[1] Principal Manufacturing Consultant, TWI [2] Professor of Civil and Structural Engineering at UMIST, Manchester.
Presented at 3rd International Seminar, "The Use of Steel Structures in Civil Construction". Belo Horizontale, Brazil, 12 - 15 September 2000, under the auspices of SME Brazil.
Abstract
The paper reviews the manufacturing processes used in the steel fabrication industry in terms of their requirements and the take up of automation and robotics. The potential for increased and wider use of robotics is explored and it is concluded that IT is vital, that there will be niche opportunities for further applications in the fabrication area, and that a wider use in specific application areas can be expected.
Keywords
Robots, steel fabrication, welding, manufacturing.
1. Introduction
At one time robots were seen as a means of significantly improving productivity in the steel fabrication industry. However, the industry has been slow to put robots to effective use. There have been a number of cases where companies have purchased robots anticipating that they would transform the company's performance but the robot was soon under covers in the corner of the fabrication shop gathering dust. Although companies have been quick to implement computer controlled equipment (e.g. numerically controlled (NC) drilling, cutting and machining) there has been a general reluctance to adopt full robotic technology. It is therefore appropriate to ask the question 'Is there a place for robots?'.
2. Review of processes in steel fabrication
Processing steel for construction applications involves some or all of the following processes: cleaning, cutting and profiling, drilling, joining, machining, protecting and always, handling. How automated are these processes at present and what is the opportunity for robot technology to reduce processing costs?
2.1 Cleaning
If the fabrication process will be of short duration, then cleaning usually comes first. However, if it will take longer then cleaning will follow welding.
Cleaning is usually a blast cleaning process which can be highly mechanised where the cross section is regular. In other circumstances a batch process is more appropriate. With (large) structures, a suited operator may direct the blasting stream.
In all of these operations it would be possible to have a robotic arm manipulate the blasting nozzle. There is probably little advantage in doing this where throughput justifies continuous type operation. However, for batch and large structure operation, robots could bring benefits. Robots might also be used to load/unload blasting systems.
2.2 Cutting and profiling
Beams are almost universally cut using large numerically controlled saws. The operators match the steel arriving at the saw with the required cutting program. These are efficient systems and good at cutting to length with straight or angled cuts.
NC controlled cutting systems cut plate into shapes used for girder stiffening and terminations. They use oxygen-fuel, plasma or laser cutting processes to produce 2D profile shapes. Specials have been developed, that are able to make the 3D to produce weld preparation profiles (e.g. the ends of tubulars).
In the automotive sector robots have allowed cutting to be used in place of press trimming or punching. They have also allowed hole forming to be performed late in the manufacturing sequence, (e.g. to cut apertures that are specific to left hand and right hand drive models). In heavier gauge materials, robot cutting has been used to prepare truck chassis components. In the ship building industry NC laser cutting produces 2D parts of high accuracy and low distortion. The laser also marks the parts, simplifying assembly and the QA/QC processes.
Robot technology may also give a 3D cutting and marking capability especially for the complex shapes used in fabrication.
2.3 Hole forming
Holes can be drilled with considerable precision in multi headed NC machines. These machines can place holes precisely in 3D on a range of section shapes. The holes are normally at right angles to the longitudinal axis of the section. Punching may be acceptable and NC machines are available. However, punching introduces stresses that are not always acceptable and there can be restrictions on thicknesses and hole size. These operations are often associated with cutting and may be included within the same machine.
Robots and laser cutting technologies can produce holes of good quality and variable shape, which may allow new design features to be used (e.g. tab and slot assembly).
2.4 Joining
Joining may include bolting, nailing, riveting, welding and bonding (and sealant addition).
In the fabrication industry these operations are most likely to be manual with the operator moving to the component. In other sectors robots are used to manipulate the process tools. In these areas the component invariably moves to the robot. To move material in this way would have a significant effect on the layout and operation of a fabrication shop.
Today robots are being used to perform many of the joining processes used in construction e.g.
- Bolting (of wheels to automobiles)
- Riveting (of pre-coated panels in the white goods sector)
- Welding (of automotive components, bridge elements and ships)
- Bonding (adhesives in automotive applications)
Indeed, where component numbers have been reasonable, robots have been used in the fabrication sector, (e.g. welding nodes to columns).
The choice of a particular welding technique depends on the type of joint, the materials involved, the welding position and on productivity and joint properties required. Manual metal arc welding is still widely used, particularly for jobbing work where a high degree of skill may be required, or where the extent of non repetitive work is high. However, the manual semi-automatic processes such as GMAW/self-shielded welding are now predominant in the industry. These processes, and the submerged arc technique, can be mechanised - which is an advantage.
Robots are used for automated welding and arc welding has always been an important robot application area, Table I. [1]
Table I Robots installed in the UK
| Year | 1982 | 1986 | 1990 | 1994 | 1997 |
|---|
| Total No. Robots |
1152 |
3492 |
6227 |
9275 |
12975 |
| Arc welding robots |
157 |
454 |
679 |
1528 |
2104 |
| % |
14 |
13 |
11 |
16 |
16 |
The limited data available indicate little use of robots by the construction sector. A Japanese survey [2] 1998, suggests that of 62184 robots only 51 were being used in Construction/Building. Of these 2 were arc welding, 40 gas welding with the others being used for handling operations.
2.5 Bending and pressing
These have, traditionally, been highly skilled manual processes. Increasingly NC operation is being introduced which has resulted in increased productivity and repeatability of performance. Robots are now being used for bending machine operations.
2.6 Rolling
Rolling is another process which has relied on highly skilled manual operators. NC operation is also being introduced for the control of this process. Size and mass are again major robot challenges.
2.7 Machining
Metal removal by machines has been an NC process for many years. The fact that material is removed, together with a need for accuracy and repeatability (nuts and bolts do not work if the threads are not compatible), rapidly led to the machines being able to do the task unaided.
Robots are increasingly used to perform machining operations (e.g. routing of aluminium components).
2.8 Protection
Robots are extensively used in the automotive industry for the application of paint and corrosion protective coatings. These systems are able to change colour from vehicle to vehicle. Robots and other machines are also being used for metal spraying activities.
As the demand grows for more complex protection systems, with their increased health and safety requirements, so the use of robots will grow.
2.9 Handling
Material handling is probably the activity that uses the most resource without adding value. The fabricator must move components which are physically large, even if they are not of great weight. Handling these components within robotic cells is the unsolved challenge. Attempts have been made but the concepts have been frustrated by the sheer diversity of product dimensions and shape.
As yet, the use of robots to perform the handling has not been seriously studied by this industry. Other industry sectors are beginning to pay more attention to flexible handling, where the robot moves the work to the appropriate machine.
3 Types of robot systems and applications
Robots are essentially machines which are programmed to carry out a series of repetitive actions. Without intelligence they simply carry out the same operations but do so repeatably. A strength, over other machines, is that they can be re-programmed to perform a different or several different tasks simultaneously. Indeed the controlling factor on production rate becomes the ability to supply work to the robot. Thus the introduction of robots has to be part of a total manufacturing plan.
A degree of intelligence can be given to robots by adding sensors, which provide information enabling the robot to take conditional action. The most common form is sensing to allow the correct relative positioning of the tool and the workpiece (joint finding).
Robots were first used for arduous jobs such as die cast machine unloading and resistance spot welding. These arm-on-a-post machines had three axes, to give XYZ position and two or three wrist movements to give orientation. Although sophisticated, they worked as dedicated automation. As the capabilities of the controllers increased so it became possible for the robots to perform multiple and complex tasks.
Arc welding presents a real challenge to robots. The robot head must travel at a controlled speed, maintaining the precise distance from the workpiece, if quality welds are to be made consistently. With larger components and where multi pass welds are needed, it is not sufficient to simply program a precise path because of the effects of tolerances and the distortions caused by welding. Sensing equipment, which enables local adjustments to the position of the welding head, becomes necessary (joint tracking and recognition).
The two main systems in use are based on: detecting arc parameters which vary as the electrode gets closer to the weld edges, or detecting the joint by laser technology.
The first arc welding tasks that were carried out by robots were highly repetitive, ( Figure 1).
3.1 Design
The key to successful application of robots has been through designs that recognise the constraints imposed by the machine. For example, an arc welding robot demands high repeatability in joint position and fit-up together with more accessibility than is required by man.
3.2 Information Technology
The recent CIMsteel project [4] , which investigated Computer Integrated Manufacture in the industry, highlighted the importance of information technology (IT) as an enabling technology.
Robots must be programmed. Where there are one-off products, the program development must be instantaneous and automatic. Furthermore, the program must be at the robot at the same time as the component(s) to be processed. Movement in and out of the cell requires IT. Also there must be feed-back to enable the system to know what is happening. This information flow requires robust links between numerous computer-based applications such as: CAD, simulation cell, administration, scheduling, planning, purchasing (ordering), maintenance control.
4 Economic considerations
4.1 Capital costs
A robot cell, able to weld construction fabrications, is likely to be large with at least two robots. System costs, Table II, are based on data obtained from robot manufacturers for a general structural steel fabrication cell, with 2 robots and 2 adjacent working envelopes of approximately 30x2x2 meters (1996 £1 = $1.55).
Table II Possible costs of a fabrication welding cell
| Items | Minimum | Maximum | Average |
|---|
| Robot & controller |
£41,350 |
£102,400 |
£60,000 |
| Welding equipment |
£11,800 |
£18,200 |
£18,000 |
| Robot floor track, 30m |
£90,000 |
£330,000 |
£190,000 |
| Robot support gantry, 3 axis |
£85,000 |
£100,000 |
£89,000 |
| Vision sensing |
£25,000 |
£50,000 |
£30,000 |
| Off-line programming system |
£4,500 |
£82,000 |
£65,000 |
| Beam manipulators, 4 off units |
£100,000 |
£180,000 |
£108,000 |
| Manipulator track, 2 off 30m |
£91,500 |
£110,000 |
£100,000 |
| Safety |
£20,000 |
£40,000 |
£30,000 |
| Engineering |
£30,000 |
£60,000 |
£40,000 |
| Training |
£8,000 |
£20,000 |
£12,000 |
| Spares |
£8,000 |
£12,000 |
£10,000 |
| TOTAL |
£515,150 |
£1,109,700 |
£756,200 |
| Maintenance |
£2,500 |
£5,000 |
£3,000 |
| Yearly service cost |
£4,000 |
£6,000 |
£5,000 |
| OPERATING COSTS |
£6,500 |
£11,000 |
£8,000 |
4.2 Productivity
Improvements in productivity arise from combinations of improved materials handling, increased processing rate and higher duty cycle. For arc welding, manual arcing efficiency ranges from 5 to 60% (mean 10%) and the robot welder from 50 to 90% (mean 70%). On average a simple arc welding cell normally has more output than 3 skilled welders. However, a more useful measure is to compare the profitability of man and machine at the same level of productivity, [5] see below.
| | Robot | Manual |
|---|
| Income |
£9,676,800 |
£9,676,800 |
| Cost of Sales |
|
|
| |
Material |
£7,257,600 |
£7,257,600 |
| |
Labour |
£126,720 |
£753,408 |
| Variable overhead |
£28,672 |
£85,341 |
| Consumables & Maintenance |
£16,000 |
£10,000 |
| Cost of employment |
£12,672 |
£75,341
10% additional |
|
| Gross Benefit |
£2,263,808 |
£1,580,451 |
| Capital outlay |
| Equipment: cell / welding sets |
£756,200 |
£18,000 |
| Other costs (integration) |
£100,000 |
|
| Other costs (jigs) |
£100,000 |
£50,000 |
| |
Total |
£956,200 |
£68,000 |
| Depreciation, 5 years |
£191, 240 |
£13,600 20% |
| Increased profit = Gross benefit - Depreciation |
| Profit |
£2,072,568 |
£1,566,851 |
| Does £505,717 additional profit justify capital spend of £738,200? |
Assumptions
- Sufficient work can be found to keep the cells working fully
- New equipment is needed for both robots and welders
| |
2 robot cell |
1 welder |
| Operator/welder/working week/year |
48 |
48 |
| Shifts/day |
3 |
3 |
| Days/week |
5 |
5 |
| Shifts/week |
15 |
15 |
| Units/shift/cell or operator |
14 |
2.33 |
| Units/day |
Beams |
42 |
|
| Tonnes |
50.4 |
|
| Staff needed to produce these 42 beams |
| Operators*/welders** |
3 |
18 |
| Supervision/Technician*** |
0.5 |
1.8 |
| Because of shift work, only 6 welding sets required |
| Operator working shifts/year |
2160 |
12960 |
| Supervisor technician shifts/year |
360 |
432 |
| Rates of pay |
|
Shift |
Annual cost |
| *Robot loaders cost |
£48 |
£103,68 |
| **Manual welders cost |
£56 |
£725,760 |
| ***Robot supervision/technician cost |
£64 |
£23,040 |
| ***Welding supervision costs |
£64 |
£27,648 |
| Labour costs |
£126,720 |
£753,408 |
| Materials |
£/tonne |
No/Yr. |
Value |
| Bought in at |
600 |
12096 |
£7,257,600 |
| Sold on at |
800 |
12096 |
£9,676,800 |
| (average unit weight of 1.2 tonnes) |
5 Future trends
Although robots could perform most fabrication welding tasks, rapid programming is vital. This will require the IT links suggested earlier. The IT cell developments at Odense Steel Shipyard have given other shipyards confidence to automate production in a similar way. We can expect a technology transfer to the fabrication sector (or the yards to commence fabricating).
Robots will be seen:
- Cutting and marking (increasingly with laser systems)
- Welding (initially using arc processes and then lasers)
- Non destructive examination (NDE)
- Coating
Important welding applications will include, web butt welds for castellated beam manufacture, attachment of stiffeners and end plates to beam sections and also column node fabrication.
The full integration of a robot in the factory will quickly transfer to an ability to do so at site. This will lead to an increased use of robots for column welding and NDT probe manipulation.
One of the most important trends, driven by a need to reduce overall building cost, will be a greater involvement by the designer, together with an integration of the design and manufacturing (robot) processes. The Japanese have begun this process through the use of modular column connections in multi-storey buildings and in the factory production of modular housing.
6 Conclusions
There is a place for robots in the fabrication sector. For welding applications it will start in two ways. One will be for producing high volumes of repeat elements such as in cleats and similar small elements. More significant development will come when the robot is part of an integrated factory, which will make it possible to fabricate larger and more complex structures economically.
The value to the fabricator will become significant when designers recognise the strengths (speed and quality) and constraints (access and need for programs) of robots.
While this is a fabrication conference it must be recognized that the greatest use of robots will probably be in their use for assembly (panelling, flooring and roofing), coating (flame proofing) and maintenance (window cleaning).
Whatever the application taken up first and widely, it will surely lead to an increased use in other areas, just as their use has spread from within the automotive to the shipbuilding sectors.
References
| N° | Author | Title |
|---|
| [1] |
|
Robot Facts 1997 Annual report of UK investment in Robot Automation in Manufacturing Industry, Published by the British Automation & Robot Association. |
| [2] |
|
JARA News Vol 10 No 3 December 1997 pp2 |
| [3] |
Weston, J 1997. |
Portable welding robots - an industrial survey. TWI members report 595/1997. |
| [4] |
|
Eureka 130 CIMsteel project reports. |
| [5] |
|
EU130 CIMsteel, Work Package 5 Deliverable D7-FCW Cost benefit appraisal, 1997 (Unpublished) |