Abstract
In response to the challenges faced by steel casting manufacturers in the cleaning of large steel casting, such as bolsters and side frames for railway freight cars, this paper proposes an intelligent and versatile solution. Based on the integration of 3D vision, robotics, safety and environmental protection, system integration, and control technologies, the solution includes various units such as a preparation station, intelligent cutting unit, manual cutting unit, intelligent grinding unit, manual welding and grinding unit, intelligent logistics unit, safety and environmental protection unit, and a sprue and riser collection system. This solution aims to form an intelligent cleaning and grinding production line for bolster and side frame steel casting. Practical production has proven that this new scheme can significantly improve operating quality, reduce manual work intensity, and be environmentally friendly and economically feasible.
1. Introduction
Large steel casting for railway freight cars, such as bolsters and side frames, are critical components that require meticulous processing. These castings are typically produced using sand casting methods, and the as-cast blanks must undergo a series of cleaning processes including sand removal, flash and riser cutting, shot blasting, carbon arc gouging, surface grinding, defect welding, and inspection before heat treatment. The traditional production organization involves synchronized and centralized operations of cutting, welding, grinding, and inspection, often interspersed with material handling using overhead cranes. This extensive, open, multi-trade, multi-personnel, and simultaneous production organization results in multiple pollution sources, various pollution types (arc light, noise, dust), high pollution concentrations, and intense pollution levels, posing significant challenges for environmental management and occupational health protection [1-2].
In terms of operational safety, the frequent overhead crane lifting and flipping of large-sized and heavy workpieces are common, and incidents such as cutting sparks and grinding wheel shattering at multiple points over long periods can lead to injuries. Moreover, the harsh working environment, high work intensity, and experience requirements contribute to difficulties in staffing.
CRRC Meishan Co., Ltd. has proposed an intelligent solution for the cleaning of large steel casting for railway freight cars, implemented through an intelligent cutting and grinding production line for bolster and side frame steel casting. This solution integrates cutting-edge technologies such as logistics, environmental protection, safety monitoring, digital 3D inspection, robotics, digital programming, and intelligent control. It is characterized by high system integration, intelligent management, and environmental friendliness, exploring a new path for the production of large steel casting in the railway industry.
2. Overview of the Cleaning Process
Before heat treatment, bolster and side frame steel casting must undergo individual processing according to the process flow of “sand removal, flash cutting, riser cutting, shot blasting, carbon arc gouging, surface grinding, internal and external surface inspection, defect welding, and inspection confirmation.” After completion, they are sent to the heat treatment process in batches by furnace. Table 1 outlines the specific contents, working hours, and pollution types of each process step.
Table 1: Cleaning Process Content, Working Hours, and Pollution Types
| Seq. | Process Name | Process Content | Working Hours | Pollution Types |
|---|---|---|---|---|
| 1 | Mechanical Sand Removal | Removal of external mold sand from castings | 15 min/box | Dust, Noise |
| 2 | Manual Cleaning | Removal of internal core sand from castings | 90 min/piece | Dust, Noise |
| 3 | Riser Cutting | Removal of fixed risers, vents, etc. | 15 min/piece | Smoke, Noise |
| 4 | Flash Cutting | Removal of flash of varying positions and sizes | 12 min/piece | Smoke, Noise |
| 5 | Shot Blasting | Removal of residual sand and oxide scale from casting surfaces | 3-5 min/piece | Dust |
| 6 | Carbon Arc Gouging | Removal of high cutting allowances, abnormal protrusions, and preparation of welding grooves | 12 min/piece | Smoke, Dust, Arc Light, Noise |
| 7 | Surface Grinding | Removal of flash, burrs, smoothing of critical surfaces, and rounding of irregularities | 60 min/piece | Grinding Dust |
| 8 | Defect Welding | Elimination of shrinkage holes, porosity, blowholes, cracks, etc. | 15 min/piece | Smoke, Arc Light, Noise |
| 9 | Inspection Confirmation | Quality inspection of castings, marking of defects, specifying operations, and recording | 15 min/piece | – |
| 10 | Workpiece Flipping | Flipping of workpieces | 1 min/piece | – |
2.1 Sand Removal
Sand removal generally involves the use of sand removal machines to remove external mold sand from castings, and manual air shovels to remove internal core sand. This process involves whole box handling, box separation, mechanical assisted sand removal, workpiece transfer, manual cleaning, and old sand recovery. For enterprises using an ester-hardened water glass sand process with a two-piece-per-box production method, the working hours for sand removal machines and manual cleaning are approximately 15 minutes per box and 1.5 hours per piece, respectively. Due to the difficulty in matching their work rhythms, manual cleaning is typically performed in a multi-station, large-area manner. With current technological means, it is still challenging to completely replace manual cleaning with mechanical methods. To enhance environmental management in manual cleaning areas, many enterprises are equipped with atomizing dust suppression devices.
2.2 Riser and Flash Cutting
To ensure casting quality, the casting process includes gates, risers, and vents at fixed positions on the workpiece, with relatively large cross-sectional areas where risers connect to the casting body. For example, the ZK6 type bolster has two gates (located at the bolster ends, in upper and lower layers, with individual cross-sections of approximately 180 mm × 20 mm), two center plate risers (each with a cross-section of approximately 180 mm × 150 mm), four wedge risers (each with a cross-section of approximately 60 mm × 60 mm), and 12 side bearing vents (each with a cross-section of approximately 40 mm × 15 mm). The ZK6 type side frame has two gates (located in the central box, each with a cross-section of approximately 160 mm × 16 mm), three central box risers (each with a cross-section of approximately 100 mm × 20 mm), two guide frame risers (each with a cross-section of approximately 160 mm × 150 mm), four slide groove vents (each with a cross-section of approximately 100 mm × 15 mm), and six large triangle vents (each with a cross-section of approximately 30 mm × 30 mm). These risers and vents are typically removed after sand removal using flame cutting methods. The manual removal of risers and flashes from bolsters and side frames using hand torches takes approximately 15 minutes per piece.
Casting flash is a type of excess metal defect caused by the fit clearance between cores, with uncertain positions and sizes. This type of defect is also removed using flame cutting methods. The manual removal of flashes from bolsters and side frames using hand torches takes approximately 10 minutes per piece.
Due to the large cross-sectional areas of risers and vents and the long residence time of high-temperature molten steel during pouring, residual sand often exists at the risers and vents. The poor thermal conductivity of surface residual sand increases the difficulty of cutting preheating, which can sometimes prevent cutting from being achieved. This issue is typically addressed by manually removing residual sand, extending preheating time, and adjusting preheating positions. The cutting allowance when removing risers and flashes using manual flame methods is generally 3-5 mm. If manual precision cutting is used, the cutting allowance can be further reduced, with skilled operators controlling it within 1-2 mm.
2.3 Shot Blasting
Shot blasting is conducted to further remove residual sand and oxide scale from casting surfaces, creating favorable conditions for subsequent surface inspection operations. A step-by-step suspension chain eight-blast wheel shot blasting machine is used, with typical blasting times set at 3-5 minutes per piece.
2.4 Carbon Arc Gouging
Carbon arc gouging is a commonly used and efficient cleaning method, primarily for removing large flashes or protrusions on casting surfaces (such as high cutting allowances and large areas of abnormal protrusions), creating conditions for surface finishing. For deeper casting defects, carbon arc gouging is also often used for elimination. However, carbon arc gouging produces loud noise, heavy smoke, and intense arc light, increasing the difficulty of environmental protection and occupational health protection. Currently, the time for gouging bolsters and side frames is approximately 12 minutes per piece.
2.5 Surface Grinding
Surface grinding of castings involves removing small flashes and burrs, shaping holes or edges for smooth transitions to reduce stress concentrations during workpiece use, and smoothing key surfaces after welding repairs to create necessary surface conditions for subsequent visual or non-destructive testing. Currently, handheld pneumatic grinding wheels are widely used, which produce loud noise and dust. The typical time for grinding bolsters and side frames is approximately 60 minutes per piece.
2.6 Defect Welding
Defect welding is one of the primary tasks in casting cleaning, aiming to eliminate discontinuous defects on the casting body, such as shrinkage holes, blowholes, and cracks. This is typically done using a direct current welding machine with handheld electrodes. The process produces welding smoke and intense arc light. Currently, the time for welding and repairing defects on bolsters and side frames is approximately 15 minutes per piece.
2.7 Inspection Confirmation
During the grinding and welding repair processes, inspectors identify defects found through visual inspection to determine whether they should be eliminated by grinding or welding and supervise and record the welding repair process.
Due to the long length and heavy weight of the workpieces, overhead cranes are commonly used for flipping to facilitate the implementation of relevant operations on each surface.
3. Overall Solution
The technical solution for the intelligent cutting and grinding production line for bolster and side frame steel casting is illustrated in Figure 1. The solution includes units such as a preparation station, intelligent cutting unit, manual cutting unit, intelligent grinding unit, manual welding and grinding unit, intelligent logistics system, safety and environmental protection unit, and sprue and riser collection system.
Figure 1: Technical Solution for the Intelligent Cutting and Grinding Production Line for Bolster and Side Frame Steel Casting
3.1 Workstation Design
Based on the current process flow, job content, working hours, pollution types, and other factors, combined with the actual factory layout, logistics and transportation, and process layout of the company, the workstation design for the production line was conducted. Due to the large area requirements, tight logistics connection with molding and sand processing, and long working hours of sand removal (including mechanical sand removal and manual cleaning), it was not included in the production line. Similarly, shot blasting and carbon arc gouging were not included in the workstation design due to existing related equipment. Robotic cutting of risers ensures flat cutting surfaces, and subsequent manual precision cutting processes effectively eliminate issues such as high cutting allowances and local abnormal protrusions [7].
During operations, surface grinding, defect welding, and inspection confirmation must be performed simultaneously, so they are integrated into a single workstation. As the production line structure necessitates the use of fixtures for workpiece clamping, flash cutting and clamping area finishing are integrated into the preparation process. Before entering the preparation process, the workpieces should undergo shot blasting to create conditions for subsequent operations.
Based on the above analysis and considerations, the intelligent cutting and grinding production line for bolster and side frame steel casting is designed with five workstations: preparation station, intelligent cutting station, manual precision cutting station, intelligent grinding station, and manual welding and grinding station. The specific situations are shown in Table 2.
Table 2: Workstation Design
| Seq. | Workstation Name | Process Content | Pollution Types |
|---|---|---|---|
| 1 | Preparation Station | Lifting workpieces from shot blasting; placing workpieces at the station; cutting flashes and burrs; finishing clamping surfaces; loading onto the production line logistics system one by one; unloading in batches from the production line logistics system. | Smoke, Dust, Noise |
| 2 | Intelligent Cutting Station | Cutting fixed positions such as gates, risers, and vents on bolsters and side frames. | Smoke, Dust, Noise |
| 3 | Manual Precision Cutting Station | Cutting flashes of varying positions and sizes; finishing cutting allowances; finishing abnormal protrusions; removing cutting slag. | Smoke, Dust, Noise |
| 4 | Intelligent Grinding Station | Grinding fixed surfaces of bolsters and side frames. | Grinding Dust |
| 5 | Manual Welding and Grinding Station | Eliminating shrinkage holes, porosity, blowholes, cracks, etc.; removing flashes, burrs, smoothing key surfaces, and rounding irregularities; inspecting casting quality, marking defects, specifying operations, and recording. | Smoke, Dust, Arc Light, Noise |
3.2 Logistics Design
Based on the workstation design, further planning of workpiece flow is the main task of logistics design [8]. The technical specifications for the manufacture of bolster and side frames stipulate that castings poured in the same furnace should undergo heat treatment in the same kiln (typically, 12 bolsters or 16 side frames can be poured in each furnace). In practice, castings from the same furnace are generally placed together, processed individually, and then transported to the heat treatment process in batches.
This solution is designed to balance management requirements and production line workspace limitations, implementing lean principles. The logistics for the preparation station and manual welding and grinding station are designed as batch flows of single furnace and multiple workpieces in a single direction to ensure overall compliance with the same furnace and kiln requirements. The intelligent cutting station, manual cutting station, and intelligent grinding station are designed as single-piece, unidirectional flows, effectively reducing the requirements on workspace and creating conditions for the adoption of automated logistics devices, as shown in Table 3. Additionally, the logistics are equipped with a sprue and riser collection device to realize the recycling and reuse of risers.
Table 3: Logistics Scheme for Workstations in the Intelligent Cutting and Grinding Production Line for Bolster and Side Frame Steel Casting
| Seq. | Pick-up Location | Logistics Method | Drop-off Location |
|---|---|---|---|
| 1 | Shot Blasting | Overhead Crane, Batch Flow | Preparation Station |
| 2 | Preparation Station | Overhead Crane, Single-piece Flow | Loading Trolley |
| 3 | Loading Trolley | Gantry Robot, Single-piece Flow | Intelligent Cutting Unit |
| 4 | Intelligent Cutting Unit | Gantry Robot, Single-piece Flow | Manual Cutting Unit |
| 5 | Manual Cutting Unit | Gantry Robot, Single-piece Flow | Intelligent Grinding Unit |
| 6 | Intelligent Grinding Unit | Gantry Robot, Single-piece Flow | Receiving Trolley |
| 7 | Receiving Trolley | Single-beam Crane, Single-piece Flow | Manual Welding and Grinding Station |
| 8 | Manual Welding and Grinding Station | Single-beam Crane, Batch Flow | Discharge Trolley |
| 9 | Discharge Trolley | Overhead Crane, Batch Flow | Logistics Channel Flat Car |
Based on the existing conditions on-site, the logistics are equipped with primary equipment such as loading trolleys, gantry robots (and their trusses), receiving trolleys, single-beam cranes, and discharge trolleys. Their operation and control logic are consistent with the aforementioned workstation operation sequence, and their operating speeds are compatible with the following takt requirements.
3.3 Takt Design
Based on the workstation design, logistics design, and combined with the previous cutting, welding, and grinding working hours and the company’s production program requirements, the overall takt for the production line is designed to be 15 minutes per piece.
4. Configuration and Requirements
4.1 Preparation Station
The primary tasks at the preparation station are to remove flashes and burrs from bolsters and side frames, finish the clamping areas of the workpieces to ensure their readiness for the production line, and handle loading and unloading operations. Based on the takt of the preparation production line and the data in Table 1, after calculation, it is determined that two workers are required. The station is equipped with cutting gas, oxygen, and compressed air for grinding. The interfaces satisfy the requirements for two people working simultaneously. The pressure and flow parameters for gas, oxygen, and compressed air are: gas (taking enhanced natural gas as an example) pressure not less than 0.3 MPa, flow rate of approximately 21.5 m³/h; oxygen pressure not less than 0.8 MPa, flow rate of approximately 25 m³/h; compressed air pressure of 0.5-0.6 MPa, flow rate of approximately 25 m³/h. According to the batch flow requirements of logistics at this workstation and the number of workpieces per furnace, one workstation rack with a capacity of not less than 16 pieces is configured. Based on the requirements for workers and work types, two cutting torches and two pneumatic grinding tools are configured.
4.2 Intelligent Cutting Unit
The primary function of this unit is to use programmable-controlled robots to drive torches to cut fixed positions such as gates, risers, and vents on bolsters and side frames, and to collect the cut waste [9-10]. Based on the positions of gates, risers, and vents on bolsters and side frames, the positions of each cutting object must be identified and located before cutting, and the workpiece attitude must be changed during cutting to meet the cutting requirements. Therefore, it is necessary to configure positioners and workpiece fixtures, vision recognition systems, robot systems, torches, cutting gas control systems, sprue and riser collection devices, etc. To satisfy logistics operations, turntables must also be configured.
Positioners and Workpiece Fixtures: The fixtures should have an appropriate flexible structure while simultaneously satisfying the clamping and safe fixing of bolsters and side frames in any attitude. The fixtures and positioners should adopt a structure that is reliably positioned and easy to disassemble, satisfying installation, use, and subsequent maintenance requirements.