Device for machining a non-circular sectioned workpiece

A machine tool employs a cam, driven in response to an angular position signal of a workpiece, to cyclically displace a cutting tool with respect to the workpiece.

BACKGROUND OF THE INVENTION 
The present invention relates to machine tools and, more particularly to a 
high speed device for automatically machining a non-circular workpiece to 
a predetermined form. Still more particularly, the present invention 
relates to a high speed device for machining a non-circular workpiece that 
is controlled by a numerical controller or computer. 
One type of conventional device for machining a non-circular workpiece 
generally requires a model of the required type and profile of the 
workpiece to be prepared to be installed in the device for replication in 
the machine tool. This method, commonly called the pantograph method, 
requires that a precise model of the workpiece be made for each new 
required shape. In addition, because the models are closely traced by a 
stylus during the machining process, they are subject to wear and must be 
replaced periodically. The constant modelling and replacement of the model 
required in this type of machine therefor requires considerable time and 
labor. 
Inaccuracies inherent in the use of models to control the machining of the 
workpiece due to delays in positioning the cutting tool also require 
extensive and labor consuming rework of the machined workpiece. 
A second type of device for machining non-circular workpieces employs lead 
screws that are controlled by a numerical controllers. This type of device 
is also subject to inaccuracies because the lead screws for positioning 
the cutting tool against the workpiece are subject to small amounts of end 
play. Such end play result in machining errors as the lead screws change 
rotational direction to move the cutting tool in and out. Such errors may 
be corrected later, or may result in scrap. Labor for correction and 
wasted scrap increase costs. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide a device for 
machining a non-circular sectioned workpiece which overcomes the draw 
backs of the prior art. 
It is a further object of the invention to eliminate the need for the 
preparation of expensive models for controlling the shape of the 
workpiece. 
It is a still further object of the invention to eliminate the error 
inherent in the use of lead screws to position the cutting tool against 
the workpiece. 
The present invention uses an eccentric cam that is controlled by a 
numerical controller to replace the X-axis lead screw for positioning the 
cutting tool against the work piece. This eliminates the shortcomings of 
both conventional means. The improvement is due to the near linearity of 
the eccentric cam drive and to its rapid response. An eccentric cam drive 
may also replace ball screw drives, which are subject to excessive wear. 
Briefly stated, the present invention provides a machine tool employing a 
cam, driven in response to an angular position signal of a workpiece, to 
cyclically displace a cutting tool with respect to the workpiece. 
According to an embodiment of the invention, there is provided a machine 
tool comprising: means for rotating a workpiece about a first axis, means 
for producing an angular position signal responsive to an angular position 
of the workpiece, a cam, means, responsive to a position of the cam, for 
displacing the cutting tool along a second axis generally at right angles 
to the first axis, and means for moving the cam in response to the angular 
position signal. 
According to a feature of the invention, there is provided a machine tool 
comprising: means for rotating a workpiece about a first axis, an encoder 
connected to produce an angular position signal responsive to an angular 
position of the workpiece, a cam, a servo motor effective for 
bi-directional rotation of the cam, a cutting tool, means for permitting 
displacement of the cutting tool along a second axis at right angles to 
the first axis, a cam follower connected to the cutting tool, means for 
maintaining contact of the cam follower with a surface of the cam, the 
contact being effective for urging the cutting tool along the second axis, 
a numerical controller, and the numerical controller producing a drive 
signal for the servo motor in response to the angular position signal, 
whereby the cutting tool is cyclically displaced along the second axis in 
synchronism with rotation of the workpiece. 
The above and other objects features and advantages of the present 
invention will become apparent from the following description read in 
conjunction with the accompanying drawings, in which like reference 
numerals designate the same elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a main shaft 5 of a lathe is rotatably supported by 
head stock 4. Main shaft 5 may be rotated by any suitable conventional 
driving means (not shown), such as an electrically powered motor or the 
like. A chuck 6 is disposed on a first end of main shaft 5 for holding a 
first end of a workpiece 1. Chuck 6 rotates with main shaft 5 to rotate 
workpiece 1. The second end of workpiece 1 is rotatably supported by lathe 
center 7. 
An encoder 8 is connected on the axis at a second side of main shaft 5. 
Encoder 8 continually monitors the rotational position of main shaft 5 and 
therefor of workpiece 1 to generate an output signal that represents the 
rotational position of workpiece 1. 
An electrically encoded output of encoder 8, is connected to a programmable 
numerical controller 3. Numerical controller 3 develops X- and Z-axis 
control drive signals from the output of encoder 8. The X- and Z-axis 
control drive signals are connected to an X-axis servo motor 9 and a 
Z-axis servo motor 17, respectively. 
The output shaft of Z-axis servo motor 17 rotates a lead screw 18 under 
control of numerical controller 3. A second end of lead screw 18 is 
rotatably threaded into a saddle 15. Saddle 15 is slidably mounted on a 
base 16 so that it can slide lengthwise (along the Z axis) with respect to 
workpiece 1. Saddle 15 is moved along the Z axis of workpiece 1 by the 
rotation of lead screw 18. 
A slide 13 is disposed in saddle 15 for movement along an X axis at right 
angles to the Z axis, as defined by the direction of movement of saddle 
15. A cutting tool 10, for machining workpiece 1, is clamped by 
conventional means to an end of slide 15 adjacent to workpiece 1. 
A cam follower 14 is attached to the opposite end of slide 13. Cam follower 
14 is maintained in constant contact with an eccentric cam 11 by springs 
12. Cam follower 14 may be a non-rotating contact element but, in the 
preferred embodiment, it is in the shape of a wheel to reduce wear. 
Eccentric cam 11 is attached to the end of rotatable cam shaft 2, which is 
drivable both clockwise and counterclockwise by the output shaft of X-axis 
servo motor 9 under the control of numerical controller 3. The rotational 
position of cam shaft 11 determines the radial position of cutting tool 10 
with respect to workpiece 1 by pressing against cam follower 14 and, 
thereby, pressing cutting tool 10 mounted on slide 15, against workpiece 
1. The depth of the cut made by cutting tool 10 in workpiece 1 is 
determined by the rotational position of eccentric cam 11. 
FIG. 2 is a curve 19 showing the displacement of cutting tool 10 along the 
X axis as the rotational angle of eccentric cam 11 is varied. The 
horizontal axis represents 360 degrees of rotation of eccentric cam 11. 
The vertical axis represents the forward displacement of cutting tool 10 
caused by eccentric cam 11 at each angle of rotation. Curve 19 therefore 
represents the displacement of cutting tool 10 through 360 degrees of 
rotation of eccentric cam 11. 
As can be seen in curve 19 of FIG. 2, the displacement of cutting tool 10 
produced by 360 degree of rotation of eccentric cam 11 is equal to 180 
degrees of a sine wave (zero-theta). This indicates that the X-axis 
displacement of cutting tool 10 would not be linear through the central 
portion of the curve. For that reason, the clockwise and counterclockwise 
rotation of eccentric cam 11 is limited between angles zero and theta 1. 
Over this range the X-axis displacement of cutting tool 10 by eccentric 
cam 11 is substantially linear. 
Referring again to FIG. 1, during operation, as workpiece 1 on main shaft 5 
is rotated from a zero degree starting point, encoder 8 continually 
monitors the rotational position of workpiece 1 and reports the angular 
data in the form of a coded electrical signal to numerical controller 3. 
In response to the received angular data, numerical controller 3 
determines the X-axis displacement required at each position to achieve 
the programmed shape in the finished workpiece 1. Suitable control signals 
are connected to X-axis servo motor 9 to rotate cam shaft 2 and eccentric 
cam 11, thereby displacing cutting tool 10 the required distance along the 
X axis. 
To achieve an elliptical cross section in workpiece 1, such as cross 
section 22 or 23 shown in FIG. 3, the rotation angle of eccentric cam 11 
is set initially to a value giving minimum displacement of cutting tool 
10. As workpiece 1 is rotated, numerical controller 3 receives rotational 
position data from encoder 8 and generates the required control signal to 
cause X-axis servo motor 9 to rotate cam shaft 2 and eccentric cam 11 
clockwise, thus displacing cam follower 14, slide 13 and cutting tool 10 
along the X axis toward workpiece 1. When workpiece 1 reaches 90 degrees 
of rotation from its zero reference angle, eccentric cam 11 is reversed. 
At 180 degrees of rotation of workpiece 1, eccentric cam is again reversed 
to move cutting tool 10 toward workpiece 1. This process is repeated in 
the angular range from 180 to 360 degrees. The amount of displacement of 
cutting tool 10 is relatively small for a near-circular ellipse such as in 
cross section 22 of FIG. 3, and is greater for an ellipse such as in cross 
section 23. 
While the workpiece is being rotated, a Z-axis drive signal from numerical 
controller 3 causes Z-axis servo motor 17 to rotate lead screw 18 in the 
direction necessary to displace saddle 15 and cutting tool 10 along the 
length of the workpiece at a controlled rate as the cutting cycle 
previously described is repeated. In this manner the elliptical cross 
section of workpiece 1 is produced along its entire length. 
Having described preferred embodiments of the invention with reference to 
the accompanying drawings, it is to be understood that the invention is 
not limited to those precise embodiments, and that various changes and 
modifications may be effected therein by one skilled in the art without 
departing from the scope or spirit of the invention as defined in the 
appended claims.