New Developments in Forging Technology

May 14, 2005
To help forgers meet the demands of automobile manufacturers, this equipment manufacturer has developed new equipment for pre-forming, automation of hammer forging, new drive concepts, and a new generation of control systems.

The customers’ demands for individual solutions are constantly increasing, especially from the automotive industry. Despite the platform strategies and modular designs of automobile manufacturers, forgers are faced with producing an increasing number of variants of aluminum forgings, increasingly complex parts, high steel costs, and sagging prices.

These developments also affect us as equipment suppliers. We have to offer different units in our product range to provide for more complex forging operations. There also is a need for increased use of suitable pre-forming units, which offer a potential for saving material, as well as producing a part with higher complexity.

Also, the number of automated lines will increase across all products. This requires forging of very different parts with a single forging line at a high degree of automation and with a competitive cost/performance ratio.

The following discussion covers new developments in pre-forming technology and automation of hammers, new drive concepts designed for reducing the contact time, and a new generation of control systems.

Pre-forming forgings
Many different processes are available from Muller Weingarten for pre-forming aluminum or steel forgings. They can be essentially subdivided according to two basic process variants:

Continuous Pre-forming — With the continuous pre-forming method, the forging is given a defined pre-shape in a single forming movement. Some of the traditionally used pre-forming units are hydraulic or mechanical presses as well as cross rolls. The continuous process offers the advantage, especially for aluminum, that the short process involves only little cooling for the component and high cycle times can be reached. A disadvantage is that the degree of forming is often limited in the pre-forming process, since only a limited amount of energy and a limited forming capability are available for the component within a single stroke (of the press) or a single revolution.

Discontinuous Pre-forming — The discontinuous pre-forming method is characterized by producing a pre-shape by means of several forming movements. The typical forming equipment used for this process is the reducer roll. However, it is also possible to use mechanical or hydraulic presses as discontinuous pre-forming units. This is usually the case if several pre-forming phases are necessary, e.g. upsetting and bending. The advantage is a high degree of forming in the pre-forming phase. A disadvantage is that it may be necessary to re-heat the workpiece, especially aluminum, before the main forging phase.

The various processes and their specific requirements for the pre-forming of aluminum forgings are described in more detail below in order to provide a deeper understanding of the different pre-forming technologies available.

Cross-roll forming

The cross-roll pre-forming method is characterized by a rotating movement of the workpiece between two rotating rolls (round dies) or vertically reciprocating plates (flat dies) rolling a contour into the workpiece with matched profiled tools. The specific tool contour produces a tapering of the workpiece, with the material being predominantly pressed either from the middle of the workpiece toward the ends or into a “non-forming zone” between the rolls.

Forming speeds for steel and aluminum can be 600 mm/sec. The resistance to flow of the workpiece material during the forming process makes it necessary to minimize any torsion of the workpiece during rolling or at least obtain a homogeneous distribution around the neutral line by suitable design of the tools.

Cross-rolls can only be used to produce rotationally symmetric pre-shapes. The degree of forming is limited because of the 360° rotary motion of the rolls, where only about 280° have a forming effect. An advantage is that the edges of the pre-formed forgings can be trimmed in the final phase of the process, which can prove beneficial for the further process with a view to constant material charges.

To obtain a reliable process for aluminum forgings, a new generation of cross-rolls was developed on the basis of the Bch design. Highest priority was given to controlled tool heating on a rotating tool holder by adding electrical heating cartridges to maintain the temperature of the tools within an acceptable range for reliable aluminum forming. The cartridges are designed so they can be automatically coupled during die changes.

The bearings and other susceptible parts of the machine must be protected against excessive heat input from the rolls. For this reason, an efficient cooling arrangement was developed for the roll bearings that prevents excessive transmission of heat from the tools to the machine bearings and the drive.

Furthermore, a quick-tool-change mechanism was developed that allows simply pulling the tools from the carrier rolls to minimize setup time, reducing downtime for the overall line.

Our clients also use our Type RBQ equipment for the production of ready-made shafts or other rotationally symmetric components with high precision requirements.

Reducer-roll forming

In the reducer-roll process, the workpiece is introduced between a rotating pair of rolls and led through them by rotation of the two rolls. In contrast with the cross-roll process, the workpiece is not rotated and therefore a suspected torsion of the pre-formed forging does not take place. Forming is usually performed in several passes with different profiles engraved on the rolls. The workpiece can be rotated from one step to the next in order to obtain a more homogeneous pre-forming result. Reducer rolls are not limited to producing rotationally symmetric pre-shapes. It is also possible to produce square symmetric shapes.

The new reducer-roll generation of the PWS series combines state-of-the-art control technology with long-term experience in mechanical engineering for forging lines. Pass selection and rotation angle are freely programmable. The feed height can be adjusted and the latest handling technologies permit fast, precise, and bidirectional reducer-roll processes, where the workpiece runs through the rolls completely in each pass and is received by the handling unit on the other side.

With regard to the application to aluminum, we were able to successfully transfer the experience gathered — with the heating technology for the tools and with cooling the machine body — from the developments in the cross-roll design described above.

The first reducer-roll delivered is additionally provided with a new spraying system and a quick-change feature for the rolls. Mller Weingarten is convinced that the reducer roll equipment will prove to be an efficient solution in the market for aluminum pre-forming just as is the cross-roll equipment. The use of a suitable reducer or cross-rolling unit is especially useful for travel-based main forging units, with a view to the constantly increasing complexity of aluminum forgings and because of the limited forming energy of these units.

Rolling pre-forming
Ever-increasing requirements for forging line process speed made it necessary to develop a low-cost alternative specifically for simple pre-shapes. The objective was to develop a technology that equals the forming and the material/structural requirements of the reducer-roll method while permitting high forming speeds at the same time.

The new process was to be suitable for integration into the press cycle of an automatic forging line, e.g. with 60 strokes/minute, and produce constant, flash-free pre-forming results.

Finally, many customers wanted a method with substantially lower investment and tooling costs compared to the reducer or cross-roll processes.

As a result of an analysis of the existing technologies, the rolling pre-forming method was conceived. Two rolls move in parallel along the workpiece, guided by two parallel contour blocks with matching profiles, displacing the material in the direction of motion. The material is upset in front of the rolls and flows in the opposite direction after rolling. This could be referred to as upset-stretch forming or extruding.

Since this is a single-axis forming process, there is very little risk of structural cracks, as opposed to the cross-roll process. The tooling costs (rolls and contour blocks) are relatively low. The maximum process speed is not determined by the automation or mechanical equipment of the unit, but depends exclusively on the maximum acceptable forming speed.

The rolling pre-forming method patented by Mller Weingarten AG is available as a standalone unit or integrated into a mechanical press and offers extremely competitive production of preformed parts.

Presses as pre-forming units

It is obviously desirable to integrate a simple pre-forming operation as the first step into the main press because of the advantages this offers for process and cycle times, production costs, and reduced cooling down of the forging. An upsetting or pre-bending operation can easily be imagined at this point.

Generally, hydraulic and mechanical presses are possible solutions for pre-forming units. The disadvantage of the travel-based mechanical press is the limited energy (energy capacity). Hydraulic presses have recently had a renaissance for some time because of their flexibility in use. The slogan “hydraulic means slow” is no longer true, which is evidenced by process speeds of up to 180 mm/min. Hence, it is not necessarily a disadvantage if a hydraulic pre-forming unit is installed before a mechanical main forging press.

The advantage is obvious. There are almost no restrictions with regard to the geometry (rotational or symmetrical) when designing the pre-forming operations. The number of pre-forming steps is only limited by the pressing force of the unit, the forming properties of the part and the specific cooling coefficient, which is an important factor in forging as mentioned above.

A multi-dimensional pre-forming process is possible, involving hydraulic or mechanical pusher tooling and subsequent bending. We consider this a modern pre-forming unit, the potential of which has been underrated in this field of application to date.

Introducing SpeedForge
In practical application of a Mller Weingarten forging crank press of type PK 3150 with a rated force of 31,500 kN and stroke rates from 30 to 60 strokes per minute, die service life increased decisively with the same parts when the stroke rate was doubled. In connection with the given sine kinematic characteristics, this is also the case when the die spraying time is reduced to the same degree. At a stroke rate of 30 per minute, a nominal spraying time of about 300 milliseconds remains for die protection; at 60 strokes/ minute, this is only 150 milliseconds.

In view of the constant pressure towards reducing the costs in the production of forgings, Mller Weingarten was urged again and again by customers and technical institutes to give more attention to the issue of reduced contact time when developing a new crank press, since there is still a potential for cost reduction inherent in closed-die forging. Furthermore, a die with less thermal stress would make it possible to produce forgings with fine structures, e.g. gear wheels, in large quantities with small tolerances and nearly ready for installation.

Many concepts were considered and assessed. The objective was to design a press with a rated force of 12,500 kN and a cycle rate of 60/minute with a very high ram speed in the bottom dead center (BDC) zone and a low ram speed in the top dead center (TDC) zone. The issue was to find a low-energy and low-wear solution while keeping the investment costs low.

Existing know-how made it possible to develop a drive concept referred to as SpeedForge, which is available as an option for every PK press. It costs only a little more than the standard drive concept and permits a freely programmable kinematic characteristic. With a cycle rate of 30 to 60 per minute, analog ram speeds at BDC of 60 to 120 strokes/minute can be freely set in combination with analog ram speeds at TDC of 6 - 12 strokes/minute.

Compared with a traditional eccentric drive, the contact time is less than half with the same cycle time. The machine’s clutch/brake assembly operates virtually without wear both in continuous mode and single-stroke mode. The press offers a kinematic characteristic with regard to the freedom of movement which is optimally suited for automation with transfer equipment.

Technological description: The press is provided with two drives. One auxiliary drive is for those sections of the kinematic characteristic where no forming operation is performed and the tried and tested PK main drive with planetary gear, high-speed flywheel and a hydraulic clutch/brake assembly with free-wheel position. The main drive is only engaged with the eccentric shaft for the actual forming operation.

The flywheel of the PK main drive has the necessary speed corresponding to the maximum ram speed at BDC. The main drive engages with the eccentric shaft just before the forming operation and disengages from the eccentric shaft after the forming operation. The ram movements before engaging and after disengaging are driven by the frequency-controlled auxiliary drive. In this phase, the clutch/brake assembly of the main drive is in free-wheel mode, i.e. neither engaged nor braked.

The auxiliary drive decelerates the ram during upstroke and accelerate the ram to the required speed during downstroke. The auxiliary drive, the eccentric shaft and the main drive have the same speed at the engaging/disengaging points, i.e. engaging/disengaging is fully synchronized, virtually without slip or wear. A kinematic characteristic as shown in the chart is thereby achieved. This can be freely programmed, depending on the cycle time required, forming travel, material flow characteristics, and transfer kinematic characteristics. To illustrate the improved kinematic characteristics, the chart shows the traditional eccentric kinematic characteristics as well.

The new SpeedForge drive concept makes it possible to use high cycle rates safely because of the improved freedom of movement for the transfer system. The energy taken from the flywheel is nearly limited to the actually required forming energy and the flywheel can be “recharged” very quickly in the disengaged phase.

Moreover, additional die spraying time is available for cooling and lubricating the dies. Of course, a customized kinematic characteristic for forging aluminum or other light metal materials can be programmed, which allows a slower forming process while maintaining a high cycle rate.

The press built with this design also includes an integrated low-energy closing mechanism for flash-free forging. It offers a closing force of 4,000 kN and implements a specially designed hydraulic concept, feeding the major part of its potential energy back into the main drive flywheel after the forming operation and before disengaging the main drive. The reduced power requirements make it possible to use a smaller drive, which is a direct benefit with regard to the investment costs for a PK press with SpeedForge.

New FCS generation

To meet market requirements, Mller Weingarten has developed a new control system generation. In addition to implementing a real-time core in the functional unit, it is now possible to integrate the system with various standards at low cost, e.g. Siemens S7 or Allen Bradley. It can be used for screw presses, hammers and mechanical presses alike.

The new concept includes thickness measurement of the forgings and the resulting energy regulation for the hydraulic short-stroke hammer as well as process control and measurement of cpk and cmk values during the forging operation. The outline curve technology for force characteristics familiar from sheet metal forming is now also available for all forging units.

This is supplemented by possible automation of hammers and screw presses, and optional extension of the system with production data acquisition functions or external modules.

There are three FCS configurations:

  • FCS Basis—Low-cost with small visualization module
  • FCS Stand Alone—Low-cost with large visualization module
  • FCS Customized—Customer-specific design with Siemens S7 or Allen Bradley module and large visualization module.

The basic functions of each unit comprise:

  • Energy measurement and control
  • Hammer speed, ram speed
  • Part thickness measurement
  • Program preselections (strokes, blow energy, die and part data, etc.)
  • Data memory for at least 100 parts/dies
  • Plain text fault display
  • All basic functions (e.g., rolling blows for hammers, production data acquisition with cpk and cmk data output for each forging, etc.)

A new user interface with a large visualization module was developed completely in a web-based design. In addition to a substantially improved remote maintenance capability via modem, this facilitates the preparation of on-line training documentation which can even be made available through the Internet. Both involve a substantial economic advantage for the customer during the implementation and commissioning of the unit or control system on his premises.

Mller Weingarten focused on the process analysis module. The performance capacity of the new MW FCS is illustrated with the following example of energy control for hammers.

With its data-scanning frequency of up to 600 kHz and MW’s sensing concept, the MW FCS is able to precisely determine the thickness of the forging with a tolerance of +/- 0.1 mm. With a stroke program of 5 strokes, for example, it is possible to control the thickness of the forging to match the target value. We discussed with our customers whether a blow-to-blow control would not erase the “traces” for assessing the process stability. The thickness of a forging and the energy actually input into the forging are closely related to each other in a successful process resulting in a part according specification. This relationship can be used to calculate coefficients from the stroke program characteristics at different energy levels, resulting in a fixed pair of coefficients in a stable process. These can be recorded as digital cmk or even cpk values and compared. After a certain number of strokes to be defined by the customer, the MW FCS calculates a correction stroke automatically. With varying coefficients, when the intervention limits to be defined are violated, this is referred to as an unstable process which can be determined only indirectly via the control variable or not at all in a blow-to-blow control design.

In the FCS, the customer decides which control method should be used.

The new control concept can be retrofitted to existing systems.

Economical forging with an automated hammer line

The forging hammer, especially with a hydraulic drive, is competitive thanks to its low capital cost and above-average energy and force capacity. Increasing demand led to an automated version with robots, especially for the production of flat parts such as connecting rods or hand tools.

The latest line delivered is equipped with a flexible gripper design that allows turning the forging during the forging process and prevents the gripper or robot from being damaged when parts stick to the upper die.

In combination with the FCS control concept, we developed a solution that is resistant to dirt, vibration, and elastic effects from the hammer. The reproducibility of this line is substantially improved in comparison with manual hammer forging.