Micro end mill tool temperature measurement and prediction

Abstract

The objective of this work is to characterize the heat transfer in micro end mill tools during machining operations. This analysis will aid in the design of heat dissipation strategies that could potentially increase tool life and machining precision. Tool temperatures, above the unmachined workpiece surface, have been measured using an infrared camera during slot milling of aluminum (6061-T6) and steel (1018) with 300 μm-diameter two-flute tungsten carbide end mills. The measured temperatures compare favorably with temperature distributions predicted by a two-dimensional, transient, heat transfer model of the tool. The heat input is estimated by applying Loewen and Shaw’s heat partitioning analysis. Analysis of heat transfer in the tool found that 46 s into a cut conduction through the length of the tool, storage in the tool, and convection from the surface account for 41.5%, 45%, and 13.5% of the heat generated during machining. Thermal expansion and cooling strategies are discussed.

Introduction

Micro end milling is becoming an important micro/meso-scale manufacturing process for producing parts and features in the range of tens to thousands of micrometers. It is suited for use in applications which require the manufacture of prototypes or small batches of micro-featured components since a part can be generated from start to finish with minimal overhead and often in a single step. Micro end milling is capable of removing material from a workpiece in all three dimensions, making it possible to generate parts with a wide range of complex features and aspect ratios. Further, a wide range of engineering materials can be machined. Polymers [1], metals and metal alloys [1], [2], [3], [4], [5], [6], [7], [8], [9], and pre-sintered powder ceramics [5] have been successfully machined by micro end milling. It has enabled the manufacture of biomedical components, micro-propellers, microfluidic devices, micro-heat sinks, micro-heat exchangers, and X-ray lithography masks [10], [11], [1].

The successful application of micro end milling requires that micron-scale features be produced with tight tolerance, quality surface finish, and limited tool wear. All variables which are strongly dependent on machining temperatures. The temperature distribution in the workpiece and tool directly affects the amount of thermal expansion that occurs. Thus, the thermal expansion may need to be compensated for, or the temperature actively controlled, in order to achieve the desired tolerances. Further, the temperature at the tool–chip and tool–workpiece interfaces strongly influences the quality of the machined surface finish and the rate of tool wear. Properly controlling the machining temperatures can help increase the tool life and subsequently reduce the cost of micro end milling a part.

Jun et al. demonstrated that an atomized spray (6–7 μm droplet diameter) of metalworking fluid achieved significantly greater workpiece cooling and tool life improvement than flood cooling as compared with dry micro end milling [12]. Temperatures below ambient were measured in the workpiece just below the cutter: a result of liquid evaporation at the surface. While the lubrication provided by a metalworking fluid may be required for certain applications, it is important to determine effective methods of removing heat during dry end milling operations.

Studies on micro-mechanical machining (i.e., small tools) and ultra-precision cutting operations (i.e., small feeds and depths-of-cut) have found that cutting temperatures are significantly lower than that for their macro-scale counterparts. Dhanorker et al. numerically simulated cutting zone temperatures ranging from 50–60 ^∘C for 2024 aluminum to 100–150 ^∘C for 4340 steel during micro end milling [13]. They attribute the low temperatures to the small chip loads (10 μm/tooth) used to prevent the micro end mills from deflecting and breaking. Moriwaki and Iwata et al. investigated machining temperatures in ultra-precision cutting operations with cubic-boron nitride, diamond, and high-speed steel (HSS) tools using small chip loads down to 0.1 μm[14], [15], [16]. They found cutting temperatures to range from 80 to 350 ^∘C, with temperatures 100 to 150 ^∘C greater for duller tools when cutting aluminum and copper. Thermal expansions of the tool and workpiece resulted in tolerance deviations of 0.5 and 1.0 μm for new and worn tools, respectively. Moriwaki et al. further investigated the effects of cutting edge radius by using HSS tools with cutting edge radii ranging from 1 to 10 μm and developing a finite element (FEM) based heat transfer model of the cutting system accounting for the relatively large edge radius. It was found that the thermal gradients in the workpiece can be much greater than in the tool due to the difference in thermal diffusivities (i.e., diamond tool versus metal workpiece).

While the cutting zone temperatures during micromachining are significantly smaller than during conventional macro-scale machining operations, significant thermal expansion can occur. In addition, it can be desirable to further reduce the temperatures in order to increase tool productivity. This paper presents an investigation of heat transfer and temperatures in the micro end mill tool by examining the region above the unmachined workpiece surface (Fig. 1). The magnitude of thermal expansion resulting from the end mill heating and cooling strategies are also investigated.