Design, development and testing of a four-component milling dynamometer for the measurement of cutting force and torque

Abstract

The cutting forces generated in metal cutting have a direct influence on generation heat, tool wear or failure, quality of machined surface and accuracy of the work piece. In this study, a milling dynamometer that can measure static and dynamic cutting forces, and torque by using strain gauge and piezo-electric accelerometer has been designed and constructed. The orientation of octagonal rings and strain gauge locations has been determined to maximise sensitivity and to minimise cross-sensitivity. The force and torque signals were captured and processed using proper data acquisition system. The dynamometer has been subjected to a series of tests to determine its static and dynamic characteristics. The results obtained showed that the dynamometer could be used reliably to measure static and dynamic cutting forces and torque.

Introduction

Force measurement in metal cutting is an essential requirement as it is related to machine part design, tool design, power consumptions, vibrations, part accuracy, etc. It is the purpose of the measurement of cutting force to be able to understand the cutting mechanism such as the effects of cutting variables on the cutting force, the machinability of the work piece, the process of chip formation, chatter and tool wear [1]. For over 100 years, metal-cutting researches attempting to understand the cutting behaviour better have investigated the cutting forces in metal cutting. It has been observed that the force values obtained by engineering calculations contain some errors compared to experimental measurements. Since the undeformed chip thickness and the direction of cutting speed vary at every moment, cutting process in milling is geometrically complex. Owing to such complexity, the cutting forces even in steady-state conditions is affected by many parameters and the variation of cutting force with time has a peculiar characteristic [2]. The need for measurement of all cutting force component arises from many factors, but probably the most important is the need for correlation with the progress of tool wear [3]. If this can be obtained, it will be possible to achieve tool wear monitoring in milling based on force variation. Another reason for the cutting forces measurement is that it is a good indicator in detecting tool wear. It is well known that during the cutting process, the cutting parameters such as cutting speed, feed rate and depth of cut often present a deviation from the calculated values. In a three-dimensional cutting operation, three force components are necessary, whereas while drilling or tapping, only a torque and thrust drill are required [4].

The strain gauge produces a clear relation between the measured quantity and the strain on a suitable spot on the spring element [5]. In most cases, the static force is obtained by a strain gauge type sensor which produces an output voltage proportional to elastic deformation.

The cutting force dynamometers must be manufactured at sufficient accuracy and high rigidity, and particularly suitable for dynamic loads [6]. Ito et al. [7] designed some strain gauge-based dynamometers that can be adapted to some machine tools and defined the criterions of their rigidity and sensitivity. In designing the dynamometer, some principles such as parallel beam type [1], [8], circular hole [9], [10], [11], piezo-electric [12], [13], etc., have been used widely.

This study outlines a strain gauge-based octagonal-ring type analogue dynamometer design and prototyping. This dynamometer is capable of measuring three-force components and can as well be used to read the torque value in drilling, tapping, etc. As the reading of analogue values manually is a difficult and tedious job, a computer connection for data acquisition has been realised.

In comparison of the piezoelectric sensors, semiconductor (silicon) strain gages and strain gages, the desirable features of piezoelectric sensors include their rugged construction, small size, high speed, and self-generated signal. On the other hand, they are sensitive to temperature variations and require special cabling and amplification. They also require special care during installation. Electrostatic pressure transducers provide high-speed responses (30 kHz with peaks to 100 kHz). A piezoelectric force sensor is almost as rigid as a comparably proportioned piece of solid steel. Semiconductor (silicon) strain gages are small in size and mass, low in cost, easily attached, and highly sensitive to strain but insensitive to ambient or process temperature variations. Strain gages require simple construction with a small mass and volume. Unfortunately, the most desirable strain gage materials are also sensitive to temperature variations and tend to change resistance as they age. For tests of short duration, this may not be a serious concern. Although the materials exhibited substantial nonlinearity and temperature sensitivity, they had gage factors more than 50 times, and sensitivity more than a 100 times. The fundamental difference between these piezoelectric crystal sensors and static-force devices such as strain gages is that the electric signal generated by the crystal decays rapidly [14], [15].

It is well known that the strain gage-based multicomponent sensors were superseded by piezoelctric ones, due to the increased rigidity which has vital function in metal-cutting tests and improved dynamic range. The peizoelectric ones are, however, more expensive, around 20:1. Hence, this paper describes a specific design of a strain gage-based multicomponent sensor due to the reached necessary rigidity and dynamic range, without the need to choose the piezoelectric option.