STP vs STEP vs IGES: Which CAD File Format Is Best for CNC Machining RFQ?
When engineers request a CNC machining quote, the file format they send can decide how quickly a supplier can review geometry, identify manufacturability risks, and prepare a reliable quotation. A clean STEP file can move directly into CAD/CAM review. A broken surface export may require repair. A PDF drawing alone may be enough for a simple plate, but not for a complex 3D pocketed part.
The most common question is simple: should you send STP, STEP, IGES, or PDF drawings for a CNC RFQ? The best answer is usually: send a STEP or STP file for geometry, and include a PDF drawing for tolerances, threads, surface finish and inspection notes.
Key Takeaways
- STP and STEP usually mean the same thing in CNC RFQ workflows. Both refer to STEP neutral CAD files, often using the .stp or .step extension.
- STEP/STP is usually the best primary file format for CNC machining quotes because it preserves solid geometry better than IGES in most modern workflows.
- IGES can still be useful for surface-heavy models or as a backup, but it may import as surfaces instead of clean solids.
- PDF drawings are still important because CAD files often do not fully communicate tolerances, thread callouts, surface finish, material notes and inspection requirements.
- The best RFQ package includes STEP/STP + PDF drawing + material + quantity + finish + QA requirements.
What Is a STEP File?
STEP stands for Standard for the Exchange of Product model data. It is a neutral CAD format used to transfer 3D geometry between different CAD systems. In CNC machining, STEP is widely accepted because it usually preserves solid bodies, surfaces, assemblies and precise geometry better than older exchange formats.
A STEP file is useful for CNC quotation because the supplier can inspect:
- Overall part size and stock estimate
- Pocket depth and cutter access
- Hole locations and intersecting features
- Curved surfaces and 3D contours
- Setup orientation and machining sides
- Thin walls, ribs and deep cavities
For most custom CNC parts, STEP should be the main 3D file sent with the RFQ.
Is STP the Same as STEP?
In practical CNC machining communication, STP and STEP usually refer to the same file standard. The difference is normally just the file extension:
| Extension | Meaning | CNC RFQ Use |
|---|---|---|
| .step | STEP file | Recommended |
| .stp | STEP file, shorter extension | Recommended |
| .iges | IGES file | Useful backup |
| .igs | IGES file, shorter extension | Useful backup |
| 2D drawing | Strongly recommended with CAD |
Some older systems used three-letter extensions, so .stp became common. Modern systems may export .step. For suppliers, both are usually acceptable.
What Is an IGES File?
IGES stands for Initial Graphics Exchange Specification. It is an older CAD exchange format that can transfer curves, surfaces and wireframe geometry. IGES is still used in some industries, especially for surface data, legacy CAD systems, molds, regulated surfaces and imported customer geometry.
However, IGES can create problems for CNC RFQs if the file imports as loose surfaces instead of a watertight solid. The supplier may need to repair gaps, stitch surfaces, check normals, or confirm whether missing faces are export errors or intentional design features.
IGES is useful when:
- The model is surface-heavy
- The customer CAD system exports poor STEP files
- The supplier requests a backup file
- Legacy data exists only in IGES format
But for most machined parts, STEP/STP should be sent first.
Why PDF Drawings Still Matter
A common mistake is sending only a STEP file and assuming it contains everything needed for quotation. Geometry is only part of the manufacturing requirement. A STEP file often does not clearly communicate:
- General tolerance standard
- Critical dimensions
- Thread standard and thread depth
- Surface roughness
- Deburring and edge break notes
- Coating or anodizing requirements
- Material grade and heat treatment
- Inspection report requirements
- Drawing revision and release status
A PDF drawing turns the 3D model into a controlled manufacturing document. For simple parts, the PDF may be enough. For complex parts, the PDF should accompany the STEP file.
Best File Package for CNC Machining RFQ
The strongest CNC RFQ package includes:
- STEP or STP file for 3D geometry
- PDF drawing for tolerances and notes
- Material grade and condition
- Quantity and expected batch size
- Surface finish and coating requirements
- QA document requirements such as CMM, MTR or FAIR
- NDA or confidentiality requirements if needed
- Revision number and contact person for engineering questions
This package allows the supplier to evaluate both geometry and manufacturing intent.
Common CAD Export Mistakes That Delay CNC Quotes
1. Exporting mesh files instead of solids
STL files are useful for 3D printing, but they are not ideal for CNC machining quotations because they approximate geometry with triangles. Unless specifically requested, send STEP/STP instead.
2. Sending mismatched STEP and PDF revisions
If the STEP file is Rev B but the PDF drawing is Rev C, the supplier must ask which file controls. Always confirm revision alignment before RFQ.
3. Missing units
Most CAD files include units, but export mistakes happen. If a part imports at 25.4 times the expected size, the quote process stops immediately. State units clearly in the drawing or RFQ note.
4. Suppressed features in export
Sometimes threads, small holes, fillets or chamfers are suppressed before export. Check the exported STEP file before sending it.
5. Overly complex assemblies
If only one part needs quotation, do not send a full assembly with unrelated components unless context is required. Send individual part files and a clear BOM when multiple parts are requested.
Which Format Should You Send?
For most CNC machining RFQs, use this rule:
| Situation | Recommended File Package |
|---|---|
| Standard machined part | STEP/STP + PDF drawing |
| Simple 2D plate | PDF may be enough, STEP helpful |
| Complex 3D contour | STEP/STP required, IGES backup helpful |
| Surface-heavy part | STEP/STP + IGES backup + PDF |
| Confidential unreleased product | STEP/STP + PDF under NDA process |
| Quality-critical part | STEP/STP + controlled PDF drawing + QA requirements |
How Andas Precision Uses CAD Files for RFQ Review
Andas Precision supports public RFQ workflows for STEP, STP, IGES, IGS and PDF drawings. When geometry is available, engineering-assisted DFM review can help identify machining risks such as thin walls, internal sharp corners, deep pockets, hole edge distance issues, tolerance conflicts and coating risks. A PDF drawing helps clarify which dimensions and notes matter before a formal quote.
For the fastest CNC quote review, send STEP/STP and PDF together with material, quantity, tolerance, finish and QA requirements.
FAQ
Is STP better than STEP for CNC machining?
No. STP and STEP usually refer to the same STEP file standard. The difference is normally the file extension. Both are commonly accepted for CNC RFQs.
Is IGES still used for CNC machining?
Yes, but usually as a backup or for surface-heavy geometry. STEP/STP is generally preferred for solid machined parts.
Can I send only a PDF drawing for CNC quote?
For simple parts, sometimes yes. For complex 3D parts, a STEP or STP file is strongly recommended because it allows accurate geometry review.
Should I send STL files for CNC machining?
Usually no. STL is a mesh format commonly used for 3D printing. CNC suppliers generally prefer STEP/STP solid models.
What is the best file package for a CNC RFQ?
The best package is STEP/STP for geometry, PDF drawing for manufacturing notes, plus material, quantity, finish, tolerances, QA requirements and revision information.