Part VI. Striking Errors:

Machine Part Impingement

Definition: This error occurs when a coin is pinned and pinched between two machine parts during the strike.  For modern US coins struck with inverted dies in a high-speed Schuler press, the two machine parts are paired feeder/ejector fingers that form the arms of a notch that cradle the planchet and (subsequently) the newly-struck coin.  The feeder/ejection fingers lie at the expanded tip of feeder/ejector arm that looks like a wrench.  Each “wrench” is one of many that fit into a rotating plate that fits the definition of a rotary or dial feeder.

Machine part impingement errors are associated with off-center strikes and with double- and multi-struck coins in which most of the strikes are off-center.  When the dies strike the off-center planchet or coin and the off-center feeder that cradles it, the two elements expand radially.  The facing edges flow together, leaving the coin with a convex facet we call a “foreign object chain strike”.  The portion of the planchet or coin that projects outside the striking chamber is pushed laterally by the expanding coin metal and expanding feeder metal.  The pole opposite the convex facet collides with the other arm of the feeder/ejector, leaving the edge of the coin with a dent and often a warped surface.

This off-center dime was pinned and pinched between two machine parts.  The right side of the coin shows a gently convex foreign object chain strike.  Here the expanding edge of the dime collided with the expanding internal edge of the feeder/ejection finger.   At the opposite pole, the unstruck portion of the dime was pushed laterally into the other feeder/ejection finger.  This collision pushed in the planchet’s edge, warping the unstruck portion of the dime.  This is an example of intra-strike damage.

This quadruple-struck nickel was pinned and pinched between two feeder/ejection fingers during the last three strikes.  After a normal first strike, the nickel received three closely-spaced off-center strikes that are well-separated from the initial strike.  This is a typical pattern for multi-strikes suffering from machine part impingement.  It’s almost certain that the misplaced and mistimed feeder/ejection arm prevents the coin from leaving the striking chamber entirely.

On the right side there is a long, relatively straight contact facet that represents a foreign object chain strike.  Once again, the culprit is one of the two feeder/ejection fingers that cradle the coin.  The left side of the coin was pushed in when the portion of the coin lying outside the striking chamber was pushed into the other feeder/ejection finger.