Forces exerted on the tool-electrode during constant-feed glass micro-drilling by spark assisted chemical engraving

The forces exerted on the tool-electrode during Spark Assisted Chemical Engraving (SACE) constant velocity-feed glass micro-drilling are measured for different machining voltages, tool feed-rates and tool sizes. A diagram of the force regions in the hole-depth vs. tool feed-rate plane is constructed for different voltages and tool sizes. Two rate limiting steps for micro-drilling were identified. For low depths, the rate limiting step is the work-piece surface heating while for high depths it is the electrolyte flushing. Based on these findings, the tool feed-rate vs. hole-depth plane of the force regions was normalized using the time needed to heat the local glass surface and the tool radius. A correlation between the force occurrence and the current signal is identified where the current shifts upwards by a constant value when a force is exerted on the tool. This finding allows the usage of the current signal to detect the contact between the tool and the glass surface. The measurement and understanding of the forces exerted on the tool-electrode that this work brings is a first step towards the development of force feedback algorithms for SACE machining. (c) 2013 Elsevier Ltd. All rights reserved.