Abstract:
A system and method for monitoring a pilger mill having a crankshaft driving rolls with a reciprocating motion to reduce a tube over a mandrel. A linear sensor coupled to the mandrel supplies a mandrel position signal. A rotary sensor coupled to the crankshaft supplies a crankshaft angle signal. A processor for combining the mandrel position signal and the crankshaft angle signal provides a mandrel motion signal characteristic of the tube reduction.

Description:
This invention relates to pilger mills, and more particularly to a system for monitoring tube reduction in the mill. 
     Pilger mills provide the means for reducing seamless pipes or tubing to be within desired dimensional tolerances. The pilgering apparatus includes a tapered mandrel on which the tube to be pilgered is mounted. Annular dies each having a peripheral circular groove are mounted to cooperate rotatably on a roll carriage or yoke. The yoke is oscillated forward and backward in a reciprocating motion along the tube axis, and the dies rotate in synchronism with the oscillations. As the dies are rotated, their grooves define a circular channel of progressively increasing or decreasing transverse cross section depending on the direction of rotation. 
     The tapered mandrel with the tube mounted on it extends through the channel defined by the rolls. The tube is advanced a short distance over the mandrel in steps following each full cycle of oscillation of the yoke. At the same time, both the tube and the mandrel are rotated about their common axis by a predetermined angle. The dies apply a high pressure to the tube during the reduction, and the dies and mandrel are formed of high strength tool steel to withstand the rolling pressure. 
     It has now been found that the axial motion of the mandrel during the pilger rolling can be correlated to the formation of tubing having a desirable or undesirable wall thickness as it is rolled. As a result, a real time system has been discovered for monitoring a pilger mill to determine if operating variables exist that cause an undesirable pattern of reduction in the tube wall thickness. 
     It is an aspect of this invention to provide a system for monitoring a pilger mill to determine if operating conditions exist that cause undesirable variation in the tube reduction. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A system for monitoring a pilger mill having a crankshaft driving rolls with a reciprocating motion to reduce a tube over a mandrel. The system is comprised of a linear sensor coupled to the mandrel for supplying a mandrel position signal. A rotary sensor coupled to the crankshaft for supplying a crankshaft angle signal. A processor for combining the mandrel position signal and the crankshaft angle signal to provide a mandrel motion signal characteristic of the tube reduction. 
     A method for monitoring the pilger mill comprises, supplying a mandrel position signal correlated with an axial position of the mandrel. Supplying a crankshaft position signal correlated with an angular position of the crankshaft. Processing the mandrel position signal and the crankshaft position signal to provide a mandrel motion signal characteristic of the tube reduction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the system for monitoring a pilger mill. 
     FIGS. 2-4 are graphs showing a representation of a waveform of the mandrel position as a function of the crankshaft position during reduction of a tube on a pilger mill. 
     FIG. 5 is a graph of the wall thickness of a tube reduced on a pilger mill. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic illustration of a pilger mill 10. The pilger mill 10 includes a thrust block 12, supporting a rod 14 which extends through a feed carriage 16 to a mandrel die 18. In operation, ingoing tubes 20 are pushed by feed carriage 16 through entry chuck 22 and between spaced rolls 24 and 26. The rolls 24 and 26 rotate and undergo a reciprocating lateral movement as indicated by arrow 28. Grooves 30 and 32 in the rolls form a circular shaped pass which corresponds to the cross section of the ingoing tube. This pass tapers smoothly over a predetermined length of the roll circumference until it reaches the size of the finished tube 34 diameter. In this way, the ingoing tube is worked to the desired degree as the rolls carry out their reciprocating movement. 
     Elongation of the ingoing tube 20 to a finished tube 34 is affected through reductions in the diameter and wall thickness. The tube is elongated between the rolls 24 and 26, and the mandrel die 18. The mandrel die 18 tapers from the size of the inside diameter of the ingoing tube 20 to the inside diameter of the finished tube 34. The tube is elongated step wise over the stationary mandrel die 18. The mandrel die 18 is tapered in the direction of rolling. The two grooved rolls 24 and 26 embrace the tube from above and below, and roll over the tube for a predetermined length, called the pass length. 
     The rolls receive their reciprocating lateral movement from a saddle, not shown, in which they are mounted. At the same time, a reciprocating rotary movement is imparted to the rolls by pinions mounted on the roll shaft and engaging with racks which are fixed to the machine frame. The reciprocating stroke of the saddle plus rolls is effected by a crank drive 38. At the completion of each stroke, the entry chuck 22, and an exit chuck 36, grip the tube and rotate it a predetermined amount so that the entire circumference is evenly reduced between the rolls 24 and 26, and the mandrel die 18. 
     We have found that the force applied by the rolls in elongating the tube causes elongation and contraction of the mandrel die 18. In addition, it was found that the elongation and contraction of the mandrel die can be correlated with the rolling stroke of the rolls to provide a function or waveform characteristic of the tube reduction. When the mandrel die elongates and contracts in a cyclical repeating pattern, i.e. a repeating waveform, the tube is reduced to a uniform wall thickness and size. However, when the mandrel die elongates and contracts in a cyclical repetition of nonuniform patterns, i.e. an erratic repetition of waveforms, the tube reduction is nonuniform, and the desired wall thickness reduction is not achieved. 
     Referring to FIG. 1, a rotary sensor 40 for determining the angular position of the crankshaft is mechanically or optically coupled to crankshaft 42, and outputs a crankshaft angle signal proportional to the angular position of the crankshaft. For example, the rotary sensor 40 can provide an analog voltage ramp corresponding to the angular position of the crankshaft. The rotary sensor 40 also outputs a voltage pulse at every zero angle position of the shaft, for example, set at top dead center corresponding to the pilger saddle moving rolls 24 and 26 to the left limit position in FIG. 1. Examples of suitable rotary sensors are an optical encoder, resolver rotary transformer that determines shaft angle, potentiometer, rotary variable differential transformer, combination of gear teeth and proximity sensor or magnetic sensor, or the like. A preferred rotary sensor is a heavy duty shaft encoder model 470, with angular position monitor model SDC-2, Drive Control Systems, Eden Prairie, Minn. 
     A linear sensor 42 is coupled to the mandrel 18, to generate a mandrel position signal proportional to the position of a free end 43 of the mandrel measured with respect to a preselected reference position, e.g. the free end when the mandrel is unloaded. A suitable linear sensor is a linear voltage displacement transducer, LVDT, such as a type 503XE-3A obtained from Schaevitz Engineering, Pennsauken, N.J. The LVDT is comprised of a magnetic core 46, and a coil assembly of one primary coil 48 and two secondary coils 50 and 52 symmetrically spaced from the primary coil. The magnetic core 46 is mounted on a non-magnetic spacer 44, such as non-magnetic stainless steel, and the non-magnetic spacer is mounted on the free end 43 of the mandrel 18. The coil assembly is mounted axially to the magnetic core so the reduced tubing can extend therebetween. 
     The crankshaft angle signal and mandrel position signal are delivered to a processor 54, such as an oscilloscope, computer or microprocessor, for processing to display as a characteristic function of the pilger rolling process. For example, the analog signals can be sent to an oscilloscope for conventional processing to display the mandrel free end position as a function of the crankshaft angle position in a plurality of waveforms. 
     In another embodiment, the crankshaft angle signal and mandrel position signal are sent to an analog to digital converter coupled to a computer. The computer is conventionally programmed, for example using Lab Windows from National Instruments, to provide the function output signal corresponding to the mandrel position as a function of crank shaft angular position. The function output signal can be displayed by conventional means such as plotting on an X-Y recorder, or a video display as a plurality of waveforms. When the voltage pulse from the rotary sensor is detected, the plot is reset to the reference crankshaft position at the current mandrel free end position. In this way, a preselect number of the pilger rolling cycles can be plotted. 
     When the waveforms are repeating in a substantially uniform manner, the tubing is being reduced in a substantially uniform manner to a uniform size. Two or more separate mandrel movement patterns can be superimposed in the graph forming, for example dual or triple repeating patterns. FIGS. 2 and 3 are plots representative of the plurality of waveforms of mandrel free end position as a function of the crank shaft angle during the pilger rolling. FIG. 2 shows waveforms having a triplicate pilger mill rolling pattern, and FIG. 3 shows waveforms having a dual repeating pattern. The tubing reduced during such pilger rolling has a uniform size, such as wall thickness, or inside diameter. 
     FIG. 4 is a plot representing nonuniform reduction of the tubing, as shown by the irregular non-repeating waveforms. Tubing reduced during such nonuniform reduction has an irregular and undesirable variation in size. For example, FIG. 5 is a graph showing the inside diameter of an elongated tube reduced in a pilger rolling apparatus. A transducer was traversed along the length of the tube to determine the inside diameter of the tube, as plotted on the ordinate, as a function of the traversing time of the transducer, as plotted on the abscissa. FIG. 5 shows tube measured in about the first 12 seconds has a nonuniform inside diameter formed during the type of reduction shown in FIG. 4, while the tubing measured thereafter has a more uniform inside diameter formed during the type of reduction shown in FIGS. 2 or 3.