Author Jeffery H. Warren
In Part One, I shared a case of a machinist blinded while using a computer numerically controlled (CNC) machining center. The equipment, which was safeguarded, contained a known hazard, but the machine manufacturer included an interlock guard to help protect the operator during the machine’s intended use.
The presence of a safeguard makes it clear that the manufacturer knew there was a hazard: tool breakage and ejection from the machine. The guard was a full enclosure with a set of interlocked doors and a maximum spindle speed of 750 rpm while the doors are open.
The equipment manufacturer included safeguards to help protect operators from the uncontrolled hazard.
Here’s what else the manufacturer knew. First, operators often desire to open the doors for various reasons. Second, there are standards that prohibit a machine from being operated at full speed with a safeguard defeated. Lastly, that risk assessments must include both intended use and reasonably foreseeable misuse.
The equipment included a door hold safety override, which negated critical safeguards.
When the machine representative enabled the door hold safety override, there was no safeguard to protect an operator, maintenance or service mechanic from the uncontrolled hazard. Additionally, at higher spindle speeds, the likelihood of the risk of tool breakage and ejection was greatly increased.
In this case, it was technologically and economically feasible to prevent the machine spindle from operating at full capacity with the doors open. The manufacturer performed a risk assessment/risk reduction analysis per ANSI B11 TR3-2000. Documentation of the risk assessment indicated they only considered ejection of parts of work material and swarf. The hazard of ejection of a broken tool was not observed to have been considered.
As designed, manufactured and installed, the machine contained an uncontrolled hazard. The hazard was a tool that broke and was ejected. The hazard and risk of tool breakage and ejection could have been controlled by limiting the spindle speed when the doors were opened or by keeping the doors closed when the machine was running at high speed. The factory representative set the machine up so that the operator could override the door interlock safety switches. When the safety switches were overridden the risk of injury from tool breakage was very high and the hazard of a tool breaking, being ejected and hitting an operator was uncontrolled.
A machine that contains an uncontrolled hazard, when it is technologically and economically feasible to control the hazard is unreasonably dangerous and therefore defective. The unreasonably dangerous and defective condition of the machine was a cause of the operator’s injury. The machine as designed violated ASME B11.23-2002 American National Standard for Machine Tools- Safety Requirements for Machining Centers, Numerically Controlled Milling, Drilling and Boring Machines, ASME B11.19-2003 American National Standard for Machine Tools- Performance Requirements for Safeguarding and ANSI B11. TR3-2000 ANSI Technical Report, Risk Assessment and Reduction- A Guide to Estimate Evaluate and Reduce Risks Associated with Machine Tools.
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.
