Rhino 3D Tip: Smash: Fast NURBS Surface Flattening Workflow

Need a fast, practical way to flatten a 3D surface into 2D for patterning or laser/CNC? Smash flattens doubly-curved NURBS surfaces into a planar shape with controlled distortion—perfect when UnrollSrf won’t solve and Squish’s mesh workflow isn’t necessary.

When to choose Smash:

• Surface is not perfectly developable and UnrollSrf returns overlaps or errors.
• You want a quick, single-click NURBS-to-2D workflow without converting to mesh.
• You accept some distortion but need the perimeter and key lengths to be close.

Before you Smash (prep for cleaner, more accurate results):

• Clean the input: use ShrinkTrimmedSrf to remove excess parameter space, and SimplifyCrv/Rebuild on trimming curves to reduce complexity.
• Minimize patchwork: MergeSrf where appropriate so you’re flattening a single, continuous face rather than multiple small patches.
• Mark references: DupEdge or Draw curves along critical measurement paths (centerlines, seam paths, datum rings) so you can compare lengths pre/post flattening.
• Check scale and tolerances: confirm model units and absolute tolerance are appropriate for downstream fabrication.

Core workflow:

• Select the surface and run Smash.
• Choose an orientation on the CPlane that maximizes usable sheet layout area.
• If the perimeter must be preserved, prioritize boundary accuracy when resolving the flattened shape. If interior path lengths matter more (e.g., gasket ID/OD), bias toward preserving those references.
• Flatten, then immediately measure key distances on the 2D result against your reference curves from the 3D model to quantify distortion.

After the Smash (validate and finish):

• Use Distance, Length, and Area to compare critical dimensions (perimeter, seam length, hole spacing).
• Where distortion is unacceptable, split the 3D surface along logical seam lines and Smash pieces separately to distribute error more evenly.
• Add notches, labels, and grain/arrows in 2D for assembly alignment. Consider FlowAlongSrf if you need to map those marks back to the 3D form.
• Export cleanly: Join edges as needed, SimplifyCrv on the 2D outlines, then Export Selected to DXF/DWG/SVG with correct units.

Pro tips:

• Try UnrollSrf first—if it works, it’s distortion-free. If it fails, Smash is your next stop. For maximum control over seams and material-like behavior, evaluate Squish on a mesh copy.
• Balance distortion: cutting the surface along high-curvature zones before Smash usually reduces edge stretch and area error.
• For soft goods, allow slight scale adjustments in non-critical zones; for metals, keep bend allowances and K-factors in your downstream CAM, not in the flattening step.
• Document your choices: store smashed results and notes on error tolerances in a dedicated “FLAT” layer set for repeatability.

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