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Quality Quandaries*: The Case of Premature Drill Wear Out

Summary: This problem occurred in a machining process for large casted metal parts. These parts are processed (by means of drilling, milling, and tapping) in a newly built production line, consisting of 21 coupled workstations, without large buffers between the stations. This lack of significant buffer capacity implicates that any disturbance in the line has a direct impact on the line output. The initial idea behind the set up of the production line was that production would run for 7.5 hours and then the tools (such as the drills) would be replaced in order to start again in a new shift. The function of the machining process is to drill seven deep holes in the metal parts. The production line processes about 80 parts per shift. The performance of the drills is continually monitored by an automated system, which measures the energy consumed by the drills (torque). The purpose of this monitoring system is to prevent the drills from breaking. The process suffered from frequent stoppages forced by the process monitoring system whenever it signaled an imminent drill break. Upon stopping the line, suspect drills would be replaced. The consequences of these frequent out-of-control signals included excessive downtime of the line (overall equipment efficiency [OEE] below 60%) and replacement of drills well before their specified life span. A team of engineers and operators investigated the problem. Before designing a remedy, they first focused on establishing the root cause. Below we describe the team’s actions and analyses, illustrating an important lesson for diagnostic problem solving. This lesson could be summarized as follows: before trying to discover a problem’s cause by experiments involving detailed candidate causes, it is important first to achieve focus; that is, to narrow down the search space to a compact area. Note: For confidentiality reasons, some of the details and data sets have been modified, but care has been taken to leave the line of reasoning and the conclusions intact.

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  • Topics: Problem Solving
  • Keywords: Brainstorming, Case study, Failure, Manufacturing, Pairwise comparisons, Tool wear
  • Author: Mooren, Jo; de Mast, Jeroen; Does, Ronald J. M. M.
  • Journal: Quality Engineering