New technology for setting up the working coordinate system in micromilling K. Popov a , S. Dimov a , A. Ivanov a , D. T. Pham a , E. Gandarias b a Manufacturing Engineering Centre, Cardiff University, Cardiff CF24 3AA, UK. b Mondragon Goi Eskola Politeknikoa, Mondragon Unibertsitatea, Loramendi 4, 20500 Arrasate, Spain Abstract A major issue in micro milling is the setting up of the origins of the Machine Working Coordinate Systems (MWCS). The existing methods for carrying out this operation have an unacceptably high error along the Z axis, due to the spindle thermal enlargement, and are effective only when the machining is “relative” to other surfaces milled with one cutting tool within one operation. Therefore, more efficient technical solutions are required for setting up MWCS and to reduce further uncertainties associated with micro-milling operations. This paper describes a new cost-effective method for setting up MWCS that applies a new technical solution for detecting the contact between the cutting tool and the workpiece. A prototype system was used to validate experimentally the accuracy and repeatability of the proposed method. The results obtained showed that the sensitivity of the system is sufficient to detect accurately the contact between the cutting tool and the workpiece, and thus to set up the MWCS origins. It was concluded that by applying this method it would be possible to minimise uncertainties introduced by the spindle thermal enlargement and touch probe run-outs in setting up micro machining operations. Also, the tests undertaken showed that the prototype system is reliable and convenient for use by machine operators. Keywords: micro-milling, on-line measuring systems, tool–workpiece coordinate system setting 1. Introduction Recent developments in machining technology and machine tool design, especially in micro-milling and a growing demand for product miniaturisation reflect the constantly increasing requirements towards the accuracy of the produced components. This leads to continuous reduction of feature sizes and correspondingly the diameter of the cutters employed for their machining. In particular, the applications that ”push” the micro-milling technology to its limits are the manufacture of micro parts for watches, keyhole surgery, housings for micro-engines, tooling inserts for micro injection moulding and hot embossing, and housings and packaging solutions for micro-optical and micro fluidics devices. A common challenge across all these application areas is the machining of micro features with dimensions smaller than 100 µm. Many researchers have contributed to the creation of the currently available process knowledge about conventional milling. Unfortunately, the size effects are dominant in micro-milling, and therefore it is not possible to benefit directly from this rich knowledge repository. To advance this technology it is necessary to study systematically the factors that affect the process reliability when it is employed for machining components incorporating micro features. Micro machining using conventional technologies, such as milling, present unique challenges. When the machining is performed with a micro cutter, diameter smaller than 200 µm, cutting forces and tool pressures present a whole new realm of problems. Tool pressure appears when the tool channels are filled with workpiece material, e.g. burrs and chips, especially during drilling operations. Under such conditions, any “drastic” changes of the cutting forces and their directions may lead to tool breakages. The spindle must be dynamically stable in order to minimise its thermal enlargement, tool change variations, and vibrations. In particular, any vibrations or run-outs at the tool tip may have adverse effects on the surface finish and accuracy of the machined micro features. To reduce these negative effects it is necessary to control as tightly as possible the whole set of machining variables, associated with different components of the Machine tool-Fixture-cutting Tool-Workpiece System (MFTWS) and the surrounding environment, and thus minimise their overall effect on part quality. In the context of part dimensions and feature sizes, relative accuracy (tolerance to feature size ratio) in micro machining brings new challenges [1]. In particular, the absolute accuracy achievable in milling micro features can be considered comparable to that in ultra-precision engineering; however their relative accuracy may not be acceptable for a range of applications. Although in conventional ultra-precision manufacturing a relative accuracy of 10 -4 -10 -6 can be attained, in micro- machining it does not exceed 10 -2 -10 -3 . For example, to achieve a relative accuracy of 10 -2 or better the absolute accuracy of a 100 µm groove has to be in the sub- micron range. Hence, it becomes necessary to re-think the meaning of precision in micro machining. There are solutions that minimise the uncertainty in MFTWS when performing micro-milling. Examples of such solutions are Tool Condition Monitoring Systems (TCMS) that improve the effectiveness of micro-milling operations. In particular, direct and indirect methods are developed for controlling the machining variables associated with the cutting tools. The direct methods are employed for detecting the actual tool status relying on optical sensing [2], while indirect ones monitor the tools through the existing relationships between their parameters [3]. There are several key areas of concern in regard to MFTWS when machining features at micro scale: 1. Environmental changes that impact on the