defines the finished opening. If smoothing reaches a neighboring wall, reduce the smooth radius, increase feature spacing, or improve field quality rather than enlarging the cutter profile.

A coarse field can still erase detail even when the cutter is long enough. Signed Distance Fields: Geometry as a Function explains the field behavior behind smooth subtraction.

Through-holes, vents, slots, trim tools, and cutters whose entry and exit faces are disposable are ideal candidates. Blind pockets require a different construction because the cutter's end may define a finished surface.

The distinction is whether a cutter face is temporary or intentional. Oversize temporary faces. Preserve faces that define the finished bottom, shoulder, or stopping plane.

A successful default preview does not prove that a parametric cutter is robust. Inspect the host and cutter branches before the final subtraction hides their relationship. Distinct diagnostic colors and a cross-section make it easier to confirm that the cutter begins outside one face, ends outside the other, and remains aligned after every parent transform.

Test the states most likely to remove the margin:

• minimum and maximum host thickness;
• transformed, mirrored, and repeated instances;
• the smallest nearby wall;
• maximum SDF smoothing;
• the lowest permitted field quality; and
• blind floors or shoulders that must remain exact.

When a cut fails, check child order, overlap, local cutting direction, final transformed bounds, smoothing reach, and field resolution before changing the finished opening. Validate the complete solved scene, not only the cutter node.

Keep finished dimensions and construction helpers separate:

partDepth        = 50
cutterOvershoot  = 1
cutterDepth      = partDepth + 2 * cutterOvershoot

Expose partDepth when it is a meaningful design choice. Keep cutterDepth derived. A released product normally treats cutterOvershoot as an internal construction value.

For a centered cutter, keep its center aligned with the part. For a one-sided primitive, use explicit entry and exit margins:

cutterDepth = partDepth + entryMargin + exitMargin
cutterStart = partStart - entryMargin

Separate the margins when one cutter end defines an intentional floor or shoulder.

A deliberate cutter margin states the real design intent: the feature passes through the host. It replaces an accidental equality between two depths with clear volume overlap that survives parameter and transform changes.

Use Signed Distance Fields: Geometry as a Function for the field mathematics behind smooth subtraction, Post-Processing in Rig Cad for ordered operations on solved geometry, and Beyond Sliders to carry construction margins and relational constraints across a complete product family.