CNC Drawing Notes Guide: What to Write on a Machining Drawing
A 3D CAD model defines shape, but a CNC machining drawing defines manufacturing intent. The drawing tells the supplier which dimensions are critical, which standards apply, what finish is required, how edges should be treated, and what inspection evidence the buyer expects. Without clear drawing notes, suppliers must guess or ask questions, which slows quotation and increases the risk of wrong assumptions.
Good CNC drawing notes do not need to be long. They need to be specific, consistent and connected to the part function. This guide explains what engineers should include on machining drawings before sending a CNC RFQ.
Key Takeaways
- A STEP file should be paired with a PDF drawing when tolerances, threads, finishes, material condition or inspection requirements matter.
- Drawing notes should separate general requirements from critical features so the supplier does not over-machine the entire part.
- Edge breaks, deburring, coating thickness and thread depth are common sources of RFQ questions and production mistakes.
- QA requirements such as CMM reports, MTRs and FAIR documentation should be stated before quotation.
Why Drawing Notes Matter Even With 3D CAD
Modern CNC suppliers can read STEP, STP and IGES files, but the CAD file usually does not explain the full manufacturing requirement. A model may show a hole, but not whether it is reamed, threaded, press-fit, cosmetic, or inspection-critical. A model may show a sharp edge, but not whether it should be broken, chamfered, deburred or protected from coating buildup.
The drawing provides context that the model cannot always communicate:
- Which dimensions control fit and function
- Which standard tolerances apply
- Which features require inspection reports
- Which surfaces are cosmetic
- Which finish or coating is required
- Which edges must be deburred or remain sharp
- Which material condition is acceptable
For accurate CNC quotes, the best package is 3D CAD plus a controlled PDF drawing.
1. Material Notes
Material notes should state the exact grade and condition. Avoid vague descriptions such as “aluminum,” “steel,” or “stainless.” Instead, specify the grade, temper or hardness condition.
Examples:
- Material: Aluminum 6061-T6
- Material: Aluminum 7075-T6, no substitution without approval
- Material: Stainless steel 316L
- Material: 4140 pre-hard, target hardness 28-32 HRC
- Material: POM black, natural finish acceptable
If a Material Test Report is required, include it in the drawing note or purchase specification:
Material certificate / MTR required with shipment.
This prevents the supplier from buying untraceable stock when traceability is required.
2. General Tolerance Notes
A general tolerance note tells the supplier what applies to dimensions that are not individually toleranced. It prevents ambiguity and avoids making every dimension unnecessarily tight.
Common examples:
- Unless otherwise specified: ISO 2768-mK
- Unless otherwise specified: linear ±0.10 mm, angular ±0.5°
- Critical dimensions marked with inspection balloon require CMM report
Do not apply tight tolerances globally unless the part truly requires it. A blanket ±0.01 mm note can increase CNC cost significantly because the supplier must assume high inspection and process control effort across the entire part.
3. Critical Dimension Notes
If a feature affects assembly, sealing, bearing fit, alignment or motion, mark it clearly. Critical dimensions should not be hidden among dozens of ordinary dimensions.
Useful notes include:
- Critical to function
- Inspect full
- CMM report required for marked dimensions
- Datum A/B/C controls alignment
- Do not change without engineering approval
A supplier can then focus inspection effort where it matters instead of treating every feature equally.
4. Surface Finish Notes
Surface finish notes should state both the process and any functional requirements. “Anodize” alone may not be enough if the part has tight fits, masking requirements or color expectations.
Examples:
- Clear anodize, Type II, cosmetic surfaces free from visible scratches
- Black anodize, mask threaded holes
- Bead blast before anodize on external cosmetic surfaces only
- Passivation required after machining
- Nickel plating, thickness 5-8 µm, dimensions apply after plating
If coating thickness affects fit, specify whether dimensions are before or after finishing.
5. Deburring and Edge Break Notes
Deburring is one of the most common missing drawing notes. If not specified, each supplier may use a different standard.
Examples:
- Deburr all edges, break sharp edges 0.1-0.3 mm
- Remove burrs, do not round sealing edges
- Edge break 0.2 mm max unless otherwise specified
- Keep marked edge sharp for assembly function
If a sharp edge is functionally required, say so. Otherwise, the supplier may break it during normal deburring.
6. Thread and Insert Notes
Thread notes should include size, pitch, depth, standard and any insert requirements. A modeled hole does not always tell the supplier whether it is tapped, clearance, reamed or helicoil-ready.
Examples:
- M6 x 1.0 - 6H, thread depth 12 mm minimum
- 1/4-20 UNC-2B, thread depth 0.50 in minimum
- Install stainless helical insert, M5 x 0.8, 1.5D length
- Threads must be clean after anodizing
- Mask threads during coating
Blind threads should include relief or minimum full thread depth. If thread quality is critical, specify the gauge or inspection method.
7. Hole and Fit Notes
Holes often require more than diameter. If a hole is for a dowel pin, bearing, shaft, fastener clearance or press fit, state the function.
Examples:
- Ø6 H7 reamed dowel pin hole
- Bearing bore, inspect roundness and diameter
- Clearance hole for M5 screw
- Press-fit pin hole, no coating buildup allowed
- Counterbore depth critical for screw head flushness
This helps the supplier choose drilling, boring, reaming or interpolation strategy correctly.
8. Inspection and QA Document Notes
If inspection documents are required, add them to the drawing or RFQ. Do not wait until after machining.
Common QA notes:
- CMM report required for ballooned dimensions
- First Article Inspection Report required
- Material Test Report required
- Certificate of Conformance required
- Coating thickness report required
- Hardness report required after heat treatment
If only selected dimensions need inspection, mark them clearly. Full inspection of every dimension may increase cost and lead time.
9. Revision and File Control Notes
The drawing revision should match the STEP file revision. If the supplier receives mismatched files, quotation stops.
Include:
- Drawing revision
- Release date
- Units
- Scale, if relevant
- Change summary for revised RFQs
- Contact person for engineering questions
A simple revision note prevents suppliers from quoting or machining an outdated model.
Example CNC Drawing Note Block
A practical note block might look like this:
Material: Aluminum 6061-T6. Unless otherwise specified: ISO 2768-mK. Deburr all edges and break sharp edges 0.1-0.3 mm. Dimensions apply after clear anodize unless noted. Mask threaded holes. MTR required. CMM report required for ballooned dimensions. Do not substitute material or finish without written approval. Drawing and STEP file must match current revision.
This type of note block gives the supplier enough information to quote more accurately and ask better DFM questions.
How Andas Precision Uses Drawing Notes
Andas Precision reviews STEP, IGES and PDF drawing packages for CNC machining RFQs. Clear drawing notes help the DFM review identify tolerance conflicts, coating risks, thread issues, edge break requirements, material traceability needs and inspection scope before formal quotation.
For the fastest review, send the STEP/STP model, PDF drawing, material, quantity, finish and QA requirements together.
FAQ
Do I need a 2D drawing if I already have a STEP file?
For simple non-critical parts, a STEP file may be enough. For production, tolerance, finish, thread or inspection requirements, a PDF drawing is strongly recommended.
What is the most important note on a CNC drawing?
Material, tolerance and finish notes are usually the most important because they directly affect price, process planning and inspection.
Should every dimension have a tight tolerance?
No. Tight tolerances should be applied only where function requires them. Use a general tolerance standard for non-critical dimensions.
How should I specify deburring?
State whether all edges should be deburred, the allowed edge break range, and any edges that must remain sharp or protected.
When should I request CMM or FAIR documents?
Request CMM or FAIR documentation when dimensional evidence is important for assembly, customer approval, regulated projects, or first production runs.