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Titanium and its alloys (Ti) are attractive for many applications due to their superior properties.However, they are regarded as hard-to-machine materials. Drilling is an important machining process since it is involved in nearly all Ti applications. It is desirable to develop cost-effective drilling processes for Ti and/or improve the cost-effectiveness of currently-available processes. Such development and improvement will be benefited by a comprehensive literature review of drilling processes for Ti. This paper presents a literature review on mechanical drilling processes for Ti, namely, twist drilling, vibration assisted twist drilling, ultrasonic machining, and rotary ultrasonic machining. It discusses cutting force, cutting temperature, tool wear and tool life, hole quality (diameter and cylindricity, surface roughness, and burr), and chip type when drilling of Ti using these processes.

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Drilling is involved in many applications of carbon fiber-reinforced plastic (CFRP) composites. Rotary ultrasonic machining (RUM) has been successfully employed to drill holes in CFRP using either cutting fluid or cold air as coolant. However, there are no reported studies to compare the results in RUM of CFRP using these two types of coolant. This article, for the first time, presents an experimental study to compare cutting force, torque, surface roughness, burning of machined surface, and tool wear in RUM of CFRP using these two types of coolant. This study will result in knowledge about machining conditions under which it is feasible to use cold air instead of cutting fluid and differences in machined hole quality produced using cold air vs. cutting fluid.