Method and lathe for machining out-of-round circumferences

#This invention relates to a method for machining the peripheries of out-of-round workpieces and to a lathe for carrying out this method. The invention is especially useful in the machining of out-of-round raw piston rings with gap sections not yet cut out.

#Lathe-like apparatus incorporating a copying device have been employed for the peripheral machining of piston rings having out-of-round inner and outer circumferences. In such apparatus, the piston rings are axially clamped together to form a stack and mounted on a driven spindle for simultaneous machining of their inner and outer circumferences. The tools are attached to the ends of pivotal tool holders provided with a radial adjustment that is controllable by a copying cam rotating in synchronism with the work spindle. In particular, an adjustable dual lever system may be used for controlling the tool for out-of-round machining of piston rings, such apparatus having been operated satisfactorily for many years.

#A disadvantage of the prior art machining apparatus is that the maximum number of revolutions of the spindle depends upon the out-of-round shape of the piston ring. Specifically, in the area of the piston ring where the gap is cut after machining and where there is a relatively slight curvature, the copying lever system does not provide a reliable, accurate transfer of the out-of-round contour of the workspindle. This occurs because, with an increase in the number of revolutions in synchronism with the copying cam, the copying roller is lifted off the cam or the mass inertia of the copying lever system is insufficient to assure accurate transfer of the out-of-round contour.

#FIG. 1 is a front view of a lathe according to the invention which uses a d.c. servomotor as the main drive.

#FIG. 2 is a plot of the angular speed of the workpiece in the lathe of FIG. 1 as a function of the angular displacement of the workpiece during one revolution thereof.

#FIG. 3 is a plot of the angular speed of the workpiece in a lathe employing a reversible pole three-phase motor drive as a function of the angular displacement of the workpiece during one revolution thereof.

#FIG. 1 is a schematic representation of a lathe for the simultaneous machining of the internal and external peripheral faces of out-of-round piston rings. The lathe includes a spindle stock 1 holding a sprindle 2 rotatably mounted therein and a counter yoke 3 supporting a rotatable socket 4 to which a piston ring stack 5 may be axially clamped. Tools 6 and 7, which are provided for the internal and external machining of the piston ring stack respectively, are connected to a pivotal rocker bar 10 by means of an arm 8 and an axial rod 9 respectively. The rocker bar 10 is mounted on two posts 12, 12' which are fastened to a longitudinal carriage 11. Additional details of the lathe are found in commonly-assigned copending application Ser. No. 924,732 filed July 14th, 1978 and are incorporated by reference herein.

#The radial vibratory movements of tools 6 and 7 caused by the out-of-round circumferential contour of the piston ring are transferred to the rocker bar 10 by a copying cam 13 which rotates in synchronism with the spindle 2 via a lever 14 having a sensing roller. The drive for the spindle is a d.c. servomotor 16 whose armature shaft 17 is connected with the spindle 2 by a belt drive 19. The servomotor is controlled by an electronic computer 20 in which a ramp function has been stored.

#In another embodiment of the invention, a servomotor 16' and an encoder 16" are mounted flush with the spindle as shown in dashed lines on FIG. 1. The encoder 16" monitors the angular position of the spindle and the direct mounting of servomotor 16' eliminates the additional elements in the belt drive which would have to be accelerated.