Tighter control of joining processes boosts non-threaded techniques

Paul Boughton
Product designers often rule out riveting and similar non-threaded fastening techniques, viewing them as suitable only for applications where there is no need for tight process control. However, as Paul Stevens reports, modern monitoring and servo control capabilities are creating new opportunities for these economical joining method,

Riveting and press-fitting are sometimes viewed as low-technology fastening methods, with little available by way of process control and monitoring. As such, these methods have fallen out of favour among some product designers and production engineers. However, modern control and monitoring technologies are paving the way for a resurgence in riveting and press-fitting, plus vision systems are now sufficiently low-cost and user-friendly that joints can be 100 per cent inspected, meaning that it is highly unlikely that sub-standard joints will be delivered to the customer. Furthermore, the tightly-controlled process ensures that the joint performance is highly predictable, resulting in more reliable products.

Recent developments, particularly in servo-controlled electrical devices, have led to the launch of both standalone actuators for integration in presses, and complete presses for use at manual assembly stations or within automated assembly processes. Typically the units give control over the force and distance, with facilities to enable data to be output for reasons of quality assurance traceability and process diagnostics.

One such product is the electro-mechanical NC (numerically-controlled) press-fit actuators from Kistler (Fig.1). These are simple to install, as they simply require mounting and the electrical connections to be made via two multi-pin plugs. Just two products are sufficient to enable Kistler to offer press forces of up to 60kN. In fact the Kistler units are based on a range of seven press-fit actuators previously manufactured by Staiger Mohilo. After Kistler acquired Staiger Mohilo in 2006, the actuators were equipped with Kistler piezo-electric force sensors, which enabled the range to be reduced from seven models to two.

Each actuator consists of a hollow-shaft servomotor that is assembled around a threaded drive spindle. This eliminates the need for gearing and provides fast response and compact external dimensions. The two units measure 475mm and 795mm in length and deliver strokes of 200mm and 400mm, respectively, for rated forces of 10kN and 60kN. Machine designers and production engineers will appreciate the stroke speed of 300mm/s, which helps to minimise cycle times, and the fact that the units can be mounted above or below a workbench.

During the stroke, the applied force and distance are monitored continuously. If there is any significant deviation from the defined process parameters, the monitoring system outputs a 'reject' signal.

Kistler says that aside from the process benefits and the knock-on benefits for the product being assembled, there are installation, maintenance and environmental benefits from using electrical rather than hydraulic power.

Automatic fastener installation

While the Kistler products are a good example of standalone actuators, the Pemserter Series3000 is an automatic fastener installation press from Penn Engineering & Manufacturing (PEM). Compared with traditional presses, the company says that its new servo electromechanical system achieves a cycle time that is 30 per cent lower (Fig.2). In addition, the press offers improved part-to-part consistency, better reliability and reduced maintenance requirements. PEM concurs with Kistler regarding the environmental benefits of eliminating hydraulic fluids.

The Pemserter Series3000 is suitable for installing self-clinching fasteners in thin sheet metal components. Maximum press force is 71.2kN and the throat depth is 610mm. Menu-driven touchscreen controls and descriptive on-screen alerts make the system user-friendly, and there is even a multimedia instructional video built into the control system. Data logging is available for quality assurance and process diagnostics. Furthermore, the system can be connected to a network for exporting data and receiving control commands.

Tox Pressotechnik is well known for its pneumo-hydraulic actuators that deliver the high-force advantages of hydraulics with the convenience of pneumatics. But in response to the current market demand for servo-electric presses, Tox Pressotechnik is now offering such devices, with press forces from 0.5 to 400kN.

Suitable for use as standalone devices without even a PLC connected, or integrated within fully automated assembly systems, the Tox Electricdrive actuators have the additional benefit of being approved for use within safety-related control systems conforming to the requirements of EN954-1 category 4, ENISO13849-1 Performance Level e, and EN/IEC62061 Safety Integrity Level(SIL)3.

Each Electricdrive system comprises an actuator, controller and Toxsoftware package. Integrated within the actuator are a servo motor, gearbox (not all models), planetary roller spindle, force sensors and resolver (from which distance measurements are derived). Customers can also specify either a holding brake or a redundant safety brake.

Recent developments in the Tox Electricdrive range include an increase in motor torque that enables cycle times to be reduced, plus the working range has been extended downwards so that the usable force range is now from 5 to 100 per cent. Improvements in the measuring system mean that accuracy is better in the latest models.

Monitoring hydraulic presses

In parallel with its electrical offerings, Tox also supplies an EPW400 monitoring system for use in conjunction with an HZW hydraulic cylinder and an integrated ZWK position transducer. Users benefit from having a system in which both the press force (or hydraulic pressure) and distance are monitored concurrently, giving a good overall picture of the joining process and an immediate alarm signal if the parameters are outside the defined process windows at up to ten points in the process.

A notable feature of the monitoring system is that it can store a maximum of 64 programs, so changing between one assembly operation and another can be very quick and simple. Changes can be made via the menu-driven touchscreen or a PLC. Interfaces are included for both CANopen (giving access to Profibus, Interbus and other industrial fieldbus networks) and Ethernet communications, plus the unit has a USB port for uploading parameter settings, downloading stored data or connecting a printer. An on-board facility for generating process statistics is useful for diagnostics and quality assurance.

For clinching applications, in which metal sheets are joined without the need for rivets, Tox offers a similar monitoring system, known as the CEP400.

In some cases the need is to upgrade an existing process, either to improve yield or to address an issue of product failures. By using the latest generation of compact force and distance sensors, it can be relatively straightforward to retrofit monitoring systems to existing assembly presses.

While it is certainly possible to design and build data acquisition and processing systems to monitor and analyse the readings from newly installed sensors, this is likely to be time-consuming and expensive compared with using an off-the-shelf system. One example of this type of dedicated press monitor is the Como (ControlMonitors) Net system from Kistler. Furthermore, not only can be data be processed locally, but it can also be transmitted to a centralised production monitoring and control system, either by a network connection or wirelessly (Fig.3). Depending on the application, an output can be used to provide an alarm signal to the operative or trigger a reject routine running on a programmable logic controller or other process control system.

The foregoing relates to in-process monitoring of the joining process itself, but Orbitform has recently launched a vision-based inspection system that identifies cracks, mis-forms and other defects. By inspecting the final form, the system can detect if an incorrect force has been applied during, for example, impact riveting, orbital forming, spin forming, radial forming or other press operations. Both aesthetic and structural defects can therefore be detected.

Orbitform's Fastener Inspection System (FIS) incorporates Cure (concurrent universal recognition engine) memory processing architecture (Fig.4). It is said to be capable of real-time process monitoring and control, with set-up achieved by means of a 'teach' routine with a known good sample. A custom crack detection image processing algorithm is applied in conjunction with a conventional vision system.

Data from the Cure system can be combined with data relating to force and distance to give a high level of quality assurance. FIS is now available on most Orbitform machines, and it can be interfaced with many other machines and systems.

FIS is a more powerful alternative to Orbitform's Watchdawg process monitoring system, which uses data from force and distance sensors to monitor the forming process and, if required, the formed height. Watchdawg can also be incorporated into a feedback loop for process control.

For example, it enables the process to be controlled on the basis of form-to-force, form-to-distance or form-to-form height.

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