Define sheet metal inspection requirements by specifying dimensions, surface finish, materials, and testing methods. Use clear acceptance criteria in your RFQ to align buyer and supplier expectations, reduce rework, and prevent costly disputes during quality review.
- List specific dimensions, tolerances, and surface finish grades in the RFQ body, not just on drawings.
- Define the inspection method for each critical feature, such as gauge, caliper, or coating adhesion test.
- State the sampling plan and rework process to avoid disputes over partial defects.
- Require a certificate of conformance for every production batch, not just the first article.
- Use clear language that separates "shall" requirements from "preferred" recommendations.
Start with the Part Function
Sheet metal inspection requirements fail when they only reference the drawing number. A drawing shows the intended shape, but it rarely explains how to verify the shape in the final shop. A bracket that holds a motor may need a flatness check. A housing that seals a fluid may need a coating adhesion test. If the RFQ does not connect the function to the check, the supplier will inspect what is easy, and the buyer will reject what is important.
Before writing requirements, identify the failure modes. Will the part warp during bending? Will the coating peel on a painted surface? Will a hole position cause an assembly mismatch? Write these risks down. Each risk becomes an inspection point.
Do not assume the supplier knows what “good” looks like. A flat part is not enough. Define the maximum flatness, the coating thickness, the hole position tolerance, and the burr limit.
Step 1: Reference the Drawing and Revision
Begin the RFQ quality section by naming the exact drawing and revision. A supplier may have an old file on the server. If you send a revision 3 drawing and reference revision 1 in the body, the supplier may build the wrong part.
State the drawing number, revision, and the date you expect the supplier to confirm receipt. This creates a paper trail. If a dispute happens later, you can prove which file was in effect when the part was ordered.
Keep the reference short. A single line in the RFQ body is enough. Do not paste the drawing text into the RFQ. The drawing is the source of truth. The RFQ tells the supplier where to look.
Step 2: Define Critical Characteristics
Not every measurement on a drawing carries the same weight. Mark critical dimensions in the RFQ. A critical dimension is one that, if out of tolerance, makes the part unusable. A non-critical dimension may still fail the part if it affects assembly, but the consequence is often a rework rather than a scrap.
List the critical characteristics in a separate table. For each item, state the feature, the tolerance, and the inspection method. This separates the “must pass” checks from the “nice to have” checks.
If a drawing does not mark critical dimensions, you must define them. Do not rely on the supplier to guess what matters. If a hole position is off by half a millimeter, will the part fit? If not, it is critical.
Step 3: Specify Tolerances and Units
The drawing should have tolerances, but the RFQ should state the units clearly. Metric and imperial systems mix in many supply chains. If the drawing uses inches and the RFQ body uses millimeters, confusion happens.
State the default units for the RFQ. If the drawing uses inches, say “all tolerances in the drawing are in inches unless otherwise noted.” If you add a tolerance in the RFQ, use the same unit as the drawing.
Do not use vague terms like “close” or “accurate.” Use numbers. A flatness tolerance of 0.5 mm is a requirement. A flatness tolerance of “within practical limits” is not.
Step 4: Define Surface Finish and Coating Checks
Surface finish requirements are where disputes start. A drawing may show a Ra value, but it rarely states the measurement method or the zone. If you require a mirror finish on a large sheet, the cost changes. If you require a coating, define the minimum thickness, the adhesion test, and the color match method.
For coatings, state the minimum thickness and the test method. A coating adhesion test may require a specific tape or cross-cut method. If you do not specify, the supplier will use the easiest method. If the coating passes the easy method but fails in use, the dispute is on you.
For bare metal, define the surface finish grade. A brush finish, a mill finish, or a polished finish each have different costs and inspection methods. State the finish grade and the measurement area.
Step 5: State the Inspection Method for Each Check
For every requirement, name the tool or method. A caliper check is different from a gauge block check. A visual inspection is different from a dye penetrant test. If you do not name the method, the supplier will choose the cheapest one that produces a pass.
For dimensional checks, state the instrument. A digital caliper, a CMM, or a pin gauge each have different capabilities. If a part requires a CMM, say so. If a pin gauge is enough, say so. This sets the cost expectation.
For visual checks, define the criteria. “No visible scratches” is vague. “No visible scratches longer than 3 mm under 45 degree lighting” is specific. If you cannot define the visual limit, use a sample piece. The sample becomes the acceptance standard.
Step 6: Define the Sampling Plan
You will not inspect every part. You need a sampling plan. State how many parts to check per lot. If the lot is 500 pieces, check 5. If the lot is 50 pieces, check 10. If you do not state the sample size, the supplier may check one part and claim the lot is good.
State what happens when a sample fails. If one of ten parts fails, does the supplier rework that part, reject the lot, or rework the whole lot? Define the rework process. If the part can be corrected without damaging the finish, allow rework. If the finish is damaged, reject.
A sampling plan prevents the supplier from hiding defects. It also prevents you from rejecting a good lot because of one bad piece.
Step 7: Require a Certificate of Conformance
For every production batch, require a certificate of conformance. The certificate should list the drawing number, the lot number, the inspection results, and the date. It should be signed by the supplier’s quality representative.
Do not accept a verbal confirmation. Do not accept a screenshot of a spreadsheet. Require a signed document. If the part fails in use, the certificate is your proof of what the supplier claimed to deliver.
If the part requires material testing, such as a mill test report for stainless steel, include that in the certificate requirement. State that the material grade must match the drawing.
Common Mistakes That Cause Disputes
The most common mistake is writing “as per drawing” and stopping. That phrase is not a requirement. It is a reference. The drawing may have no tolerance for a critical feature. The drawing may have no surface finish grade. The drawing may have no coating thickness.
Another mistake is using “shall” and “will” interchangeably. Use “shall” for requirements. Use “may” for recommendations. If you write “the supplier shall provide a coating,” that is a requirement. If you write “the supplier may provide a coating,” that is optional.
Do not bury the requirements in the last paragraph of a long email. Put the quality section in the RFQ body. If the quality section is in an attachment, the supplier may miss it. If the quality section is in a separate document, the supplier may use an old version.
Do not assume the supplier will flag a conflict. If the drawing and the RFQ conflict, the supplier will follow the drawing. If you want a different requirement, state it clearly in the RFQ and ask the supplier to confirm.
Final Verification Step
Before sending the RFQ, read the quality section as a supplier would. Can you build the part from the text? Can you inspect the part from the text? If you cannot, the supplier cannot.
Check the units. Check the tolerances. Check the inspection methods. Check the sampling plan. Check the certificate requirement. If any item is missing, add it.
Send the RFQ. Ask the supplier to confirm receipt and to flag any ambiguity. If the supplier asks a question, answer it in writing. If the supplier does not ask a question, ask if they have any questions. Silence is not confirmation.
If the part is new, require a first article inspection. If the part is existing, require a production sample. The first article inspection verifies the process. The production sample verifies the part. Do not skip either.
Summary Table of Inspection Requirements
| Requirement Type | What to State | Example |
|---|---|---|
| Dimensional | Feature, tolerance, method | Hole position, +/- 0.2 mm, CMM |
| Surface Finish | Grade, Ra value, zone | Mirror finish, Ra 0.4 um, 10x10 mm |
| Coating | Minimum thickness, adhesion test | Powder coat, 150 um min, cross-cut |
| Material | Grade, mill test report | 304 stainless, MTR required |
| Sampling | Lot size, sample size | Lot of 500, check 5 pieces |
| Certificate | Lot number, results, signature | CoC with signed quality rep |
Frequently asked questions
Can I rely on the supplier to identify critical dimensions?
No. Suppliers identify risks based on their experience, but their experience may not match your application. Define critical dimensions in the RFQ to ensure the supplier inspects the features that matter most.
What if the drawing does not have tolerances?
State the tolerances in the RFQ. If the drawing is silent, the supplier will apply their default. If their default does not meet your requirement, the part will fail.
Do I need a separate quality document?
No. Put the quality requirements in the RFQ body. A separate document creates version control problems. The RFQ is the contract. The quality requirements must be in the contract.
How do I handle visual inspection disputes?
Use a sample piece. The sample becomes the acceptance standard. If the sample is approved, parts that match the sample are approved. This removes subjectivity.
Can I use "as per drawing" in the RFQ?
You can, but it is not enough. "As per drawing" is a reference, not a requirement. State the specific checks, tolerances, and methods that the drawing may not cover.
