Lathe Chuck Injury Due to Faulty Lathe Operation

One of the more interesting cases that we have been assigned had to do with a personal injury suffered as a result of an airborne chuck.  The injury occurred after an individual purchased a chuck and spindle adapter for use with a Shop Smith lathe that was manufactured back in the late 40s or early 50s (see photos below).

 

Chuck and pin wrench used to tighten chuck onto spindle adapter

Antique Shop Smith Lathe

After attaching the spindle and mounting the chuck, the owner started the lathe and tested the operation.  Everything went well until the machine was turned off.  As soon as it was, the spindle shaft stopped but the chuck kept spinning and spun itself off the adapter.  The chuck went airborne, bounced off parts of the lathe and hit the owner’s hand causing severe injury.  The investigation came about as a result of the owner’s allegation of a defective product, ie, the chuck was defectively designed and had no means of stopping if it separated from the spindle adapter.  The chuck is designed to be tightened against the spindle adapter using special wrenches.  According to the owner, he claimed that he “tightened the chuck as tight as he could” before starting the lathe.  During the examination of the lathe, the lathe was started and run without the chuck attached.  When the lathe was turned off, it immediately became clear that there was something wrong when the spindle shaft did NOT coast to a stop.  It was also clear that the abrupt stop provided the torque necessary to cause the chuck to spin off the spindle adapter.  In addition, it was also noted that if the chuck was to stay attached to the spindle adapter, the torque applied during tightening had to be greater than the torque causing separation.  After working the math, it was determined that the owner could not have tightened the chuck was much as he claimed.  As a result, it was further determined that the owner unfortunately caused his own injury by ignoring the problem of abrupt stopping of the spindle shaft.  The lawsuit was subsequently dismissed.