New Dimension for Orbital Forming Technology

Press ReleaseHS 05

Combined Exploitation of Hot and Cold Forming Properties March 2006
Orbital Forming Presses from Heinrich Schmid Ltd. Produce Near-Net-Shape Differential Bevel Gears

New Dimension for Orbital Forming Technology

Switzerland’s Heinrich Schmid Ltd. has been developing and building orbital forming presses since 1980. The recently applied combination of orbitally cold forming hot forged preforms provides significant advantages over other more complex and expensive forging or machining processes.Moreover, substantially larger workpieces than hitherto can now be processed. Differential bevel gears for India’s commercial vehicle and tractor market are now being produced by this new combination of processes. Components previously manufactured by conventional machining methods were no longer price competitive in that market.

A number of well-known processes are used to make differential bevel gears for trucks and tractors. The new combination of hot forging and orbital cold forming is a cost-effective alternative to the machining of hot forged and/or preturned blanks, orprecision hot forging with secondary cold calibration. The processes not only differ in terms of complexity and cost, but in the quality of the finished product, particularly with regard to static fatigue resistance, bending resistance and tooth quality.

Traditionally, complex precision hot forging is used to achieve 98% tooth geometry in differential bevel gears.Following the hot forging stage, the final profile/contour is achieved by calibration on a corresponding press. Alternatively, a simple hot forged blank is machined by chip forming processes to obtain the final shape.Precision hot forging with subsequent calibration is expensive because of the die maintenance costs involved if an accurate perform is to be obtained.Investments in calibrating presses are high because inordinately high force is required to achieve an extremelylow degree of deformation. Machining preforms or billets is time consuming and the surface finish of gear teeth no longer meets today’s quality requirements. Moreover, the material costs can be as much as 70% higher than in the forging/forming processes. In the face of today’s price pressures, it is no longer possible to justify excessive material costs.

Minimal effort required to achieve net shape

By combining straightforward hot forging with subsequent orbital cold forming, it is possible to exploit the best features of two formerly competing processes, thereby achieving an overall more economic process and a superior final result. The bevel gear performs are still hot forged but, by using a much simplified forging sequence, are only brought to around 65% of the net shape. A single subsequent orbital cold forming operation converts the preforms to their final geometry. The gear teeth themselves achieve net shape and require no further machining or finishing.

Orbital forming technology is simple and reliable

The relatively unknown orbital forming technology is an incremental cold forming process in which the forming force is only applied to a partial surface of the workpiece. In competing extrusion processes, the friction between workpiece and die hinders the radial displacement of the material. Surface tension is highest in the center of the workpiece and decreases towards the unrestrained periphery.The greater the friction, the higher the maximum strain. In conventional extrusion processes, the maximum strain can become several times higher than the yield stress.

In orbital forming, the situation is different. Whereas the lower die thrusts the billet against the upper die, the upper die executes a circular tumbling motion at a given angle from the orbital axis. Depending on the application, the orbital angle is set between zero and two degrees. The workpiece material is literally “kneaded” into the die. As force is applied to a limited area of the workpiece only, the resultant friction is considerably lower than in conventional extrusion. Consequently, the material displaces radially (rolling-sliding friction rather than just sliding friction) without significant resistance. Theoccurring maximum stress only slightly exceeds the yield stress of the workpiece. Given the smaller contact area and the more favourable friction conditions, required forming force is at least ten times lower than conventional deformation. Thanks to the tumbling motion of the upper die, elevated degrees of deformation with absolute radiographic integrity can be achieved. The operational parameters and the machine design concept define orbital forming as a single stage forming process. One stroke of the ram executes one forming operation. Deformation, however, is achieved through several orbital cycles during that single stroke – hence, the term incremental forming. As die making is relatively economical, the orbital forming process is also profitable with small batch production.

Compared to extrusion processes, orbital forming offers the following benefits:

Greater degree of deformation in a single operation

Considerably smaller presses required

Reduced load on forming dies

More parts thanks to longer die life

Significantly lower generation of noise and vibration; no impact

Ready integration into productioncells

Fine module gear teeth with minimal tip radiuses

Thanks to the benefits of the tumbling upper die, it is possible to obtain relatively fine gear modules with small tip radiuses that exhibit excellent fill characterisitics. In the production of coupling discs, for example, R0.4 was achieved. This excellent result stems from the cyclic incremental deformation that occurs across the entire workpiece surface and the resultant low friction level between die and workpiece. Notwithstanding the minimal tooth tip radiuses, die life of several ten thousand pieces is achieved.

Non-impact orbital forming at room temperature allow chip forming secondary operations to be carried out in the immediate vicinity of the press, In the case of differential bevel gears, for example, downstream operations such as turning and broaching can be integrated into an end-to-end production line.

Superior quality at lower cost

In the production of satellite bevel gears for differential gearboxes destined for passenger cars, mechanical properties obtained through machining, precision forging and orbital forming were investigated in some depth. Compared to machining, the teeth of precision hot forged gears exhibited some 20 percent better resistance to static rupture and around 10 percent higher resistance to bending fatigue. Orbital forming produced a 30 percent improvement in static rupture resistance, while resistance to bending fatigue was as much as 40 percent higher than the machined variant. Tooth pitch accuracy to DIN3965 achieved an ISO class 7 tolerance quality for orbital forming, thereby exceeding the required IT 8. Precision hot forging is unable to improve on IT8, while the machined gears only achieve IT9. Compared to machining, the cost of orbitally formed bevel gears was 20 percent lower. Moreover, with a surface finish of Ra = 0.3, flank quality was vastly superior to the Ra = 2.0 that results from machining.

Orbital forming technology is eminently suitabe for components that are more or less rotationally symmetric such as flanges, coupling elements and, in particular for bevel gears. Used primarily by sub-contractors to the automotive industry, the potential range of application is significantly broader. Typical orbital forming applications are currently, for example, the reverse gear shaft and differential gearbox bevel gears produced by Visteon for Ford, steering racks for variable pitch steering systems made by ZF Steering Systems, ThyssenKrupp Presta SteerTec and Delphi. An orbital forming press recently installed in India is producing differential bevel gears for agricultural machinery and commercial vehicles.

Limitations of orbital forming technology

It is the height of positive contours on the upper face of the component that define the technology envelope. The contour height, in turn, is influenced by two factors. One of them is the „torque“ that acts on the workpiece being formed; the other is the die space available within the orbiting head. In order to obtain accurate parts, the contour of the upper die must be corrected to compensate for the orbital angle. The geometric differences between the die and workpiece surfaces do not allow a straightforward „rolling off“ the workpiece, but result rather in a roll and slide movement. Rollen und Gleiten. This generates a rotational effect on the workpiece and additional thrust on the die elements. The higher the contour profile, the greater the rotational action of the „torque“.

The other limiting factor is the height of the upper die because the orbital point is located in the center of the bell-shaped orbital head, in which the upper die is installed.

Competing processes now complementary

This new range of applications in conjunction with hot forging has created a complementary situation between two hitherto competing processes; with the end-user as the beneficiary. In addition to the obvious savings in cost, time and floorspace, the human resources required are also substantially lower than for machining processes. The major benefit, however, is the expansion of the spectrum of potential applications for the orbital forming technology. Nowadays, completely different workpieces can be considered.

((Box))
Orbital forming developed to market maturity
In addition to building orbital forming presses, the Swiss company, founded 1914, is also an internationally leading supplier of fineblanking press lines. Heinrich Schmid, son of the founder and after whom the company is named, was introduced to orbital forming by a Polish engineer in 1976. Since that time, the company has perfected the technology to market maturity level. Nowadays, Schmid builds and sells orbital forming presses in the 300 to 800 ton ram capacity range. Presskraft an. A typical Swiss SME, the company’s 65 highly-skilled specialists build presses that are exported across the globe. Thanks to the new range of potential applications, Heinrich Schmid Ltd. is confident that the recent growth in demand for orbital forming presses will continue for years to come.

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