Method for preventing burr formation during electron beam drilling

In drilling a hole, slot or other passageway through a workpiece wall having a first penetrated and last penetrated surface by localized impingement of an energy beam on said first surface, wherein molten workpiece spatter is expelled from the hole during drilling and deposits on and welds to said first surface around the hole perimeter in the form of one or more burrs, the step of substantially preventing burr formation by:

• coating the first penetrated surface prior to drilling with an expandable layer of stop-off material so that hot molten spatter expelled from the hole during drilling of the workpiece wall is deposited on said layer, the stop-off material being characterized by high temperature resistance such that, although the energy beam readily penetrates through the layer to strike said first surface, the hot molten spatter deposited thereon does not penetrate through the layer to weld to said first surface and further that the heat generated in the workpiece wall during drilling does not significantly degrade the layer so as to cause loss of adhesion from said first surface, said stop-off material being further characterized as innocuous to the workpiece properties.