As early as 1923/1924, the British company MORRIS MOTORS developed a cycle line for the mechanical processing of cylinder heads. It was purely mechanically controlled, had a cycle time of four minutes and completed 53 operations. However, the control system proved to be unreliable and still required 21 workers to operate it. However, the general goal of saving on manpower has not changed to this day. But there are new factory concepts under the term Industry 4.0, which can also be used to effectively process a batch size of one. At its core, this involves comprehensive data networking. A workpiece to be manufactured finds its own way through the production plant.

What is behind the conventional term interlinking? It is the connection of automated machines or workstations to form an efficient system structure with a continuous workpiece flow. This is achieved by means of devices for automatic workpiece transfer, but also by connecting the machines via an information and energy flow that enables overall control.

Figure 2: Parts handling on a deep-drawing press. 1 linear unit, 2 stroke/rotation unit, 3 press ram, 4 removal unit, 5 drawing tool, 6 press frame. © Hesse

Interlinking type
The interlinking of the machines can be rigid, loose (time asynchronous) and target-coded. In the case of loose interlinking, workpiece stocks are created between the machines. If a machine upstream or downstream fails (short-term malfunction), the part of the production line that is still intact can continue to work for a limited time thanks to the storage. For this reason, loosely linked machines are always more efficient than permanently linked machines. For workpiece storage, a distinction is made between main and secondary storage. The basic structure is shown in Figure 1. The main lock storage must be designed in such a way that workpieces can be connected to workpiece carriers in the event of a gap in the flow.

In forming technology, presses are connected to form lines. Continuous conveyors are only used to a limited extent for the workpiece flow. Press-integrated or stand-alone robots are suitable. Figure 2 shows an example. Three-axis robot arms bring blanks into the working zone of the machine, remove the deep-drawn part and then place it in the following machine for trimming.

Figure 3: Multi-machine operation with central robot. 1 Robot footprint, 2 Enclosing rectangular area, 3 Measuring grid for precise planning, 4 Machine tool, 5 Conveyor belt, 6 Pallet station, 7 Gripper changing station, B Operating station, R Arm reach. © Hesse

A prioritized axis of movement is defined to control the machine. In this case, this would be the press ram. Smooth interaction over time requires all overtravel paths and signal delay times to be taken into account. A freedom of movement model is useful here as the basis for a movement regime for all processes.

Multi-machine operation
Several operating points of work machines, which are arranged in a circle or line, are approached by an industrial robot as required. This leads to good utilization of the robot. The organizational goal is to achieve a minimum of waiting times for both the machines and the robot. In the case of a robot workstation with a central industrial robot, the question arises as to where it should be positioned. A simple graphical method for determining the location is shown in Figure 3. An arc with the arm reach R of the robot is drawn around each operating position B to be approached. The circular arcs enclose an area. Each grid intersection point within the average set is a potential location. It is only necessary to check whether a collision-free passage of a work cycle is possible. The collision-free locations are selected according to the criteria

Figure 4: Conversion-optimized assembly station for a choice of four small assemblies. 1 Loading and unloading position, 2 automatic assembly unit, 3 top plate, 4 rotary indexing table 180°, 5 assembly device, 6 index bush, 7 index bolt, 8 working force, A-D base part holder. © Hesse

• Minimum cycle time
• Load on the industrial robot
• Safety and accessibility

an optimum location is selected. You can generate further variants of the machine installation and thus also find further alternative solutions.

Convertible small parts assembly
How can a small assembly fixture (Fig. 4) be quickly converted? In the example, the indexing plate of the device is designed for four different assemblies. You can see a station for inserting, checking and removing. Opposite is the automatic assembly station, for example a press. The rotary table always cycles through 180 degrees. Changing over to a different assembly is simple; the top plate is lifted off and replaced in a staggered position.

Stefan Hesse