Planning and simulation of the ring rolling process for improved productivity

Int. J. Mach.ToolsManufact. Vol.33, No. 2. pp. 153-173. 1993. Printed in Great Britain

0890-6955/9356.00 + .00 © 1993PergamonPressLtd

PLANNING AND SIMULATION OF THE RING ROLLING PROCESS FOR IMPROVED PRODUCTIVITY RAJIV SHIVPURIt a n d ERDEN ERUq:~

(Received 1 February 1991; in final form 1 November 1991) Abstraet--Rolling of rectangular rings often leads to flow related defects such as fish tail or dog bone. This paper presents a computer aided program, ERCRNGROL, for the planning and simulation of the rectangular ring rolling on a ractial-axial mill to achieve a defect free ring. Given the final dimensions of the ring, the process planning section of this program allows for the determination of the dimensions for the rolled ring, the ring blank and the cut billet. Based on these dimensions, the "simulation" section of this program calculates the values of the process parameters including rolling forces and torques, wall thickness and height of the ring, and ring: temperature as a function of rolling time. To compare the predictions of this program with the production results, a ring rolling mill was instrumented and production data recorded for 61 rectangular rings with varying dimensions. These comparisons indicate that when the rolling schedules of the production mill diffe,r considerably from those of the simulations, flow related defects start developing and the operator has to make large corrective actions to avoid rolling a defective ring. Therefore, this computer aided program will be of great value for ring mill operators in process planting, and in the determination of desired rolling schectules for reduced rejection rates.

INTRODUCTION

RING rolling is a forming process used to produce seamless rings such as bearing races, and aeroengine rings and fairings. A concise summary of ring rolling technology has been provided in previous papers by the authors [1,2]. A typical ring rolling machine has two sets of rolls; the radial set reduces the radial thickness of the ring, whereas the axial set controls the height of the ring. Due to the reductions in the ring cross-sectional area, the diameter of the ring grows during the process. Once the final diameter is reached, the rolling process stops. A schematic illustration of rolling with a radial-axial machine is given in Fig. 1. The feed rate and the rotational speeds of the radial and axial rolls primarily affect the deformation rate in the roll gaps. 'The drive specifications under no-load conditions and the rolling force, torque and power required by the process, influence these feed rates. Workpiece handling and downtimes, associated with tool replacement due to surface wear or a new set-up, directly affect the productivity of the operation. During the rolling operation, the machine must supply the maximum rolling force, torque and power required by the process. The efficient and economic use of existing ring rolling processes or planning of new investments for production of seamless annular parts require a thorough knowledge of deformation characteristics and capacities. In addition, deformation patterns in a given cross-section, their relation to material type, and pertinent process parameters and their relationships are also critical to the prediction of surface defects and residual stresses that lead to greater yield loss and ring instability problems. As can be observed in Fig. 2, even for a relatively simple geometry like a weld neck flange, the forming paths are not unique. Often, the optimum path is a desired goal. In addition, the ring rolling machine can be part of a production line that may also include forging, piercing and rotary forging units. Consequently, for the maximum output, the enti:re production line may have to be considered. To optimize process parameters, the operator must be provided with more information than a forming path tDepartment of Industrial and Systems Engineering, The Ohio State University, 1971 Nell Avenue, Columbus, OH 43210, U.S.A. and Author to whom correspondence should be addressed. :[:Department of Engineering Mechanics, The Ohio State University, 1971 Nell Avenue, Columbus, OH 43210, U.S.A. HTH 33:2-D 153

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"that works". Therefore, in order to produce rolled rings to closer dimensional tolerances and better mechanical properties, the entire schedule of a ring rolling process must be appropriately planned. The motivations behind such planning are numerous but optimization of production time, material and energy input and the prevention of defects are the major goals. A ring rolling process should be able to produce rings within tolerances, and without defects. However, this becomes difficult due to variations in the operation. Some of the variabilities that the machine controls have to compensate include: (a) temperature and flow rate dependent properties of the workpiece materials; (b) eccentricities and non-uniform properties of the incoming ring blank; and (c) different initial temperature of blanks depending on their relative positions in the furnace.

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Improved Ring Rolling Productivity

155

The use of computers in ring rolling has provided the manufacturers with flexible machines that are adaptable to various rolling strategies and production philosophies. Continuous batc]aes requiring dedicated machines or smaller batch sizes can both be handled better with the help of computers. Small batches benefit from the memory features, for a rolling schedule which worked earlier can be retrieved from the data banks, whereas larger batches are rolled to the same size repeatedly with minimal operator assistance. The flexibility of such a system is apparent when various materials and different ring geometries constantly require modification of the rolling strategies in order to obtain defect free rings. Computers also assist in the process planning and optimization functions. The basis of optimization is to minimize a chosen parameter such as rolling time, which is achieved when various strategies are compared. To decide on a particular rolling strategy as a reference and to maintain it over a number of rings demands the repeatability inherent in the numerical controls. By using transducers linked to the computer, monitoring of the ring dimensions becomes easier and quicker without any operator inputs. However, the operator can override a routine that the computer may have initiated if he deems it necessary [3,411. The researchers at the Engineering Research Center for Net Shape Manufacturing at the Ohio State University have been involved with the development of a computer program for the ring rolling process so that (a) blank dimensions and weight can be calculated, (b) defects can be avoided and (c) an optimum rolling strategy can be established. The primary objective of the research presented in this paper has been to increase process productivity by planning and simulation techniques. Initially a survey of state-of-the-art in ring rolling machines and processes was conducted [5] and then a ring rolling software E R C R N G R O L was developed [6]. This paper presents details of this software and results of the experimental trials conducted to verify the predictions of E R C R N G R O L . PAST WORK IN PLANNING AND CONTROL

Blank preparation Due to the constancy of workpiece volume in plastic forming, the final cross-section of the ring is determined by the choice of blank dimensions and the deformation sequence imposed by the ring rolling machine. The blank can be intelligently shaped to improve profile filling and to reduce defect formation as well. Therefore, blank sizing and preshaping constitute an important part of process planning. The blanks for the ring rolling process are usually prepared from round bar stock in four steps: cropping; upsetting; piercing; and punching, as illustrated in Fig. 3. Reasons for the variation in blank volume are discussed in detail in Ref. [5]. Recently, a ring rolling process line was designed which maintains constant blank weight by adjusting the web thickness during the piercing stage; the idea is "to punch more volume from a heavy block (billet), or to punch less volume from a light block" [7]. By placing a hot billet weighing machine between the descaler press and the blanking press, the incoming billet weight is measured. The measured billet weight helps determine the volume to be punched to maintain constant blank weight throughout the entire batch. Most of the reported work on preforming blanks is experimental, and relies heavily on experience and trial-and-error approaches [8]. Recently, preform design for rectangular profiles, by backward tracing of the rolling simulation, has been attempted at the Technical University of Aachen, Germany [9,10]. Computers in process control Production control schemes implemented on the ring rolling machines are of the four types: force cont~rol schemes; feed rate control schemes; diameter growth rate control schemes; and schemes utilizing real time process data. The following discussion on these various schemes focuses primarily on the radial-axial machines.

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In the force control schemes, the forces applied on the radial and axial rolls are controlled during the process. A schematic plot of radial force as a function of time is shown in Fig. 4. From this figure, it can be seen that in Phase 1, the mandrel is driven into the ring at maximum speed until the rolling force reaches its maximum value. The control system maintains this condition for most of the rolling cycle in Phase 2. In Phase 3, just before attaining the final diameter, the rolling force is steadily reduced to a small fraction of its maximum value. Finally, Phase 4 is started in which a few revolutions are carried with minimal force to remove any non-circularities. Should the diameter growth rates or the wall reductions become excessive during the process, the operator can intervene in order to avoid ring instability. Noda et al. [7] have further improved this system by including computer controls in monitoring the rolling forces. They have introduced two methods, one is the "step control" method which changes the rolling force only at preset points of the outer diameter and the other is the "DDC (direct digital control)" in which the program changes the rolling force at certain intervals according to a predetermined rolling strategy. Flow defects such as fishtailing can be avoided by controlling the deformation in the radial and axial gaps. For rectangular profiles with high aspect ratios, Vieregge [11] suggests that fish-tailing and other defect formations can be avoided when the ratio of radial to axial feed per revolution is kept equal to the ratio of axial to radial dimensions, or if the relation Ah Ab

b h

is followed throughout the process. Hence, primary reduction is achieved across the smaller of the section dimensions. Along with constancy of volume condition, this relation allows for process monitoring and blank design with growth effective results. Further details of this approach are provided later in the ERCRNGROL program description. Koppers [12] found that the cavities were formed when radial and axial reductions were not coordinated with each other. His experimental studies suggest that the roll gap ratios Cr (radial) and ca (axial), defined as the ratios of average instantaneous contact length to average material thickness between the rolls, are the major influencing factors in defect formation. Koppers suggests that, in order to roll defect free rings,

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the cr value must be larger than 0.35 if the height is kept constant, and similarly ca must be larger than 0.35 if the wall thickness is kept constant. It is not as essential to attain specific c values in radial/axial ring rolling, because defects can be avoided in the cross-section through simultaneous control of radial and axial deformations, if the reductions in both gaps are balanced. The controls that help the rolls maintain a desired feed rate profile have also been studied in detail. Yun and Cho [13] recommended a rolling strategy in four sequential stages to ensure closer dimensional tolerances and better mechanical properties in the rings: (1) feed rate control of the mandrel motion; (2) growth rate control of the ring diameter; (3) feed rate control of the mandrel motion; and (4) position control of mandrel motion. They suggested optimal and suboptimal control systems for determining the rolling force necessary for this purpose [13,14]. This control system automatically adapts to the rolling force variations that may arise due to temperature related flow stress variations. Choi and Cho later applied their control systems to ring rolling machines, which were programmed to track a desired outer diameter growth rate profile [15], as follows. During the process, the parameter that is often measured directly by the tracer roll is the outer diameter. This makes controlling the final dimensions easier as compared to, for example, controlling the mandrel advance or the rolling force which are very much dependent on the material resistance. For example, the surface conditions and flow stress of the ring, which critically influence the deformed state, are very sensitive to a variation in temperature. This affects the rolling force, thus altering the mandrel motion. Stability problems arise when the force is reduced below the geometry dependent critical rolling force, at a point when better control over the dimensions is desired. In addition, the ring diameter would continue to grow if the mandrel force is larger than the critical. Therefore, it is more advantageous to choose diameter growth as the key control parameter. Some diameter growth rate control schemes follow a preset rolling schedule [16-18]. Machine limits determine whether such a reduction curve can be followed and if the machine limits are exceeded, the process is slowed down to an acceptable range while still following the reduction curve. Doege and Abotour [19] have also developed a process control program which determines the appropriate radial and axial feed rates while maintaining the required outside diameter growth rate. A suitable ratio of the feed rates is chosen at every moment and the resulting deformations are determined by using an upper bound slab analysis technique. The scheme of using the measured process data as a feedback to the process control system was largely exploited by Koppers et al. [10,12]. In their routines, the data measured during the process provides the feedback to update constitutive models, which are then stored for future reference. This production control scheme suggests a process schedule similar to the earlier diameter growth rate scheme, but it is not limited to the diameter growth rate as the control criterion. A predetermined schedule may not be the fastest, although it may work. In this case, the philosophy is process optimization and rolling in the fastest possible manner. The rolling variables are

158

R. SHIVPURIand E. ERU(

constantly monitored and compared with the machine limits and the fastest possible schedule without causing defects is determined during the rolling process. Yang has developed a computer aided manufacturing system in which real time data are continually compared with calculated values [20]. The observed force and torque calculations are based on approximate results of force polygon diagrams, which are conveniently adaptable to a microcomputer because of the small computation time involved. PROCESS PLANNING AND SIMULATION SOFTWARE: ERCRNGROL

E R C R N G R O L is an interactive computer software that allows a user to determine size and weight of the rough ring and blank from finish ring dimensions, as well as simulating the rolling of a ring under various operating conditions. Input data on ring dimensions, material grade and quantity of rings to be produced are entered into the program. The results can be saved on a disk file for future recall, and plotted to provide a hard copy scaled picture of the finished ring and the starting blank. The results of the simulation (time histories of important system parameters) can be plotted to obtain a visual picture of the process before production of rings, and to compare with the production data after completion of the process. Some 6f the mathematics of the simulation algorithm are based on the work of Dr Vieregge of Wagner-Dortmund, Germany [11]. The initial Battelle version of the program was corrected for programming errors and implemented on a VAX-based operating system. In addition, this program was modified so that its predictions match production data better. PROGRAM DESCRIPTION: PROCESS PLANNING

An engineer in a production ring rolling plant basically performs the following steps for process planning prior to actual rolling: (i) obtains finished ring data from the customer (ring dimensions, material and quantity); (ii) adds rolling envelope, scale and shrinkage allowances, and tolerances (rough ring sizing) to the finished ring dimensions to obtain rolled ring geometry; (iii) from the rolled ring geometry, calculates the blank (donut) geometry; (iv) from the blank geometry, calculates billet dimensions and weight, including losses due to descaling, sawing and punch out; (v) selects suitable production sequences and machine settings (rolling strategy); and (vi) prepares shop orders. As shown in Fig. 5, the E R C R N G R O L algorithm is constructed to assist the process engineer in ring rolling plant, in the process planning and the machine setting stages. The program calculates rolled ring dimensions, blank dimensions and billet dimensions. In all these cases, the values are calculated by three methods: the calculation method; the table-driven method; and the manual method. The latter two methods require prior skill and data base and will not be covered in this paper. The following discussion presents only the calculation method.

Calculation of rolled ring size The calculation method for rough sizing is based on DIN 7527 (German Standards). The tolerance allowance (Z) for each of the ring surfaces is expressed as: Z = C1 + (?2 (finish o.d.) + C3 (max of height or thickness) where C1 = 0.100 in (allowance for unavoidable defects), C2 = 0.004 in (growth factor for ring diameter) and C3 = 0.01 in (growth factor for ring height or wall thickness, whichever is larger).

Improved Ring Rolling Productivity

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159

Machinedring dimension$~~N~ Materialgrade / / Quantity J

• Rolledring dimensions(cold) • Blankdimensions(hot) • Billet dimensions/wt.(cold)

• Forces/torque/dim.time history • Optimumrolling strategy • Rollingtime

FIG. 5. Flow diagram for ERCRNGROL.

The rough dimensions are obtained from finish dimensions by adding the rough allowance to the three finish dimensions. The rough ring size calculated is checked for the size and weight limitations of the mill system specified. If not, the engineer must specify a different mill system. Calculation of blank size The first step, in finding the blank dimensions is to find the punch size required by using the blank sizing table. The first entries in this table are the height to be added to the rough height as a function of the mill system. Each mill system has a fixed value that is added to the rough height to give the blank height. Total reduction in height and blank weiglat determines the punch size to be used. The punch size fixes the blank internal diameter (i.d.).

160

R. SHIVPURIand E. ERug

The blank geometry is calculated from the relationship as follows [11]. Given the final ring dimensions, one can trace back to the blank dimensions using the relation in equation (1). Such reduction schedules lead to the hyperbolic curves illustrated in Fig. 6 for different sectional dimensions. In the following discussion, subscripts i and o denote inner and outer diameters, respectively, and the subscripts 1 and 2 stand for blank and the finished ring dimensions.

hi

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(b2t 2 (bit2

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o f billet size

The "cut weight" of the cold billet is calculated assuming losses due to the hole punch-out and the formation of scale in the furnace heating as follows: Cut Weight = Blank Weight + Weight of the punch-out + Scaling Factor * Blank Weight.

PROGRAM DESCRIPTION: PROCESS SIMULATION Once the dimensions of the billet, blank and ring have been calculated, the process simulation begins. The main objective of the "simulation" section of the program is to calculate time histories of key process parameters such as reduction in wall thickness, reduction of height, ring temperature, radial force and torque, and axial force and torque. These time histories not only help in the determination of rolling schedules but hR/bR:

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Improved Ring Rolling Productivity

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serve to check if the maximum force and torque capacities of the rolling mill have been exceeded. Therefore in this section of the program the process is matched to the rolling equipment. In this moduJe, the ring rolling process is simulated by dividing it into small time steps (increments) based on the data generated in the previous parts of the program. The entire rolling schedule consists of four major stages: Stage Stage Stage Stage

0: 1: 2: 3:

start of simulation--application of radial force; radial rolling only; radial and axial rolling; radial and axial rolling to final size, with reduced loads.

The ring rolling simulation begins with the initialization of the required variables in the simulation program. Proper material values are obtained from the material data files, and the flow stress values calculated from strain rate and temperature values expected durinb~ rolling. Other important input data to this section are the equipment data, customer order data and manufacturing cost data. The modular construction of the E R C R N G R O L program is shown in Fig. 7. The ring sizing and the ring simulation sections are included in the simulation module of the central block.

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Simulation and cost estimation

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An important objective of the simulation program is to optimize the rolling strategy. The primary goal of the rolling strategy is to maximize the diameter growth rate through cross-sectional reductions without introducing surface defects and ring growth instabilities. Consequently, the deformations imposed in the radial and axial passes are controlled while keeping in the diameter growth rate as high as possible. Given the constant volume of the ring material, this incremental relationship results in a functional expression between ring height and width. Although equation (1) is effective in blank design, a rolling schedule based on it requires very large reductions at the beginning of the process. Hence, a constant ratio of reduction is adopted which gives the relation: d2o + d2i _ blhl b2h~"

(3)

2"dLi + b l

It is suggested that the constant C should be larger for large initial ring wall thickness to roll-ring contact length ratio, therefore allowing the ring to reach its final height before the final wall thickness is reached. This adjustment for the constant value reduces the demand on the process control system in the final phase of rolling and allows for a softer unloading of the ring cross-section "soft landing". Typical rolling curves (schedules) derived from the above relationships for washertype and sleeve-type geometries are shown in Fig. 8. For square blanks rolled to square rings this results in a linear rolling curve. In practical ring rolling, rolling curves other than above are often used based on the final blank dimensions and the starting stock geometry. These composite rolling curves consist of linear and hyperbolic sections and their choice is based on past experience. This program at present does not include composite rolling curves for their selection is entirely based on the rolling practices of a particular rolling plant and is not generic enough. In E R C R N G R O L , the simulation allows for a minimum of three revolutions of the ring with only radial rolls engaged to round the blank and enable the ring to be totally under the axial rolls prior to the start of axial rolling, as illustrated in Phase 2 of Fig. 9. Because there is only radial reduction in this phase, the reduction ratio is set to zero. During radial/axial rolling, however, a smooth transition is achieved by incrementing the reduction ratio by the increments of 0.2 until it equals: Ah _ h - h 2 Ab b-b 2

(4) "

The critical nature of starting blank size and weight is underscored by the fact that theoretically there is only one starting blank configuration.

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The rolling forces and the torques are calculated by simple formulae using the projected roll-ring contact area and the roll pressures determined by the slab analysis. Ring temperarare calculations include radiative, the heat losses to the atmosphere and the heat generation due to plastic deformation, but ignore the small convective losses to the atmosphere and the contact heat losses to the rolls. This is reasonable for the ring rolling process where the majority of the ring surface is open to the atmosphere and the ring enters the deformation zone repeatedly. The E R C R N G R O L software can provide plots of simulation variables vs time, or vs o.d. Fig. 10 shows a sample of simulation plots for the process variables: ring wall thickness; ring height; ring temperature; radial torque; radial force; axial torque; and axial force. PRODUCTION EXPERIMENTS

In the summer of 1988, experimental runs were conducted at an industrial plant that uses Thyssen-Wagner's R A W 160/120 rolling machine [5]. This force controlled

164

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machine was not programmable. On this machine neither the rolling strategy nor the reduction schedule are predetermined. It is built with intrinsic controls that provide the rev/min adjustments of the axial rolls (Fig. 11). These limited controls also decide on the radial motion of the axial unit as illustrated in Fig. 9. All the reductions and centering arm motions are consequences of the applied rolling forces and the centering force. The torques required are also dependent variables. This machine requires a highly skilled operator who would roll the rings based on previous experience, and his skills and intuition to keep the rings defect free. Usually, the operator chooses to roll at this machine's maximum axial or radial force capacity. The centering force is adjusted based on ring behavior. If the ring grows too fast, there exists the danger of overrolling past the final dimensions of the ring. Therefore, rolling forces are reduced towards the end to ease manual sizing. If the ring is allowed to grow larger than the

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Improved Ring Rolling Productivity

165

final o.d. then it would have to be scrapped, whereas an under-rolled ring can always be rerolled. The billets used in the tests were cut from blooms with rectangular or circular crosssections of various sizes. Sawing was followed by heating and blanking. The loading and unloading between stations, and positioning of the billet while it was being formed in the press was maintained by fork-lifts. The time needed to thoroughly soak the billets in the furnace is dependent on the size of the billet and the furnace. Usually, the billets are loaded the day before, in the late afternoon. The furnaces are turned on early in the morning to give enough heating time, before blanking. If the billets are not soaked at the time of blanking, they are put back in the furnace for reheating before rolling. When they are hot enough, the process is pu~Lch-and-go, i.e. rolling immediately follows piercing. Punch diameter selection and blank dimensions are determined as described above. Some of the other considerations in blank preparation are given below. (a) If a square stock is used it has to be rounded on a press. A typical press sequence for a billet of 16S square cross-section is illustrated in Fig. 12(a). (b) The punching of the hole is achieved by using a standard indent punch attached to the press. When different punch sizes are required, instead of disabling the press by changing the indent punch, a cork-shaped punch is put in place by the forklift operator

(a)

(1)

(3)

(2)

< >''r" ( ) ' i'" R o t a t e 45 °

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(b)

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Colder on the bottom

I

(c)

FIG. 12. (a) Press sequence for blanking from a rectangular cross section. (b) Blank shape variation based on the temperature distribution. (c) Typical blank shape for a case of short stock.

166

R. SHIVPURIand E. ERU~:

and the press is brought down to bury the punch into the blank. Then the blank has to be turned upside down to shake the punch off. (c) When larger diameter stock is unavailable, billets are cut longer to provide the required blank volume. Such tall billets may buckle plastically in upsetting. If the blank is not symmetric or if the material distribution is not as desired, it may be necessary to "roll on the punch" which entails squaring the edges of the billet while the punch is still inside. Again, the punch needs to be dropped upon rounding. This procedure allows the operator to improve on corner filling while rolling. (d) If the billets are not thoroughly soaked and may be hotter on top, the upsetting will cause the billet to expand easier near the upper die (Fig. 12(b)) in a non-symmetric fashion. The subsequent punching will drive the material out on the bottom resulting in a blank shape fairly symmetric in height. Reheating may be required. (e) As a general rule of thumb, the operator wants the flat surface on top, after squaring, to be as wide as the final wall thickness. Otherwise, the corners may not fill and rolling on the punch may be necessary to distribute the material more evenly. The billet height is reduced to within one inch of the specified blank height in the initial upsetting. The decided blank height is imposed while squaring before piercing. (f) When the proposed stock size is changed in favor of a larger cross-section, the cut lengths accordingly have to be reduced to have the required volume. Shorter billets cannot be upset as severely in the press. The ensuing punch motion creates a nonsymmetric cross-section as in Fig. 12(c) with a larger diameter at the bottom than on the top. Such a blank shape will cause the blank to climb upon radial rolling and damage to the tracer roll. To avoid such a potentially costly mistake, the ring can be turned upside down before rolling to avoid its climbing or it may be rolled on the punch. (g) In case the web is too thin, or it is too hot, or if the punch or rod comes down off-center leaving more clearance on one side, the web hinges on one edge and does not separate from the ring; then, there is the danger of smearing the web between the die and piercing rod, locking it in place. In these cases, the blank has to be removed and the web must be cleared by a torch. SIMULATIONS AND DISCUSSIONS OF RESULTS

Data collected at the production plant during the test runs included 61 rings of which nine rings with complete data were chosen for simulation. However, in this paper, results from four typical rings (two sleeve type and two disk type) are presented: CN 7242, CN 7375, CN 7346 and CN 7457. They were chosen because they represent samples of good cases, CN 7242 and CN 7346, as well as bad cases, CN 7375 and CN 7457. Data on the other rings can be found in Ref. [5]. The simulations on ERCRNGROL were run strictly based on the dimensions given in the order forms. It can be seen in the plots for reduction schedules, height vs time and wall thickness vs time in Figs 13-16, that the production data usually have different starting points as compared to simulations. When more material is cut than the specified billet volume, the operator evenly distributes the extra material around the required cross-section, leading to a different blank cross-'section than that suggested. Other reasons for such variation were described earlier. This is apparent in reduction schedule CN 7346. Also, use of punches and mandrels other than specified, changes the proposed blank dimensions and rolling forces. The operator may choose to use different punch and mandrel if the batch size does not justify changing the earlier set-up. The fundamental difference between the data collected and the simulations, apart from deviation from the order forms, is the rolling strategies followed. During simulation, E R C R N G R O L enforces a hyperbolic reduction schedule that depends on the initial dimensions of the ring. In production, the operator gets suggested blank dimensions based on a similar criterion. However, hyperbolic relationships are not identically imposed during rolling on the force controlled RAW 160/120 and the reduction schedule followed depends on the operator's skill. He continuously monitors the process cor-

Improved Ring Rolling Productivity

167

recting for the input variations in blank parameters. These may include variations in stock size based on availability, blank temperature, room temperature and mandrel size. Although the minimization of rolling time is important, the overall production time for the ring includes retooling, heating and loading times as well as forming. Hence, the operator may choose not to use the suggested mandrel size if the batch size does not justify the time required for retooling. All the simulation plots for height vs time and the reduction schedules have an initial increase in height. This increase is due to the initial rounding phase incorporated in ERCRNGROL, during which rolling is at maximum power in the radial gap only. The axial rolls do not engage until the ring is completely under the axial rolls (Phase 2, Fig. 9) in simulation. In production, such is not the case and axial reduction is initiated as soon as the axial unit arrives at its closest position to the main roll. This leads to immediate reductions in height in the experiments, which explains the differences in the plots during the initial phases of each run. The discussions of results for four selected rings are grouped into sleeve-type and ring-type rings because these rings are subjected to different reduction schedules.

Discussion of sleeve-type rings The rings are classified into sleeve-type if the final height is larger than the final width. The discussion on the two sleeve-type rings selected for comparison is given below. CN 7242. During experiments, the operator applies a large reduction in the wall height at the beginning of the operation (Fig. 13(a) and (b)). This results in a high initial axial force in the first 25 s (Fig. 13(g)) and owing to spread, a slightly higher wall thickness during the process (Fig. 13(c)). The higher wall thickness provides for a higher radial force curve (Fig. 13(e)). The radial and axial torque predictions are not satisfactory: higher experimental values for radial torque (Fig. 13(f)) and lower for axial torque (Fig. 13(h)). These discrepancies can be linked to the differences in the assumed friction factor and the actual interface conditions and area values. The good agreement in the rolling schedules (Fig. 13(a)) results in close estimates for the radial and axial forces (Fig. 13(e) and (g)). The lower temperatures (Fig. 13(d)) during experimental rolling can be accountable for the slightly higher radial rolling force values (Fig. 13(e)). These compa:risons indicate that when the experimental rolling schedule matches that calculated by ERCRNGROL the ring rolling proceeds smoothly and the values of the experimental process parameters are in reasonable agreement with those calculated by the program, q-he simplified force and temperature analysis seem satisfactory for the simulation of the sleeve type of ring. CN 7375. Irt this experiment too, the operator initially decreased the ring height at a slightly greater rate than calculated by the program ERCRNGROL (Fig. 13(b)). The starting experimental wall thickness, being larger than the reduction in wall thickness initially, is also larger (Fig. 13(c)). However, after 40 s, it is seen that the operator holds the height constant while adjusting the wall thickness very slowly (Fig. 14(b) and (c)). Therefore, in the experiment, more severe reductions in height and wall thickness were taken earfier in the run, whereas the simulation suggests a smoother reduction schedule. The ring temperatures being closely matched, this disagreement in rolling schedules results in the mismatch of rolling forces (Fig. 14(e) and (g)) and torques (Fig. 14(f) and (h)). The program seems to over-predict the force values while torque values find lower discrepancy. Discussion of disk-type rings The rings were classified as disk type if the final width is larger than the final height. CN 7346. The starting blank and the final rolled ring dimensions for this ring were different for the experiments and the simulation, although both followed the same customer order. The slightly higher starting value will result in greater material waste

168

R. SHIVPUKIand E. ERu~

(a)

(b) R e d u c t i o n s c h e d u l e s for Ring 7 2 4 2

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21

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(c)

17

I

6

8

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Wail thickness

12

I

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14

(in)

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(d) Wall t h i c k n e s s vs time for Ring 7242

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(e) Radial f o r c e vs t i m e for Ring 7242

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(g)

(h) A x i a l force vs t i m e for Ring 7242

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200,000

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100

150

200

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FIG. 13. Plots comparing predicted and experimental values for various process parameters for ring CN 7242.

Improved Ring Rolling Productivity

(a)

169

(b)

Reduction schedules for Ring 7375 14

Height vs time for Ring 7375 14

--

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FIG. 14. Plots comparing predicted and experimental values for various process parameters for ring CN 7375. ITIR 33:2-E

170

R. SmvPt.mland E. ERu~," (a)

(b) R e d u c t i o n s c h e d u l e s for Ring 7346

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40

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80

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(f)

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F

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40

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80

100

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60

100

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FIG. 15. Plotscomparingpredictedand experimentalvaluesfor variousprocessparametersfor ring CN 7346.

Improved Ring Roiling Productivity

171

and higher finish dimensions will increase the final machining time. This comparison indicates that the experienced operator erred on the side of caution resulting in increased costs. These costs could have been saved by following the computer calculated schedule. Although the reaction schedules do not agree initially because the ring dimensions in the experiment are larger (Fig. 15(b) and (c)), they result in similar force plots (Fig. 15(e) and (g)) and torque plots (Fig. 15(f) and (h)). Initially, high radial forces for the simulation result in the rapid reduction of ring wall thickness while the axial rolls are not engaged (first three revolutions) during the first 20 s, which is then kept at a constant value while decreasing the wall height gradually. Main reduction is achieved in the axial gap as expected for a disk-type ring. This comparison shows that for this geometry, the initial condition of radial rolling only in the simulation program is not reasonable and a modification is suggested. The temperature predictions are reasonable, though the rolling times for the simulation are much larger as the final ring dimensions in simulations are smaller (larger reductions needed). CN 7457. This ring has almost a square cross-section with the initial and final experimental dimensions being lower than the simulation values (Fig. 16(a), (b) and (c)). This resulted in a ring with lower than expected machining allowances and possibilities of rejection at inspection. Also, note that in this case, the starting temperature in the experiment is about 200°C lower (Fig. 16(d)). The warmer rolling temperatures for the simulation contribute to the lower rolling forces (Fig. 16(e) and (g)). The kink in the experimental reduction schedule and wall thickness plots (Fig. 16(a) and (c)) can be traced back to the drop in radial force at about 75 s. The ring mill operator realized that he was reducing the wall thickness too fast and suddenly reduced the radial force. The higher axial force and the lower radial force for the experiment during rolling times of 75-120 s resulted in the reduction schedule curve being kinked and large deviations of experimental results from the predictions. Within this range, a steeper curve favoring a reduction in height is obtained while maintaining the wall thickness constant. This example is a case when the experiment deviated considerably from simulation due to operator errors. CONCLUSIONS AND RECOMMENDATIONS

This paper presents a computer aided approach to process planning and simulation for rolling rectangular rings. Details of a computer program E R C R N G R O L are provided and representative output shown. Experiments were conduct.ed on a production ring rolling mill to verify the applicability of this program to both the sleeve- and disctypes of rings, and to ascertain the effects of departure from the rolling schedules calculated by this program. Comparing the results of simulations and experiments, it was found that the experiments in which the actual production schedule comes close to the calculated schedule, the force and torque data agree quite well. However, the experiments in which the production schedule differs from that of the simulation, and in which the starting blank volume in the experiment does not agree with that of the order forms used in simulation, the force and torque data differ. Initially, the E R C R N G R O L simulation program provides for three revolutions of the ring under radial rolling only for rounding purposes and to let the ring diameter grow to a predetermined value before the axial rolls engage. This restriction was found to result in a non-optimum rolling schedule in the disc-type rings with wall thickness much larger than height; radial rolls engage earlier and reduce wall thickness considerably before the axial rolls engage. In these rings the major reduction is in the height direction and the above schedule can result in defects. Based on the above comparisons, it is reasonable to conclude that the process planning software; E R C R N G R O L can provide assistance in the planning of the ring rolling process arid in the selection of the rolling schedules, and also that significant variations from the suggested schedule can result in rings with dimensional defects, including final dimensions that are out of tolerance.

172

R. SHIVPURI and E. ERug

(a)

(b) Reduction schedules for Ring 7457

14

Height vs time for Ring 7457

--

14

--

_E~.

12

[] RNGROL 2 • Experiment 2

12

y

8

!

i

o R. RO, 2

8

6 9

10

11

12

13

100

14

Wall t h i c k n e s s (in)

I 200

Time ( s e c )

(c)

(d) Wall thickness vs time for Ring 7457

Temperature vs time for Ring 7457

14

2400 I 2300!

[] RNGROL 8 • Experiment 8

--

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o~'2200 i

12 •

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~ \

2100

2000 .2

~1900

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1700

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1600

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Time ( s e e )

200

Time ( s e c )

(f)

(e)

Radial torque vs time for Ring 7457

Radial force vs time for Ring 7457 300,000

30,000 - -

--

o--

[] RNGROL 4 * Experiment 4

v200,000

_ _ ~ ~ ~20,000

\

o o m

100,0oo

~ RxNpGerRiOmLe5t5

I

_

I

I

lOO Time (sec)

200

10,000

?. / I lOO

200

Time (see)

(g)

(h) Axial force vs time for Ring 7457

200,000

Axial torque vs time for Ring 7457 30,000

__

._ / ~ 20,000

~'150,000 --

m RNGROL 7 • Experiment 7

o~

lOO,OOO 50,000

0

~.Co ,o,ooo [] RNGROL 6 • Experiment 6

"~ <:

I

I

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200

I

0

100 Time (sec)

I

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FIG. 16. Plots comparing predicted and experimental values for various process parameters for ring CN 7457.

Improved Ring Rolling Productivity

173

Acknowledgements--This study was conducted at the National Science Foundation sponsored Engineering Research Center for Net Shape Manufacturing at the Ohio State University. Dr Taylan Altan, the Center's Director, provided encouragement and support throughout the program, and Srini Gengusamy and Ray Keeton of Ajax Rolled Ring Company, Detroit were very helpful during the production experiments. Thanks are also due to Dr Nuri Akgerman, of Battelle Columbus Labs for providing information and assistance during the initial ph~tse of this research program.

REFERENCES [1] E. ERut~ and R. SHIVPURI,A summary of ring rolling technology--I. Recent trends in machines, processes and production lines, Int. J. Mach. Tools Manufact. 32, 379 (1992). [2] E. ERUq and R. SHIVPURI,A summary of ring rolling technology--II. Recent trends in process modeling, simulation, planning and control, Int. J. Mach. Tools Manufact. 32, 399 (1992). [3] Thyssen Machinenbau-Wagner Dortmund, Radial-axial ring rolling mills with forward looking technique, 6th EMO exhibit, Hannover (1985). [4] Thyssen Machinenbau-Wagner Dortmund, Modern equipment for the manufacture of seamless rings of any size, any shape and in any quantity, FIAs Forming Equipment Symposium, Forging Industry Association, Cleveland, Ohio (1985). [5] E. ERUq and R. SmvPoRI, Ring rolling: development in machines and processes, Engineering Research Center at The Ohio State University, Report No ERC/NSM-88-04 (1988). [6] R. SHIVPUR1,E. ERU~, Y. C. SHIAU and T. ALTAN, Ring rolling process optimization via simulation, Proc. Conf. Near Net Shape Mfg, Columbus, Ohio (1988). [7] T. NODA, K. TSUMARAand Y. OKAGATA,Process controlled ring rolling line, Proc. of 3rd Int. Conf. on Rotary Metalworking Processes, pp. 239-250, Kyoto, Japan (1984). [8] K. H. BESELER Modern ring rolling practice, Metal Forming 36, 44-50 (1969). [9] R. KoPP, U. KOPPERSand H. WIEGELS,New control system for ring rolling, Proc. 2nd Int. Conf. on Technology of ,Plasticity, pp. 803-807, Stuttgart, F.R.G. (1987). [10] U. KOPPERS,H. WIEGELS,P. DREINHOFF,J. HENKELand R. KoPP, Methods for reducing material and energy input in ring rolling, Stahl und Eisen 106, 789-795 (1986). [11] G. VmREGGE, Hot rolling of rings, unpublished work (1981). [12] U. KOPI'EgS, Geometry, Kinematics and Statics in Rolling of Rings with Rectangular Cross Sections, Stahleisen mbH, Dusseldorf (1987). [13] J. S. YON and H. S. CHO, Optimal control system design for ring rolling process, Proc. 1st Int. Conf. on Technology of Plasticity, pp. 1322-1327, Tokyo, Japan (1984). [14] J. S. YON and H. S. CHO, A suboptimal design approach to the ring diameter control for ring rolling processes, J. Dyn. Syst., Meas. Control 107, 207-212 (1985). [15] H. D. Cool and H. S. CHO, An adaptive controller for ring rolling process, Winter Annual Meeting, ASME, Anaheim, California (1986). [16] H. J. MARCZINSKI,Ring rolling mills: state of development, Metall. Met. Forming 43, 171-177 (1976). [17] Thyssen Machinenbau-Wagner Dortmund, Wagner's CAR System, Technical Report No 2/84 (1984). [18] Thyssen Machinenbau-Wagner Dortmund, Precision forgings produced on axial closed die rolling lines, FIAs Forge Fair '88, Cincinnati, Ohio (1988). [19] E. DOEGEand M. ABOTOUR,Simulation of ring rolling process, Proc. 2nd Conf. Technology of Plasticity, pp. 817-824, Stuttgart, F.R.G. (1987). [20] n . Y. YANG, Development of a new computer aided manufacturing system for the hot ring rolling process, Proc. 3rd Int. Conf. Rotary Metalworking Processes, pp. 229-238, Kyoto, Japan (1984).