COLD FORMING

Present horizontal multi-stage cold formers for the production of complex shaped parts made from wire cannot deny their origins in the former bolt- and nut formers.
Over the years, the customer benefit was increased again and again by adding more and more versatile technologies and comprehensive components to the unchanged basic concept.
Meanwhile, the formerly quite uncomplicated and reliable machines have partly mutated into highly complex systems, liable to malfunctions, cost intensive and space-hogging. Further steps in this direction for the benefit of the customer seem to end in nowhere. Therefore It was time to completely question the present concept, which was established under an entirely different set of conditions, and to change it under the consideration of today’s requirements, experiences and technical opportunities.
However, this step requires a holistic approach and must involve all aspects with regard to the market, to machine users, to product developers, to tool and process developers, to tool setters and machine operators, to machine developers as well as to experts for drive technology and software.
These are the main requirements on a new generation of part formers in order to increase the customer benefit:

The individual features of the new concept:

- Improvement of product accuracy
- Improvement of flexibility in terms of product geometry and tool design
- Reduction in changeover times and simplification of the changeover process with more reproducible results
- Improved reliability and reduced maintenance
- Reduced space and energy consumption
- Reduced investment

Die case for ultra high pre-stress

With a background of many years of experience and knowledge as machine user, machine developer, tool and process developer, and utilising today’s increasingly powerful servomotor and direct drive technology together with modern materials, functional coatings, design- and processing methods, the vision of a new machine concept came into being. That new concept will meet all these mentioned requirements.
The implementation of all individual ideas, however, requires comprehensive detailed developments and can only be achieved step by step.
In this process, machines according to the existing type of construction will still have their qualification depending on customer requirement and product range.
In many cases, a new solution to one of these features was only possible by applying the possibilities and conditions that another new developed feature offers.
Direct drive of the crankshaft via servo torque motor
Advantages:
- No need for the maintenance intensive clutch- brake combination
- Full forging force even at start and at creep speed
- Freely programmable speed-travel characteristics
- Very small braking angle
- Compact and maintenance free
- High efficiency
The conventional drive mechanism may, however, not simply be replaced by a direct drive without changing the kinematics.
As in case of direct drive without flywheel, the motor itself must supply the torque which is required for the forging load process. Compared with a comparable conventional drive mechanism with flywheel the required peak power of the direct drive would be higher to such an extent, which is economically not justifiable,.
By means of the now possible discontinuous speed of the crankshaft, the ram stroke can be decreased considerably, thus reducing the required torque accordingly. That is now possible because the required time for the transverse transport of the parts between dies and punches does not need to be created by the excess travel of the ram anymore, but by reduction of the speed of the crankshaft at the rear dead centre. A short ram stroke also makes the machine design more compact and stiff.
The required torque can be additionally reduced by changing the crank kinematics from “pushing connecting rod” (crankshaft behind the punches) to “pulling connecting rod” (crankshaft behind the dies).
The high current demand of the direct drive in the moment of forging can be buffered by means of an electric flywheel storage, which is also used for storing the brake energy. This is a separately located motor with flywheel which is kept on speed by a current demand from the grid, which is limited to rated power. By decreasing the speed it then provides the required current peak generatively via the DC-circuit coupling of the converters.
Two counter-directional rotating crankshafts located behind the dies and one above the other, with crank pivots at both ends and discoidal connecting rods.
Advantages:
- Favorable travel-time-course by the kinematics of the “pulling connecting rod” with very short con-rod length to reduce the required torque in connection with servo direct drive
- No vertical force components acting on the ram guide.
- Simple mass balancing
- Very compact machine design
- Insensitive to eccentric load – the massive punch block is guided directly in the die block in a very stiff way and free of clearance by solid guide columns at both sides of the die block, which transfer the press force from the crankshafts to the punch block – no conventional ram guiding necessary
- Very simple and easy going product discharge possible, separately for each station by means of short gravity chutes, longitudinally arranged side by side directly underneath the punch block
- Exact mechanical overload protection by accordingly pre-tensioned tie rods inside the stiff guide columns – if the set tension force is exceeded, the tie rods elastically elongate some millimetres with only low increase in force and absorb the energy until the stop of the crankshaft.
The latter point is a requirement to allow the use of roller bearings for the crank mechanism. This in turn forms the preliminary requirement to ensure the maximum press force during creep speed and also to avoid that the press jams in the front dead centre. It is also the prerequisite for a high efficiency and ram movement without axial clearance.
With standard sliding-bearings, very slow speed results in greatly increased bearing friction due to the loss of the hydrodynamic lubricating effect. This means that a large proportion of the torque is lost to overcome the bearing friction.
Impact cutter with impact mass driven by servo motor
Advantages:
- Constant shear velocity of up to 10 m/s, independent from the speed of the coldformer
- Plane and precise shearing surface with minimal deformation and hardening even in case of soft materials
- Simple and low-maintenance mechanical construction, no mechanical drive coupling with the press
- Low energy demand
The main problem at known impact cutters – either driven mechanically, hydraulically, pneumatically or electrically – is to absorb and destroy surplus energy, which is not consumed by the shearing process but is needed for an adequate speed at the end of the shearing process.
The developed new cutter solves this problem in a simple way according to the invention: In contrast to other types, here the impact mass (hammer) and the counter mass (shear slide with cutting knife) are almost of equal mass. According to the laws of elastic impact, this results in the following procedure:
The hammer is accelerated by the servo motor and hits the shear slide, which is in its starting position, with the programmed speed. After a travel in the range of tenth of a millimetre, which is defined by the spring stiffness of the impact elements, the hammer comes to standstill and the slide is accelerated to almost the same speed. After the material has been sheared off, the remaining speed of the cutting slide is slowed down by means of spring elements, which then accelerate the slide in opposite direction. Shortly before the slide has reached its starting position, with the remaining energy it impacts the hammer, which is still resting there. In this process, the cutting slide comes to standstill directly at its start position and the hammer is accelerated backwards with the remaining energy. The servo motor now slows down the hammer with regaining the energy and repositioning it at its starting position.
The synchronisation with the press is accomplished by an electrical starting signal at the appropriate moment.
The cut-off blank is ejected out of the knife by the following material. In order to pick up the cut-off blank with the transfer fingers, the material feed motion stops shortly after a stroke according to the width of the knife and is then finished. In case of blank lengths shorter than the width of the knife, the stroke is finished in opposite direction.
Linear feed with linear-servo-motor driven slide and servo-motor-actuated wire clamps
Advantages:
- Change of feed lengths from 0 to max without mechanical adjustments, only by programming at a precision of 0.01 mm.
- Length corrections possible during running
- No variations in the cut-off length due to variations of feed counter force from decoiling and straightening
- No wire gripper change when changing the wire diameter
- Very simple and wear-free mechanical design, no mechanical drive coupling with the press, no hydraulics
- Low energy demand
The compact and complete feed module (illustration 1) may be very easily mounted to existing equipment. The synchronisation with the coldformer is accomplished by means of a rotary encoder from the crankshaft.
At smaller modules, the actuation of the wire grippers is accomplished pneumatically via quick shift valves and fast-action short stroke cylinders with excenter transmission.
Servo-motor-actuated transfer fingers and servo-motor-driven transfer slide
Advantages:
- Changes in the timing and movement of the individual opening and closing of the fingers and the transverse motion without mechanical adjustments, just by programming
- Simple monitoring of the correct gripping without additional sensors, only by the information of position and force in the servo-system
- Very fast motion of fingers and transverse movement, which results in the simplification of tools and the possibility of picking up short parts also directly out of the punch (instead of rotating fingers)
- Simple, low-maintenance and wear resistant mechanical construction
- No mechanical drive coupling with the press and thus very easy to swivel aside.
The compact and complete transfer module (illustration 2) may be very easily mounted to existing coldformers.
The movement relative to the ram remains always equal, irrespective of the ram speed, due to the electronic cam functions with the crankshaft as master axle.
Special new designed kinematics open and move the fingers over a maximum punch diameter according to 95% of the die pitch, whereby they do not alter the center point of gripping at the beginning of the opening motion.
The very simply designed and inexpensive fingers can be quickly changed without adjustment gauge due to precisely repeated fixation.
Mechanical synchronous ejectors which are directly coupled with the ram motion
Advantages:
- Movement precisely synchronous with the ram movement, irrespective of stroke length and timing settings
- Simply adjustable kick-out stroke
- Identical system with the same maximum strokes and forces on the punch and die side
- Simple mechanical construction with very direct, short and stiff flow of forces, resulting in high kick-out forces and low deflections
- Simple and adjustable mechanical force limitation and monitoring without predetermined breaking parts.
The die-ejector bridge, which is located behind the die block, is solidly connected with the ram; the punch-ejector bridge, which is located behind the ram, is solidly connected with the machine frame.
The limitation of the different station-specific kick-out strokes is accomplished by decoupling from the ram movement, which is triggered by the cam-guided folding of the folding levers which are elongated during the synchronous ejection stroke.
Setting of the die-side kick-out pin position by externally preset quick-change cartridges behind each die
Advantages:
- Elimination of the threaded kick out sleeves in the machine frame, making a more compact design
- Elimination of the servo-motor-adjustment system, without extending the changeover time
- Increased stiffness and precision of the kick out pin support
- More precise reproduction of the settings
The adjustment cartridges can be easily removed from above without having to take out the dies. For quick changeover they exist in twin configuration.
Axial punch adjustment via hydrostatic pressure support behind each punch
Advantages:
- Elimination of the adjusting wedges which have some wear and also weaken the ram
- Individual setting of overload protection for each station
- Precise monitoring and measuring of absolute force at every station.
Identical tool dimensions at die- and punch side
Advantages:
- High flexibility with product geometry, process layout, tool design and standardisation
Die and punch block with hinged upper halves, which completely release the tool package after motorised opening
Advantages:
- The die and punch packages can be easily lifted and removed directly upwards – completely as well as individually - by means of an associated lifting device with quick couplings and a simple crane. Thus, the complex and expensive block exchange systems become redundant.
- A manual or hydraulic radial tool clamping is not required anymore. The axial tool fixation is accomplished by means of a small ring groove at the lower halve of the block. On the punch side, it changes its axial position with the axial adjustment of the punch. On the die side, with opening the upper half of the block the transfer module, which is mounted on it, is swivelled aside at the same time.
The request of tool developers for larger die diameters as prerequisite for favourable die pre-stressing conditions and a good tool life may not be fulfilled by machine manufacturers without negative influence on the size of the whole machine, and thus reducing the max. production speed and increasing the cost of the equipment.
In case of tool life problems due to insufficient radial pre-stress, special designs such as multiple stress rings or stripwound containers are applied. These are using the available case volume in terms of pre-stress better than a single stress ring (according to the equation of Lamé).
But also modern stripwound containers can not supply considerable improvements in case of unfavourable diameter ratios of Do/Di < 4. Due to the necessary outer casing and the inner winding core, not much volume remains for the winding strip which generates the pre-stress. If the stripwound container must also absorb high axial forces, accordingly constructive measures reduce the maximum winding strip volume additionally.
Therefore, we are just about to develop a solid one-piece die case, where a special manufacturing and treatment process generates a state of residual stress which is similar to the effect of a stripwound case (high tangential compressive residual stresses at the inner diameter of the ring, which turns into tangential tensile residual stresses towards the outer diameter).
The stress ring can bear a press fit interference of more than 1% without plastic settings even after multiple core exchanges and that at a ratio between outer diameter and inner diameter of the stress ring of just 2.

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