Machinery's Handbook, 31st Edition
1156
MICROMACHINING MICROMACHINING Introduction
Recent technological advancement and market need for product miniaturization de- mand three-dimensional (3D) microcomponents. Although microelectronic manufac- turing techniques can produce pseudo 3D microdevices using silicon and other semi- conducting materials, such materials are neither robust nor biocompatible for demanding applications in aerospace, medical, sensor, defense, petroleum, and transportation. Ex- amples of robust applications include microdrilling holes for fuel or ink injection nozzles, electronic printed circuit boards, microfabrication of watch components, air bearings, cooling holes in turbomachinery, high aspect ratio features on tool steel molds and dies, etc. There are alternative nontraditional processes to produce microfeatures on robust engineering materials such as laser micromachining, electrical discharge microdrilling, electrochemical micromachining, chemical etching, electron/ion beam machining; how- ever, these processes are either cost prohibitive, limited to conductive materials, or infe- rior when comparing resulting surface integrity, subsurface damage, high aspect ratio, or microfeature quality. Microfabrication with traditional processes such as micromilling, microdrilling, microturning… are still the preferred choice in most applications. There is no standard that defines micromachining, but most researchers uses cutting tools to produce components with key dimensions less than 1 mm (0.040 inch) or when depth of cut is comparable to tool sharpness or tool grain size in their micromachining studies. Realizing the needs for traditional micromanufacturing, there are more commercially available machine tools and microtools in the market. However, costly equipment, lack of in-depth understanding of micromachining, and limited guidelines for effective use of microtools are still the bottleneck for full application of micromachining. Universities and research institutes worldwide have started theoretical investigation of micromachining and produced positive results from the academic point of view. Without practical guide lines on micromachining, technicians and machinists probably would make wrong and costly decisions when simply extending macroscale machining practices into microscale machining applications—a microtool simply breaks at even conservative macroscale parameters for speeds, feeds, and depth of cut.This section, while complementing other chapters in this Handbook, focuses on practicality, based on proven theories and published data, to help decision makers to understand the requirements for micromachining, and as a guide to people on the shop floor to quickly and confidently begin using the recommended parameters and techniques. Both US standard and SI metric units are included for conve nience. Examples of how to use the data and equations are given throughout this chapter. Machine Tool Requirements To obtain the same surface speed as in macromachining, a machine tool must: a) Be capable of rotating a workpiece or tool at high speeds of 25,000 rpm or above b) Control spindle runout to submicron level c) Have a very robust mechanical and thermal structure that is not affected by vibration or thermal drift d) Have high resolution tool positioning and feeding mechanisms Success in micromachining depends on tool quality and precision of the machine tool. Machine spindle runout, tool concentricity and tool positioning accuracy must be in the neighborhood of 1⁄100th of tool diameter or less for successful operation. Tolerance stack up for spindle runout, tool eccentricity, and wandering of a microdrill causes cyclic bend ing of the tool that lead to catastrophic failure. At a low rotatational speeds, the displace ment of a spindle can be monitored with a sensitive mechanical indicator. However, this option is not applicable for machines that operates at or above a few thousands rpm. Other non-contact techniques using capacitance, magnetism, or light would be more appropriate. Fig. 1a shows an example of spindle runout measuring setup. A laser beam is
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online