DFM & Design

The CNC Drawing Checklist:
12 Things to Get Right Before You Hit Send

4 min read · Quality Prototype Engineering Team

We've quoted tens of thousands of parts. The drawings that sail through quoting and into production without a hitch all have the same things in common. Here they are — bookmark this page and check it before every send.

01
Title block is complete and current
Part name, part number, revision, material, finish, scale, and drawn-by fields are all filled in. The revision matches the file you're sending. Nothing says "this drawing wasn't reviewed" like a blank title block or a mismatched rev.
02
Material specified to grade and temper
Not "aluminium." Not "stainless." AL6061-T6. SUS316L. Ti-6Al-4V. If the grade matters for your application (and it almost always does), write the full designation. If you genuinely don't care, write "6061 or equivalent, engineer's discretion" — not a blank field.
03
General tolerance standard called out
Add a general tolerance note: "Unless otherwise stated, all dimensions per ISO 2768-m" (or -f for finer, or -c for coarser). This tells the machinist what to assume for every uncalled feature. Without it, they'll ask — and that's a day lost.
04
Critical tolerances explicitly called out
Every feature that has a functional requirement tighter than your general tolerance standard must be explicitly dimensioned and toleranced. Bore diameters, mating surfaces, sealing faces, thread depths. If a machinist misses it because it was "implied," that's on the drawing.
05
Surface finish specified (Ra value or symbol)
Use ISO surface finish symbols or explicit Ra values. "Smooth" and "fine finish" mean different things to different people. Ra 1.6µm, Ra 0.8µm, Ra 0.4µm — be specific. Mark "as machined" on surfaces where finish doesn't matter. This saves money and removes ambiguity.
06
Thread callouts are complete
Every threaded feature needs: thread standard (M, UNC, UNF, BSP), diameter, pitch, class of fit, and depth. "M6 threaded hole" is incomplete. "M6×1.0 6H, depth 12mm" is complete. For blind holes, specify full thread depth separately from drill depth.
07
Datum references are clear and unambiguous
If you're using GD&T, your datums A, B, C should be clearly marked and logically chosen (primary datum should be the most stable, largest surface). If you're using coordinate dimensioning, make sure every dimension chains back to a clear reference edge or face.
08
No over-constrained dimensions
Check that you haven't dimension chains that add up to more than the overall size. Over-constrained drawings either don't make physical sense, or leave the machinist guessing which dimension is the one to sacrifice when tolerances stack. Use reference dimensions (parentheses) for redundant info.
09
All views needed to fully define the geometry are present
Top, front, and side views cover most parts, but complex geometry needs section views, detail views, and auxiliary views. If a feature can't be fully described by the views on the drawing, add a view. If you can't tell the wall thickness from the drawing, neither can the machinist.
10
Post-machining operations are called out
Anodising, plating, heat treatment, passivation, marking — anything that happens after machining needs to be specified. If you need Type II clear anodise followed by laser marking, both operations must appear on the drawing or in a separate finishing spec document referenced on the drawing.
11
Inspection requirements are specified for critical features
If you need a CMM report, a FAI report, or 100% inspection on a specific feature, call it out. "All dimensions to be verified" and "critical dimensions only" produce very different inspection scopes and very different costs. Be specific about what you need verified and reported.
12
You've read the drawing yourself, front to back, as if you'd never seen the part
This is the most consistently skipped step and the one that catches the most problems. Can you reconstruct the complete 3D geometry from the drawing alone? Can you identify every dimension needed to machine every feature? If not, someone else will have the same problem — and they'll ask you about it by email, one question at a time.
Bonus: send both the STEP and the PDF.
The STEP gives us the geometry. The PDF gives us your intent. Together they eliminate 90% of questions before they're asked.

What happens if you miss something?

Realistically: your quote gets delayed, or the part gets made to a different spec than you intended, or you get an email with questions. None of these are catastrophic. But on a tight timeline, every day matters.

We do our best to flag incomplete specs before we quote — our DFM review catches most issues. But the cleaner the drawing you send, the faster everything moves.

Ready to send?
Run through the checklist, attach your STEP + PDF, and send it our way. Quote in 12 hours, parts in 5–10 days.

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