Method and Machine Tool for a Circular Machining of Eccentric Shaft Portions

(1) Field of the Invention

This invention relates to a method for a circular machining of eccentric shaft portions, particularly crankpins of crankshafts, comprising an externally cutting, rotating or oscillating tool, such as a grinding wheel, a tool of a superfinishing device, or an end mill, which tool is longitudinally positioned to engage the shaft portion and machines the same as an angular feed movement relative to the workpiece is imparted to the tool.

(2) Description of the Prior Art

To permit a higher surface finish to be obtained and smaller dimensional tolerances to be adhered to, it is often necessary to machine workpieces in a plurality of operations. In most cases the workpieces are milled or turned and then finished by grinding or superfinishing. These finishing operations are performed by externally cutting tools, which rotate or oscillate. So far, to use of such driven tools for machining eccentric shaft portions, such as the crankpins of a crankshaft, has been time-consuming and complicated and has required expensive machine tools because an angular feed movement is to be imparted to the workpiece as it is machined and for this purpose the workpiece must be rotated about the axis of the eccentric portion which is to be machined. For this purpose the workpiece must be eccentrically gripped and its gripping arrangement must be changed when a different portion is to be machined. Besides, expensive means are required for driving the workpiece and the rotation of the workpiece involves a rotary unbalance, which adversely affects the machining conditions.

It is an object of the invention to eliminate these disadvantages and to provide a method which is of the kind described first hereinbefore and which permits a circular machining of eccentric shaft portions in a relatively fast, efficient and inexpensive manner. A machine tool which is highly suitable for carrying out that process and as simple as possible is also to be provided.

This object is accomplished in accordance with the invention in that the driven tool is moved along a circular path which surrounds the workpiece as the latter is machined, the workpiece is held in position as it is machined, and when the tool has been longitudinally positioned relative to the workpiece the tool and the workpiece are transversely displaced relative to each other until the center of the desired cross-section of the shaft portion to be machined coincides with the center of the circular path of the tool, which is subsequently engaged with the workpiece. Because the workpiece is held in position as it is machined and the tool performs also the circular feed movement, optimum cutting and machining conditions can be obtained with a relatively simple structure. Besides the movement of the tool along a circular path can be effected in a simpler manner and with higher accuracy than a rotation of the workpiece with rotary unbalance. Whereas it was previously necessary to eccentrically grip the tool and to frequently change that grip, it is now sufficient to effect a transverse displacement, which can be rather easily performed within a short time. For this purpose the workpiece which is held against rotation may be displaced whereas the tool is held in position or the workpiece may be held in position and the tool may be transversely displaced. In no case is it necessary to change the grip of the workpiece or to rotate the latter as it is machined, whether a centered or eccentric shaft portion is to be machined.

A machine tool which is particularly suitable for carrying out the method according to the invention comprises two gripping heads and a tool carriage, which are mounted on a machine bed and longitudinally displaceable relative to each other. The tool carriage may consist of a compound slide or the gripping heads may be provided with chucks which are displaceable in planes that are normal to each other and to the axis of the workpiece. The tool carriage comprises a carrying ring, which is known per se and adapted to be driven, and a radially movable slide is guided in said carrying ring and carries a machining unit which comprises the tool and means for driving the tool. The rotation of such carrying ring will virtually automatically impart to the tool a movement along a circular path and the tool can easily be moved radially into engagement with the workpiece by the radially movable slide. As the latter carries the machining unit, which comprises the tool and the means for driving the tool, the tool, in spite of the provision of the special means for driving the tool, can be moved into engagement with the workpiece and fed relative to the workpiece without any restriction and the tool proper will be stably held. The carrying ring must be so large that it can surround the workpiece and can be displaced relative to the workpiece also in an axial direction. The longitudinal positioning is effected in a conventional manner and in dependence on the concept of the machine tool in that the carriage is moved or in that the two gripping heads which grip the workpiece are displaced jointly. When the tool has been positioned, the axis of the carrying ring can be quickly and easily moved to coincide with the axis of the desired cross-section of the shaft portion which is to be machined. For this purpose, a compound slide may be used to move the carrying ring in the desired direction, which is normal to the axis of the workpiece, or the gripping heads may be displaced in synchronism to move the workpiece parallel to itself in the desired direction to the extent of its eccentricity. Compound tool slides are well known and, as shown, for example, in U.S. Pat. Nos. 3,880,025 and 4,326,323, comprise horizontally and vertically movable slides. The same result can be obtained if the carrying ring is movable relative to the machine bed in a first direction that is normal to the longitudinal axis of the workpiece, and chucks in which the workpiece is gripped are movable relative to the machine bed in a second direction, which is normal to the first direction and to the longitudinal axis of the workpiece. The radially movable slide can then be moved to engage the tool with the workpiece and the carrying ring can subsequently be rotated to impart to the rotating or oscillating tool the circular feed movement required for the desired machining of the shaft portion. When the machining proper has been effected, the parts are returned to their initial positions and the machine is then ready to perform the next operation.

Illustrative embodiments of the invention are shown by way of example on the drawing, in which

FIG. 1 is a side elevation showing a machine tool according to the invention,

FIGS. 2 and 3 are, respectively, in an end view and axial sectional view on a larger scale a machining unit used in that machine tool and

FIGS. 4 and 5 are axial sectional views which are similar to FIG. 3 and show two other illustrative embodiments of such a machining unit.

A machine tool 3 for an efficient circular machining of eccentric shaft portions, such as the crankpins 1 of a crankshaft 2, comprises a machine bed 4, two gripping heads 5, 6 for holding the workpiece between them, two back rests 7, 8, which also support the workpiece, and compound tool slide 10 carried by a carriage 9, which is movable along the machine bed 4. The illustrated compound tool slide 10 comprises horizontally movable slide 10b mounted on carriage 9 and having two vertical guides 10a vertically slidably supporting vertical slide 9a. A carrying ring 11 is rotatably mounted in vertical slide 9a of the tool slide 10 and is adapted to be driven so that it rotates about its axis. A radially movable slide 12 is adjustably mounted in the carrying ring 11 and carries a machining unit 13, which consists of a tool and means for driving the tool.

As is indicated in FIGS. 2 and 3, the machining unit 13 carried by the radially movable slide 12a, which is guided in the carrying ring 11, may comprise a grinding wheel 14, a belt drive 15 and a drive motor 16. An adjusting screw 18 driven by a motor 17 and in threaded engagement with an adjusting nut 19 of the radially movable slide 12a may be provided for adjusting the slide 12a so as to engage the grinding wheel 14 with the shaft portion that is to be machined.

The machining unit shown in FIG. 4 consists of a superfinishing device 20, which is mounted in the radially movable slide 12b and includes a tool consisting of a superfinishing stone 21, which is oscillated by a diagrammatically indicated pneumatic drive 22. In this case too, the radially movable slide 12 is movable by an adjusting drive comprising a motor 17, an adjusting screw 18 and an adjusting nut 19. Vibration-damping inserts 23 are provided to prevent a transmission of vibration to the carrying ring 11.

As is apparent from FIG. 5, the machining unit carried by the radially movable slide 12c may comprise a milling tool, such as an end mill 24, which is mounted on a driving spindle 25. The latter is rotatably mounted in the radially movable slide 12c and is driven by a motor 26. In this case the radially movable slide 12c can be adjusted by a pinion 28, which is driven by the motor 27 and in mesh with a rack 29 of the radially movable slide 12c.

By means of the carrying ring 11, the machining unit can be moved along a circular path around the workpiece so that eccentric shaft portions can be machined to a circular shape in a simple manner in that the carrying ring which has been longitudinally positioned is transversely displaced to a position in which it is coaxial to the desired cross-section of the eccentric shaft portion to be machined. Then the carrying ring can be used in an efficient manner to impart the circular feed movement to the tool. As the carrying ring 11 is rotated, the radially movable slide 12 guided in the carrying ring and the machining unit carried by the radially movable slide are moved around the workpiece so that the latter can be machined as desired without difficulty. The radially movable slide can be adjusted as is required for that machining. Simple ring conduits 30 are provided between the carrying ring 11 and the machine tool 10 and serve to power and control the machining unit 13 carried by the radially movable slide 12 and the drive means for adjusting the radially movable slide.