How should buyers specify tolerances for custom CNC milling quotes?
How should buyers specify tolerances for custom CNC milling quotes?
For custom CNC milling quotes, state the general tolerance authority and units, identify critical features, define datums and GD&T, and specify material, finish, final acceptance state, inspection method, sampling, and records. Tie each special requirement to a feature or drawing zone. Ask the supplier to list assumptions and exceptions rather than pricing maximum precision across every dimension.
Ambiguous callouts prevent comparable quotes because suppliers may assume different process controls, inspection scopes, or acceptance states. An unnecessarily tight default can price risk into unrelated features. Place tolerance decisions inside the CNC machining order workflow, and use the drawing's machining tolerances to define acceptance rather than rely on a supplier's unstated shop standard.
1. Define the General Tolerance Authority and Exceptions
A general tolerance should govern only dimensions without an individual requirement. State the governing standard or drawing rule, edition, class where applicable, units, decimal conventions, and exceptions. Do not substitute an assumed shop default for a contractual acceptance requirement.
Feature-specific limits override the general note and should be tied to function. Tightening an unrelated default narrows manufacturing and inspection margins across the part, so quotations may include controls that the assembly never uses.
Drawing Item | RFQ Specification | Quote Decision |
|---|---|---|
Unspecified ordinary dimension | General rule, units, class, and exceptions | Sets default acceptance without inventing precision |
Mating size | Feature ID, limits or fit, and final state | Drives process and gage selection |
Sealing surface | Size, form, texture, and post-finish state | Separates geometry from surface function |
Alignment feature | Datums, basic dimensions, and geometric control | Defines the relationship and its verification |
2. Rank Critical Characteristics by Assembly Function
Identify critical characteristics by feature ID, mating component, and failure consequence. Fit, sealing, motion, load transfer, alignment, and cosmetic zones can require different controls. A dimension is not critical merely because it is easy to measure or carries many decimal places.
The RFQ should distinguish controlled characteristics from reference or ordinary dimensions. Request a quote response that identifies the planned control and inspection evidence for each critical item, plus any requirement the supplier cannot interpret or achieve as written.
3. Make the Datum Reference Frame Manufacturable and Inspectable
A datum reference frame must identify datum features, precedence, and the degrees of freedom they constrain. Datum features should be accessible and stable in the specified acceptance state. A convenient setup surface is not automatically the drawing datum.
For a multi-face part, state whether coating, heat treatment, or release from workholding occurs before final datum-based inspection. If a datum feature can change during processing, the supplier needs a transfer and re-establishment plan before quoting.
This separation of local size from shared reference geometry is central to dimensional and geometric tolerances.
4. Use GD&T to Express Relationships, Not to Replace Function
Size limits do not control every functional relationship. A bore can pass diameter but fail location; a mounting face can pass thickness but fail form or orientation. The drawing must separate size, form, orientation, location, and profile according to assembly function.
Use the edition and system named on the drawing. ASME Y14.5 governs dimensioning and tolerancing in the ASME system; ISO 1101 and ISO 5459 cover geometrical tolerancing and datum systems in ISO GPS. Include basic dimensions and modifiers only where function supports them, and clarify any ambiguous frame before the quote becomes binding.
Functional Need | Drawing Control | Quote Confirmation |
|---|---|---|
Mating size | Size limits or fit | Material, temperature, and final acceptance state |
Hole-pattern relationship | Position to a datum reference frame | Datum access, basic dimensions, modifiers, and inspection |
Sealing or mounting face | Form control plus texture when required | Separate form, roughness, and support state |
Angled interface | Perpendicularity, angularity, or profile | Datum order and measurement method |
Complex contour | Profile to a defined datum system | Model authority, tolerance zone, and inspection coverage |
5. Specify Surface Texture and Acceptance State Separately
Dimensional tolerance does not define surface texture. For functional surfaces, state the parameter, limit, governing evaluation standard, measurement direction, sampling locations, and whether acceptance occurs before or after coating or finishing. A roughness value alone may not describe lay, defects, waviness, or cosmetic appearance.
Map each requirement to the relevant surface and function. Use the existing surface roughness specification and agreed quality control method for technical acceptance; define cosmetic zones and visual criteria separately when appearance also matters.
6. Lock Material, Stock Form, and Process State Before Quoting
Specify the material standard, exact grade, condition, and stock form, plus heat treatment and surface processes. Even Aluminum 6061-T6 or SUS316 does not define stock form, supplier standard, dimensional state, or final finish by itself.
State whether tolerances apply before or after heat treatment, anodizing, plating, coating, or workholding release. Stock residual stress, thermal processing, and material removal can change finished geometry, so the quote needs the same accepted state that production inspection will use.
7. Define Inspection Evidence, Sampling, and Decision Rules
For each reported characteristic, identify the feature, acceptance state, measurement method, sampling level, record format, and decision rule. A request for “CMM inspection” is incomplete when datum alignment, coverage, and required output are undefined.
Not every feature needs a formal report. Separate setup checks from final acceptance, and distinguish first-article, per-lot, periodic, and full-inspection requirements. Measurement uncertainty and any rule for results near a limit should be agreed before pricing.
Use the linked inspection reports and CMM inspection reports pages to identify available document types, then place the exact deliverable and sampling requirement in the RFQ.
8. Control File Revision, Authority, and Conflict Precedence
State whether the 2D drawing, model-based definition, or another controlled specification governs acceptance. The 3D model can define geometry, while notes, dimensions, and tolerances may reside in another file. Matching revision identifiers and an explicit precedence rule prevent silent conflicts.
A STEP model plus controlled PDF is one workable package when both share a revision; another neutral or native format may be used by agreement. Confirm the accepted file set for CNC machining orders and require clarification when files disagree.
9. RFQ Completeness Check Before Supplier Review
RFQ Field | Required Definition |
|---|---|
General tolerance authority | Standard or rule, edition, class, units, and exceptions |
Critical feature register | Feature IDs, function, limits, final state, and records |
Datum and GD&T scheme | Datum order, basic dimensions, modifiers, and acceptance state |
Surface requirements | Texture parameter, limit, direction, locations, and process state |
Material definition | Standard, grade, condition, stock form, and final processes |
Inspection deliverables | Method, alignment, sampling, decision rule, and record format |
Controlled data package | File revisions, governing authority, precedence, and conflict process |
10. Release the RFQ Only After Assumptions Are Closed
A quote is comparable only when each supplier prices the same revisions, tolerance authority, critical features, datum scheme, material state, surface requirements, and inspection scope. Require written assumptions, exceptions, and proposed clarifications before accepting a manufacturing route.
Release the order after the drawing and quote agree on final acceptance state, sampling, records, and conflict precedence. This prevents a low quote based on omitted controls from being compared with a complete, validated route.
