Bolts and screws are among the most fundamental components in modern engineering — found in everything from consumer electronics to aerospace structures. Yet behind every fastener lies a highly disciplined manufacturing and quality control process governed by international standards. This article provides an in-depth walkthrough of the full production workflow for industrial bolts and screws, based on a professional Process Control Plan (PCP) effective from 2010-06-11 (Version 0B), used in precision fastener manufacturing.
The process is divided into seven major stages, each with defined inspection methods, sampling frequencies, acceptance criteria, reference standards, documentation requirements, and non-conformance handling procedures. Three quality roles are involved throughout: IQC (Incoming Quality Control), IPQC (In-Process Quality Control), and QA/FQC (Final/Outgoing Quality Assurance).
Stage 1: Incoming Wire Material Inspection (IQC)
Everything starts with the raw material — wire coils that will eventually become bolts and screws. Before any processing begins, the incoming wire undergoes two separate inspection sub-steps managed by the IQC team.
1a. Full Incoming Inspection
Equipment & Method: Visual measurement, dimensional gauging with calipers, and spectrographic analysis.
Inspection scope:
- Dimensions & material composition — verified against the incoming inspection control program GB/T 4356-2002 using calipers and a spectrographic analyzer
- Surface appearance & markings — visually checked for absence of visible cracks, scratches, or cuts
Acceptance criterion: C = 0 (zero defects tolerated)
Reference documents: Incoming Inspection Work Standard; Supplier Material Quality Certificate; Spectrographic Analysis Report
Records: Spectrographic analysis report
Non-conformance action: Return to supplier
1b. Wire Drawing / Straightening Inspection
Equipment & Method: Visual measurement, material hardness testing (pull test).
Inspection scope (full inspection):
- Dimensions & material composition — verified against the Wire Material Work Order using calipers
- Surface appearance & markings — visually checked for absence of scratches, cuts, or deformation
Acceptance criterion: C = 0
Reference documents: Wire Material Work Order
Records: Self-inspection checklist; Wire material patrol inspection log
Non-conformance action: Rework
Stage 2: Cold Heading / Forming (IPQC)
This is where the raw wire is transformed into the basic fastener shape. Multi-station cold heading machines — typically one-die two-punch configurations — use high-speed mechanical force to press wire blanks into bolt or screw heads without any cutting or heat.
Equipment: Multi-station cold heading machine (one-die two-punch)
Sampling plan: Every hour, pull 20 pieces for inspection
Inspection type: Appearance inspection
Inspection criteria:
- No cracking, splitting, burring, or edge collapse on the head
- No missing or incomplete head geometry
- Compliance with GB/T 5779.1-2000 (surface discontinuity standard for bolts and screws)
Inspection tool: Visual (naked eye)
Acceptance criterion: Ac = 1
Reference documents: Process Inspection Work Standard; Work Instruction
Records: First-piece inspection record; Self-inspection checklist; CPK report
Non-conformance action: Sort, scrap, and analyze root cause
⚠️ This stage is designated a Critical Process in the control plan, requiring heightened monitoring and mandatory CPK (Process Capability) reporting.
Stage 3: Thread Rolling / Tapping — Critical Process (IPQC)
Thread formation is the most technically demanding and quality-critical step in fastener manufacturing. The control plan designates this as a Key Engineering Process (重要工程项目). Two sub-processes are involved: thread rolling (搓牙) and die tapping (切边/冲针).
3a. Die Cutting / Edge Trimming
Equipment: Trimming die / punch pin
Sampling plan: Every hour, pull 10 pieces for full dimensional inspection
Reference standard: Work Instruction & Production Order
Dimensions measured and tools used:
- Shank diameter (光径/对边) — Caliper
- Head thickness (头厚) — Head caliper
- Screw body length (光杆长) — Caliper
- Overall outer diameter (外径) — Micrometer
- Tail length (束尾长) — Caliper
Acceptance criterion: C = 0
Records: First-piece inspection record; Self-inspection checklist; CPK report
Non-conformance action: Sort, analyze root cause, and re-adjust tooling
3b. Thread Rolling (搓牙)
Equipment: Thread rolling machine
Sampling plan: Every hour, pull 20 pieces for appearance inspection
Inspection criteria:
- No head damage — head surface must remain intact after thread rolling
- Thread surface must be clean and free of burrs
- No incomplete thread profiles or unrolled thread sections
- Corner angles must be fully formed — no sharp unfinished edges
Inspection tool: Visual
Acceptance criterion: Ac = 1
Records: First-piece inspection record; Self-inspection checklist
Non-conformance action: Full inspection, scrap, and analyze root cause
3c. Thread Gauge / Die Plate Inspection (牙板)
Equipment: Tapping die plate
Sampling plan: Every hour, pull 10 pieces
Dimensions measured:
- Outer diameter (外径) — Micrometer
- Thread length (牙长) — Caliper
- Thread pitch accuracy (螺纹精度通止) — Thread ring gauge
Acceptance criterion: C = 0
Non-conformance action: Analyze root cause and re-adjust tooling
Stage 4: Grinding and Cleaning
After forming and threading, fasteners undergo surface grinding and cleaning to remove oxides, burrs, lubricant residue, and machining debris. This step uses dedicated cleaning lines and grinding machines, preparing the surface for semi-finished inspection, plating, or coating. While this stage does not carry a separate sampling row in the control plan, it is a mandatory process step ensuring surface cleanliness meets downstream requirements.
Stage 5: Semi-Finished Product Inspection (FQC)
Before final packaging or surface treatment, a comprehensive semi-finished goods inspection is conducted by the FQC team per international sampling standards.
Equipment: Cleaning line / grinding machine
Sampling plan: Per ISO 3269-2000 single sampling, 32 pieces per lot
5a. Appearance & Marking Inspection
- Markings, surface finish, and visual appearance verified per GB/T 5779.1-2000
- Inspection tool: Visual
- Acceptance level: AQL = 1.0
5b. Dimensional & Functional Testing
- Full dimensional verification against engineering drawings
- Functional testing (thread engagement, fit)
- Inspection tools: Calipers, micrometers, thread gauges
- Acceptance level: Ac = 1
Records: Semi-finished inspection record
Non-conformance action: Sort / return / scrap / report — responsible party analyzes root cause and implements corrective action
Stage 6: Final Inspection & Reliability Testing (QA)
Finished fasteners undergo a three-part QA final inspection before shipment, managed by the QA team covering corrosion resistance, mechanical performance, and packaging compliance.
6a. Salt Spray / Corrosion Resistance Testing
Applicable to: New steel grades and surface-treated fasteners
Equipment: Salt spray test chamber
Reference standard: ASTM B117
Acceptance criterion: Grade 10
Records: Salt spray test report
Non-conformance action: Scrap
Salt spray testing simulates corrosive environments to verify that surface treatments (zinc plating, black oxide, etc.) provide adequate protection. Grade 10 per ASTM B117 represents a high level of corrosion resistance.
6b. Mechanical Properties Testing
Sample size: 3 pieces
Equipment: Tensile testing machine; Hardness tester
Tests performed:
- Tensile / pull-out strength (抗拉强度)
- Hardness (硬度)
- Torque / torsional strength (扭力)
Reference standard: Product mechanical properties standard; GB/T 3098.6-2000
Acceptance criterion: C = 0
Records: Final inspection report
Non-conformance action: Scrap with failure analysis report
6c. Packaging Inspection
Method: Visual inspection
Sampling plan: Per ISO 2859 single normal inspection
Inspection scope:
- Appearance and labeling accuracy
- Packaging method compliance
- Dimensional verification of packaging
Acceptance level: AQL = 1.0
Records: Final inspection report
Non-conformance action: Scrap or rework
Quality Standards Reference Table
| Standard | Scope & Application |
|---|---|
| GB/T 4356-2002 | Incoming wire inspection control program |
| GB/T 5779.1-2000 | Surface discontinuities — bolts, screws, and studs |
| ISO 3269-2000 | Fastener acceptance inspection — sampling procedures |
| ISO 2859 | Sampling procedures for inspection by attributes |
| ASTM B117 | Salt spray (fog) corrosion resistance testing |
| GB/T 3098.6-2000 | Mechanical properties of fasteners (set screws) |
Acceptance Criteria Summary
| Stage | Criterion | Meaning |
|---|---|---|
| Incoming wire (IQC) | C = 0 | Zero defects — any failure triggers return to supplier |
| Cold heading (IPQC) | Ac = 1 | Max 1 defect per 20-piece sample before lot rejection |
| Thread rolling (IPQC) | Ac = 1 | Max 1 defect per 20-piece sample before lot rejection |
| Die cutting / tapping (IPQC) | C = 0 | Zero dimensional non-conformances tolerated |
| Semi-finished (FQC) | AQL = 1.0 / Ac = 1 | Statistical sampling per ISO 3269, 32-piece sample |
| Salt spray (QA) | Grade 10 per ASTM B117 | High corrosion resistance required |
| Mechanical properties (QA) | C = 0 | Zero failures on tensile, hardness, torque |
| Packaging (QA) | AQL = 1.0 | Statistical sampling per ISO 2859 |
Documentation & Traceability
A key feature of this control plan is its emphasis on full traceability. Every stage generates mandatory records:
- Incoming: Supplier material certificates, spectrographic analysis reports
- In-process: First-piece inspection records, self-inspection checklists, CPK process capability reports, patrol inspection logs
- Final: Semi-finished inspection records, salt spray test reports, final inspection reports
These records enable root-cause analysis when non-conformances occur and support customer audit requirements for certified fastener supply chains.
Conclusion
Manufacturing a reliable bolt or screw is far more complex than it appears. From spectrographic wire analysis at incoming inspection to salt spray corrosion testing and mechanical pull-out strength verification at final QA, every fastener passes through six major process stages, multiple sub-inspections, and dozens of individual quality checkpoints — all governed by Chinese national standards (GB/T), ISO international standards, and ASTM test methods.
This level of rigor — with defined sampling frequencies, calibrated measurement tools, mandatory CPK reporting at critical processes, and documented non-conformance handling — is what separates precision industrial fasteners from commodity hardware. It is why quality-conscious engineers, procurement teams, and OEM manufacturers specify certified fasteners for critical structural, mechanical, and safety applications.
At Fullerkreg Fasteners, our production and quality control processes follow this same disciplined framework, ensuring every DIN/ISO standard fastener we supply meets the dimensional, mechanical, and surface integrity requirements your application demands.