Sinker EDM of a Hex Socket in 3D

A spark doesn't care how hard the screw is

A broach has to cut the metal, and its life falls away as the work gets harder; a common rule of thumb puts the limit near 35 HRC. A finished ASTM A574 socket head cap screw is 37 to 44 HRC and an F912 set screw 45 to 53 HRC, which is why broached sockets are made before heat treatment. A spark only needs metal that conducts, so sinker EDM can make or enlarge a socket in a screw that is already hardened, or in tough alloys such as aged alloy 718. It is slow, it leaves a thin heat-affected skin, and some aerospace purchase terms forbid it without the buyer's approval, so check the drawing and the order first.

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Screw drawn to ASME B18.3-2012: 1/2-13 socket head cap screw at the maximum head, 0.750 in. across by 0.500 in. high, with a 3/8 in. socket and 0.245 in. minimum key engagement. Pilot drawn at the hex across-flats size, 0.375 in., leaving the six corners. Ram (sinker) EDM after published EDM handbooks and patents: a graphite hex electrode made undersize by the overcut (0.0008 to 0.025 in. a side), the servo lowering it to the first spark and on to a set depth, one spark at a time where the gap is narrowest, jump flushing and corner wear, in one pass with no finishing pass (our practice); the recast layer and heat-affected zone are drawn as a schematic. Socket gauging after ASME B18.3; hardness from ASTM A574 and F912; the 35 HRC broach limit is a rule of thumb quoted in the trade press; alloy 718 from ASTM B637. The spark gap and electrode wear are drawn exaggerated, the sparks far fewer than the real thousands a second, and the sinking as a time-lapse with no times given. The machine (column, servo ram, X/Y table carrying the work tank, pump and filter unit, generator) and the gauges are generic.