Double margins refer to an additional margin ground just behind the leading margin on each drill land.
Double margin drills generally range to one inch in diameter while triple margin drills kick in above one inch in diameter.
One function of these additional margins is to stabilize the drill when going thru drill bushings. You now have four location points on the drill instead of two inside of the bushing, providing increased bushing support of excess of 200%. Controlled back taper is also tighter aiding additional support that gives better hole location, a “rounder” hole and better hole finish.
Double margin drills can also perhaps eliminate a reaming operation if you have a slightly wide reamed hole tolerance. When using short drill bushings alignment can be improved considerably with the addition of extra margins.
The trailing margin does not actually cut a chip but burnishes the part making a smoother finish and contributes to a uniform hole size.
Double margin drills are also effective on special multi-diameter tools when the large diameter will aid the small diameters centrality and helps give a more concentric drilled hole as well as any chamfered sections.
Double margin drills also reduce tool, bushing and spindle wear due to its bushing contact and rigidity.
In Deep hole drilling double margins help, as the margins will support the drill when going deep!
Some of the operations that can be reduced using double margin drills include center drilling and spot drilling.