A technical drawing for CNC machining should communicate the requirements needed to make and inspect the finished part. Pair it with a matching CAD model where appropriate, and clearly identify material, critical dimensions, tolerances, threads, finishes, and revision.
The goal is not to fill a sheet with annotations. It is to remove the decisions a supplier would otherwise have to guess.
A model can show a hole perfectly while leaving its fit, thread, surface condition, or inspection requirement unclear. A useful drawing closes those gaps.
Do you need a 2D drawing if you have a CAD model?
Not every CNC order requires a separate 2D drawing. Some suppliers can quote straightforward geometry using a model and agreed default requirements. For example, Protolabs Network accepts CAD files for quotation and recommends drawings to communicate requirements such as threads, specific tolerances, and finishes. Protolabs Network drawing guide
Distinguish a plain geometric model from model-based definition, where the digital product data also communicates manufacturing requirements. A suitable annotated model can carry information that would otherwise appear on a drawing. ASME Y14.41 addresses the preparation and revision of digital product-definition data. ASME Y14.41 overview
For many custom-part orders, a matching 3D model and readable PDF drawing remain a practical combination. Confirm the supplier's accepted formats and how it will use each file.
Think of the package this way:
File or information | Main purpose |
|---|---|
3D model | Communicate nominal geometry for review and manufacturing preparation |
Drawing or accepted digital annotations | Define requirements that geometry alone does not establish |
Purchase or quotation information | State quantity, delivery needs, documentation, and other order conditions |
The files should complement one another and describe the same released part.
Start with part identity, units, and revision
Give the supplier a clear way to identify the component. Include the part number, descriptive name, drawing revision, and units. Use matching identifiers in the drawing and model filenames.
State the material and finish in the designated fields or notes. If the drawing has several sheets, make sure the recipient can identify the complete set.
A revision should also be understandable. When a bore changes, record the change through your document-control process and update the affected files. Xometry's drawing guidance recommends revision records that identify the change and its date. Xometry technical drawing practices
As a practical release check, open the exported PDF and STEP file yourself. Confirm that the export contains the intended part and the latest geometry. A correct native CAD file does not prove that the files attached to an RFQ are current.
Choose views that make the geometry clear
Use enough views to explain the part without crowding the sheet. An isometric view helps the reader recognize the shape; orthographic views show the part from defined directions for dimensioning.
For hidden geometry, a section view can clarify the interior. A detail view can enlarge a small feature that would be difficult to read at the main scale. Dassault Systèmes describes these as useful elements of CNC technical drawings. Dassault Systèmes drawing guide
Consider a bracket with a recessed mounting hole. A top view may show two circles, but a section makes the recess depth and remaining material easier to understand.
Identify the projection method and scale, and make sure the exported PDF remains legible. Do not require the manufacturer to measure a printed image to recover a missing controlled dimension.
Dimension the features that control function
Begin with how the part fits and works. Which surfaces locate it? Which holes align with another component? Which dimensions control clearance or sealing?
Separate size from feature relationships
A hole diameter specifies its size. It does not, by itself, define the hole's location or orientation relative to the assembly.
Geometric dimensioning and tolerancing, or GD&T, provides a structured language for requirements such as form, orientation, and position. ASME Y14.5 defines symbols and interpretation rules for that language. ASME Y14.5 overview
A datum is a theoretically exact reference used to establish a measurement or tolerance framework; the drawing identifies physical datum features from which that reference is established. Choose references that reflect the part's functional relationships.
For a mounting plate, the seating face and locating features may matter more than an unused outside edge. Have the responsible designer define the required relationships rather than asking the shop to invent them.
State the tolerance system
Use individual tolerances where necessary and a clearly identified general tolerance provision where appropriate. Confirm that its scope covers the dimensions you intend it to cover.
Do not copy a supplier's default tolerance into a drawing without checking that it suits the part. A loose outside dimension and a precision bearing seat serve different functions.
For deeper guidance on achievable accuracy, use the CNC milling accuracy guide. This drawing review should capture the selected requirements, not promise a universal tolerance.
Make hole and thread requirements complete
Check that each hole's purpose is clear. A clearance hole, a locating bore, and a threaded hole can need different information even when their model geometry looks similar.
For threaded features, define the thread designation, pitch or threads per inch as applicable, tolerance class, and required extent. For a blind thread, distinguish the usable full-thread depth from the hole depth. Those are not interchangeable.
A shallow blind hole also needs enough room for the chosen manufacturing method. Discuss the available depth and any nearby features when space is restricted. Protolabs' machining guide illustrates thread size and depth information used in manufacturing preparation. Protolabs machining guide
The drawing should normally communicate the required finished thread. If a particular process is mandatory, state it explicitly. Otherwise, the supplier can review the manufacturing route against the requirement. Our thread milling vs tapping article explains that process decision.
Specify material, finish, and edge requirements
“Aluminum, black finish” leaves substantial room for interpretation. Identify the required grade and condition, then define the finish and its applicable requirements.
Check these details:
- The alloy or polymer grade, temper, heat treatment, or condition where relevant.
- The finish specification, coating type, color, thickness, and other necessary acceptance requirements.
- Surfaces that must be masked or kept free of coating.
- Whether critical dimensions apply before or after finishing.
- Surface roughness requirements on the faces that need them.
- Edge breaks, burr restrictions, and cosmetic surfaces.
Surface roughness and coating type are separate requirements. Naming anodizing does not fully specify the texture beneath it. See the CNC surface finish guide for the broader comparison.
For a precision bore, coating growth may affect fit. If the bore is to remain uncoated, identify it clearly and confirm that the exposed surface is acceptable for the application. The Type II vs Type III anodizing guide discusses dimensional allowance and masking.
Avoid a vague deburring note when an edge performs a specific function. For example, a sealing edge may need a defined condition rather than an unrestricted edge break.
A mounting-plate example: what the model cannot settle alone
Imagine a hypothetical aluminum mounting plate with a central bore and four corner holes. A plain model shows the shape, but it does not necessarily answer these questions:
Feature or decision | Requirement to resolve |
|---|---|
Central bore | Clearance opening, bearing fit, or another function? |
Corner holes | Clearance holes, threads, or locating holes? |
Seating face | Which face contacts the assembly, and what controls its form? |
Finish | Which surfaces are coated, masked, or cosmetic? |
Acceptance | Which dimensions are inspected, and in what finishing state? |
The correct values depend on the mating parts and operating requirements. Do not invent a tight tolerance because a feature looks important.
For instance, if the center opening only clears a cable, a precision bore fit may add effort without improving function. If it locates a bearing, size alone may be insufficient; the responsible engineer must also evaluate the related geometric requirements.
This is a drawing-review example, not a ready-to-manufacture plate specification.
Agree on inspection and required records
Define the evidence needed to approve the order before quotation. A request for “inspection included” may not establish the report format or the features to be reported.
Identify critical characteristics and any required dimensional reports, material certificates, finish records, or first-article documentation. If a customer-specific format is required, provide it with the request.
For flexible parts, agree whether acceptance applies in the free state or under a defined restraint. A fixture that forces a part flat can produce a different result from an unloaded measurement. The thin wall CNC machining guide explains why that distinction matters.
Discuss requirements that are difficult to measure before production starts. A dimension is only useful for acceptance if both parties understand what is being evaluated.
Resolve model and drawing conflicts before release
Do not assume that the drawing always overrides the model, or that the latest email automatically changes a controlled requirement. The agreed product-definition and order rules determine authority.
If two files conflict, resolve the discrepancy with the design owner and release a consistent package. Document any accepted change so the supplier does not have to infer approval from a conversation.
A practical example is a drawing revised to enlarge a bore while the attached STEP file retains the old diameter. Renaming the old model does not fix the mismatch; regenerate and verify the export.
Keep obsolete files out of the active quotation package, or clearly identify them as superseded when they must be retained for reference.
CNC machining drawing checklist
Before sending the files, confirm:
- Identity: Part number, revision, and quantity are clear and consistent.
- Geometry: The model and drawing agree, with sufficient views and sections.
- Units and conventions: Units, projection, and applicable drawing standards are identified.
- Dimensions: Functional sizes and relationships are defined without conflicting requirements.
- Tolerances: Individual and general provisions have clear scope.
- Threads and holes: Type, size, extent, and other required details are complete.
- Material and finish: Grade, condition, coating, texture, and masked areas are specified as needed.
- Acceptance: Inspection state, critical characteristics, and required records are agreed.
- Release: The exported files open correctly and represent the intended revision.
For geometry changes that can make the part easier to manufacture, use the DFM guide. Keep the drawing aligned with any changes you approve.
Frequently asked questions
Is a STEP file enough for a CNC quote?
Sometimes. A supplier may quote straightforward geometry using agreed defaults. Additional drawing or annotation information is needed when the model does not communicate the requirements that affect price or acceptance.
Must every dimension have an individual tolerance?
No. A suitable general tolerance provision can cover dimensions within its stated scope. Use individual requirements where the part's function needs them, and avoid conflicting instructions.
Do you need GD&T on every part?
Not necessarily. Use it when it communicates the required geometry and relationships effectively. The notation must be correct and based on the applicable standard.
Which wins if the CAD model and drawing disagree?
Follow the agreed document-authority rules and resolve the conflict before manufacture. There is no safe universal assumption that applies to every supplier, contract, or product-definition system.
Send a package the supplier can act on
A useful technical drawing makes the finished-part requirements clear. Combine it with the appropriate model, verify the revision, and resolve any ambiguity around fits, threads, finishes, and inspection before release.
PiPrecision is a Shenzhen CNC machining manufacturer supporting global customers with milling, turning, and coordinated finishing from prototypes to production. Send your CAD model and drawing for review, together with quantity and required documentation. The team can then review manufacturability and identify questions that need resolving before quotation or production.