This paper reports the modeling and experimental work done for the design of a microgripper using surface tension forces for the handling of submillimetric balls of a watch bearing. Its originality lies in the adaptation of existing capillary forces models to this microassembly case study and in the exhaustive characterization work required as a first step towards an automated assembly. Picking and placing operations have been experimentally studied, and solutions are proposed to tackle the typical related problems. The component feeding should be studied in detail.
This paper describes the study of a gripper using the surface tension effects to pick and place the 0.5mm or 0.3mm diameter balls of a millimetric watch bearing. Two liquid supply strategies have been tested (pressure drive and tip dip). The effects of the coating (through the measurement of the contact angles), the presence of an internal channel and the size of the gripper have been studied. Analytical and numerical force models have been developped and validated thanks to a test bed allowing the measurement of the developped force (typically of the order of 100µN) with a resolution of 1µN. A complete pick and place cycle has been performed using the 0.5mm diameter gripper. Such a test has still to be done in the future with the 0.3mm diameter gripper.
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Abstract:Assembly of high precision products is often done manually. The main reasons are the complexity of automation and the production volumes that often remain small to medium. Watches, medical devices and sensors are some examples of products requiring high precision assembly: often expensive products with high margins. It is interesting to notice that achieving higher assembly yield allows for relative quick pay-back of equipment. This is also the reason why western European countries remain competitive in this field. In this paper, the important points to remember when selecting a solution to efficiently assist operators in their assembly tasks are highlighted. Good assistance should lead to higher yields, higher throughputs and better quality. One should take into account assembly processes and their difficulties, as well as production volume and economic profitability.Key words: micro-assembly; precision assembly; man-machine cooperation; semiautomatic assembly INTRODUCTIONPrecision assembly is often done manually due to difficult processes, high equipment cost and short product life cycle. But manually assembly results in low yields and thus high assembly cost, which drives to find better solutions. The issue is to provide efficient assistance to assembly operators with automatic or semi-automatic devices: some examples are high precision axis with manual actuation like the Tresky cell's, semi-automatic assembly cells, telemanipulation with haptic devices (Ferreira 2003), intuitive programming, and vision enhancing.It is a priori difficult to decide how to assist the operator, and to choose the part of the assembly process to automate. This decision shouldn't rely on WHAT IS THE BEST WAY TO INCREASE EFFICIENCY IN PRECISION ASSEMBLY?2 some kind of dogmatic principle, but on a thorough analysis and on long and short term economic considerations. HISTORY AND MACRO ASSEMBLYIn the past, all assembly operations were entirely manual. Mechanization was introduced to achieve lower cycle times, as well as robotics some decades later. Machines achieved simple feeding operations, and simple assembly operations with straight movements where men could easily be replaced. Operators did the more complex operations, such as orienting, adjusting, tuning, and inspecting.Design for Assembly rules (Boothroyd, 1991) were introduced in the 80 thies by classifying assembly processes from the easiest (or fastest) to the most difficult for automatic assembly. Soon people understood that easy for automatic assembly also meant easy for manual assembly. From that point, the design rule for new products was: make them easy to assemble automatically, even if you do it by hand.The easiest assembly operation is a simple straight insertion along one axis, the preferred one being vertical. It is the well known peg-in-hole situation. Assembly with two or more axis, in hidden positions becomes difficult for machines, but also trying for operators.A problem occurs in simple insertion when there is a slight misalignment between the insertio...
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