Author Jeffery H. Warren
Machine equipment manufacturers are called upon to study potential risks of equipment before selling it in the marketplace. In addition, ANSI machine tool standards further address equipment hazards and risks that cannot be eliminated by design. In one case I investigated, the depth of that critical risk assessment became an important factor in a claim involving a machinist blinded while using a computer numerically controlled (CNC) machining center.
The computer numerically controlled (CNC) machining center installed in the dragster engine shop.
The large CNC machining center was installed in an engine shop that manufactures large racing engines for top fuel dragsters. The equipment included a safety interlock to protect the operator from potential tool breakage and ejection from the machine. While ANSI machine tool standards echoed the hazard risk, both ANSI and the manufacturer recognized the inability to fully eliminate the risk. The manufacturer guarded the machine from the known hazard with interlocked doors and slow spindle speeds while the doors were open.
During the installation of this particular piece of equipment, a manufacturer’s representative changed a control system setting that allowed the operator to override the safety interlock. With the door hold safety override enabled, the machine operator could, through the operator’s console, override the safety switches on the doors so that the machine would operate at spindle speeds up to 10,000 rpm with the doors open.
At the time of the incident in question, the operator was setting up the machining center to machine an engine head, a deep hole drilling operation. The operator was manually stepping the machine through the work piece program with the doors separated approximately 6 inches for clearer viewing while the tool was operating at the programmed speed of 8000 rpm. When the operator initiated the high-speed spindle rotation, the tool holder bent and broke. The separated shank was ejected from the machine and struck the operator in the head between his eyes above his nose causing severe injures, including the loss of his sight.
The ejected tool broke the operator’s safety glasses as it struck him in the head.
The fractured tool holder.
With the presence of a protective interlock guard to protect the operator during the machine’s intended use, one might believe that, in this case, the hazard was reasonably well controlled. In Part Two, we’ll discuss why that was not the case at all.
Jeffery H. Warren, PhD, PE, CSP, is the chief engineer and CEO at Warren specializing in mechanical, machine design, and safety. His deep expertise in machine design and safety analysis makes him a frequent presenter, trainer and expert witness. In addition to investigating more than 2000 claims involving property damage and injuries related to machinery and equipment since 1987, Jeff has an undergraduate degree in Mechanical Engineering from the University of North Carolina as well as a Master of Science and a Doctorate in Mechanical Engineering from Virginia Polytechnic Institute and State University — both with machine design emphasis.
