How to Prepare CAD Files for CNC Machining RFQ: File Formats, Tolerances & Checklist (2026)
How to Prepare CAD Files for CNC Machining RFQ: File Formats, Tolerances & Checklist (2026)
Key insight: 60% of CNC RFQs are delayed by 1–3 days because of incomplete or unclear CAD files. The #1 issue: missing tolerance specifications. The #2 issue: wrong file format. Sending a STEP file (.stp/.step) with critical dimensions marked on a companion PDF can cut quote turnaround time by 50% and reduce the chance of quoting errors.
CAD File Format Guide: What CNC Shops Actually Need
| Format | Rating | Contains | Best For |
|---|---|---|---|
| STEP (.stp / .step) | ★★★★★ | Solid 3D geometry, assembly structure | Primary format — always send this |
| IGES (.igs / .iges) | ★★★★☆ | Surface 3D geometry | Legacy files, surface models |
| Parasolid (.x_t / .x_b) | ★★★★★ | Solid 3D geometry, high precision | Siemens NX, SolidEdge users |
| DXF / DWG | ★★★☆☆ | 2D drawings, flat patterns | Sheet metal, laser cutting, 2D parts |
| PDF (dimensioned) | ★★★☆☆ | 2D drawing with dimensions, notes | Companion to 3D file — critical dims, tolerances |
| STL | ★★☆☆☆ | Mesh / faceted geometry | 3D printing only — NOT for CNC quotes |
| Native CAD (SLDPRT, .prt, .ipt) | ★★★★☆ | Full parametric model | Only if shop uses same CAD software |
8-Point CNC RFQ Checklist
Include these 8 items with every RFQ. Suppliers report this reduces back-and-forth by 80%.
| # | Item | Example | Why It Matters |
|---|---|---|---|
| 1 | 3D CAD file (STEP) | bracket_v3.stp | Primary geometry for CAM programming |
| 2 | 2D drawing (PDF) | bracket_v3.pdf | Critical dimensions, tolerances, notes |
| 3 | Material specification | Al 6061-T6 per ASTM B209 | Determines tooling, speeds, feeds, cost |
| 4 | Quantity | 10 prototypes + 500 production | Affects setup amortization, per-part cost |
| 5 | Critical tolerances | Bore Ø20 ±0.01; others ±0.1 | Drives inspection time and machining strategy |
| 6 | Surface finish | As-machined, or anodized clear | Adds cost and lead time if post-processing needed |
| 7 | Thread specifications | M5×0.8 6H, 12mm deep | Prevents wrong thread form or depth |
| 8 | Application context | “Load-bearing bracket, 50kg static” | Helps supplier suggest DFM improvements |
Common CAD File Mistakes (and Their Cost)
| Mistake | Delay | Fix |
|---|---|---|
| Sending STL instead of STEP | 1–2 days (request correct format) | Export as STEP AP214 or AP242 from any CAD package |
| Missing thread callouts | 1 day (supplier asks) | Model threads or note them on the drawing |
| No tolerance on critical dims | 2–3 days (supplier assumes worst case) | Mark functional dimensions with tolerance bands |
| Unspecified units (mm vs inch) | 1 day + possible scrap | Always state units on drawing and in email |
| No material temper (e.g., “aluminum”) | 1 day | Full spec: “Al 6061-T6” not “aluminum” |
| Over-constrained model | 1–2 days (CAM rework) | Remove unnecessary constraints before export |
Software-Specific STEP Export Guide
- SolidWorks: File → Save As → STEP AP214 (*.step) → Options: check “Export sketch entities”
- Fusion 360: File → Export → Type: STEP Files (*.stp) → uncheck “Send to 3D Print Utility”
- Inventor: File → Export → CAD Format → STEP → Options: AP214
- Onshape: Right-click Part Studio tab → Export → STEP
- FreeCAD: Select body → File → Export → STEP with colors (*.step)
CNC Precision Tech — China CNC machining sourcing and export support partner.
Industry RFQ Preparation Examples
Use these examples to decide which extra notes to include with STEP files and 2D drawings for application-specific CNC sourcing.
- Automation Equipment CNC Parts – Fixture plates, gripper parts, guide blocks, shafts, mixed-material RFQs, and repeat batches.
- Optical and Camera CNC Components – Lens mounts, camera housings, adapter rings, black anodizing, burr control, and alignment datums.
- Drone and UAV CNC Parts – Lightweight brackets, gimbal mounts, motor adapters, housings, anodizing, and RFQ notes.
- Battery and Energy Equipment CNC Parts – Cooling plates, insulating spacers, fixtures, mixed metal and plastic RFQs, and no-burr areas.