What buyers should verify first
A reliable decision starts with evidence that matches the drawing, production volume and business risk. Use the table below during RFQ review; replace assumptions with named documents, owners and acceptance criteria.
| Review area | What good control looks like | Buyer action |
|---|---|---|
| General untoleranced dimensions | Use a stated general standard agreed on the drawing | Avoid verbal assumptions |
| Critical bores/shafts | Can be tighter with controlled finishing and gauging | Confirm process capability |
| True position/profile | Datum scheme and fixturing dominate | Review setup plan |
| Thin walls | Distortion can exceed machine positioning error | Plan stress and clamping |
| Finished surfaces | Coating growth may change final dimensions | Define before/after finish |
1. Separate machine accuracy from process capability
Ask for data from a similar feature, material and batch size. A one-off inspection result does not prove that a process stays centered as tools wear and temperature changes.
For sourcing approval, convert this point into a written requirement: identify the responsible party, evidence to retain, acceptance rule and response when the rule is not met. This keeps a sample success from being mistaken for a capable production system.
2. Define the measurement agreement
State datum simulation, instrument, environment, reporting precision and whether dimensions apply before or after surface treatment. Many disputes are measurement-method disputes.
For sourcing approval, convert this point into a written requirement: identify the responsible party, evidence to retain, acceptance rule and response when the rule is not met. This keeps a sample success from being mistaken for a capable production system.
3. Build a critical-characteristic list
Mark only dimensions that affect fit, sealing, motion or safety. Give the supplier freedom on nonfunctional dimensions to reduce tool changes, inspection time and scrap.
For sourcing approval, convert this point into a written requirement: identify the responsible party, evidence to retain, acceptance rule and response when the rule is not met. This keeps a sample success from being mistaken for a capable production system.
4. Plan for drift
Use first-off approval, periodic checks, tool-life limits and reaction rules. For higher volumes, request capability evidence on selected CTQs rather than an oversized final report.
For sourcing approval, convert this point into a written requirement: identify the responsible party, evidence to retain, acceptance rule and response when the rule is not met. This keeps a sample success from being mistaken for a capable production system.
5. Prototype the process, not just the shape
A prototype cut slowly from soft jaws may not represent production. Ask how fixtures, tools, feeds and inspection will change at volume.
For sourcing approval, convert this point into a written requirement: identify the responsible party, evidence to retain, acceptance rule and response when the rule is not met. This keeps a sample success from being mistaken for a capable production system.
Practical buyer scenario
A bearing bore may need a controlled finishing pass and bore gauge, while the outer cosmetic envelope can use a general tolerance. This concentrates control where assembly risk exists.
The useful output is a controlled plan, not a promise. Record the drawing revision, assumptions, open questions, required evidence and approval gate in the RFQ or purchase order. For critical applications, obtain engineering, quality and regulatory review appropriate to the product.
RFQ checklist
- Controlled 3D model and 2D drawing with matching revision
- Material grade, condition or temper, and any certificate requirement
- Quantity per release, annual demand and target delivery date
- Critical-to-quality characteristics and agreed measurement method
- Surface finish, cosmetic zones, masking and post-finish dimensions
- Packaging, labeling, Incoterm and destination
- First-article, sampling, traceability and corrective-action expectations
Frequently asked questions
Can every dimension be ±0.01 mm?
Usually not economically, and sometimes not physically, across different sizes and geometries. Tighten only functional features.
Does a CMM guarantee accuracy?
No. Programming, fixturing, datum alignment, probe qualification and environment still matter.
Should tolerances apply before anodizing?
State this explicitly; coating growth can affect fits and threads.
What data should I request?
First-article results and, for repeat production, trend or capability data on selected critical features.
Sources and further reading
Technical note: Examples in this guide are planning guidance, not a universal manufacturing specification. Final capability, certification, tolerances, treatment and acceptance criteria must be confirmed for the exact drawing and contract.
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