Sheet Metal Quality Control & Inspection

We understand that your biggest concern is usually not whether a supplier can make the part at all. If one supplier cannot do it, you can always find another. In Chinese manufacturing, there is a saying: “If one supplier can’t do it, plenty of others can.”
So the real issue is not whether the part can be made. The real issue is this: the sample passes, but problems appear once mass production begins. That is what truly causes frustration.

Based on our internal data, around 65% of quality issues occur during mass production, where material batch variation, machine parameter drift, and process deviation are much more likely to appear. At SR MFG, we address these risks through a systematic quality control approach designed to prevent problems at the source.

Springback is an inherent characteristic of sheet metal fabrication. Different materials, thicknesses, and bend radii produce very different levels of springback. If compensation is not controlled accurately, bend angle deviations will accumulate.

Here is how we manage springback control:

1. Material springback database
   For commonly used materials such as SPCC, SUS304, and AL6061, we maintain a springback database covering different thicknesses and bend radii, with compensation accuracy controlled within ±0.3°.
2. First article verification of compensation values
   Before each production run, the first article is checked to confirm that bend angles are within tolerance, and compensation parameters are fine-tuned based on actual results.
3. Dedicated angle measurement tools
   We use digital angle gauges with an accuracy of ±0.1° and profile measurement equipment with an accuracy of ±0.01° to ensure accurate bend angle verification.
4. Ongoing bend angle stability monitoring
   Bend angles are sampled every 50 pieces. If a deviation greater than 0.2° is found, production is stopped immediately for adjustment.

The heat-affected zone created during welding can lead to sheet metal distortion and unattractive weld appearance. These are common customer complaints and account for approximately 15% of quality issues. Welding distortion can affect overall dimensions, while poor weld appearance can damage the visual quality of the product.

We control welding quality through the following measures:

1. Standardized welding parameters
   We develop standardized Welding Procedure Specifications (WPS) based on sheet thickness, material, and joint design, controlling current, voltage, and welding speed to reduce heat input.
2. Use of anti-distortion tooling
   For welds prone to distortion, we apply preset counter-deformation, typically around 1.2 times the expected deformation, so the part returns to the correct shape after welding.
3. Post-weld stress relief
   For higher-requirement parts, we perform stress-relief heat treatment after welding, typically at 300–350°C for 1–2 hours, to reduce residual stress.
4. Weld appearance grading inspection
   Weld appearance is evaluated according to ISO 5817. For Grade B welds, undercut, porosity, and cracks are not acceptable.

Surface finishing is the final step in sheet metal fabrication and serves as the product’s protective outer layer. It is also the stage where customers are most likely to notice defects. Color inconsistency, scratches, or poor coating adhesion can directly affect both appearance and protective performance.

We control surface finishing quality in the following ways:

1. Standardized pretreatment process
   Degreasing, rust removal, phosphating, or passivation are all tightly controlled to ensure the surface reaches the required cleanliness level, typically Sa2.5.
2. Real-time coating thickness monitoring
   Coating thickness is checked during processing using coating thickness gauges to ensure powder coating thickness of 60–80 μm and liquid coating thickness of 40–60 μm.
3. Color difference testing
   We use color meters to compare finished parts against approved color samples and keep color variation within ΔE ≤ 2.0 to meet appearance requirements.
4. Cross-hatch adhesion testing
   The first article of each batch undergoes a cross-hatch adhesion test in accordance with GB/T 9286, with coating detachment limited to 5% or less, meeting ISO Class 1 requirements.

The accuracy of PEM nuts, threaded holes, and related features has a direct impact on assembly and future maintenance. Improper fastener installation, stripped threads, or hole position errors can all lead to assembly difficulty or fastening failure.

We control these risks as follows:

1. Standardized insertion force for PEM hardware
   For PEM hardware ranging from M3 to M12, we define standard insertion forces, typically 5–20 kN, to ensure secure installation without looseness.
2. 100% thread inspection with go/no-go gauges
   Every threaded hole is checked using go and no-go gauges to ensure the go gauge passes fully and the no-go gauge does not, in accordance with GB/T 3934.
3. Post-installation perpendicularity check
   The perpendicularity of installed hardware relative to the sheet surface is checked with angle measuring tools, with deviation controlled to ≤1° to prevent misaligned screw installation.
4. Mistake-proofing for directional hardware
   For PEM hardware with directional requirements, we apply poka-yoke design features and clearly indicate installation direction on the drawing.

This may be the customer’s biggest concern of all. The sample passes inspection and everything looks fine—but once mass production starts, dimensional issues, appearance defects, and other problems begin to appear. In most cases, the root cause is that the sample was made under different conditions from the actual production run.

At SR MFG, we control this risk in the following ways:

1. Prototype parts are made using production processes
   During the sample stage, we use the same production equipment and process parameters intended for mass production, rather than relying on temporary hand-built or sample-only methods.
2. Pilot run validation
   After sample approval, we carry out a pilot run of 10–50 pieces to verify process stability before full production begins.
3. Process capability analysis
   We perform Cpk analysis on critical dimensions and require Cpk ≥ 1.33 to ensure sufficient process capability for mass production.
4. SPC control chart monitoring
   Critical processes are monitored using SPC control charts so that abnormal trends can be detected and corrected early.

Poor traceability is a common weakness among many suppliers. When a problem occurs, the root cause cannot be clearly identified, and responsibility becomes difficult to define.

Our approach is different:

1. Batch traceability system
   Every product is marked with a production batch number that can be traced back to the raw material batch, operator, equipment used, and inspection records.
2. Digital inspection records
   All inspection data is entered into our MES system, stored permanently, and available for historical review at any time.
3. Closed-loop quality issue response
   We operate a rapid-response quality system that provides root cause analysis within 8 hours and a corrective action plan within 24 hours.
4. Customer complaint case database
   We maintain a database of customer complaint cases, conduct root cause analysis on recurring issues, and use those findings to prevent similar problems from happening again.

Quality traceability is at the core of effective quality control. When a quality issue occurs, it allows us to quickly identify the root cause, determine responsibility, and take corrective action without delay.

Material batch traceability → Drawing revision control → Production record retention → Inspection record retention 

A customer reports that the hole diameter on a certain batch is undersized. The traceability system is then used to investigate:

Nonconformance handling is another essential part of the quality system. The goal is to ensure that once an issue is found, it is addressed quickly, corrected effectively, and prevented from happening again.

SR MFG Nonconformance Handling Process

Issue identified → Identification and segregation → Nonconformance report issued → Root cause analysis → Corrective and preventive actions defined → Implementation → Effectiveness verification → Record retention

Response Time for Nonconformance Handling

• Issue identified: immediately upon detection
• Root cause analysis: within 24 hours
• Corrective action plan: within 48 hours
• Implementation: carried out according to plan
• Effectiveness verification: within 7 days after implementation

A batch of parts develops cracks after welding. The response process is as follows: