BACKGROUND OF THE INVENTION

This invention relates to the art of thread cutting and, more particularly, to an improved thread cutting die and method of making the same.

It is of course well known to provide threads on the end of a workpiece through relative rotational and axial displacement between the workpiece and thread cutting dies supported on a thread cutting die head. The die head may be manually rotated relative to the workpiece, or mounted on the tool carriage of a power driven threading machine in which the workpiece to be threaded rotates relative to the die head. Such a power-driven thread cutting machine is shown, for example, in U.S. Pat. No. 4,613,260 to Hayes, et al., the disclosure of which is hereby incorporated herein by reference for background information. The thread cutting dies are made from high speed steel bar stock and haste thread cutting teeth on one end thereof which extend in the direction of relative rotation between the die head and workpiece. It is known that the coating of such dies, including the teeth, with an ultra hard material such as titanium nitride having a hardness greater than that of high speed steels improves the cutting performance of the dies and extends the life thereof, especially in high volume output situations and in threading hard to cut materials such as stainless steel.

SUMMARY OF THE INVENTION

While coated thread cutting dies heretofore available provide the foregoing advantages, it has been discovered that further improvement in connection with the quality of the threads cut is achieved by removing the coating from the faces of the teeth of the cutting die prior to using the latter. Particularly in this respect, vibration or chatter is reduced, especially in connection with dry thread cutting operations, whereby the quality of the thread is improved relative to that achieved with the coating on the tooth faces. In connection with manufacturing the thread cutting die, the teeth are preferably formed by grinding and, after the coating is applied to extend over the entire tooth form and shank of the cutting die, the coating is removed from the teeth faces such as by a regrinding operation. Alternatively, the coating can be applied to the die so as to exclude coating of the faces of the teeth. In either event, the coating remains on the shank of the cutting die and thus provides the increased hardness at the leading edge of the teeth with respect to the direction of cutting to promote improved performance and tool life relative to uncoated high speed steel cutting dies. At the same time, the improved performance is further enhanced by the removal or absence of the protective coating from the faces of the thread cutting teeth.

It is accordingly an outstanding object of the present invention to provide an improved thread cutting die of the character having thread cutting teeth formed on one end of a block of high speed steel and a protective coating of a material having a hardness greater than that of the high speed steel.

Another object is the provision of an improved thread cutting die of the foregoing character wherein the faces of the thread cutting teeth are free of the coating material.

Yet a further object is the provision of a method of making a thread cutting die of the foregoing character by applying a coating of ultra hard material such as titanium nitride to all parts of the die except the faces of the cutting teeth.

Still another object is the provision of a method of producing a thread cutting die of the foregoing character wherein the coating material is applied to the teeth and is removed from the teeth prior to use of the thread cutting die.

Another object is the provision of a method of the foregoing character wherein the teeth are provided on the high speed steel block by grinding and wherein the coating material is subsequently removed by regrinding the teeth.