Identification and control for micro-drilling productivity enhancement

Micro-hole drilling (holes less than 0.5 mm in diameter with aspect ratios larger than 10) is gaining increased attention in a wide spectrum of precision production industries. Alternative methods such as EDM, laser drilling, etc. can sometimes replace mechanical micro-hole drilling, but are not acceptable in PCB manufacture because they yield inferior hole quality and accuracy. The major difficulties in micro-hole drilling are related to the wandering motions during the inlet stage, high aspect ratios, high temperature, etc. However, of all the difficulties, the most undesirable ones are the increases in drilling force and torque as the drill penetrates deeper into the hole. This is mainly caused by chip-related effects. Peck-drilling is thus widely used for deep hole drilling despite the fact that it leads to low productivity. Therefore, in this paper, a method for cutting force regulation is proposed to achieve continuous drilling. A proportional plus derivative (PD) and a sliding mode control algorithm will be implemented and compared for controlling the spindle rotational frequency. Experimental results will show that sliding mode control reduces the nominal torque and cutting force and their variations better than PD control, resulting in a number of advantages, such as an increase in drill life, fast stabilization of the wandering motion, and precise positioning of the holes.     

微细孔超高速钻削电主轴的开发

随着对电器产品的小型、轻量、高集成、高可靠性要求的提高，在印刷电路板上进行微细孔加工越来越广泛。为了加工出直径小于1mm的微细孔，我们开发出了一种能稳定运转的超高速电主轴，其最高转速可达150，000r/min。本文重点介绍这种超高速微细孔钻削电主轴设计的关键技术，包括气体静压轴承的设计、内装电机的冷却与散热等。     

Using workpiece vibration cutting for micro-drilling

The purpose of this paper is to investigate the effects of assisted vibration on the drilling quality of aluminium alloy (Al 6061-T6) and structure steel (SS41). In the past, research methodology of vibration drilling on small-diameter holes has mainly involved vibrating from the spindle side. In this paper, a new approach to obtain the desired vibration is proposed from the workpiece side, by a self-made, vibrating worktable. Through extensive experiments with a twist drill size of 0.5 mm, we found that hole oversize, displacement of the hole centre, and surface roughness of the drilled wall could be improved with the increase of vibrating frequency and amplitude. Roundness of the drilled hole could also be improved when high amplitude and proper frequency are imposed.     

Micro-drilling of monocrystalline silicon using a cutting tool

The micro-drilling of monocrystalline silicon using a cutting tool was tested with the aim of fabricating three-dimensional and high aspect ratio micro-shapes. Micro-tools with a D-shaped cross-section and cutting edge radius of 0.5 μm were fabricated by wire electrodischarge grinding (WEDG). The results showed that, with a depth of cut of 0.1 μm, ductile-regime cutting was realized, and that a tool clearance angle larger than 0° was necessary to prevent fractures at the hole entrance. The smallest machinable hole was of 6.7 μm diameter, which is the smallest not just in the present study, but of all holes drilled using a cutting tool so far. Furthermore, an aspect ratio of more than four was obtained in the drilling of a 22 μm diameter and 90 μm deep hole.     

Effect of backstitch tool path on micro-drilling of printed circuit board

As electronic components have become smaller, micro-drilling tools have been developed and used to drill holes in the hundred-micrometer range on printed circuit boards. To improve the productivity of the drilling process, printed circuit boards are generally stacked in several layers and drilled simultaneously. In this process, however, misalignment of the drilled holes on the top and bottom layers occurs, and this consequently degrades the overall product quality. To solve this problem, a new tool path strategy is proposed, which we refer to as the backstitch tool path. The basic scheme of the suggested strategy is to make a balance on the substrate. We compared this approach with a conventional tool path by examining the hole positioning error, drilling thrust force, drilling torque, and drilling duration under various drilling conditions. The effects of each process parameters were analyzed, and the suggested backstitch tool path has a positive effect on low bending stiffness tool, too much adjacent holes, and higher in-feed rate, respectively. Applying the backstitch tool path strategy to a micro-drilling operation improved the productivity and product quality.     

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天然气水合物是一种非常规的、洁净天然气资源，开采难度大。文章主要介绍了水合物实验研究设备——天然气水合物合成及微钻实验装置。该设备为科研人员在实验室内进行天然气水合物形成条件，模拟自然环境下进行天然气水合物钻采以及进行钻井工艺实验研究提供了良好的实验基础。文章介绍了设备的主要组成部分、材料选择及各部件的主要工作机理，设备可适用的实验内容等。     

Micro-drilling of alumina green bodies with diamond grit abrasive micro-drills

Since ceramic plates containing many micro-holes are used for MCPs (Micro-channel plates) for electron amplification, catalytic converters, filters, electrical insulators and thermal conductors in integrated circuits, the efficient drilling of micro-holes in ceramic plates is important for productivity and cost. Since ceramics have poor machinability due to their low thermal conductivity, high hardness and high brittleness, in this work, alumina green bodies rather than sintered alumina were drilled to manufacture ceramic MCPs, followed by sintering the machined green bodies. Alumina green bodies were drilled with electro-deposited diamond grit WC micro-drills, and the cutting force with respect to drilling time was measured to determine a suitable micro-drilling condition. From the measurement of the micro-drill tip wear during micro-drilling of alumina green bodies, a model for the cutting force during micro-drilling was constructed.     

Micro-drilling of Ti–6Al–4V alloy: The effects of cooling/lubricating

This paper presents a series of experimental investigations of the effects of various machining conditions [dry, flooded, minimum quantity lubrication (MQL), and cryogenic] and cutting parameters (cutting speed and feed rate) on thrust force, torque, tool wear, burr formation, and surface roughness in micro-drilling of Ti–6Al–4V alloy. A set of uncoated carbide twist drills with a diameter of 700 μm were used for making holes in the workpiece material. Both machining conditions and cutting parameters were found to influence the thrust force and torque. The thrust force and torque are higher in cryogenic cooling. It was found that the MQL condition produced the highest engagement torque amplitude in comparison to the other coolant–lubrication conditions. The maximum average torque value was obtained in the dry drilling process. There was no substantial effect of various coolant–lubrication conditions on burr height. However, it was observed that the burr height was at a minimum level in cryogenic drilling. Increasing feed rate and decreasing spindle speed increased the entry and exit burr height. The minimum surface roughness values were obtained in the flood cooling condition. In the dry drilling process, increased cutting speed resulted in reduced hardness on the subsurface of the drilled hole. This indicates that the surface and subsurface of the drilled hole were subject to softening in the dry micro-drilling process. The softening at the subsurface of drilled holes under different cooling and lubrication conditions is much smaller compared to the dry micro-drilling process.     

Comparison of tool wear mechanisms and surface integrity for dry and wet micro-drilling of nickel-base superalloys

The study focuses on the surface integrity and wear mechanisms associated with mechanical micro-drilling of nickel-base superalloy (Inconel 718) under dry and wet cutting conditions. Mechanical and metallurgical characterization was undertaken using scanning electron microscopy (SEM), backscatter electron microscopy (BSE), electron backscatter diffraction microscopy (EBSD), transmission electron microscopy (TEM), focused ion beam (FIB) microscopy, nanoindentation, energy dispersive spectroscopy (EDS) and elemental analysis techniques. The surface integrity results revealed large scale near surface deformations with high dislocation density along with nanocrystalline grain structures both under wet cutting conditions, with evidence of recrystallisation and lower dislocation density for dry cutting. Cutting conditions play a significant role in determining the depth of the affected layer, the frequency of misorientations, the microstructures and the stored energy found there. The cutting temperature and use of coolant play a key role in the formation of the altered surfaces. Abrasion, diffusion and micro-chipping were found to be the main wear mechanisms for wet cutting compared to abrasion, high adhesion, macro-chipping and catastrophic failure for dry cutting. Adhesion of work-piece material to the tool associated with abrasion and diffusion processes is the main contributor to wear phenomena. The results are important in guiding the choice of cutting conditions for acceptable surface integrity.     

Evaluation of micro-drilling technologies for metal-injection-molded 420 stainless steel

Metal-injection-molded (MIM) 420 stainless steel is a commonly used material for high-value products such as fuel injector nozzles. However, the trade-offs involved in using different micro-drilling processes on this material are not well-documented in literature. This article presents a micro-drilling study of MIM 420 stainless steel using four candidate processes: micro-electrical discharge drilling (micro-EDD), ultrasonically assisted micro-EDD, micro-mechanical drilling (micro-MD), and ultrasonically assisted micro-MD. The micro-EDD results shows that the use of ultrasonic vibrations significantly improves the overall process time, spark erosion efficiency, and material removal rate of the process. However, this improvement comes at the expense of increased tool wear and surface roughness, especially while machining under high-discharge-energy conditions. The micro-MD results show that the use of ultrasonic vibrations is beneficial in lowering the thrust force, drilling torque, and tool wear at chipload values greater than the minimum chip thickness of the material. However, the ultrasonic vibrations do not have a notable effect on the surface roughness or on the size of the exit burrs. The results obtained from this study have been used to develop a Likert-type comparison scale to enable application-specific selection of micro-drilling processes for MIM 420 stainless steel.