PSI - Issue 13

Peter Monka et al. / Procedia Structural Integrity 13 (2018) 959–964 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Despite the constant progress in industry, the threaded joints are still one of the most frequently used solutions for assembling mechanical parts, because it leads to an assembly having a high strength and high rigidity. Moreover, the usage of thread allows components disassembly for maintenance and recycling. According to Cao, T. and Sutherland, J.W. (2002), threading is divided into two types, internal and external. Concerning the manufacturing of internal threads, two main manufacturing processes are available: cut tapping and form tapping. The choice of one of these processes has to result from considerations and technologist’s decision, who takes into account process aspects, and also from an analysis of the specifications and characteristics of the internal thread. Internal threading is done using a tool called a „tap“ in a hole drilled to a specific diameter for the thread size and pitch that is necessary to cut. Taps come in various configurations and materials, the most common being High Speed Steel (HSS) for softer materials and Cobalt for hard materials like stainless steel. Some manufacturers apply coatings designed to increase the cutting ability and life of their products, beneath that, it is usually one of the two materials for most applications. The study presented in the article is focused on a durability of taps making the internal threads by cutting.

Nomenclature FFT

Fast Fourier Transformation

HSS CNC

High Speed Steel

Computer Numerical Control

2. State of the art

Tapping is a difficult operation due to the large number of involved cutting edges and complicated synchronization necessary between rotational and feed movements of the tap. A task is particularly difficult at high speeds. Given that tapping is usually performed at the end of the product flow, when a work piece has reached high added value, it is of interest to predict when these problems will occur in order to avoid them. (Ahn et al., 2003) Tap is multi-wedge cutting tools with an active part in required profile in which a groove is ground. This groove forms the tool face and divides the base screw surface of the tool for a specified number of teeth. At the same time, it is used to remove the chips from the cutting site. The cutting part of tap has the shape of a cone, the bevel of the cone corresponding to the angle of adjustment of the major cutting edge. The calibration part of the tap serves to guide the tool and calibrate the thread shape. It decreases toward to the clamping shank, which ensures its conicity, thereby reducing the friction between the tap and already machined thread. Conicity is selected in the range from 0.05 to 0.1 mm on length of 100 mm. By grinding of the tap on the tool face, the transition area between the cutting and the calibration portion of the cone moves more towards the shank, i.e. to the part with reduced diameter of the tool, which results in a reduction in the diameter of the cut thread. (Smith, 2008) The cutting tool operates efficiently only when the cutting wedge has a suitable shape characterized by geometric parameters, i.e. no loaded area of the cutting wedge exhibits the critical wear defined by the particular operating conditions according to the characteristics defined by standards, as it is e.g. in STN ISO 3685. This standard defines tool wear as a change in tool shape compared with the original shape due to material loss or deformation during cutting. However, the indicator of the cutting tool wear may not be only a change in the geometry and in dimensions of the cutting wedge as it is formulated by the standard. (Valicek, 2008) At the finishing operations, when the wear of the surface getting worse with increasing wear, the criterion of the tool wear may be an achieved roughness of the machined surface or an achieved accuracy of the dimensional and geometrical tolerances. In roughing operations, the quality of the machined surface is not indicative and the tool wear indicator can be the dynamics of the machining process, the ability of cutting wedge to control the chip removal or dimensional wear of such size when there is a risk of sudden fracture of the cutting wedge. (Yao, 2014) When looking at the durability of cutting tools in general, cutting speed, feed rate, cutting depth and geometry of the cutting tool can be considered as the most important factors (excluding cutting material and workpiece material).