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54ТЕХНИЧЕСКИЕ НАУКИ
DEVELOPMENT OF ROTARY FORGING MACHINES: FROM IDEA TO ADDITIVE TECHNOLOGIES
Aksenov L.B.
Peter the Great St.Petersburg Polytechnic University, Professor
Kunkin S.N.
Peter the Great St.Petersburg Polytechnic University, Associate Professor
ABSTRACT
Rotary forging is a relatively new manufacturing process with potential for cost-effective applications. The idea of rotary forging was appeared in the early 20-th century. Currently, machines for rotary forging are manufactured in all industrialized countries and used worldwide in industry. Technological capabilities of machines for rotary forging are determined by the kinematic schemes of these machines, which very differ. However, not all potential schemes machines, which defined by the motion of forming tool, are actually in using. Presented morphological analysis of rotary forging machines demonstrates the variety of those machines. It gives an opportunity to find optimal design for machine, based on the geometry of the manufactured parts that provides the greatest efficiency of production.
Keywords: rotary forging, orbital forming, kinematic, morphological analysis
Introduction
Large amounts of axisymmetric parts are used in various branches of industry. The nomenclatures of these parts are very diverse and are regulated by various standards. The production of these parts is carried out by various technologies, but they have a low utilization
rate of metal, especially in the manufacture of details as rings and flanges. Many technologies are based on the hot pressing with subsequent additional processing. Technology of rotary forging is designed for manufacturing of the axisymmetric parts from tube and rod workpieces (Fig. 1).
Fig.1. Raw material for rotary forging and rotary forged parts
This technology is representative the processes part of the workpiece (Fig. 2), which reduces the con-with the local deformation of the processed metal. It tact area, the magnitude of the contact stresses, and, ac-means that in contact with the deforming tool is only cordingly, the required forming force.
a. b.
Fig. 2. Contact area between forming tool and formed workpiece at rotary forging of rod (a) and hollow (b) parts
The first modern rotary forging machine was developed in the U.S. in 1918 by Edwin E. Slick [1]. In 1929, H.F. Massey, founder of B & S Massey, Ltd., designed and patented a vertically operated rotary forging press [2]. Currently, machines for rotary forging are manufactured in all industrialized countries and are used in many countries over the world. Wide review about development of rotary forging machine was done by R.Shivpuri [3]. This deep, detailed analysis was made 25 years ago and therefore does not cover more recent development of designs, technologies and simulation in this field.
In recent time the research of technological processes of rotary forging and their computer simulation have been significantly expanded. The aim of this work is to systematize the possible kinematic schemes machines for rotary forging and connection of kinematics
of these machines with their technological capacity. The importance of such analysis was defined in [4]. The subsequent development of rotary forging machines and technologies allows obtaining some new results. Three different types of rotary forging machines at St.Petersburg Polytechnic University (St.PPU) were used for long time to get experience in designs of kinematics and technologies [5].
The basic kinematic principle of rotary forging machine
The process of rotary forging is carried out at a relative rotation of the workpiece and the forming tool. All machines for rotary forging realize one or more motions defined by the Euler angles [6]: nutation, precession, and spin (Fig. 3).
Fig. 3. Family of rotary forging machines defined by their tool motions [6]
This leads to the theoretical possibility of the creation of the seven groups of machines for rotary forming: P, S, N, NP, NS, PS, NPS. Only two types of rotary forging machines have found wide industrial application. There are: precession machines (often called orbital presses or rocking die) and machines with spinning workpieces or forging tool. In machines for orbital forming (Fig. 4a) the workpiece does not rotate, and the tool performs relatively workpiece set of complex
movements, which are the sum of the rotational motions around the three axes (Fig. 4b). Wide range of movement of the tool on the end surface of the work-piece provides the opportunity to implement many types of technological processes (Fig.4c). In the past, these types of rotary forging machines were mostly used.
a. b. c.
Fig.4. Scheme of orbital press (a), types forming roll motions on face of workpice (b) and different technological operations which can be realized at orbital press (c)
However, orbital presses have a very complicated mechanical drive. At the same time the angle of inclination of the forming roll does not exceed 2-3 degrees, which makes it impossible to realize the benefits of these machines in the direction of reduction of technological force. Therefore, along with orbital presses, rotary forging machine appeared with simpler kinematics. So, are widely used machine with spinning
workpiece (or forming roll) which were called axial rotary forging machines. Scheme of orbital forming is presented in Fig. 5a, while in Fig. 5b scheme of axial rotary forging with a rotating workpiece and forging roll move progressively along the axis of the work-piece. The rotation of the workpiece is performed by the drive of the machine, while forging roll is driven by its own drive or by the workpiece due to friction [7, 8].
a. b.
Fig. 5. Schemes of orbital forming (a) and axial rotary forging (b).
Axial motion of forming rolls and workpiece metal deformation per one revolution of workpiece. along axis of blank rotation determines the degree of Forming carried out under the action of axial forces by
different ways: by motion only forming rolls, by the motion only of workpiece or synchronou rolls and workpiece towards each other. Th tures of axial rotary forging are as follows:
motion only of workpiece or synchronous motion of rotates around its axis and acts on the end face the rolls and workpiece towards each other. The main fea- workpiece;
upper die is tilted to the axis of the workpiece, it > around its axis and acts on the end face th iece;
workpiece has the drive and rotates on its axis.
a. b.
Рис.6. Schemes of axial rotary forging by conical (a) and cylindrical (b) rolls: 1 - die holder, 2 - die, 3 - workpiece, 4 - mandrel, 5 - forging roll, 6 - rotary forged part, 7 - cross roller.
Axial rotary forging machines can use conical roll (Fig. 6a) with angle of inclination 10-25 degree and cylindrical roll(s) with inclination 85-90 degree (Fig. 6b).
In practice, the rotary forging with conical rolls is more common technology compared to rotary forging by cylindrical roller, as it provides wider technological capabilities. However, conical rolls have more complicated geometry, and their dimensions are linked to the dimensions of formed parts. Diameters of cylindrical rolls in principle do not depend on the size of formed parts. So it is possible to use several cylindrical rolls simultaneously for forming. That creates a uniform technological load on the machine. Technologically cylindrical rolls are preferred for forming the outer surfaces and large parts. The schemes of machines for axial rotary forging with conical roll and cylindrical rolls are presented in Fig. 7.
The simplest type of rotary forging machine is the machine with the drive for blank rotation and passive rolls receiving the rotation from the blank due to friction forces on the contact surface. These machines were designed and manufactured at St.PTU. The machines working by such way have a number of advantages:
• the center of mass of the tool is not rotated about the vertical axis and, therefore, the forming process can be greatly intensified by increasing the speed of rotation of the blank and deformation of the blank per revolution;
• reduced demands to the stiffness of the frame and to the mass of foundation;
• reduced noise levels and also risk of damage for machine elements due to the reduction of low and high frequency vibrations.
a. b.
Fig. 7. Schemes of rotary forging machines with conical (a) and cylindrical (b) rolls: 1 - press frame; 2 - ram; 3 - rotary drive; 4 -bearing unit; 5 - ejector; 6 - displacement drive; 7 - inclination
drive; 8 - transverse roll.
Rotary forging machine consists of two main technological units - upper and low. The low unit contains the spindle with a drive for workpiece rotation, bearing units and pusher. The upper unit has the drive for transverse displacement of forming rolls relative of work-piece axis the drive for rolls rotation.
Drive for forming rolls is necessary part if mass and inertia of upper unit (including the spindle and bearing units) are large enough. In the case of rotary forging flanges from thin-walled tubes (for example.
with wall thickness S = 3 mm and less) at the initial moment of flanging-outward contact area between forming rolls and workpiece has a minimum value and friction force on the contact area is too small to form metal in right direction. This phenomenon can lead to negative consequences: the process of rotary forming becomes impossible for realization as the blank loses its stability and it is crumpled by forging roll due to lack sufficient friction forces for implementation of flang-ing-outward at the early stages of process (Fig. 8).
Fig. 8.Instability of tube-blank at axial rotary forging
Possibilities to keep under control metal flow at rotary forging are very limited. Different types of applicable tools (cross-rollers, forming rolls with shoulders, mandrels) can limit the metal flow in a certain directions, and after forming of some area of the workpiece to redirect the metal in the place, where the forming of workpiece is not yet ended [7-12]. Such technology requires considerable force, as the most volume of the metal in the final stage of forming presents a rigid body having a strain-state with three dimension compression.
More effectively to change metal flow direction to use changing the direction of the friction forces, which acts on the contact area between forming rolls and forming metal [13, 14]. It was determined that there is possibility to change the direction of the friction forces by displacement of cylindrical rollers relatively traditional layout. To receive the outer flanges it is required
that metal flow should be in outer direction of the blank. For this cylindrical rollers should be placed with some displacement 5 relative transverse axis of the work piece (Fig. 9a). In this case direction of friction forces will be in the required direction for rotary flanging-out-ward.
Magnitude and direction of friction forces on the contact area at the rotary forging by cylindrical rolls are determined by sliding speed of roll relatively to the surface of the workpiece. Along with slipping of deformed metal relative forming rolls caused by kinematic characteristics of rotary outward process, metal slips also along the contact area in the tangential and radial directions due to motion of forming metal. As a result at the contact area components of the velocity are imposed on each other and, depending on its sign, will be added or subtracted.
a. b.
Fig. 9.The rotary outward-flanging by cylindrical rolls (a) and conical roll (b) with displaced axes
The offset of the rolls leads to an increase in velocity component of movement of the metal which is directed from the centre of the workpiece. This phenomenon is difficult to explore on the basis of the simulation process, as the programme shows in this case the complete velocity vector, in which the main part is the part caused by the rotation of the workpiece [10].
It should be noted that the tangential kinematic slip is few times greater, than slip caused by a deformation of blank in the tangential direction. Thus, there are two zones with different conditions of friction on contact area at cold axial rotary flanging by cylindrical rolls:
• the zone of an advancing rolls is located on the flange on the outside of the diameter of the engagement. Forming metal slides on this part of contact area in direction of rotation of forming roll due to the fact that workpiece moves faster than the rolls on this part of contact area;
• the zone of lag is located on the inner side of the diameter of engagement. In this zone a metal slides along contact area and it is determined by the lag of the workpiece from the forming rolls.
The ability to control the forces of friction on the contact surface expands the technological possibilities of a rotary forging, helps to reduce the value of contact stresses and positive influences on process of metal flow and the forming of flanges. Described technology of axial rotary forging with displaced rolls and rational modes of feed provides steady process of flanging and rotary forging of thick flanges. The flanging process by cylindrical rolls depends on such an important parameter as relation the wall thickness of the tube-blank against to its diameter So/Dy (Fig.9a), and can be recommended for So/Dy < 0.04. Similarly, for operation of the flanging by conical roll, it is recommended to set the peak of the conical roll not on the axis of symmetry of the workpiece but with an offset of 5 = (0.1 - 0.3)R (Fig. 9b).
Rational choice of technological parameters of rotary forging and first of all, the displacement of rolls and feed allows avoiding losses of stability at rotary forging and sticking of metal on the rolls. This allows, using simple rotary forging machines with passive motion of forming rolls, to form the wide flanges with thickness equals to initial wall thickness of tube-blanks (Fig. 10) and also thick flanges [13, 14].
Periods of development of machines and technologies for rotary forging
It is possible to identify five periods in the development of machines and technologies for rolling (Fig. 8):
• the initial period (1-st), characterized by patenting ideas and the creation of the first machines that implement 3 kinematic schemes (orbital forming machines);
• the commercial production (2-nd) of orbital forming machines and determination of their technological capabilities mainly for hot rolling;
• the development of new types of rotary forging machines (3-rd), including machines with simple kinematics and rotating workpiece (axial rotary forging machines), the development of the processes of cold rotary forging;
• the period of expansion (4-th) of technological capabilities of rotary forging with the cylindrical rolls [5, 8], the application of rotary forging for powder materials, assembly procedures, use of variable inclination of the forging rolls during forming [11], the use of an ejector as an additional tool and another; computer simulation of various rotary forging processes [9, 12, 15].
• the present period (5-th) saw the creation of multifunctional machines capable of implementing various kinematics via the full individual CNC control [16]. These machines are primarily intended for the processing of composites and other exotic materials, and also materials used in the aerospace industry.
1-st Period: 2-nd Period: 3-nd Period: 4-th Period: S-th Period:
patenting commercial development of expansion Multi kinematics
and first machines production of new types of of technological machines with
orbital forming rotary forging capabilities of CNC
machines machines rotary forging
-►
1916- 193X 193X-195X 195X- 198X 198X-200X
Fig. 10. Periods of development of rotary forging machines
200X - Present time
Morphological analysis of kinematic for rotary forging machines
It is possible to collect all known machines for rotary forging in one multidimensional morphological matrix (Table 1). The main independent parameters of the morphological analysis are: X1 - the type of the forging roll, X2 - types of drive for workpiece and forging roll(s), X3 - type of forging roll(s) motion (kinematics of machine), X4 -the temperature mode of rotary
forging, X5 - the ability of displacement of the forging rolls relative to the axis of the workpiece is applied for technology of outward-flanging, X6 - the possibilities to use ejector as a part of forming process, X7 - axial translational movement of workpiece or forging roll. The table gives information about the number of potential machines for rotary forging. Formally, can be synthesized 1008 different types of machines, but really not more than two dozen really are used in industry.
Only four types are realized in these machines of seven possible (marked in grey in the table 1).
Table 1
Morphological matrix of rotaiy forging machines
The traditional method of design technology is usually determined by the existing machine for rotary forging and geometry of the part, which must be made. This method is not always optimal, as there are just a few opportunities to change the parameters of the machine and process optimization because of the technology of rotary forging and type of tool is very limited by the parameters of the machine.
For industry, the main goal is production its own parts and, therefore, companies are interested in the purchase of machines, most adapted for this production and are not interested in the acquisition of universal machines, which are quite expensive, and may not be optimal for a specific production. It is therefore wise to start choosing the type of machine with an analysis of the geometry of the part, intended for the production method of rotary forging. Then make the choice of technological design and only then, using morphological analysis, go to the synthesis of kinematic scheme of the machine.
So, as example, for manufacturing of flanges from thin-walled tubes with technology outward-flanging the preferred machine is 2-1-3-1-1-2-1 (see the table 1). Design in the sequence "from part to technology, from technology to the tool and then from tool to the machine" was successfully implemented for realization of new technologies [17] and in the industry in some cases of the mass production of the same and similar types of parts [18]. The modern rotary forging machines with changeable kinematics and CNC control open new perspective for additive rotary forging technologies.
Resume
• Opportunities to create new, efficient machines for rotary forging not used completely. Only four types of machines have been used out of a possible seven, determined by kinematic of the forming tool.
• Morphological analysis of rotary forging machine allows expanding the understanding of the object
of research, which leads to a search for an optimal solution to the extended set of possible variants. Shown that it is essentially possible to create several hundred different machines for rotary forging.
• The morphological analysis creates the conditions for an optimal design of machines for different groups of manufactured parts and that is why increases the efficiency of technological processes.
References
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