This compression connector was made by collapsing a 5/8" long piece of
"3/8-Inch" black iron pipe over a die.  The die was made from 1/2" diameter round 
cold rolled steel by milling symmetrical flats on both sides so that a flat 
section 0.295" thick remained.  This flat tongue is about 1"long and the 1/2" rod 
was turned down to about .495" dia. to allow insertion of the tongue into the 
"3/8-Inch" pipe before collapsing.  The tongue was sawed off the rod in later use.  
The 0.295" thickness allows a #1 copper wire (0.289"Dia) to fit in the collapsed 
pipe/connector with some freedom.

Black iron pipe is not seamless and has a welded seam along one side which is 
somewhat weaker than the rest.  This weak weld seam should be placed over one of 
the flats of the die before collapsing or else the weld will tend to fail and 
produce a corner rather than a smooth curve on one end of the connector.  The 
bolt is a 1/4-28x1/2"L stainless steel in this case.  The clamp will compress 
over a #1 copper wire with three #12 wires lying in the end of the curve however 
it would need a longer bolt or shoe under the bolt to tighten down on the #1 wire 
alone. Copper tubing and a thread forming tap to increase thread engagement might 
be a useful variation.  

No attempt is made to make this clamp part of the high conductivity path between 
the connected wires.  Plating the clamp and/or wrapping the wires with copper 
foil and a shoe under the bolt to protect the foil and distribute force on the 
soft copper wire might increase conductivity. Use of antioxident grease between 
the conductors before joining would undoubtedly help maintain reliability.