DESIGNING A TOOL FOR COLD KNURLING OF FINS

Andrej Olejnik Moscow State Technological University “Stankin“, Institute of Information Systems and Technologies, Department of Management and Informatics in Technical Systems Moscow, Russia Alexey Kapitanov Moscow State Technological University “Stankin“, Department of Automated Information Processing and Control Systems, Moscow, Russia Islam Alexandrov Institute of Design-Technology Informatics of the Russian Academy of Sciences, Moscow, Russia


INTRODUCTION
One of the most progressive directions in the development of modern engineering is the desire to reduce the specifi c gravity of machining and to obtain a billet approaching in shape and size to the fi nished product [1][2][3]. This is largely due to a decrease in the relative role of subtractive technologies for processing materials by cutting due to the development of additive and hybrid technologies of shaping, the development of pressure processing methods [4,5]. Given this thesis, engineers and technologists face many unsolved problems associated with increasing the effi ciency and productivity of solutions, while reducing and minimizing implementation costs. One of the applied problems that need to be solved in the key to increasing the effi ciency of the technological process while improving the quality of products is the production of developed heat transfer surfaces with a decrease in overall dimensions. Today, compact and efficient heat transfer tools include products with transverse fi nned surfaces -fi nned tubes [6]. Executions of transversely fi nned surfaces differ in manufacturing technology and in geometric, mass, thermo-physical and thermo-aerodynamic parameters [7,8]. Cross-fi nned pipes are produced by the following methods: special casting methods [9][10][11], installing or rolling on alongside with brazing spiral-ribbon fi ns [12], pressing the fi ns along the guide [13,14], knurling bimetallic (aluminum and copper) fi ns on a steel supporting pipe [15][16][17], electric arc or high frequency currents (HFC) welding of fi ns [18]. In the case of cold knurling of fi nned surfaces, the axial, radial, and circumferential components of the cold knurling force have a signifi cant effect on the nature of the deformed metal fl ow. The purpose of this work was to develop a tool for determining the forces acting in the cold knurling process and study their infl uence on the characteristics of the fi nal products. The types of fi nned tubes with the highest manufacturability have been introduced massively, the manufacture of which is possible in mass production on highly effi cient and technologically advanced equipment -these are tubes with fi nned non-ferrous metals and alloys pressed into a spiral groove (aluminum, copper), bimetallic tubes with spiral-screw fi nning, as well as pipes with spiral-screw fi ns welded using high-frequency currents [17][18][19]. At the same time, these types of fi nned pipes have signifi cant technological and operational shortcomings, namely low thermal and diffusion contact of the fi ns with the supporting pipe and the high cost of metals and alloys used for the fi rst two types in production, low degree of surface development (reduced fi nning coeffi cient), as well as the high complexity and cost of production equipment of automatic lines for fi ns of pipes with spiral-helical welded fi ns. The estimated cost per linear meter of fi nned tubes can reach 20 or more US dollars, which reduces the competitiveness and potential of their mass and serial production. The increased cost and energy intensity of the corresponding production and technological equipment does not make it possible to increase production capacities and the necessary output volumes. Products manufactured by these enterprises have many signifi cant structural and operational advantages. In particular, they have reduced aerodynamic drag in comparison with round pipes, which makes it possible to reduce production and operation costs, and produce designs in which movement of inter-fi n and annular airis carried out by natural traction [20][21][22][23]. However, the technology involved in manufacturing this type of fi nned pipe is expensive; it is high in energy consumption and production is relatively slow. This can be associated with either the electromechanical winding of spiral-ribbon fi ns or the installation of rectangular fi ns on an oval-section supporting pipe and subsequent brazing to a support pipe by immersion in molten zinc [24].

LITERATURE REVIEW
Modern production has various technological processes at its disposal, aimed at connecting with subsequent affi xing of the fi ns to the outer surface of the supporting pipe. These processes have both positive and negative qualities [6][7][8]. The gradation of methods for producing fi ns based on patent searches (on patent documentation funds) allows us to distinguish the following groups: obtaining fi ns by precision casting; winding with a pretension of a non-ferrous metal tape (usually aluminum) onto a supporting alloyed base in the form of a steel pipe; crimping steel washers on a supporting pipe; continuous arc or resistance welding of steel tape or washers; and connection of the fi ns of steel pipes with high-frequency currents [9,10,12,14,18]. Existing methods, in turn, can be differentiated into the following subclasses: fi nning, mechanical fi nning [25]; and fi ns obtained using welding methods (arc, contact, or high-frequency currents) [18]. As a basis, steel or cast-iron pipes are more often used as a supporting pipe, less commonly used non-ferrous metals. To aid heat transfer from the coolant in contact with the carrier pipe to the surrounding space (through the developed fi n surface to the outer surface of the carrier pipe), fi ns are fi xed transversely or longitudinally. The fi ns are mainly made of aluminum and copper and less often of brass and carbon steel. Along with this, the following types of manufacturing can be distinguished: 1. Use of a solid billet. This involves the use of special casting methods or the extrusion method from a solid billet [10,26,27]. Products made by this method have enhanced operational properties and durability because there is no diffusion connection between the fi ns and the supporting tube. However, despite its positive qualities, this manufacturing type is expensive and technologically complex. Consequently, it is not widely used. 2. Using a composite billet. This type involves using two metals when manufacturing fi nned tubes and using the thermos-physical properties of each metal to improve the technical characteristics of the fi nal product. The combination of a high level of thermal conductivity and mechanical strength leads to an increase in the anticorrosion and performance properties of the fi nal product. A material, preferably carbon or stainless steel, is selected as the basis for the carrier pipe considering physico-mechanical and temperature characteristics. The most effective material for making a fi nned surface is copper, which has high heat transfer properties, though aluminum is more economically effi cient, having thermal properties comparable to copper [6]. Worldwide, many enterprises, multi-unit enterprises, and factories produce fi nned pipes. Many industries that manufacture fi nned tubes for heat exchangers use the high-frequency currents (HFC) method [18]. However, this method has many operational and technological disadvantages: 1. Process speed: The process speed is much higher relative to other manufacturing methods, but modern economic development and market conditions require scaling and an increase in output. In the process used, increasing productivity is a diffi cult task. 2. Energy consumption. The consumption of technological and production equipment used in the manufacture of the HFC method is estimated at 264 kW. When using a pipe with a diameter of 30 mm and a rotation speed of 200 rpm, the production speed is 3.79 m/min, which corresponds to a welding speed of 0.49 m/s. Thus, more than 1.25 kW/h is required per 1 m of a fi nned pipe. Finned pipes are mass-produced; therefore, the energy consumption with this method at 1.25 kW/h is estimated as extremely high. 3. Restriction on materials. The HFC method is not optimized for the use of various types of metals, as it is a special solution for steel fi ns (support pipe is made of steel; fi ns are made of steel tape). Steel is not the perfect material for heat transfer. In terms of heat transfer, the use of steel and steel alloys is not the In addition to thermos-physical drawbacks, steel has high specifi c gravity and low corrosion resistance. This factor is restricting. 4. The accumulation of micro defects during operation due to a variable temperature gradient during cooling. The metal has a short heating stage, followed by rapid cooling, mainly by water, which leads to micro-cracks in the contact zone. As a result, the service life is reduced. 5. Reduced contact strength between the fi n and the carrier pipe. The next factor in the rapid cooling of the fi nning welding seam is the appearance of a zone with increased hardness, which leads to a decrease in the welding seam quality and loss in heat transfer effi ciency. The possibility of creating fi nned surfaces for heat exchangers can be achieved by fundamentally different technologies. However, the effectiveness of their func-tioning will substantially depend on the quality of the surfaces [20,28,29]. The modern progressive method is the plastic deformation of metal in a hot or cold state [30]. Plastic deformation of the metal during knurling is associated with the use of signifi cant pressures of the working tool and large radial, circumferential and axial knurling forces [25]. Using the method of plastic deformation of the metal reduces processing waste. In addition, it reduces the consumption of electricity, wear on the tools, and labor costs. Products obtained by this method have a material structure that provides improved mechanical and operational characteristics, compared with products obtained by cutting [31][32][33]. It should be emphasized that the use of developments that lead even to a slight decrease in metal consumption and the cost of fi nned surfaces with large requirements for heat exchange equipment across industrial sectors can lead to signifi cant cost savings, expanding the application of resources and energy-saving technologies. Cold rolling of products with a helical surface is accompanied by a signifi cant deformation of the metal. The maximum allowable deformation during cold rolling is determined by the plastic properties of the billet material [34]. In the manufacture of products with a helical surface, the geometry of the rolled profi le depends not only on the geometry of the tool but is also determined by the laws of the peripheral fl ow of metal in roll passes. During rolling, the billet's metal is deformed by the intake cone of the rolls, the ring or screw gauges of which hamper the axial fl ow of the metal. The deformation of the billet's metal is the result of radial and axial compression by the protrusions of the roll profi le. The form-making of the rolled profi le occurs mainly due to the fl ow of the outer layers of the billet's metal. The manufacture of capacitors with low fi ns by cold knurling is not associated with the use of large loads. The high productivity of this process and the relatively small production costs make it the most promising. During cold knurling, an increase in the metal durability of the rib's surface layer because of hardening is achieved, and an improved metal texture is provided [31][32][33]. This significantly increases the wear resistance and strength of the knurled fi ns compared to milled fi ns If various defects often arise on the working surface during hot knurling as a result of heating and cooling the billet such as folds, scales, cracks, temperature stresses, which are primarily responsible for its low fatigue strength-then these defects are absent during cold knurling [30,35]. During cold knurling, it is possible to obtain-a knurled crown with predetermined geometric parameters attributed to plastic redistribution of the billet metal that takes place because of the working profi le of the knurled tool crown. The metal extruded by the knurling roller head fl ows upward, thus simultaneously forming the fi n head due to the feeding the rollers or the billet. In addition, the deformed metal fl ows axially in both directions, forming a wave ahead of itself, which ultimately fl ows to both ends.

METHODOLOGY
Two methods of plastic deformation possess universality and high productivity: rolling with roll-forming tools and rolling in stands with shaped rolls. During rolling, the metal of the billet is deformed by the intake cone of the rolls, the ring or screw gauges of which impede the axial fl ow of the metal. The deformation of this metal is attributed to radial and axial compression by the protrusions of the roll profi le. The form-making of the rolled profi le mainly occurs due to the fl ow of the outer layers of the billet's metal. Fins meanwhile, grow asymmetrically. The fi n side, which coincides with the axial movement direction of the billet (right side), grows faster than the opposite/left side [36,37]. At the intermediate stages of the radial unit compression, the right side has a step, the angle of which differs from that of the rolls' profi le. As demonstrated by the experimental knurling of billets made of brass grades CW610N EN (annealed and not  annealed) and CW508L EN, it is possible to eliminate fl owed metal formation by installing dividing tools from both ends that limit the metal fl ow. At the same time, it was observed that using additional compression of the knurled billets package with an axial force in the range from 60 to 80 N signifi cantly reduces or eliminates metal outfl ow at the ends of the billets (see Figure 1). Concurrently, the size of the burrs at the ends directly depends on the axial compression force of the knurled billet package. Figure 1 illustrates the product width increase as Δh=H 1 -H where H and H 1 denote billet width and knurled roll, respectively. The process of cold knurling of capacitor fi ns is most rational to carry out on capstan lathes with the help of knurling tools, which operate according to the scheme of three-sided metal compression by three knurled rollers with axial feed, which provides the highest knurling performance [38,39]. The tool (Figure 2, 3) has increased body rigidity and individual components, while the use of a pneumatic clamping tool allows the axial clamping of a knurling package of billets with great axial forces. There fore, the tool can be used in severe conditions of cold knurling of fi ns with a low fi nning height. The knurling tool housing 20 is installed in the hole of lathe's machining turret and is attached to it by four bolts 19. Three knurling heads 2 are installed in three symmetrically arranged radial guides, in the openings of which on the rolling bearings 4 and the rollers 5 three knurling rollers 3 are installed. Each knurling head is attached to the tool body with four bolts. Radial movement of the knurling heads when adjusting the tool is carried out by stop screws 1, which, besides, prevent radial wipping of the heads under the action of knurling forces. In the inner bore of the knurling tool casing, sleeves 12 with a rotating heel 10 are installed in the bushings 13 and 18, which serves to compress the knurled package of billets.
To ensure the fl ow of the deformed metal in the radial direction and to limit its undesirable fl ow in the axial direction, an axial clamp of the billet package is provided with a pneumatic cylinder. When the piston stroke is up to 50 mm and the air pressure in the network is 4 kg/ cm2, a force of 420 kg is provided on the rod. When the air pressure in the network exceeds 6 kg/cm2, the force on the rod exceeds 600 kg. The nature of the deformed metal fl ow during the cold knurling of the capacitor fi ns is signifi cantly infl uenced by the axial, radial, and circumferential components of the cold knurling force. In this regard, it is very important to know the infl uence of each of them on the nature of the fl ow of a deformable metal. A tool for cold knurling of fi ns was designed to determine the value of these components. It was manufactured according to the two-sided compression of metal by two symmetrically located knurling rollers ( Figure 5, 6). One of the two knurling heads is a three-component dynamometer. A free-fl oating tool holder with a knurling roller is mounted on the sleeve, where three strain pins with wire strain gauges are located. Balls installed in the head help in transferring the forces arising during knurling to the load cells. The values of the components of these forces are recorded using an oscilloscope-recorder. The knurling tool (see Figures 4,5) consists of a rigid support housing 1, in the radial grooves of which are attached to two power housings 12 with eight bolts (it. 30).
In the internal bore of the housing, strain pins 8, 16  ). An indicator tool (parts 2 and 5) is used to determine the total amount of clamping of the rolled-out heads y arising due to large knurling forces, as well as the elasticity of the part -tool -adaptation -lathe system. The fins made of brass CW610N EN -1 (annealed) and CW508L EN, as well as bronze AERIS 1355, were subjected to cold knurling. Knurled rollers had a purity of working profiles within 9-10 classes. The wipping value y in the process of cold knurling depends on the rigidity of the machine-tool system and is in the following range: 0.03-0.09 mm for brass CW610N EN -1 annealed; 0.02-0.13 mm -for brass CW610N EN -1 not annealed; 0.04-0.12 mm -for brass CW610N EN. Given the wipping value y and depending on the cold knurling modes, it is possible to determine the billet diameter for knurling ( Figure 7). It can also be calculated theoretically by the formula (1): where m is the normal fi n package; z is the number of the knurled fi ns; K is the allowance of the external diameter of the knurled surface K=(0,4-0,5)m; ξ is the correction coeffi cient. The tolerance on the knurling diameter is d=±0,01 mm, and on the radial runout of the outer billet's diameter d should not exceed 0.01 mm. The results of comparative tests for the bending strength of milled, machined and knurled fi ns are shown in Figure  7. As can be seen from the fi gure, the bending force for knurled fi ns was 50-60% higher than for fi ns obtained by cutting methods. Experimental knurling confi rmed that the strength of the fi ns with this method increases on average by 25-40% [40].

DISCUSSION
Cold plastic deformation has a signifi cant effect on the physicomechanical properties, macro-and microstructure of the treated metal. Because of cold deformation, the initial metal, which had properties that are approximately the same in different directions and randomly oriented structure, receives a directionally oriented structure of a fi brous nature and increased anisotropic mechanical characteristics. The change in the properties of the metal depends on the degree of plastic deformation, with an increase in which all the characteristics of the metal resistance to deformation increase, namely, strength, hardness, and yield of metal increase. Simultaneously, the plasticity values decrease, which are elongation and toughness. The infl uence of cold plastic deformation on the surface quality of parts is especially signifi cant. When rolling the fi ns, a signifi cant change in the structure of the outer layer of the billet's metal occurs. The structure gets a fi brous structure; metal fi bers are oriented along with the worm profi le and are highly densifi ed in the cavity. At the recommended fi ns rolling modes, no defects such as folds, back fi ns, etc. are observed. Cavity opening or discontinuities in the axial zone of the rolled fi n are possible when heating the billet for rolling over its entire cross-section and applying additional compression along the billet's diameter with a fi nally formed profi le. In this case, the metal pressure on the rolls increases rapidly and, in some cases, a discontinuity of the base metal appears or a cavity is opened. Discontinuities and cavity opening are also facilitated by an increase in radial single compressions, an increase in the heating temperature of the billet during knurling, and an increase in the rolls' width. Due to the fi ns' knurling, metal hardening occurs over the entire cross-section of the fi ns' profi le. This is because the conditions of the fi ns rolling process are similar to those of the thermomechanical processing of metals.

CONCLUSIONS
The manuscript proposes the design of the tool for determining the forces acting in the process of cold knurling and investigates the effect of forces on the characteristics of manufactured fi nned surfaces. The authors tested the developed tool and determined, with its help, the dependence of the increment of the product width on the axial compression force during knurling. Fins made from various grades of brass were subjected to cold knurling. The authors carried out comparative tests of knurled, milled, and machined samples and fi ns. It has been shown that an important feature of cold-knurled capacitor fi ns is the formation of a reinforced textured structure over the entire cross-section, while the milled fi ns have no fi brous structure. Hardening of the surface layer during knurling due to deformation and the fi brous structure of the metal structure increase the operational strength under cyclic loads by an estimated 1.2 times compared with machine cutting.