STEP vs. STL

STEP (ISO 10303) is a neutral CAD exchange format that preserves exact geometry (as a B-rep), plus PMI and other product data, in a form other CAD and manufacturing software can read precisely. STL, by contrast, approximates a shape as a mesh of triangles, discarding exact curves, surfaces and product data entirely; it was designed for 3D printing and visualization, not precision manufacturing exchange.

Why STEP, not STL, is used for CNC manufacturing

For CNC manufacturing, feature recognition and precise machining depend on exact geometry, which is why STEP is the standard exchange format for machined parts, while STL is common for 3D printing or visualization where a triangulated approximation is acceptable.

Where the distinction matters

Implied by the file format a part is delivered in; a STEP file signals exact geometry is available, an STL-only file signals it isn't.

Common mistakes

Accepting an STL file as sufficient for precision CNC quoting or programming; the triangulated approximation lacks the exact geometry (true circles, exact fillets) that manufacturing calculations and feature recognition rely on.

Can an STL file be converted to STEP?

Not meaningfully; STL never had the exact geometry to begin with, so converting it back only produces another approximation, not the original precise shape.

Does STEP support PMI, and does STL?

STEP (via AP242) can carry PMI and other product data; STL carries only triangulated shape data, nothing else.

Why is STL common for 3D printing but not CNC quoting?

Because 3D printing builds up material layer by layer from an approximated shape, while CNC quoting and machining need exact geometry to reason about features, tolerances and toolpaths.