Automatic center punch

Automatic center punch

An automatic center punch is a hand tool used to produce a dimple in a workpiece (for example, a piece of metal). It performs the same function as an ordinary center punch but without the need for a hammer. When pressed against the workpiece, it stores energy in a spring, eventually releasing it as an impulse that drives the punch, producing the dimple.


Within the body of the punch, there are three principal moving parts arranged in line:

* The punch rod
* The intermediate rod
* The hammer mass

The hammer mass is spring-loaded from the back of the punch by a large spring. (The spring's compression can be adjusted by loosening or tightening the end cap at the back-most portion of the punch.) There's a hole drilled a short distance into the front center portion of the hammer mass.

The intermediate rod provides the automation. It's designed so that its resting position is skewed and it bears on the hammer mass slightly off-center, on that part of the front of the hammer mass where the hole isn't. As the punch is pressed against the workpiece, pressing the punch rod inwards, the intermediate rod bears on the hammer mass and pushes the hammer mass back against its spring, storing energy in the hammer spring.

At a certain point, a spring and ramp mechanism associated with the intermediate rod straightens out the intermediate rod ("unskews" it). Now centered, the end of the intermediate rod falls into the central hole in the hammer mass and the hammer mass, driven by its spring and no longer held back by the intermediate rod, starts accelerating towards the front end of the punch. But the hole in the hammer mass is only so deep so eventually the moving hammer mass bottoms out against the tail end of the intermediate rod and the impulse of the hammer mass is transmitted through the intermediate rod, through the punch rod, and into the workpiece.

Use in advertising

In the late 1960s, an automatic center punch was prominently featured in the advertisements for "Xerex", a U.S. brand of antifreeze. The can of Xerex antifreeze was punctured several times with an automatic center punch and the antifreeze proceeded to spray out of the can. But the anti-leak agents in the antifreeze then plugged the holes in the can and the spray of antifreeze stopped.

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