Carbon Fiber Reinforced Polymers (CFRP) are a class of advanced materials widely used in versatile applications including aerospace and automotive industries due to their exceptional physical and mechanical properties. Owing to the heterogenous nature of the composites, it is often a challenging task to machine them unlike metals. Drilling in particular, the most commonly used process for component assembly is critical especially in the aerospace sector which demands parts of highest quality and surface integrity. Conventionally, all composites are machined under dry conditions. While there are drawbacks related to dry drilling, for example, poor surface roughness, there is a need to develop processes which yield good quality parts. This thesis investigates the machining performance when drilling CFRP under cryogenic, MQL and hybrid (CryoMQL) modes and comparing with dry drilling in terms of the machining forces, delamination, diameter error and surface integrity assessment including surface roughness, hardness and sub-surface damage analysis. Additionally, the effect of varying the feed rate on the machining performance is examined. From the study, it is concluded that drilling using coolant/ lubricant outperforms dry drilling by producing better quality parts. Also, varying the feed rate proved to be advantageous over drilling at constant feed.

In recent years, there has been a substantial growth in the application of carbon fiber reinforced plastic (CFRP) composite materials in automobile and aerospace industries due to their superior properties such as lightweight, high strength, excellent corrosion resistance, and minimal fatigue concerns. The present study evaluates the drilling performance of woven carbon fiber reinforced plastics under both dry and cryogenic cooling conditions using uncoated solid carbide drill with a through-hole for coolant application. The effects of the cooling conditions and the cutting parameters on drilling performance in drilling CFRP were evaluated in terms of generated thrust force, torque, cutting edge radius, outer corner flank wear, hole quality (including surface roughness, diameter error, roundness, delamination, burr formation, sub-surface quality). Both cooling conditions and cutting parameters were found to influence the thrust force and torque at different levels. The thrust force and the torque are higher in cryogenic cooling under all cutting parameters. In most of the cases, cryogenic drilling gives better bore-hole quality with lower surface roughness, more accurate diameter, less burr generation, better sub-surface quality, etc. Also, the tool-wear rates measured in drilling shows that cryogenic drilling produces less tool-wear than dry drilling does.

This research studies microdrilling of biocompatible materials including commercially pure titanium, 316L stainless steel, polyether ether ketone (PEEK) and aluminum 6061-T6. A microdrilling technique that uses progressive pecking and micromist coolant is developed that allows drilling of 127 micrometers diameter microholes with an aspect ratio of 10:1. The drilling parameters, dominant wear pattern, hole positioning accuracy and effect of AlTiN tool coating are experimentally determined. The experimental data trend agrees with classical Taylor's machining equation. Despite of fragile and long microdrills, the progressive pecking cycle and micromist allowed deep hole drilling on all the tested materials. Drill wear is more pronounced at outer cutting edge due to higher cutting speeds. However, when drilling 316L stainless steel attrition wear at chisel edge is dominant. Hole quality degradation due to formation of built up edge at the drill tip is observed. Coated drill improves tool life by 122% and enhances hole quality when drilling 316L stainless steel. The hole positioning accuracy is improved by 115% and total hole diameter variation decreased from 0.11% to 0.003% per mm of drilling distance.