How to Create Drawings for CNC Machine Work
A shop-floor guide to drawings for CNC machine jobs: which views to include, how to set datums and tolerances, and where most drawings send a job back for clarification. Written for design engineers and buyers who need a part cut on the first pass.
In this article
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Key takeaways
What a CNC Shop Actually Needs From a Drawing
A drawing is a contract. The machinist never sees your intent, only the geometry, tolerances and notes on the sheet. If the drawing is ambiguous, the shop either quotes conservatively or emails you back, and both cost you time. Most of the delay we see at GreatLight is not cutting time; it is a 24-hour loop of clarification questions that a clean sheet would have avoided.
A usable drawing answers four questions in the first ten seconds. What material, what revision, how many pieces, and what finish. Everything else is detail. Put those four in the title block and in the notes block, and half of the back-and-forth disappears.
You do not need a perfect drawing to get a quote. A 3D STEP file plus a marked-up PDF is enough for us to run a DFM check and send pricing within 12 hours. But you do need a controlled drawing before production, because that is the document the operator holds at the machine and the inspector holds at the CMM.
- 1Title blockPart number, revision, material spec, quantity, scale, units, date.
- 2GeometryOrthographic views plus one isometric for orientation.
- 3TolerancesDimensional, geometric and surface finish.
- 4NotesDeburr, edge break, finish, marking, packaging.
Choosing Views and Projection for Drawings for CNC Machine Parts
Start with the orientation the part is machined in, not the orientation it is used in. If the first operation is a face mill on the largest flat, make that face the front view. The operator reads the drawing against the vise or the fixture, and a view that matches the setup removes a mental rotation step. On a 5-axis part, add one view that shows the tool access direction so the programmer can see which faces are reachable.
Use third-angle projection for North American and Asian suppliers, first-angle for most of Europe. Never mix them on one sheet. State the projection symbol in the title block so there is no debate at the machine.
Section views solve internal features faster than a stack of hidden lines. A single A-A section through a bore, a counterbore and an O-ring groove tells the whole story; four hidden-line views do not. Keep hidden lines out of the main views when a section exists, the sheet stays readable at 1:1 print size.
Break the rules when the part is simple. A turned shaft with one thread and two diameters often needs nothing more than a single view with dimensions along the axis. Extra views on a simple part add clutter and hide the one dimension that matters.
Datums, GD&T and Tolerance Stack-Up
Datum selection decides how the part is measured, and measuring is where arguments start. Choose the mounting face as the primary datum A, a locating hole or slot as B, and a second hole or edge as C. Then dimension every critical feature from those datums instead of chaining dimensions across the part. Chained dimensions accumulate error; datum dimensions do not.
Use geometric tolerances where the function demands it, not everywhere. Position tolerance on a bolt circle, perpendicularity on a mating face, and flatness on a sealing surface cover most parts. A position callout of Ø0.1 mm at MMC is a common, manufacturable value. Pushing to Ø0.02 mm on a 200 mm bolt circle raises cost sharply and rarely helps the assembly.
Tolerance stack-up is arithmetic, so do it before you release the drawing. Add the worst-case contributions of every interface in the assembly. If the stack exceeds the clearance you need, loosen the least functional dimension rather than tightening all of them. One loose dimension is cheaper than five tight ones.
Remember that our standard machining tolerance is ±0.005 mm, and surface finish can reach Ra 0.2–0.8 μm when a process truly needs it. Those numbers are achievable, not automatic. Mark the features that need them and leave the rest at general tolerance.
Notes, Finish and Material Callouts
The notes block is where drawings for CNC machine work usually fall short. Write the material to a real specification: 6061-T6, 316L, 17-4PH, Ti-6Al-4V, PEEK. Grade matters because 6061 and 7075 machine differently and price differently. A generic aluminium callout forces us to guess and quote the safe option.
Surface finish belongs on the drawing as a number. Ra 1.6–3.2 μm is a standard as-machined finish for most milled surfaces. Ra 0.8–1.6 μm suits sealing faces and sliding contact. Ra 0.2–0.8 μm is a lapping or fine-boring operation and should be applied only to the specific face that needs it.
Anodize, plating and coating callouts need a type and a thickness. Clear anodize, hardcoat anodize type III, electroless nickel, black oxide and powder coat are different processes with different dimensional effects. Hardcoat can add 25–50 μm per surface on a radius, which matters on a thread or a press fit.
Marking and packaging notes save money too. Laser marking needs a minimum character height of 1.5 mm to stay legible. If a part is cosmetic, say so and name the faces that must not be scratched, or bead blasting and tumbling will do their job on the wrong surface.
Step by Step: Building the Drawing
From model to released sheet
- 1Lock the modelFinish the 3D model and set units to mm or inch, not both. Save a STEP AP214 for the shop and keep the native file in revision control.
- 2Pick the machining orientationChoose the primary face and the first setup. Make that face the front view so the drawing matches the vise or fixture.
- 3Add three views plus isoFront, top, side and one isometric. Add section views through internal bores instead of stacking hidden lines.
- 4Set datums A, B, CPrimary mounting face, primary locating hole, secondary hole or edge. Dimension critical features from datums, not in a chain.
- 5Apply tolerances and GD&TGeneral tolerance in the title block, specific ± or position callouts only on functional features. Ø0.1 mm at MMC is a typical position value.
- 6Add finish and process notesRa value per surface family, coating type and thickness, deburr requirement, marking height minimum 1.5 mm.
- 7Run a DFM checkLook for deep pockets under 3× tool diameter, sharp internal corners, and tolerances tighter than the process can hold. Fix before release.
- 8Release with revision controlOne PDF and one STEP per revision. Note the revision letter in the title block and never reissue a file under the same letter.
Common Drawing Callouts and What They Cost
Values our shop works to
| Callout | Typical value | When it is worth it |
|---|---|---|
| General tolerance | ±0.1 mm | Non-critical faces, clearance holes, cosmetic edges |
| Machining tolerance | ±0.005 mm | Bearing seats, mating bores, locating features |
| Surface finish | Ra 1.6–3.2 μm | General milled and turned surfaces |
| Sealing surface | Ra 0.8–1.6 μm | O-ring grooves, gasket faces, sliding contact |
| Fine finish | Ra 0.2–0.8 μm | Optical seats, seal faces, precision journals |
| Position tolerance | Ø0.1 mm at MMC | Bolt circles, dowel pin patterns, mating plates |
| Minimum wall | 0.8 mm aluminium | Lightweight covers, heat sinks, brackets |
| Laser marking | 1.5 mm character height | Traceability codes, part numbers, logos |
A Clean Drawing Is the Cheapest Tooling You Own
Get the datums, the tolerance intent and the finish notes right, and the part comes off the machine correct the first time. Everything else is a phone call you did not have to make.
Frequently Asked Questions
Can I get a quote from a 3D model only, without a drawing?
Yes. Send a STEP file and we run a free DFM analysis and return pricing, usually within 12 hours. The model gives us geometry and the DFM report flags thin walls, deep pockets and tight tolerances.
A drawing is still needed before production. It carries the material grade, finish, tolerance intent and revision control that the operator and inspector work from.
How tight a tolerance should I put on my drawing?
Put the tightest tolerance only on the features that touch something else. A bearing bore, a dowel hole or a sealing face may need ±0.005 mm or a Ø0.1 mm position callout.
Everything else can sit at a general tolerance of ±0.1 mm. Tightening a non-functional face adds inspection time and cost without improving the assembly.
Third-angle or first-angle projection?
Use third-angle if your supplier is in North America or Asia, first-angle if they are in Europe. The important part is stating it in the title block with the projection symbol.
If you are unsure, add a small isometric view. A 3D orientation view removes almost all projection ambiguity.
What file formats do you accept?
STEP and IGES for 3D geometry, DXF and DWG for flat sheet metal profiles, and PDF for the controlled drawing. Native SolidWorks, Creo or NX files are also fine.
Keep the PDF and the STEP on the same revision letter. A mismatched pair is the most common cause of a wrong first article.
Do you sign an NDA before I send drawings?
Yes. An NDA is available on request, and uploads are handled as secure and confidential. We can sign your template or provide ours.
For early-stage concepts, a marked-up PDF with key dimensions removed is enough for an initial feasibility check.
How do I call out a finish on only one face?
Use a surface finish symbol with a leader pointing to that specific face, and add a note such as do not polish remaining surfaces.
If the whole part is cosmetic, add a note naming the protected faces. Bead blasting and tumbling reach every surface, so an unnamed face will be processed.
Send Your Drawing or Model for a Free DFM Check
Upload a STEP file or a PDF drawing and get pricing plus a manufacturability review within 12 hours. No minimum order quantity, from one prototype to 10,000+ parts.
12-hour quote±0.005 mm100% inspectionNDA on request
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