Increasing the Accuracy of Pressed Thin-Walled Copper Pipes

Abstract

A new tool has been developed and successfully tested for the adjustment of horizontal hydraulic presses used to make bars and profiles. The tool makes it possible to use the method of reverse pressing to obtain high-precision thick-walled copper pipes. The rigid but easily disassembled tool guarantees the required adjustment accuracy both during the initial moment of the pressing operation and in the following stages up to and including the formation of the press-residue. This makes it possible to produce longer pipes, which in turn minimizes metal waste because it facilitates cutting of the tubular semifinished product. The product can be cut to lengths that are multiples of the height of the jackets of continuous-caster molds or ingot molds. Thin-walled copper pipes are often used as semifinished products to make parts such as bushings, drums, ingot molds, jackets for continuous-caster molds, and other finished products. The pipes are used instead of solid castings or forged semifinished products, which are not well-suited for the applications just mentioned. Methods have been described (1, 2) for the reverse pressing of the pipes on horizontal hydraulic presses (HHPs) used to produce bars and profiles. The pressing operation is performed using an assembled tool that includes a solid punch (with a mandrel) or a hollow punch. An annular gap is formed between the die and the mandrel; the ingot that is formed in this pro- cess can be hollow or solid. The technological capabilities of the aforementioned methods are limited, since the outside diam- eter of the pipe being pressed is limited by the dimensions of the cavity of the punch, while the capabilities of combination dies are constrained by certain features of their design. In the traditional variant of the reverse pressing of pipes (3), one side of the container is closed by a plug, and the metal of the ingot - which is pierced by a punch fitted with a press-ring on its end - flows into the gap between the container and the press-ring. The following problems are encountered when such tool- ing is used for the reverse pressing of large-diameter pipes, especially when the pipes are made of nonferrous metals. First of all, it is particularly difficult to attain an acceptable level of accuracy on the pressed pipes when the scheme just described is used (3). The main problem is the variation of the thickness of the pipe wall. Such variations occur for the following reasons. 1. One of the most important design characteristics of HPPs - especially presses that are not equipped with a pierc- ing system - is their centering action. The execution of this operation determines many of the product's parameters, includ- ing its dimensional accuracy. According to the recommendations in (4), the degree of non-coaxiality between the container and the punch can be within the range 1.5-6.0 mm for a 7.5-120-MN horizontal hydraulic press. This range is quite broad, even without allowance for the effect of negative factors on the pressing operation. However, it is difficult to satisfy this norm