MEASURING SYSTEMS IN
MEASURING SYSTEMS IN TUBE MILLS MEASURING SYSTEMS IN TUBE MILLS Tube Mills and Typical Tube Contours Concentric tube Center of outside diameter = Center of IMS system Center of inside diameter Max. wall thickness Rotary piercing mill and press In the first forming step in tube production the solid bloom is „pierced“, i.e. a rolling or pressing process turns the solid material into hollow material (hollow bloom). In the rotary piercing method, which is the most widespread process in use today for the first rolling stage, the tensile stresses occurring inside the bloom, caused by the two staggered rolls (skew rolls) running against each other, are used to tear open a hole shortly before the piercing mandrel. The process is therefore less a mechanical „drilling“ than a „widening“ or „pulling on“. In the pressing method, which is often used in the preliminary stage of Pilger Mills, a die is pressed into the bloom under high pressure so that a hole is formed in the bloom by transverse impact. Eccentricity In the ideal case both piercing methods should form a concentric hollow bloom, i.e. the center of the inside radius is identical to the center of the outside radius. Eccentric tube Min. wall thickness As a result of the rotation of the bloom during the rolling process, the eccentricity has a spiral shape in dependence on the feed angle of the skewed rolls. The eccentricity formed in the bloom press only changes in amplitude, and not in orientation, along the length of the tube. MPM and Sizing Mill In an MPM (Multistand Pipe Mill) and Sizing Mill two-roll stands arranged alternately at 90° to each other are used. In the MPM the rolls roll against a mandrel bar, i.e. the rolls reduce the wall thickness while retaining a constant inside diameter. The outside contour only becomes round in the so-called extractor mill (MPM extractor), which is not reached by the mandrel (retained mandrel), but then previous irregularities in the inside contour are moved. Typical shapes occurring here are an internal oval and/ or an internal quadrangle (cloverleaf). The causes of this are: Uneven or offset rolls Rolls that are opened too wide or closed too far The shape is typically constant along the complete length of the tube. Push-Bench, Stretch Reducing Mill, Sizing Mill, PQF ® - and FQM- Mills As an alternative to the aforementioned two-roll mills, three-roll mills are used also. The Push-Bench process differs from the Stretch Reducing and Sizing Mill in that the rolls roll against an internal tool, the so-called mandrel bar, similarly to in an MPM mill. The mill stands are not driven – the mandrel bar with the hollow bloom attached to is pushed. Most modern rolling mills, the PQF ® or FQM Mills, are three-roll mills too and are using the same principle as the MPM mill to ensure the appropriate elongation of the material. Here also the retained mandrel bar will work as an internal tool. The Stretch Reducing and Sizing Mill produce the outside diameter and wall thickness of the finished tube. This is done by arranging numerous three-roll stands, which can be driven jointly or separately, one behind the other. In Stretch Reducing Mills with individual or group drives the wall thickness can be controlled locally using an automation system. All mentioned mill types produce a typical shape of an internal triangle or internal hexagon. Measured value display Very different gauge types can be used in the three-stage rolling process depending on the measuring task, which is reflected by different displays. In the field of mass measurement the mean wall thickness along the tube length is shown. As additional functionality the measuring system can determine the lengths of the thickened ends on stretch-reduced finished tubes in order to control a rotary saw by use of these tube length positions. Using numerous gauges in the passline, it is possible to monitor the control intervention of the automation system easily. This is done by comparing the measured data on the shell, which is stored temporarily, with the measured data on the finished tube. In addition to information on the mean wall characteristics, the Multi-Channel gauges determine the internal contour of the tube up to the sixth order (internal hexagon) depending on the number of channels (5, 9 or 13-Channel). Internal tube contour of an PQF ® tube Various external influences, however, cause the two centers to be moved from this ideal position more or less strongly, resulting in an eccentric tube contour. The following main influences lead to an eccentric tube geometry: Internal oval and internal quadrangle Internal triangle and internal hexagon Unequal temperature distribution in the hollow bloom Imprecise centering of the bloom Worn or defective piercing mandrels Evaluation “thickened ends” 8 9
