How large a bit before needing to predrill?

I'm no expert but shouldn't you at least pre-drill to the outside of the end drill size web diameter?
The web is the center point called the chisel edge of the drill, the web gives the bit it's strength.
Just my 2 cents.
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Drills are designed for producing rough holes for the most part, if one were to always pilot at the chisel edge diameter as the drill size becomes very small the pilot would approach zero, what is the chisel edge width of a .020" diameter twist drill?

As mentioned above, drill straight through with the largest tool that the machine will push. The power of the machine is the limiting factor, other then this there is no good reason to step drill a rough hole if it can be avoided.
 
I hand sharpen my drill bits, its difficult at best, impossible for me to get the flanks exactly equal. If I need a hole close to the required size without boring or reaming, drilling an undersized (pilot) hole close to the required size first followed by the required dia gets me on size. If the cutting edges aren't equal and no pilot hole is drilled the bit will swing around the chisel point and drill oversize with the radius of the hole set by the longer cutting edge.
You can see the result when a bit breaks through and leaves a reduced dia hole at the end.

Greg
 
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