Defects in sheet metal products are determined by non-conformity with agreed requirements: dimensions, geometry, coating condition, completeness, and fitness for use. A scratch, waviness, or angle deviation requires verification against the drawing and acceptance criteria. It is not always possible to determine the cause of damage or the decision regarding the part by its appearance alone.
Below are the signs to check in panels, cassettes, lamellas, and other sheet metal parts, how to record non-conformities, and what helps reduce their occurrence.
Reject and defect · Inspection table · Causes · Prevention · Actions during acceptance · Questions
Reject, defect, and non-conformity: what to distinguish
The American Society for Quality glossary distinguishes between a non-fulfillment of a specified requirement and a defect related to the expected use of a product. For practical acceptance, it is important to record not just the word “defect,” but the specific sign, requirement, and inspection result.
- Non-conformity: the actual parameter does not meet the requirement. For example, the hole position falls outside the agreed tolerance.
- Defect: a flaw relating to fitness for the intended use. Its significance depends on the part’s function.
- Rejected product: in this article, a part that, based on the evaluation results, cannot be accepted in its current condition. Further decisions may involve agreed rework, repair, or replacement.
Obvious signs are detected during visual inspection and measurements. Less noticeable non-conformities may require special testing or manifest over time. A hidden defect does not mean it is fundamentally impossible to detect before operation: the result depends on the method and scope of inspection.
Consequences for appearance, assembly, corrosion protection, and structural performance are evaluated separately. A seemingly minor crack near a fastener can be more important than a noticeable cosmetic unevenness on an unloaded surface.
Typical defects and how to check them
This is a practical inspection scheme, not a universal tolerance table. Values, measurement methods, and permissible defects must be taken from the specific product’s documentation.
| Sign | What to check |
|---|---|
| Length, width, or angle do not match | Dimensions according to the current drawing, agreed tolerances, measurement bases, and the state of the part without external load. |
| Thickness deviation | Nominal thickness, tolerances, control points, and the contribution of coatings to the measurement result. |
| Dent, distortion, waviness | Geometry under agreed inspection conditions; packaging, transportation, supports, and fasteners if the part is already mounted. |
| Holes missing or displaced | Drawing version, quantity, diameter, and coordinates of holes; compatibility with fasteners. |
| Crack, fold, burr, tool mark | Cutting or bending zone, edge condition, actual radius, and surface; potential impact on assembly and use. |
| Scratch, chip, coating peeling | Location and extent of damage, base condition, surface acceptance conditions, and agreed evaluation method. |
| Difference in color, gloss, or texture | Agreed physical sample, lighting, orientation of adjacent parts, and batch data; if necessary, instrumental control. |
Waviness is not explained by a single cause
For wide flat areas of thin metal, the phenomenon of oil canning — visible waviness — is characteristic. The Coated Metals Group technical bulletin lists rolling residual stresses, forming, panel handling, uneven supports, fastening, and thermal movements among possible factors. Lighting can change the visibility of the waves.
Therefore, a photograph from one angle does not provide sufficient grounds to identify the source of the problem. The part, installation conditions, and agreed appearance requirements are evaluated; the general explanation of oil canning does not override these requirements.
Coiled metal and strip
Where do non-conformities come from
Material and design
Check the grade, thickness, and condition of the metal, coating properties, and compliance of the raw material with documentation. Not only the quality of the material is important, but its suitability for a specific shape and loads. A correctly manufactured part may also fail to perform its task if its geometry, supports, or fasteners are chosen incorrectly.
The base criteria are discussed in the article on steel for sheet metal. Low price or country of origin by themselves do not prove the cause of a defect.
Cutting, bending, and equipment setup
The result depends on the tool’s compatibility with the material, its condition, settings, and the sequence of operations. SSAB’s explanation on bending covers radius, thickness, and springback. Numerical parameters for high-strength steels from this publication cannot be automatically transferred to any thin sheet.
Springback after load removal is a physical behavior of the material, not inherently proof of a defect. Non-conformity arises when the final geometry does not meet the specified requirements. A conclusion about quality is not made solely based on the age, price, or automation level of the machine.
Coating, storage, and installation
Scratches, peeling, or corrosion can have various causes. Compare the condition of the raw material and the finished part, surface preparation, forming conditions, packaging, moisture during storage, fastening, and operating environment.
ECCA describes testing of prepainted metal according to product-specific requirements: in particular, control of coating thickness, gloss, and behavior during bending. Visual inspection does not replace required testing. The mere fact of surface change after installation does not establish at which stage the cause arose.
How to reduce the reject rate
Practical measures cover the entire journey of a part: from agreeing on requirements to checking the finished product.
- Before production: agree on the current drawing, material, tolerances, surface, and completeness; verify the design’s suitability for the application conditions.
- During raw material acceptance: verify batch documents, marking, geometry, and surface condition.
- Before the series: check the settings and the first part, including critical dimensions and bends.
- Between operations: monitor parameters that may change during cutting, forming, or coating application; separate questionable parts.
- Before shipment: check the agreed product volume, completeness, marking, packaging, and inspection records.
- After discovering a problem: establish the cause, agree on an action for the specific batch, and verify the result of the process correction.
The scope of inspection is agreed upon based on risk and order requirements. Acceptance sampling of a batch, which ASQ considers as acceptance sampling, does not mean individual inspection of every part and does not provide grounds to promise zero defects.
What the buyer should do during acceptance
- Verify the name, quantity, marking, and documents. Check the packaging and accessible surfaces.
- If a non-conformity is found, mark and separate the part. Do not hide the problem with subsequent installation or self-repainting until a decision is agreed upon.
- Take a general photo, a close-up, and a photo with a scale; save the batch number and part designation on the drawing.
- Record the requirement, the actual result, and the measurement method. Use a tool suitable for the specified tolerance.
- Send the data to the supplier and agree on re-inspection, measurement, testing, rework, or replacement.
In the presence of cracks near fasteners or doubts about the structural performance, postpone the use of such a part until a technical evaluation. The cause of damage and the applicability of warranty conditions are considered based on the actual data of a specific order; the general article of this conclusion does not substitute for it.
When ordering Mehbud facades, ceilings, and fences, it is useful to agree on acceptance criteria in advance. Finishing requirements can be clarified in the article on painting sheet metal.
Frequently Asked Questions
Does every scratch mean a defect?
The decision depends on the agreed surface condition, the size and location of the damage, and its impact on use. First, record the sign and check the corresponding requirement.
Can hidden defects be detected before installation?
Some — yes, if appropriate measurement or testing methods are applied. Routine inspection may not be enough; the list of required checks depends on the product.
Does corrosion after installation prove a manufacturing defect?
By itself — no. It is necessary to evaluate the material, coating, edges, storage and installation conditions, and operating environment.
Can a non-conformity be fixed on site?
Only after agreeing on the method, materials, and verifying the result for this part. The possibility of repair does not mean that any correction will preserve the initial characteristics.
Related articles
How to bend sheet metal at home
How to bend sheet metal at home and in the workshop? We review tools, bending methods, material thickness, and safe techniques.
Using mobile apps to calculate the area of facades, fences, and ceilings
Digitalize architectural measurements: use LiDAR, ARKit, and mobile apps like Polycam to accurately calculate facade, fence, and ceiling areas.
Zinc: Everything You Need to Know About This Metal
Main properties of zinc, its history, compounds, and alloys. We explain the mechanisms of steel protection, differences between galvanizing methods, and material inspection criteria.