What Makes a Good Part Drawing
A good drawing tells the shop everything it needs. No phone calls. No emails. No guessing. The machinist reads your drawing and knows exactly what to make.
A drawing is not just a picture. It carries your design intent. It shows which features matter and how tight they need to be. It picks the material, the finish, and the quality level.
Think of your drawing as a contract. The shop builds what the drawing says. If the drawing is wrong, the parts will be wrong too.
Always send a PDF drawing with your CAD file. The CAD file shows shape. The drawing shows intent. You need both.
Required Elements on Every Drawing
Every part drawing needs these items. Missing any one of them can delay your order.
- Title block: Part name, number, revision, material, and finish.
- Dimensions: All sizes needed to make the part.
- Tolerances: How much each size can vary.
- Material callout: The exact alloy or grade.
- Finish spec: Surface roughness and any coatings.
- Notes: Special instructions, threads, and heat treat.
- Views: Enough views to show all features.
Material Callout
Be specific with your material. "Aluminum" is not enough. Write "6061-T6 Aluminum" instead. The alloy and temper affect strength, machinability, and cost.
Finish Spec
State the surface finish in Ra (roughness average). A standard CNC finish is 125 Ra. Smoother finishes like 32 Ra or 16 Ra cost more. Call out any coatings like anodizing or plating.
Dimensioning for CNC
Good dimensioning saves time and money. Bad dimensioning causes confusion and scrap parts.
Use Datum-Based Dimensions
Pick one corner or face as your datum. Measure all features from that datum. This matches how the CNC machine works. It also makes inspection easier.
Avoid Chain Dimensions
Chain dimensions stack up errors. Each link in the chain adds tolerance. Use baseline dimensions from your datum instead. This keeps total error low.
Dimension to the Right Features
Dimension what matters for function. If two holes must line up, dimension the distance between them. Do not force the shop to calculate it from other sizes.
Never use "scale to fit" as a dimension. Always write the exact number. Machinists do not scale drawings.
Title Block Contents
The title block sits in the bottom-right corner. It holds the key info about your part. Here is what to include.
- Part name: A short name that describes the part.
- Part number: A unique ID for tracking.
- Revision: Start with Rev A. Bump for each change.
- Material: Full alloy and temper (e.g., 6061-T6).
- Finish: Surface treatment or coating.
- General tolerance: Default tolerance for unlabeled dims.
- Scale: The drawing scale (e.g., 1:1, 2:1).
- Units: Inches or millimeters.
- Company name: Your company.
- Date and approval: When it was drawn and approved.
Revision Control
Use revision letters that start at A. Each time you change the drawing, bump the letter. Add a revision block. List the rev letter, date, change, and who approved it.
Always send the latest revision to your shop. Old revisions cause wrong parts. Mark old drawings as "SUPERSEDED" so no one uses them.
Common Drawing Mistakes
We see these errors every week. Avoiding them will save you time and money.
- Missing tolerances: Every critical dimension needs a tolerance. Do not rely on general tolerances for features that matter.
- No material callout: "Steel" is not a material spec. Write the exact grade.
- Over-tolerancing: Tight tolerances cost more. Only tighten what needs to be tight.
- Missing views: If the shop cannot see a feature, they cannot make it. Add section views for internal features.
- No finish spec: The shop does not know if you want 125 Ra or 16 Ra unless you say so.
- Wrong units: Mixing inches and millimeters causes errors. Pick one and state it clearly.
- No revision number: Without a rev number, the shop cannot tell if they have the latest version.
A clear drawing is faster to quote, faster to machine, and less likely to have errors. Spend the time up front. It pays off every time.
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Upload CAD for Instant QuoteFrequently Asked Questions
Do I need a drawing if I have a CAD file?
Yes. A CAD file shows shape. A drawing shows intent. The drawing tells the shop which dimensions matter, what tolerances to hold, and what material to use. Without a drawing, the shop has to guess.
What tolerances should I use for CNC parts?
Use +/- 0.005" as a general tolerance. Use +/- 0.001" only where fit or function needs it. Tighter tolerances cost more. They need slower cuts and more inspection. Only tighten what matters.
What file format should my drawing be in?
PDF is the most common format. It looks the same on any device. Send the CAD file too if your shop needs it. Common CAD formats are STEP, IGES, SolidWorks, and Fusion 360.
What goes in a title block?
A title block includes the part name, part number, revision letter, material, finish, general tolerances, scale, units, your company name, date, and approval.
How do I handle revision control?
Use revision letters starting with A. Each change bumps the letter. Add a revision block that lists the rev, date, change, and who approved it. Always send the latest version.
