Mechanical Engineering Experts Evaluate Fatal Injury Caused by Runaway Machinery


Mechanical Engineering Expert WitnessThis case involves a female employee of a large contract manufacturing firm who suffered fatal injuries while on the job. On the date of the incident in question, the woman was reviewing the programming of several CNC lathes and milling machines in her capacity as a managing engineer. The woman was in the process of inspecting an active lathe when the work-piece began to rotate at an unusually high rate. The woman was looking at the work-piece through the safety window when the work-piece disintegrated, shooting chunks of metal at a very high speed. One of the chunks of metal was able to pierce the safety screen, at which point it struck the woman in the head, killing her instantly. It was alleged that the CNC machine malfunctioned, due to the fact that the work-piece was able to spin at such a high rate of speed, and the fact that the safety screen failed to prevent the woman’s fatal injuries.

Question(s) For Expert Witness

  • 1. Do you have extensive experience with computer numerical control machines?
  • 2. What is your experience working with similar machines?
  • 3. Have you worked on cases involving CNC machinery malfunctioning?

Expert Witness Response E-008230

I have worked with N/C equipment, including lathes and many types of manufacturing machinery as a forensic engineer with multiple different firms. In conjunction with other experts on N/C, I have had cause to examine the actual N/C program code for similar machines to determine cause for accidents, or just to improve production techniques. I have done several N/C failures in recent years, mostly due to lightning strikes and power related issues. There are a number of areas where failure could have occurred: The first may have been in the N/C programming itself, setting the wrong speed for an operation. Second, there are mechanical/electrical failures that did not keep the spindle speed at the specified RPM, such as feedback circuits. Third, mechanical failures could have caused the chuck to eject the piece. Finally, since most N/C operations can be manually overridden, there is always the possibility of operator error. A number of people including factory employees, N/C service and maintenance personnel, and others would have to be interviewed to obtain the full story.

Expert Witness Response E-015387

The CNC metal lathe is a mostly self-contained system of electro-mechanical devices controlled simultaneously through different levels of logic executed by a user-defined program that defines the sequence of events. There are also several entities with potentially differing levels of liability in the case of a mishap. These entities include: the machine tool builder & supplier; the purchaser (capital ownership); the management including engineering and maintenance; and the end user or operator. In my 30 years of experience I have had one runaway condition in a CNC machine, which happened while I was working for a large machine tool company. These situations are possible, and can be tested for if the conditions can be replicated. Most often there is a limitation in the user-defined setup, programming, and operation of the machine. I would like to investigate whether the setup (hard tooling) was robust enough for the operating conditions and whether the programmed instructions were appropriate. There are machine design aspects, machine build conformance requirements, machine control configuration implications, all of which can be investigated for proper interlocks for safety, as long as the machine remains in an un-altered condition.

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