Brass Cylinder Housing Machining for Lock Assembly Cases
Cylinder Housing Production Challenge
A commercial lockset manufacturer required 180,000 lock cylinder housings per year for their new Grade 2 key-in-knob lockset, a high-volume product destined for the hospitality and education markets. The cylinder housing was machined from C36000 free-cutting brass hexagonal bar stock (AFNOR 24mm AF hex), and housed all locking mechanism components including the pin tumbler cartridge, the cam driver, the retainer clip, and the keyway. The housing design included a 24 mm hex body, a 12.5 mm diameter pilot bore for the pin cartridge installation, a 4.0 mm wide keyway slot, two M4 threaded mounting holes, a cam pocket, and a snap-ring groove for retainer installation.
The specific dimensional requirements were classified as IT7 precision for the pilot bore (12.50 mm ±0.015 mm), IT8 for the keyway width and depth, and positional tolerances of ±0.03 mm for the two M4 threaded holes relative to the keyway datum. The pilot bore surface finish was specified at Ra 0.8 μm to ensure smooth insertion and rotation of the pin cartridge without wear or binding over 100,000 operating cycles. The annual volume of 180,000 units required a robust, automated manufacturing cell with minimal operator intervention.
Manufacturing Cell Design and Process Flow
We designed a dedicated manufacturing cell around two Mazak Quick Turn Nexus 200-II CNC turning centers and one Brother Speedio S700X1 vertical machining center. The turning centers were equipped with bar feeders and subspindles for first and second-operation turning, and the machining center performed all milling, drilling, and tapping operations on the hex body. The process flow was arranged in a lean U-shaped cell with automated part transfer between stations.
| Workstation | Machine | Operations Performed | Cycle Time |
|---|---|---|---|
| Station 1 - Bar turning | Mazak Nexus 200-II (1) | Face, center drill, rough bore φ11.5 mm, turn pilot OD, form hex relief | 65 sec |
| Station 1b - Subspindle | Mazak Nexus 200-II (1) | Part off, back-face counterbore, snap ring groove turning | 42 sec (overlaps Station 1) |
| Station 2 - Pin side turning | Mazak Nexus 200-II (2) | Second end face, bore finish ream φ12.50 mm, chamfer internal edges | 55 sec |
| Station 2b - Subspindle | Mazak Nexus 200-II (2) | Thread bore φ3.3 mm for M4 tap (pre-drill), back chamfer | 35 sec (overlaps Station 2) |
| Station 3 - Hex body milling | Brother S700X1 | Keyway slot rough and finish (4.0 mm), mill cam pocket, drill/tap M4 holes (2x), mill snap-ring ledge | 150 sec |
| Station 4 - Deburr & wash | Manual station with robotic assist | Deburr all external edges (0.1-0.3 mm chamfer), hot water wash, air blow dry | 20 sec |
| Station 5 - Final inspection | Vision system + air gage | Bore 100% air gage, keyway 100% vision, CMM sample 1/50 | 12 sec (inline) |
The total cell throughput was one finished cylinder housing every 68 seconds, yielding a production rate of approximately 52 parts per hour per cell. With two shifts and 85% OEE, a single cell produced approximately 14,000 parts per month, and three identical cells met the annual requirement of 180,000 units with a 12% capacity buffer.
Precision Bore and Keyway Machining
The 12.50 mm pilot bore was the most critical feature. The bore serves as the mounting receptacle for the pin cartridge and must be concentric with the external hex within 0.03 mm TIR to ensure proper alignment of the pin tumblers with the keyway. The bore was produced in three stages: rough drilling to 11.5 mm on the first turning operation, semi-finish boring to 12.40 mm on the second turning operation, and finish reaming to 12.500 mm using a PCD-tipped reamer with a floating holder.
| Feature | Specification | Process Capability | Tooling | Inspection Method |
|---|---|---|---|---|
| Pilot bore (12.5 mm) | 12.500 ±0.015 mm | Cpk 1.55, range 12.493-12.508 mm | PCD reamer, floating holder | Air gage, 0.5 μm resolution |
| Bore roundness | 0.008 mm LMC | 0.003-0.006 mm | Same reamer | Roundness gage, sample 1/100 |
| Bore surface finish | Ra 0.8 μm max | Ra 0.42-0.65 μm | PCD reamer, 0.05 mm/rev | Contact profilometer |
| Keyway width (4.0 mm) | 4.00 ±0.025 mm | Cpk 1.48, range 3.98-4.02 mm | Ø4 mm carbide end mill + slot broach | Vision, 1 μm pixel resolution |
| Keyway depth | 2.00 ±0.03 mm | Cpk 1.52, range 1.98-2.02 mm | Same end mill + depth stop | Vision + laser depth sensor |
| M4 thread position (2x) | ±0.03 mm relative to keyway | Cpk 1.62, range ±0.018 mm | Carbide drill + roll-form tap | CMM, 3-2-1 alignment |
| M4 thread quality | 6H tolerance | Pass 100% GO/NO-GO | Roll-form tap (OSG) | Thread plug gage |
| Cam pocket depth | 3.00 ±0.05 mm | Cpk 1.35, range 2.97-3.03 mm | Ø6 mm carbide end mill | Vision + CMM |
The keyway slot required a two-step broaching process because the CNC end milling operation could not achieve the 0.025 mm width tolerance consistently at production speeds. The end mill produced the slot to 3.90 ±0.05 mm in roughing, and a carbide slot broach then pulled through to the final 4.00 ±0.025 mm width at a rate of 3.5 m/min. The broach produced a superior surface finish of Ra 0.8 μm on the keyway walls, critical for smooth key insertion and preventing picking vulnerabilities.
Production Ramp-Up and Continuous Improvement
During the initial production ramp-up from 10,000 to 180,000 units per year, three key issues required attention. The first was the M4 roll-form tap life, which initially achieved only 4,000 threads before chipping. Changing from a standard roll-form tap to an OSG A-TAP with optimized flute geometry for brass doubled the tool life to 9,000 threads.
The second issue was coolant management. Brass machining produces fine chips that can clog coolant filtration systems. We installed a magnetic separator and paper band filter that removed 95% of the brass fines from the coolant, extending coolant life from 3 months to 12 months and preventing surface finish degradation from recirculated chip particles.
The third improvement was in the material handling between stations. Initially, parts were manually transferred from the turning center to the machining center, creating a bottleneck. We implemented a gantry robot system that automatically picked parts from the turning center output conveyor and loaded them onto the machining center fixture, reducing the inter-station wait time from 12 seconds to 4 seconds and increasing cell throughput by 11%.
The overall cost per cylinder housing was $1.45, with material (C36000 brass hex bar at $4.50/kg) accounting for 55% of the cost. The tooling cost per part was $0.12, and quality inspection added $0.05 per part. The process demonstrated that with proper cell design, automation, and tool selection, high-volume brass cylinder housing machining can achieve IT7 precision at a cost that supports the competitive commercial lockset market.
