A 3D CAD model shows the intended shape of a part, but shape alone does not always tell a machine shop what the finished part must meet. A 2D engineering drawing can communicate tolerances, threads, surface finishes, and other requirements that may not be clear from the model. For many CNC machining projects, sending both gives the shop the clearest starting point.
Quick Decision: Drawing, Model, or Both?
- Use a drawing to communicate manufacturing and inspection requirements, including critical dimensions, tolerances, notes, and finishes.
- Use a 3D model to communicate nominal geometry, especially when contours, pockets, or feature relationships are difficult to interpret from drawing views.
- Send both when available. A matching model and drawing give the shop a useful view of the shape and the requirements governing the finished part.
A drawing alone can be sufficient for a straightforward part if its views, dimensions, and notes define the geometry and requirements clearly enough for the shop to quote and manufacture it. A model alone may be a useful starting point when the design is early or the geometry is complex, but the shop still needs the functional requirements. If those requirements are not yet defined, discuss them before treating the quote or part specification as final.
What a 3D CAD Model Tells the Shop
A model represents a part's nominal geometry: the intended shape and size before allowable variation is considered. It can show holes, pockets, curved surfaces, and the relationship between features in a form that is easier to visualize than several separate views. Depending on the file format and the shop's workflow, it may also provide geometry for programming.
For example, a model of a contoured housing can help the shop understand wall shapes, pocket depths, and areas that may be difficult to reach with a cutting tool. That context can help the shop evaluate stock, workholding, and a possible machining approach. It does not, by itself, establish every requirement for the finished housing.
A modeled bore may show its nominal diameter without stating the acceptable size range or fit. A hole may appear in the right location without clearly identifying its thread or the accuracy needed for assembly. Some models contain manufacturing information through model-based definition, but requirements that are absent or ambiguous should not be inferred from geometry. Tell the shop where the governing information is recorded.
Choosing a file format
STEP is a common neutral format to offer when available. If you send a native CAD file, identify the software and version used to create it, and ask whether the format suits the shop's workflow. Before sending, check that the export opens as the intended part and that its units are correct. For an assembly, identify the components to be quoted and provide the requirements for each.
What a 2D Drawing Adds
An engineering drawing records requirements that a shop needs to make and verify the part. Depending on the design, it can specify dimensions, tolerances, geometric relationships, material, threads, surface finishes, and special notes. It also gives engineers and purchasing teams a common reference for discussing what the quote covers.
A useful drawing makes functional requirements clear without applying restrictive callouts to every surface. For instance, a bearing bore may need a carefully specified fit while an outside edge has a less demanding role. Identify the features that affect assembly, sealing, movement, or other functions, and specify the tolerances they actually need. Include general tolerances for features without individual callouts where appropriate.
Tighter or more complex requirements can affect machining methods, setups, measurement, and price. They may be necessary, but the shop should not have to guess which features are important or derive a tolerance from the decimal places displayed in CAD. If a requirement is uncertain, explain how the feature works or mates and discuss it with the shop before finalizing the specification.
Make notes and finishes actionable
Identify the surfaces affected by a finish requirement and give the finish value or other acceptance criteria. A note such as “smooth finish” is open to interpretation. Likewise, specify thread size, type, and depth where applicable; a plain modeled hole does not necessarily communicate a thread.
For coatings or other post-machining treatments, identify the affected surfaces and whether specified dimensions apply before or after treatment. Include relevant material specifications, edge treatment, and other notes when the design requires them. Ask the shop about any outside process rather than assuming it is included in the quote.
When One File May Be Enough
The right submission depends on whether the available information defines both the shape and the requirements, not just on the file type. A simple spacer, for example, may be fully described by a legible drawing with sufficient views, dimensions, material, tolerances, and any necessary finish notes. A shop can review that drawing without a model, although it may still have questions about the job.
For a part with complex freeform surfaces, a model can convey geometry more efficiently than a drawing crowded with views and dimensions. If the model contains all agreed manufacturing information, a model-based definition workflow may be suitable. Confirm the format and document hierarchy with the shop rather than assuming that every model's embedded information will be read or used in the same way.
If only a geometry model is ready, send it with the material, quantity, intended use, and any known critical requirements. Explain what remains undecided. The shop can then identify the information needed for a useful quote. Conversely, if only a drawing exists, ask whether it provides enough geometric detail or whether a model would help with a particular feature.
How to Handle Drawing and Model Conflicts
Before submitting both files, check that they describe the same part and revision. Compare changed features, units, and other details likely to affect the job. Do not knowingly submit mismatched files as a finished package: correct the discrepancy or clearly identify it as unresolved and request clarification before the shop relies on the information.
State the governing document and revision in the RFQ. A drawing is often the controlling specification when supplied with a model, but a project may use an agreed model-based definition or another document hierarchy. If a mismatch is discovered, identify the affected feature, confirm which requirement controls, and issue consistent revised files or a clear written clarification. The shop should ask rather than assume which version is right.
This matters commercially as well as technically. A quote based on an older hole pattern or a different finish may not reflect the intended part. If a design changes after a quote, identify the revision and the change so the shop can determine whether the price or plan needs review.
Build a Practical CNC Machining RFQ
A useful RFQ does not need a long form, but it does need enough information for the shop to understand the part and the requested order. Start with the essentials, then add special requirements only when they apply. If an item is unknown, say so rather than leaving the shop to assume it.
Start with the essentials
- Part files: Provide a legible drawing or a STEP/CAD model when available; send both if you have matching versions. Identify the part and revision.
- Material: State the material and any necessary grade, condition, or specification. Mention whether an alternative is acceptable.
- Quantity: Give the number of parts to quote. If you want prices for different quantities, list each separately.
- Critical requirements: Identify functional tolerances, threads, and required finishes in the drawing or other agreed specification. Explain any important requirements that are still being developed.
- Desired delivery date: State when parts are needed and whether the date means shop completion or arrival at the destination.
For an early prototype, distinguish requirements needed for the test from details that may change. For a repeat order, identify expected releases or future quantities as estimates unless they are committed. This context helps the shop understand what to quote without implying that an unfinished design is production-ready.
Add requirements that apply to your project
Include inspection reports or other verification documents if they are required, specifying the features or reporting format involved. Similarly, describe outside processing, coating, marking, packaging, or special deburring only when needed. If a customer requires a particular standard, certification, or documentation, provide the exact requirement and ask the shop to confirm whether it can meet it. Do not assume a quote automatically includes a particular report or compliance service.
Finally, note what is flexible. An acceptable alternate material or a flexible delivery date gives the shop useful options; a sealing face or mating feature that cannot change deserves clear attention. This distinction can make the discussion more productive for both engineering and purchasing.
Request a CNC Machining Quote from Anco Precision
Ready to source your part? Request a CNC machining quote from Anco Precision. Send your drawing or STEP/CAD file when available, along with the material, quantity, critical requirements, and desired delivery date. Include inspection or outside-processing requirements if they apply, and flag any information that still needs clarification.