A Subland Drill is considered to be two conventional twist drills with different diameters combined into one tool.  Subland construction has built in concentricity between diameters for the length of flutes, whereas a step drill may be eccentric due to grinding of the small diameter out of the large diameter.
The lands of a subland tool extend back the full flute length to the shank making it easier to evacuate the chips from the drill cavity.  Sublands are easier to resharpen due to its independent flutes while a step drill tends to weaken at the intersection of the small and large diameter after so many regrinds.  When this happens to a step drill, you must cut off the small diameter and recreate it again.  Additionally, sublands have their own web for each diameter and do not have to be web thinned like a step drill.
Subland tools eliminate operations, reduces set up time, perform two or more operations at once, and produce both quality and accurate holes.

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