Reducing the vibration when cutting internal threads by mills equipped with replaceable hard-alloy plates

Abstract

In the past decade, Russian manufacturing has made expanded use of domestic and imported numer� ically controlled machine tools. Therefore, the machining of complex surfaces on such machine tools is of great interest today. The cutting of internal threads with planetary tool motion is one such method. Thread cutting is a complex and laborious process. Cutting internal threads is even more com� plex, on account of the spatial constraints on tool operation. The most effective alternative to traditional thread cutting is the cutting of internal threads by mills equipped with replaceable hardalloy plates of TMC type, produced by Kennametal, Vardex, Iscar, and Emuge. In comparison with other technology, this method reduces the machine time, thanks to high cutting speed and fast supply, as well as the possibility of com� plete (over the depth of the profile) machining of the thread in a single pass. The formation of short and eas� ily removable chip reduces the time for chip removal in maintenance. Regardless of the thread diameter, a sin� gle tool may machine left and right threads and also internal and external threads. In the event of tool frac� ture, it is easily extracted from the working space. The specified thread precision and configuration may be obtained by a single tool, by introducing appropriate corrections to the control program of the numerically controlled machine tool. In a hole of finite depth, a thread may be cut practically over the whole length, without the need for a channel for tool extraction. This method performs well in machining gray and high� strength iron castings, and also castings of aluminum alloys, corrosionresistant steel, and titanium (1). Nevertheless, analysis of experimental experience shows that, even when all the factors ensuring suffi� cient rigidity in the tool system are present, increased selfoscillation may be observed. This is accompanied by premature wear and shear of the tool's cutting edge. The vibrations that appear in thread cutting create significant problems in tool operation, with impair� ment of the technological capabilities and the effi� ciency. In the present work, we consider two means of reducing the vibrational loads on the tool's cutting edge: (1) investigation of the dynamic characteristics in machining an internal thread by means of planetary tool motion, so as to ensure a more stable cutting pro� cess; (2) investigation of the damping processes of a composite hardalloy plate for a thread mill. In the first case, we take into account that milling is accompanied by variable cross sections of the cut aris� ing in tool motion. This facilitates selfoscillation. The thickness a of the cut layer in one pass of the mill tooth varies from zero to the maximum value amax. The width B of the cutting edge (the cut width) is either constant or (in traditional machining) increases in proportion to the thickness of the cut layer. In cutting an internal thread by means of planetary tool motion, the cut width increases in inverse proportion to the thickness of the cut layer. In other words, when B is a maximum, the cut thickness a is a minimum (Fig. 1).