Robotic multi-axis electric discharge machining of exotic hard-to-cut materials
Creators: name: De Almeida, Sergio Tadeu
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401406
subject: Machining

description: Exotic materials have unique resistance and properties that make them suitable for high end-application such as aerospace, military, automotive, and medical applications. However, the same characteristics that make them suitable for demanding applications – under traditional machining approaches – will come together with other properties such as ultra-hardness, high toughness, and hardening behaviour, which will attain high cutting forces and vibrations, resulting in low material rate removal (MRR) and poor surface roughness (SR). Furthermore, exotic materials are an expensive and finite resource whose suitability in terms of material usage efficiency is not always considered during the already challenging machining task.

As a solution Electric Discharge Machining (EDM) stands out as a non-traditional machining process that shapes complex profiles of conductive hard-to-cut materials, achieving high dimensional accuracy and superior surface finishing. Furthermore, the EDM process has no mechanical contact, and the resulting cutting forces are nearly neglectable. However, to this day, EDM is found in rigid bed-based CNC machines whose traditional design structure frequently restricts the workpiece design freedom in terms of size, shape, and complexity due to reduced machining envelope constraints and few available axes. As a result, the process becomes expensive, often conducted with a series of small and segmented workpiece sizes demanding custom-made fixtures. At the same time, advantageous monolithic parts made of exotic materials remain difficult, if not impossible.

On the other hand, traditional machining processes such as drilling and milling using six-axis industrial robots (IR) have been intensively investigated due to the robot´s ability to deliver cost efficiency, improved envelope and high manufacture flexibility by reprogrammable application and virtually unlimited external multiple-axis combination. However, due to the intrinsic design of IRs, they offer low structure stiffness resulting in severe limitations in machining exotic hard-to-cut materials, often facing prohibitive vibration and chatter, resulting in low accuracy, poor finishing, and lack of repeatability. Thus, IRs are not comparable to CNC machining outcomes, and IRs machining benefits are constrained to soft materials and low accuracy applications. 

Therefore, the core objective of the present thesis is to achieve the ability to machine monolithic and complex workpieces of any conductive hard-to-cut material to better use the expensive and non-renewable resource of exotic materials and improve the performance of high-end industries such as aerospace and defence.

Following an in-depth literature review of EDM and robotic machining fields, it identifies and extracts the proposed combination's benefits and potentially harmful outcomes. By concurrently maximizing benefits and mitigating risks, the research develops a synergistic combination of a novel machining technic combining carefully selected wire and Milling EDM variants with a robotic seven-axis machining system. As a result, the research expects to contribute to overcoming limitations in both fields, delivering a process able to machine large and complex workpieces made of any conductive material – independently of its hardness – yet with minor forces to deliver an accurate multi-axis robotic machining system.

Since the combination of robotic machining and EDM technic configures a novel system, it is mainly composed of equally novel sub-systems encompassing design and control, culminating in seven innovations listed below to be later presented.

Innovation 1.  The first two robotic Wire-EDM end-effectors (CHAPTER 4:)
Innovation 2.  The first flexible Wire-EDM electrode feed system design and control  (CHAPTER 5:)
Innovation 3.  The first robotic Milling EDM end-effector (CHAPTER 6:)
Innovation 4.  The first FEA approach to evaluate the risk of EDM robotic vibration (CHAPTER 7:)
Innovation 5.  The first Robotic EDM Digital-Twin for Offline Programming (CHAPTER 8:) 
Innovation 6.  The first robotic EDM online control for pulse and robot speed (CHAPTER 9:)
Innovation 7.  The first robotic Milling-EDM system with offline wear compensation (CHAPTER 10:)