The present invention relates to the art of power driven threading machines and, more particularly, to a thread cutting die head having improved control for the receding of thread cutting dies in connection with cutting a tapered thread on a workpiece.
Power driven thread cutting machines are of course well known and basically comprise a rotatable spindle and chuck assembly for rotating a workpiece to be threaded, and a thread cutting die head supported on a tool carriage for axial displacement relative to the workpiece to advance thread cutting dies supported by the die head into thread cutting engagement with the end of the workpiece. Generally, the die head assembly is pivotally supported on one side of the tool carriage for displacement between stored and use positions. In the use position, the axis of the die head is aligned with the workpiece axis, and the side of the die head opposite the pivotal mounting engages the corresponding side of the tool carriage to axially and vertically support the die head in the use position.
It is likewise well known in connection with cutting a tapered thread on a workpiece to release the thread cutting dies at the end of the thread cutting operation for displacement radially outwardly of the workpiece, or to provide for the thread cutting dies to progressively recede radially outwardly relative to the workpiece near the end of or throughout the thread cutting operation, followed by displacement away from the workpiece. In such die heads, the thread cutting dies are supported for radial displacement inwardly and outwardly relative to the die head axis and are so displaced by a cam plate coaxial with and pivotal about the die head axis. The cam plate is interengaged with the thread cutting dies in a manner whereby displacement of the cam plate in opposite directions about the die head axis displaces the thread cutting dies radially inwardly and outwardly thereof. The cam plate is spring biased to displace the thread cutting dies radially outwardly, and the thread cutting operation is initiated with the cam plate latched in a given angular position against the spring bias. Release of the cam plate provides for the spring bias against the cam plate to displace the latter to in turn displace the thread cutting dies radially outwardly of the workpiece. If such release of the cam plate is at the end of a thread cutting operation, the displacement of the cam plate and thus receding of the thread cutting dies, takes place immediately to retract the thread cutting dies to their outermost positions in which they are spaced from the workpiece. If such release is to provide for the progressive receding of the thread cutting dies near the end of or during the entire threading operation, angular displacement of the cam plate under the bias of the cam plate spring is controlled to achieve such progressive receding until the end of the thread cutting operation at which point the cam plate is immediately released for further angular displacement by the biasing spring to retract the thread cutting dies to their outermost positions away from the workpiece.
Heretofore, progressive release of the cam plate near the end of a thread cutting operation, or during the entire threading operation has been achieved through the use of a lever actuated latch arrangement including a lever pivotally mounted on the die head and extending into the path of movement of a workpiece being threaded. The lever is engaged and displaced by the workpiece and in turn either disengages from the cam plate, immediately or progressively, or displaces another component which immediately releases or controls the progressive release of the cam plate to achieve gradual receding of the thread cutting dies near the end of or during the threading operation. Prior art arrangements for achieving progressive release of the cam plate in the foregoing manner near the end of the threading operation are shown, for example, in U.S. Pat. No. 4,288,181 to Sakaguchi et al, and Japanese Patent Application No. 57-20101. Prior art arrangements for achieving the progressive release of the cam plate in the foregoing manner during the entire threading operation are shown, for example, in U.S. Pat. No. 4,880,340 to Taki et al, Japanese Patent Laid-Open No. 57-66817 to Sakamoto, and Japanese UM Laid-Open No. 60-7931 to Gotov.
In the prior art arrangements providing for progressive release of the cam plate, the relative displacement between the component parts of the release mechanism is directionally transverse sliding displacement during which considerable force is applied between the relatively sliding surfaces resulting in rapid wear and, thus a loss in the accuracy and/or uniformity of threads being cut. More particularly in this respect, the lever or the component displaced thereby moves axially of the die head and is engaged by the cam plate or a component thereon which is biased by the cam plate biasing spring to pivot about the die head axis and thus transverse to the direction of movement of the lever or the component displaced thereby. Thus, during a threading operation, the relative displacement between the interengaging surfaces of the component parts is simultaneously axially and circumferentially with respect to the die head axis. It will be appreciated that such directionally transverse sliding interengagement between the component parts, which is repeated for each threading operation, results in rapid wearing of the component parts. Both the point of release of the cam plate at the end of a threading operation and control of the receding action of the thread cutting dies by the progressive release of the cam plate during a threading operation are important with respect to achieving accurate thread cutting and/or consistently uniform thread cutting. It will be appreciated that as the slidably engaging component parts of the release mechanism progressively wear, such accuracy and/or consistency is progressively reduced.