Length-wise tool wear compensation for micro electric discharge drilling of blind holes Ganesh Malayath, Srikanth Katta, Ajay M. Sidpara ⇑ , Sankha Deb Mechanical Engineering Department, Indian Institute of Technology Kharagpur, Kharagpur, India article info Article history: Received 1 October 2018 Received in revised form 26 November 2018 Accepted 12 December 2018 Available online 13 December 2018 Keywords: Micro EDM drilling Blind hole Tool wear compensation Image processing abstract Blind hole drilling using micro electric discharge machining (EDM) faces the difficulty to achieve intended depth as the tool electrode erodes continuously. A practical solution for this problem is to give a lengthwise compensation during the machining process. There are two kinds of compensation strate- gies exist. Online strategies compensate for the reduction in length by counting the discharge pulses and mapping it with a material removal model. Offline strategies often use the periodical measurements of tool length or electro thermal models to predict the anticipated tool wear for a particular depth. Periodical measurements are much simpler and effective compared to the discharge counting techniques, but the machining operation has to be stopped for the measurements. In this paper, a new tool wear com- pensation method is realized which integrates the micro EDM machine tool to an image processing mod- ule and computer controlled tool wear prediction algorithm. The proposed system is capable of updating the part program constantly according to the forecasted tool wear. A series of blind holes of 200 mm intended depth is drilled using the proposed methodology and compared it with hole drilling without compensation along with the other compensation methods. The proposed method gives 0.3–1% deviation from the intended depth of the hole. This will help in drilling blind holes with a minimum relative error with respect to the intended depth which will be useful for fabrication of series of holes or pattern. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction Drilling is one of the basic machining processes to create holes in different materials for different applications. Micro-drilling is an essential operation in the fabrication of small and high aspect ratio micro holes for micro-fluidic systems [1], fuel injection nozzles [2], micro dies [3], starting holes for wire electric discharge machining (EDM) [4], etc. Making accurate blind micro holes is a necessary in fabrication of micro molds which are further used for making micro patterns. Reverse EDM (R-EDM) uses tool electrodes with through or blind micro holes to fabricate micropillars on workpiece surface [5,6]. Unlike through holes, blind holes in R-EDM electrode demands high depth accuracy as the machined pillar height will be entirely depends on it. To ensure the uniform height of electrodes, the depth of micro hole has to be uniform and accurate. Blind micro holes are used to improve the carrying capacity of biomate- rials like Ti6Al4V [7]. Light addressable potentiometric sensors with micro blind holes have demonstrated the improvement in performance regarding increased photocurrent amplitude, sensi- tivity, mechanical strength and linearity [8]. Micro EDM drilling can be used to drill blind micro holes in the silicon wafer to be used as part of MEMS devices and 3D IC/Si integrations [9,10]. Metallic micro fluidic circuit demands micro holes with exact depth equal to the channel depth to facilitate inlet and outlet flow. Considering the case of machining of micro holes, the scope of using conven- tional drilling process is very limited. Even though commercial tools are available up to 50 mm or less, the tools are susceptible to early breakage due to tool rigidity problems. Micro EDM drilling is a dependable alternative in this case. In EDM, the material is removed by melting and evaporation. Therefore, there is no need for any physical contact between the tool and workpiece. It elimi- nates the problems of tool rigidity during machining. Apart from the advantages of using EDM, shape and dimensions of the features are very much dependent on the shape accuracy of the tool elec- trode. During machining, the tool electrode also wears out in lengthwise and sidewise direction. This will cause dimensional inaccuracies and distort the shape of the final hole to be made. Due to this reason, machining blind holes using micro EDM drilling often fails to achieve the intended depth. The constant reduction in length of the tool electrode will result in a reduced depth of the hole. A solution to this problem is to add a lengthwise compensa- tion to the total tool travel during drilling. This becomes a very tricky due to the inability to measure and monitor the machined https://doi.org/10.1016/j.measurement.2018.12.047 0263-2241/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: ajaymsidpara@mech.iitkgp.ac.in (A.M. Sidpara). Measurement 134 (2019) 888–896 Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement