Introduction

In the 1980’s the technology of cold forming control based on the measure of produced strength was widespread on double-stroke formers. In the first stage, this technology was supported by the use of small-size piezoelectric sensors, which could be easily installed on older machines still in service. The benefits of this process control system were clear, and derived from observable results. Satisfied users were perhaps not overly concerned with the theory behind this success, or in metrological tests to prove a relation between product dimensional defects and detectable stress size deviation.

The method protected the tools housed in the machine, and drastically reduced the number of personnel assigned to machine control as well as the risk of producing defective lots. Later on, the technology was further improved to detect any splinters in the punches used to produce cross-recess screws (Phillips, Pozidrive, etc.) and similar parts, thus avoiding defects on the head of the finished part. Today, this type of control has become the standard for starting a production cycle on a double-stroke former, even in the absence of an operator.

Control on double-stroke formers

The first Eldes control systems, entirely based on Italian technology, were developed in 1982. The company’s Prometeus series for forming process control has undergone gradual development, passing from double-stroke to progressive formers. This evolution permits the latest model (Figure 1), even when it is radically overhauled, to retain its compatibility with previous models.

Figure 1: The Prometeus 2K equipment. In addition to these instruments, Eldes recommends installation of piezoelectric sensors in a symmetric structure to ensure superior immunity from electrical troubles

The new model can be applied to new and existing equipment, allowing machine operators to adapt readily to its use. The method of machine supervision, based on stress control, can also be applied to rolling machines, thus forestalling several operational problems. In comparison, general control of the wave shape as established from the deflection of a part placed in the grip does not - as soon as this wave exceeds the tolerance range - provide any clear indication on the origin of the defect.

Figure 2: Synthetic visualisation simultaneously displaying the machine data and those referred to station control

Full coverage of operations

Thanks to its fruitful cooperation with the Italian firm TLM, specialists in the manufacture of rolling machines, Eldes was able to develop such inspection techniques as feeding repeatability and rolling stress duration (Figure 3) to ensure correct threading and, indirectly, finished parts kick-off. Dovetailing these timing measures with the usual amplitude controls on the wave shape allows complete supervised coverage of operations and clear diagnosis of defects requiring that the machine be stopped. This method also assists set-up in all operations.

Figure 3: Representation of feeding control and rolling duration. Control setting is flexible,
permitting an advance from self-tuition to detailed personalisation

Evaluating the control system

While rolling stress control systems are now fairly well known, in standard fastener production such a system is generally considered useful only when it does not lead to machine stoppage in the event of an isolated defect, but rather scraps out-of-tolerance parts "on the fly". Control equipment such as the Titanus (Figure 4) can be furnished with a device for surveying the vertical stress of the fingers, including the sizes of the positioning coordinates of the finger block, and - at a later stage - for perfecting the product.

Figure 4: Titanus 2K is control equipment designed for rolling machines with linear grips. Visual display and wave shape control are by means of two sensors for reconstructing the wave trend during the entire stroke, and detecting minimum diameter defects and differences arising from thermal treatment

For progressive forming, in which visualisation of the wave shape is particularly useful, the use of stress control systems is not as widespread as it is with double-stroke formers. This is at least partially explained by the position of the control method on the side frame. Although intended to be accessed only in the event of significant changes in overall stress, this is more readily comprehended and apparently less “invasive”.

The use of sensors

Sensors built onto the die supporting plate caused serious interference problems when first put into operation in production plants. Eldes thereupon introduced sensors housed in the punch adjustment wedges, a solution that proved efficient in terms of size. However, it cannot be applied for all purposes, since the outlet cables are exposed to the risk of damage when the operator must adjust the punches. This solution is still employed when it is necessary to precisely control a low-strength working process such as part punching (Figure 5) or in special cases, e.g. “turned hexagon” (Figure 6).

Figure 5: The wave shapes in a four-station former equipped with sensors on the wedges. This permits clear observation of the work being performed at the four stages: preparation, head, extrusion, kick-off

Figure 6: Anti-rotation control for the hexagon. Through sensors located on the station, and by confining the analysis process within a suitable time segment, the system can sense any slight angular defect

To handle interferences, an intermediate solution places the sensors on the side, behind the wedges. This is today’s preferred solution, particularly appreciated by both manufacturers (because it can be easily standardised and is compatible with the machine structure) and users (because it does not create any obstacle in the working area). Figure 7 shows this process in a five-station former. Ram C moves within the machine body. In the active stage of the cycle, it brings the punches P to compress the material to be processed, which finds itself in the dies M.

Figure 7: Control system in a five-station former

Since the entire working stress on each station is conveyed through the adjustment wedges R, the sensors S - suitably positioned near the wedges - are prepared to receive an indirect measure of this stress. The electrical signal produced by them is almost proportional to the developed strength. The control system must collect from each sensor a signal to describe the developed distortion stress, which should (to the extent possible) be free from the interference generated by the stress (Figure 8) on the adjacent stations.

Figure 8: Stress signal wave shapes simultaneously visualized on five forming stations - 5/6-station formers

Mr Paolo Belloto, general manager of Eldes, told EuroWire: “The supervision method through sensors on the shoulders of the press cannot still be considered an alternative to the type of control described here. It is true that an extension of stress analysis to the whole wave shape would allow some improvements in the control of working defects, in comparison with the initial method from which it derives. Although we acknowledge that this is a very useful accessory, and necessary if the performance it provides is not included in a more complete control system, in our opinion it cannot in general be considered a step forward in forming supervision methods. Working operations on multi-station formers are becoming increasingly complex. In the future, the professional skills of a company will be developed on these more challenging machines."

Figure 9: The tolerances of stress control can usually be established
separately for any single station, or globally for all stations

Mr Belloto also said: “Evolution in measurement technology is ongoing. The control of overall strength will be judged ultimately in performance. If the technology of detailed analysis of the strength exerted on each station has not yet reached an optimum level, it has, however achieved some results that invite us to overcome the obstacles remaining. The ground between the borders of two specialized fields, such as mechanics and electronics, runs the risk of not being cultivated enough if both disciplines do not collaborate from the stage of the birth of ideas. Too often they limit themselves to interaction with one another at a later stage. Electronics, particularly, should avail itself of appropriate calculation methods to reduce interference. But it also needs the informed input of working mechanics”.

Figure 10: Prometeus 2K/65 is Eldes control equipment specially designed for progressive 5/6-station formers

According to this view, for their part mechanics would stand to discover that the introduction of specially developed parts, or parts worked with higher technological requirements to make measurements more reliable, would be very useful. Further, the installation - in addition to sensors on each station - of additional sensors on the machine shoulders cannot be neglected. These allow the operator to directly and periodically reset those installed on the individual stations.

Conclusion

Mr Belloto declared, “If we wish to ensure our competitiveness, I do not believe that we can afford to fear complexity. As regards the machinery of the future, we can plausibly foresee an appropriate amount of electronics enabling users of formers and rolling machines to relate by means of a single interface to a variety of different functions. These will include part length control, stress control, management of alarms, and machine adjustments - not excluding automatism”.

Mr Paolo Belloto, general manager of Eldes

Company Profile

Eldes s.n.c di Bellotto Paolo & C. specialises in the development and design of monitoring systems based on the measurement of the force developed in the production process by cold forming machines. The company’s portfolio includes equipment for cold headers such as the Prometeus series 2K, available in 2, 4 or 6 station versions and designed for the control of peak values, waveforms and special points, as well as KN/Tons valorisation with simultaneous display of the waveform of all the stations. An industrial PC-based is available, version Theseus 2k, with SVGA TFT LCD 12” display, and Windows/Linux O.S.

The company also offers equipment for thread rollers, Titanus 2K, available in versions with 2 to 4 sensors and suitable for controlling peak values, waveforms, introduction instants, duration, expulsion, and driving of a selection device. Furthermore, Eldes’s portfolio includes machine terminals Echo 2K and Prodat programmes for the centralisation of the production data, as well as Themis control station, designed for the acquisition of dimensional measurements and statistical process control of the production.