STEP vs STL vs DXF for custom automotive parts

STEP, the format machine shops want

STEP (.step / .stp) carries true solid geometry: exact surfaces, exact cylinders, exact radii. It opens in essentially every professional CAD and CAM package, which is why it's the default exchange format for CNC machining. If a machine shop asks for 'a 3D file' with no further detail, send STEP.

What STEP does not carry well is your feature history, a shop can machine it and modify it, but not roll back your design steps.

STL, the format for 3D printing

STL describes the surface of a part as a mesh of triangles. It is an approximation: curves become many small flats, and the resolution is baked in at export. That's fine for 3D printing, which slices the surface anyway, and poor for machining, where exact geometry matters.

An STL also carries no units, no tolerances and no editable features. Treat it as an output, not as a source model.

DXF, the format for flat, 2D cutting

DXF holds 2D vector geometry, which is exactly what laser cutters, waterjets, plasma tables and CNC routers consume. For a bent sheet metal bracket, the useful deliverable is the flat pattern DXF with bend allowances already accounted for, plus a bend drawing showing angles and directions.

IGES, the older neutral format

IGES (.igs) predates STEP and is still requested by shops running older software. It's usually surface-based rather than solid, so it can arrive with small gaps that need healing. Send it when asked; otherwise prefer STEP.

PDF drawings, still essential

A dimensioned 2D drawing is how tolerances, thread callouts, surface finish, material and critical dimensions get communicated. A 3D model says what shape the part is; the drawing says what actually has to be held. Shops quote more confidently and more cheaply when the critical few dimensions are called out and everything else is left open.

Quick reference

• CNC machining: STEP + PDF drawing.
• 3D printing: STL (or STEP, which most slicers now accept).
• Laser, waterjet, plasma, router: DXF flat pattern + bend drawing.
• Older shop software: IGES on request.
• Quoting and inspection: dimensioned PDF drawing.
• Future revisions: the editable source model.

Ask for all of them

Formats cost nothing to export once the model exists. A package that includes STEP, STL, DXF where relevant, a PDF drawing and the editable source means you can approach any shop without going back to your designer first.