High-Volume Lock Bolt Progressive Die Stamping Case Study
Production Challenge for High-Volume Lock Bolts
A major North American lock hardware brand requested a quotation for 5 million lock bolts annually, to be produced from 2.5 mm thick SAE 1010 cold-rolled steel strip. The lock bolt design featured a 16 mm wide body with a 10 mm deadbolt extension, two side notches for latch retention spring engagement, and a keyed hole pattern on the rear face for attachment to the actuator mechanism. The flatness requirement was 0.10 mm overall, with the critical thickness dimension at the bolt nose held to ±0.05 mm. The customer was currently using a multi-operation approach with individual stamping stations and spot welding for a secondary bracket attachment, running at a combined cycle time of approximately 15 seconds per part.
Our engineering team proposed converting the entire lock bolt assembly to a single progressive die stamping process, including the secondary bracket as a formed feature within the strip rather than a separate welded component. The elimination of the spot welding operation alone would reduce per-part cost by approximately 35%, while the progressive die approach would increase production speed from 4 parts per minute to 120 parts per minute.
Progressive Die Design and Process Flow
The progressive die was designed with 14 stations across a die length of 1,200 mm, mounted in a 250-ton high-speed press with variable speed control. The material feed pitch was set at 35 mm per stroke, using a servo-driven roll feed system with ±0.02 mm feed accuracy. Each station performed a progressive operation, building up the complete lock bolt geometry by the final station.
| Station | Operation | Tool Type | Tolerance Achieved |
|---|---|---|---|
| 1 | Pierce pilot holes (2x) | Carbide punch/bushing | ±0.02 mm (hole position) |
| 2 | Idle station | N/A | Material alignment check |
| 3 | >Notch side profiles (2x) | Carbide notching punch | ±0.04 mm (notch width) |
| 4 | Partial form key-hole pocket | D2 tool steel form punch | Depth ±0.05 mm |
| 5 | Bracket form-up (90°) | D2 tool steel, spring-loaded | Angle ±0.5° |
| 6 | Pierce actuator holes (4x) | Carbide punch/bushing | ±0.03 mm (hole position) |
| 7 | Idle station | N/A | Sensor check for slug removal |
| 8 | Trim excess material from bracket form | Carbide trim punch | Edge finish Ra 1.6 |
| 9 | Pierce latch retention spring slots (2x) | Carbide punch with stripper | Slot width ±0.03 mm |
| 10 | Final form – bolt nose chamfer | Form punch with carbide insert | Chamfer angle ±0.5° |
| 11 | Pierce anti-drill pin hole | Carbide micro punch (Ø1.5 mm) | Hole Ø ±0.02 mm |
| 12 | Idle station | N/A | Vision inspection station |
| 13 | Cutoff – separate part from strip | Carbide cutoff punch | Shear edge, burr < 0.03 mm |
| 14 | Part ejection with air blow | N/A | Part orientation check |
The tool design emphasized carbide inserts for all piercing and cutting stations to achieve the target tool life of 2 million hits between major regrinds. D2 tool steel was specified for forming stations where impact toughness was more critical than wear resistance. Each pilot hole station included slug detection sensors to prevent double-hit conditions, and optical sensors at stations 7 and 12 monitored material progression and part formation quality in real time.
Production Results and Quality Metrics
After die tryout and optimization, the production run achieved consistent operation at 120 strokes per minute, producing 7,200 lock bolts per hour. The die ran for three shifts with planned maintenance intervals of 200,000 strokes for lubrication system checks and station sensor calibration.
| Metric | Target | Achieved | Improvement vs. Previous Process |
|---|---|---|---|
| Cycle time per part | 0.5 sec | 0.5 sec (120 SPM) | 30x faster (was 15 sec) |
| Bolt body flatness | 0.10 mm max | 0.06-0.09 mm | 2x better |
| Nose thickness tolerance | ±0.05 mm | ±0.03 mm (Cp 1.55) | 40% tighter variation |
| Bracket form angle | 90° ±1° | 90° ±0.3° | Eliminated bracket welding |
| Reject rate | 0.5% | 0.12% | 76% fewer rejects |
| Tool regrind interval | 1 million hits | 2.2 million hits | 2.2x longer tool life |
| Annual output (2 dies) | 5 million parts | 5.8 million parts | 16% capacity buffer |
The elimination of the secondary bracket as a separate spot-welded component proved to be the most significant improvement. The brackets are now formed integrally from the strip material at station 5, saving $0.12 per part in material and $0.08 per part in welding labor. The annual savings totaled $1.0 million at 5 million parts.
Technical Challenges and Solutions
Several technical challenges emerged during the ramp-up phase. The most persistent was micro-cracking at the bracket form bend radius, caused by the cold-rolled steel reaching its forming limit at the 1.5x material thickness bend radius. We addressed this by adding a stress-relief annealing pass through the coil before stamping, heating the strip to 650°C for 30 seconds followed by controlled cooling. This increased material elongation from 28% to 35% and eliminated the cracking issue entirely.
Burr height at the cutoff station was another concern, trending upward after approximately 800,000 hits as the carbide cutoff punch began to wear. We installed a burr monitoring system using a laser profilometer that measured burr height on one part per 5,000. When burr height exceeded 0.03 mm threshold, the system flagged the operator for a punch rotation or replacement within the next 50,000 strokes, preventing out-of-spec parts from reaching the customer.
Conclusion
This case demonstrates that converting a multi-operation lock bolt assembly into a single progressive die stamping process delivers order-of-magnitude improvements in throughput, cost, and quality. The 14-station die running at 120 SPM achieved the customer's annual requirement of 5 million units with capacity to spare, while the per-part cost reduction of approximately 50% created long-term competitive advantage for the customer in the lock hardware market.
