Wire & Cable Technology International - January/February 2008 - (Page 60)

Expediency of Roller Dies Application in Wire Drawing continued where R is the radius of a roller in pass of reduction, do is the initial diameter of a wire, d1 is the final diameter of a wire, h1 is the height of intermediate wire section and b1 is the width of intermediate wire section. The friction is inevitable during processing metals by pressure. Increased friction can result in deformation of the tool, loss of dimensional management, bad quality of a surface, cold welding and increase of deformation heterogeneity. The picture of action of friction forces in a stationary die and in a roller die is different. In roller dies part of friction forces in a deformation channel is replaced by external friction forces in the bearings of rollers. Therefore it is necessary for us to develop a satisfactory model of inclusion of friction forces for a case of roller dies. In roller dies, external friction forces between the metal and mobile details of a die such as rollers distinctly decrease. Under the action of external friction forces during the established drawing process, the rollers are in uniform motion. If we do not take into account the twisting moment necessary to overcome friction in bearings of rollers, the uniformity of motion of rollers shows that the total moment of friction forces working on a roller on a contact surface is equal to zero. The metal being drawn in the deformation zone flows in the direction of drawing with velocities growing from the input to the output. This growth at the given sizes of a roller depends entirely on the degree of deformation. Any point on a roller surface at its rotation also advances with growing velocity in the direction of drawing. This change of velocity, as is well known, occurs as expressed in Equation 9:4 Where is the velocity of a point A on a contact surface of a roller (Figure 2), is the peripheral velocity of a roller and is the corner formed by the radius passing through the point A and the radius situated at an output plane of the metal from deformation zone. Depending on the position of each point on a contact surface of the metal, velocity can be equal to, less or more than the velocity of its corresponding point of a roller as seen in Equation 10 and Equation 11: Fig. 2 — Powered conditions scheme in a roller die. At drawing velocity or the velocity of metal output from the deformation zone, the velocity of input in it will be equal to draw where the drawing is for a pass. Therefore, an average velocity of metal motion in the deformation zone will be determined as Equation 12: Then the average velocity of mutual movement of a roller and the metal can be calculated on the basis of the formula seen in Equation 13: In a die with motionless contact surfaces, the average velocity of mutual movement is determined by the average velocity of metal motion in deformation zone (as seen in Equation 12). Therefore, it is possible to consider that the / times losses on external friction in roller dies will be less than in a traditional one. According to this, drawing force in a roller die can be approximately determined by the formula seen in Equation 4, in which friction coefficient is = A* , where A* is determined by the expression seen in Equation 14: If at points of a contact surface velocities of the metal and the roller are equal, there will not be losses on external friction. If in some section, for example I, of the contact surface the inequality as seen in Equation 10 is observed. The metal remains behind the roller and friction forces appear (their resultant Tl), which brake the rotation of a roller. And as the roller moves uniformly, friction forces appear in the section II (their resultant Tll), which promote the movement of this roller and the inequality appears. The metal being drawn leaves behind the roller. The result of friction forces of both groups should be numerically equal to themselves and the losses on friction will be expressed only by work on mutual moving of metal and rollers which transforms to heat. The equality of the resulting friction forces allows us to consider at some approximation that the sections of advance and gap are equal to themselves, and that owing to small nib angles ( nib) the peripheral velocity of rollers is equal to average velocity of motion of the metal in deformation zone. 60 Wire & Cable Technology International/January 2008 The given expression shows that coefficient A is much less than one. It explains the decrease of losses on external friction in a roller die in comparison with a traditional die. WCTI Part 2 of this article in the next issue of Wire & Cable Technology International will explain the practical use of the mathematical apparatus. References: 1 Gokyu I, Okubo T. Tetsu-to-hagone Overseas, Vol. 4, No. 1, March 1964. 2 Ekkelenkamp J.H., Khosrovabadi P.B. Wire Journal I, December, 1989. p.22, 27-28,30,35-40. 3 Walter A. Backofen. Deformation Processing, Addison-Wesley Longman, Incorporated, 1972. 4 Perlin I.L., Ermanok M.Z., " Teorija volocenija ". Moskva, Metallurgija. 1971, p. 448.

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Wire & Cable Technology International - January/February 2008 Editorial Wire & Cable Events Industry News Global Economy Drives Specialty Cable Demand Corporate News Change is Tough! Top Products of 2007 New Magnesium Hydroxides Enabling Low-Smoke Cable Compounds Straight From the Show - IWCS Conference™ A History of Long-Term Thinking Ensures a Pioneering Spirit Expediency of Roller Dies Application in Wire Drawing - Part 1 WireDrawing 101 Strain Rate Transforming Our Ideas Into Your Success Straight From The Show - Wire Southeast ASIA 2007 Capstans, Dancers, Accumulators Roundup Spotlight On Quality Control Pioneering Virtual Technology for Extrusion Electrically Heated Furnace Cuts Energy Costs/Reduces Carbon Footprint Machinery Accessories Materials Polymer Developments in the Cables Industry Wire & Connector Top Products of 2007 Just Load, Click and Print Superior Flame Resistance for Minimal Smoke, CO & Acid Gas High-Flex Round Cable Added to On-Line Design Tools Failed Bridge Cables Replaced With Smaller, Stronger Ones Advertisers Index Classified Advertising Internet Spotlight Editorial Index

Wire & Cable Technology International - January/February 2008

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