In recent years, the market demand for commercial vehicle bevel gear sets has grown significantly, accompanied by a steady increase in product variety. The range of bevel gears required for heavy, medium, and light-duty vehicles expanded to over 22 distinct types. To accelerate the implementation of intelligent manufacturing, alleviate the labor intensity associated with manual loading and unloading (especially for larger bevel gears weighing over 20kg), and enhance the machining quality of the drive pinion, our company embarked on an ambitious upgrade project. The goal was to transform two existing semi-automated flow lines for drive pinion machining into a highly automated Flexible Manufacturing Line (FML) through a “machine-for-human” replacement strategy, utilizing our in-house stock of CNC horizontal lathes and gear hobbing machines.
The original production flow, which these lines followed, is conceptually outlined below. The first line consisted of two WIA L280 CNC horizontal lathes with FANUC 0iTD systems, two YKX3132M CNC gear hobbing machines with SINUMERIK 802D solution line systems, two Oerlikon C50 bevel gear cutting machines with SINUMERIK 840D power line systems, and two SIMATIC S7-300 PLC-controlled loop conveyors. The second line had a similar configuration but with one less C50 machine and one less loop conveyor.
FML System Design and Architecture
Drawing from successful FML implementations for other components like steering knuckles and differential carriers, and after evaluating production efficiency and stability requirements, we integrated one KAWASAKI BX200L six-axis articulated robot with an 8-meter travel range into each production line. This robot became the central material handling unit. The comprehensive scope of the FML development required the integration and modification of numerous subsystems, as detailed in the table below.
| Development Area | Key Tasks and Objectives |
|---|---|
| Four WIA L280 CNC Lathes | Backup of original PMC data & part programs; pneumatic circuit design & I/O signal assignment; development of new Robot-capable PMC logic; modification of part programs; creation of auto-door operation manuals. |
| Four YKX3132M CNC Gear Hobbing Machines | Backup of original PLC data & part programs; pneumatic circuit design & I/O signal assignment; development of new Robot-capable 802Dsl-PLC logic; modification of part programs. |
| Three Loop Conveyor Systems | Backup of original S7-300 PLC programs; I/O signal assignment & circuit design; development of new Robot-capable S7-300 PLC logic. |
| Robot End-Effector Tooling | Selection of dual-gripper tooling and determination of optimal robot unloading poses for each machine station. |
| Auxiliary Handling Equipment | Design and fabrication of the raw material feeding conveyor, part reorientation station, and dual-station sampling/inspection units. |
| Robot 7th Axis (Linear Track) | Design, installation, and commissioning of the servo-driven linear track for robot mobility. |
| Central Control System | Design and layout of the master control platform; programming of the central S7-200 PLC; development of HMI software on a SMART 700 touch panel with fault alarm messaging. |
| Process-Specific Fixturing | Design of chip management solutions for turning and multi-variant adaptable pallets for raw material feeding. |
The upgraded FML layout enables a fully automated workflow: the robot picks raw bevel gear blanks from the feeding conveyor, loads/unloads the four lathes and four hobbing machines, places finished hobbed bevel gears onto the loop conveyor for transfer to the C50’s integrated loader, performs out-of-line sampling for inspection, and manages all interlocks and safety signals. The central HMI provides real-time monitoring, production counting, tool life management, and instant SMS alerts for line stoppages.
Key Implementation Details of the Flexible Manufacturing Line
1. Robotic Integration for CNC Horizontal Lathes
The CNC lathes perform the rough and finish turning of various shaft diameters, back cones, and arcs. To achieve robotic (RT) integration, we extended the machine’s original hardware and software framework. The primary modifications included adding an automatic door cylinder, designing corresponding pneumatic and electrical circuits, developing new PMC ladder logic for robot interaction and automatic door control, and modifying the part programs for autonomous operation.
The first critical step was data security. Using the FANUC Ladder-III software via a peer-to-peer Ethernet connection, we uploaded the active PMC program for backup and analysis. Part programs and CNC parameters were saved to a CF card. The new pneumatic system for the automatic door, based on an SMC double-acting cylinder and pilot-operated solenoid valve, was then integrated. I/O signals for robot communication were mapped to unused addresses in the existing PMC I/O table.
The core of the integration lay in developing new PMC logic. Three key functional blocks were created:
1. RT-Machine Handshake Logic: This block manages the communication protocol, allowing the robot to query the lathe’s status (ready, alarm state, cycle complete, door open) and enabling the lathe to start its cycle automatically upon a robot command.
2. Automatic Door Control Logic: This enables the machine door to be opened (M61) and closed (M62) automatically from within the part program when in FML mode, with position confirmation via limit switches.
3. RT-Controlled Tailstock Logic: This allows the robot to command the tailstock center to advance (clamp) and retract (unclamp), with feedback signals confirming the action’s completion to the robot, ensuring secure part handling for these bevel gears.
2. Robotic Integration for CNC Gear Hobbing Machines
The gear hobbing machines perform the spline cutting operation on the bevel gear blanks. Their integration followed a similar but distinct path due to the different CNC system (SINUMERIK 802Dsl). We performed a backup of the machine’s built-in PLC program using the PLC802 tool and saved part programs via CF card. The pneumatic circuit for the automatic door was analogous to the lathe’s setup, with an additional circuit for an automatic chip blowing valve.
The new PLC logic for the hobbing machines, written in the 802Dsl’s integrated PLC environment, included four major functions:
1. RT-Machine Handshake Logic: Similar to the lathe, this facilitates communication for coordinated FML operation.
2. Automatic Door Control Logic: Controlled via M46 (open) and M47 (close) codes.
3. Automatic Chip Blowing Logic: A significant improvement, this uses M54 (start blow) and M55 (stop blow) to automatically clean the work area after machining, replacing manual cleaning and improving consistency for the bevel gears.
4. RT-Controlled Tailstock with 2-Step Clamping: This advanced function allows the robot to initiate a two-step clamping sequence for the workpiece tailstock, enhancing grip security for the machining forces involved in hobbing.
3. Robotic Integration for Loop Conveyor Systems
The Oerlikon C50 bevel gear cutters feature integrated loading robots. Therefore, the external KAWASAKI robot interfaces with the C50’s system via the loop conveyor. The robot places hobbed bevel gears onto the conveyor pallet, which then transports them to the pickup point for the C50’s internal loader. The existing S7-300 PLC program of the conveyor was uploaded via STEP 7 software for analysis. New I/O signals were assigned to handle the new “FML Mode,” including signals for “pallet at robot unload position” and “robot unload completed.” New PLC logic was written to manage these signals and integrate the conveyor’s operation seamlessly into the robotic cell’s cycle.
4. Robotic Tooling and Process Poses
To maximize handling efficiency for the family of bevel gears, a SCHUNK pneumatic gripper with a dual-gripper configuration and a 30kg payload capacity was selected. Custom V-shaped polypropylene pads were fitted to the jaws to prevent marring the gear surfaces. Integrated air blow nozzles clean the gripping area before each handling operation. Through manual teaching on the robot pendant, precise unloading poses for the lathes, hobbing machines, and conveyor were defined and programmed, ensuring smooth, collision-free motion paths within the robot’s working envelope.
5. Design of Auxiliary Systems
Several custom auxiliary stations were designed and built:
– Raw Material Feeding Conveyor: A 32-position palletized conveyor with a stepper motor drive allows for 3-4 hours of unmanned operation. Different pallet inserts accommodate various bevel gear blank sizes. A secondary positioning station at the robot pickup location ensures precise placement into the lathe chuck.
– Part Reorientation Station: Due to robot wrist axis travel limits, a pneumatic station was added to reorient the workpiece 180 degrees after hobbing before the robot places it on the loop conveyor.
– Dual-Station Sampling Unit: Two independent, pneumatically actuated units allow for scheduled or random sampling of work-in-process bevel gears from the turning and hobbing operations for quality verification without stopping the line.
6. Robot 7th Axis and Central Control
A servo-motor driven, dual-rack-and-pinion linear track (7th axis) was installed, providing the robot with the 8-meter reach required to service all machines. All communication between the robot and the track is digital for high reliability. The heart of the FML is the central control platform based on a Siemens S7-200 PLC. It acts as the system coordinator, handling signal routing between the robot, all machine tools, and auxiliary equipment. Its program structure is modular, consisting of a main organizational block (OB1) and several subroutines (SBRx) for specific functions like robot handshaking, sampling station control, and tool life monitoring. The operator interface, developed on a SMART 700 HMI, provides comprehensive visualization of the entire line’s status, production data, and immediate fault alerts.
Development Challenges and Technical Hurdles
The development of this bevel gear FML presented several significant challenges, the resolution of which was key to the project’s success. A summary of the primary challenges and the required technical responses is shown below.
| Challenge Category | Specific Difficulty | Resolution & Action Taken |
|---|---|---|
| Technical Obfuscation | Password-protected or uncommented PLC programs on third-party equipment (C50, loop conveyor). | Required reverse-engineering of PLC logic and, where necessary, password recovery procedures to understand and modify control sequences. |
| Diverse and proprietary programming environments. | Required mastery of multiple software suites: Ladder-III (FANUC), PLC802 (802Dsl), STEP 7 (840D/S7-300), and Micro/WIN (S7-200). | |
| System Heterogeneity | Integration of 11 machines with three different control architectures (FANUC, SINUMERIK 802Dsl/840D, SIMATIC S7). | Necessitated developing unique communication interfaces and logic for each control type, as detailed in the implementation sections. |
| Comprehensive Skill Set | The project spanned mechanical design (pneumatics, fixtures), electrical circuit design, and advanced software programming for multiple platforms. | Required a cross-functional team or individuals with multi-disciplinary expertise in CNC systems, PLC programming, robotics, and mechanical integration. |
The complexity of integrating diverse systems can be conceptualized by considering the number of unique communication interfaces (N) required, which is a function of the different control system types (C) and the various machine roles (M). For our line, the challenge was non-trivial: $$ N = \sum_{i=1}^{C} M_i \times K_i $$ where $K_i$ represents the unique protocol or method for system $i$. Successfully managing this complexity was fundamental.
Operational Results and Benefits Achieved
The commissioning and operation of the Flexible Manufacturing Line for bevel gears delivered transformative results across multiple metrics, quantitatively and qualitatively enhancing production capabilities.
Technical Capability Upgrades:
• The four CNC lathes gained fully automatic door control (M61/M62) and robot-commanded tailstock motion, transitioning from standalone to FML-interlocked operation.
• The four gear hobbing machines were upgraded with automatic doors (M46/M47), automatic chip blowing (M54/M55), a two-step tailstock clamping cycle, and FML interlock control.
• The loop conveyors were enhanced with a new “FML Mode” and precise robot interface signals for synchronized part transfer.
Production and Economic Impact:
The transition from a semi-automated flow line to a robotic FML yielded substantial gains. The most significant improvements can be summarized by the following performance equations:
1. Productivity Increase: The unmanned automation and reduced cycle time increased daily output. If $Q_{old}$ is the original daily output and $\eta$ is the efficiency gain factor, the new output $Q_{new}$ is:
$$ Q_{new} = Q_{old} \times (1 + \eta) $$
For our line, $\eta \approx 0.33$, raising output from 120 to 160 pieces per line per day.
2. Labor Cost Savings: The reduction in direct labor per shift ($\Delta L$) multiplied by the number of shifts ($s$) and annual cost per operator ($C_{op}$) yields annual savings ($S$):
$$ S = \Delta L \times s \times C_{op} $$
With $\Delta L = 2$ operators per line, $s = 2$ shifts, and $C_{op} = \$70,000$, the annual saving for two lines is:
$$ S = 2 \times 2 \times 70,000 = \$280,000 $$
3. Quality and Ergonomics: The elimination of manual handling of heavy bevel gear blanks and work-in-process pieces removed a major source of physical strain and potential safety incidents. Automated, consistent loading also improved process repeatability, contributing to more stable machining quality for the bevel gears.
| Performance Metric | Before FML (Semi-Auto Line) | After FML Implementation | Improvement |
|---|---|---|---|
| Daily Output per Line (pieces) | 120 | 150-160 | +25% to +33% |
| Operators per Line per Shift | 3 | 1 | Reduction of 2 |
| Manual Handling | Full manual loading/unloading | Fully automated by robot | Eliminated |
| Process Integration | Isolated machine operation | Fully synchronized FML with central control | Enabled unmanned production |
In conclusion, the successful development of this Flexible Manufacturing Line for commercial vehicle bevel gears demonstrates a viable and highly effective model for modernizing legacy production systems. By integrating a central robotic handler with strategically upgraded machine tools and auxiliary systems, we achieved a significant leap in automation, productivity, and working conditions. The methodologies applied—spanning pneumatic design, multi-platform PLC/PMC programming, robotic integration, and custom fixture design—provide a comprehensive technical blueprint. This approach is not only directly applicable within the automotive gear manufacturing sector but also offers valuable insights for similar transformation projects in aerospace, rail, and marine industries seeking to upgrade their machining lines for complex components like bevel gears.