How to Prepare CAD Files for a CNC RFQ

A practical checklist for preparing CAD files that will help you get accurate quotes and faster production.

One of the most common causes of delays and misunderstandings in CNC projects is improper or incomplete CAD files. Taking time to prepare your files correctly from the start saves time, reduces cost, and ensures your parts are manufactured exactly as intended.

1. Use the Right File Format

Preferred formats (in order):

• STEP / STP — The best choice. Preserves 3D geometry and is format-independent.
• IGES / IGS — Older format, sometimes used for legacy systems.
• STL — Use only for 3D printing or simple geometry. Not ideal for precision machining.
• Native CAD files — SolidWorks, Inventor, Pro/E, CATIA. We can open these directly.

Also include: A 2D PDF drawing with all dimensions and tolerances clearly marked.

2. Check Your 3D Model

• No missing surfaces — Every feature must be a closed solid, not an open shell.
• No intersecting geometry — Surfaces should meet cleanly, not overlap or intersect.
• No duplicate faces — Remove overlapping or duplicate surfaces.
• Correct wall thickness — Verify all walls are at least 1.5mm for aluminum, thicker for steel.
• No extremely thin features — Thin walls below 1mm may deform during machining.

3. Define Clear Datums and Origins

Specify datums clearly for inspection and fixturing:

• Primary datum — First surface to locate the part
• Secondary datum — Second surface, perpendicular to primary
• Tertiary datum — Third surface to fully locate the part

Set the model origin at a logical location (e.g., corner of a bounding box) for easier programming.

4. Specify Tolerances on Your Drawings

Do not leave everything at default tolerance:

• Critical dimensions: Specify ±0.01mm to ±0.02mm
• Non-critical dimensions: Use general tolerance ±0.1mm
• Tolerances on drawings, not 3D model: 3D models cannot store tolerances — use 2D drawings

5. List All Material Requirements

Clearly state:

• Material type — Aluminum 6061-T6, SS 304, etc.
• Material specification — If using a specific standard (ASTM, DIN, etc.)
• Finish requirement — As-machined, anodized, etc.
• Any special requirements — Food-safe, aerospace grade, etc.

6. Include Thread Information

For threaded holes, specify:

• Thread size — M8, 1/4-20, etc.
• Thread depth — Full thread or partial
• Thread standard — ISO, UNC, UNF, etc.

7. Check for Undercuts and Deep Cavities

Be aware of features that are difficult to machine:

• Deep pockets — Depth greater than 4× tool diameter is challenging
• Narrow openings — Opening width should be at least 1.5× depth
• Undercuts — Require special tooling or multiple setups

8. Include Assembly Information

If your part is part of an assembly:

• Show how it fits — Include drawings of mating parts if critical
• Specify mating dimensions — Clearances, interference fits, etc.
• Indicate critical interfaces — Which features must match other parts

9. Provide a 2D Drawing with Full Details

A 2D drawing should include:

• All dimensions with tolerances
• Datums and reference dimensions
• Surface finish requirements (Ra values)
• Geometric tolerances (GD&T) if needed
• Material and finish specifications
• Quantity required
• Any special instructions

10. Communicate Early

If you are unsure about any aspect of your design, send us the files before finalizing. We can provide a DFM (Design for Manufacturability) review and suggest improvements.

Summary Checklist