Preparing CAD Models and Drawings for a CNC RFQ

Key decisions

• Create one authoritative revision and state how model, drawing and notes relate.
• Put tolerance and finish requirements on the features that determine function.
• Request the same material, inspection and delivery scope from every quoting supplier.

Establish a clear source of product definition

An RFQ often contains a STEP file, a PDF drawing, an older assembly and screenshots showing desired changes. If their authority is not clear, the supplier must choose between conflicting instructions or delay the quote. Give the part one number and revision, identify reference-only files, and list any known difference between the model and drawing.

The drawing or a properly defined model-based dataset communicates material, tolerances, datums, surface requirements and acceptance notes. ASME’s Y14 standards provide a framework for these requirements, including dimensioning and tolerancing and digital product definition. Name the applicable standard and edition when required. An ordinary unannotated solid model is not automatically a complete model-based manufacturing specification.

Check the exported geometry, not just the native CAD file

Open the actual file that will be sent. Confirm units, overall dimensions, body count and revision. A model that looks correct in the originating software may arrive as multiple surfaces, lose small features or present an unexpected scale after export. For an assembly, identify the components to quote and distinguish purchased or reference items from manufactured parts.

Where model-based product information is used, confirm that the receiving system can interpret the information as intended. NIST’s PMI validation work distinguishes graphical presentation from information that software can interpret semantically. A dimension visible on screen is not proof that it remains machine-readable after translation. If the chosen exchange route cannot preserve acceptance information reliably, provide an agreed readable companion definition.

Build a small, complete RFQ package

The package should contain the current information required for a comparable manufacturing review. Give each document a defined role so an email note does not contradict the drawing.

Explain which features control assembly

A useful drawing does more than add tight tolerances everywhere. It identifies the surfaces and features that locate, support, seal or move against other components. Build a datum scheme around those functions, and choose geometric controls where size tolerances alone do not describe the required relationship. A bore can be the correct diameter while its axis is misplaced relative to a mounting face.

Separate reference dimensions from acceptance requirements and avoid redundant closed dimension chains that imply conflicting limits. For an internal pocket, check that specified radii are consistent with the model and the mating component. If a sharp internal corner is functionally necessary, say so; if clearance relief is acceptable, show the approved form. The manufacturer should propose alternatives for approval rather than infer an unrecorded design change.

Threads and holes need more than a cylindrical surface

For threads, provide the standard, nominal size, pitch, tolerance class and usable engagement length. Distinguish a through feature from a blind feature, and clarify required thread depth separately from drill depth where relevant. A cosmetic thread display in CAD does not define a complete thread requirement. Mark whether the final thread is checked after coating or protected by masking.

For a blind hole, consider the available space for drill-point geometry, thread runout and chip evacuation. Do not prescribe a perfectly flat bottom unless the function needs it; equally, do not allow an assumed drill point to intrude into a protected wall. If a cross-hole creates an internal burr-sensitive intersection, identify that intersection and the acceptance method explicitly.

Resolve model-and-drawing conflicts before pricing

A short conflict list is more effective than leaving a supplier to interpret a crowded revision history. State the intended correction, update the affected files together and retain a record of the released package. The following example shows why the physical assembly requirement belongs with the geometry.

Define the material and the condition of the accepted part

Use the required alloy or polymer grade, condition and applicable product specification. If substitutions are allowed, define the approval procedure and essential properties. A generic note such as aluminum or black plastic cannot establish the supply or qualification scope. Include material certification or traceability requirements at the RFQ stage if receiving inspection depends on them.

For finishing, identify treatment specification, thickness where required, appearance zones and masking. State whether critical sizes are accepted after the final treatment. A photograph can support a color or texture discussion but cannot define coating chemistry or deposition on a hidden surface. Keep any approved appearance sample connected to its revision or identifier so repeat orders have an unambiguous reference.

Ask for evidence and quantities that match the purchasing decision

An inspection request should identify what is measured, on how many pieces, in what condition and with what record. Avoid treating inspection as one undifferentiated deliverable. A report on selected samples, a first-article report and a check of every critical dimension on every part have different scopes. If the receiving team uses a required format or fixture, provide it before quotations are compared.

Separate development samples, pilot quantity and realistic production releases. Give the requested destination and timing without treating an estimate as a supplier commitment. For a confidential project, establish the required information-sharing arrangement and verified contact route before transmitting protected geometry. Once quotations arrive, compare the same revision, material, finish, evidence and delivery scope.

• Open the exported CAD and PDF and compare them feature by feature at critical interfaces.
• Confirm part number, revision, units and document precedence.
• Specify material, condition, threads, datums, finish and edge requirements.
• Identify the inspection scope, sample approval stage and required records.
• Ask Beigain to list assumptions and proposed deviations clearly in the quotation.

Questions about this topic

Technical references

• ASME: Y14 standards — Primary standards overview for drawing, geometric tolerancing and digital product definition.
• NIST: MBE PMI validation and conformance testing — Primary explanation of validating product and manufacturing information across digital workflows.

Continue reading

• Preparing a CNC RFQ that suppliers can quote accurately
• Defining CNC inspection requirements before production
• How to Order Custom CNC Parts
• Request a Manufacturing Quote