Twist drill

An improved twist drill which comprises:

• a pair of spiral flutes, joined by a central web, extending from a shank at a first end of said drill to first and further cutting blades at a further end of said drill, each of said cutting blades extending radially from a chisel point on the axis of said drill to a cutting lip at the peripheral edge of said flutes, each of said cutting blades defining a leading cutting edge and a trailing cutting edge;
• said first cutting blade being provided with a nonconverging groove spiralling about said axis of said drill across said first cutting blade from said leading cutting edge to said trailing edge thereof at a selected position between, but removed from, said chisel point and said cutting lip of said first cutting blade;
• said groove defining an inner edge toward said chisel point and an outer edge toward said cutting lip of said first blade, said inner edge having no portion at a greater radius from said axis of said drill than said inner edge at said leading cutting edge, and said outer edge of said groove having no portion at a smaller radius from said axis of said drill than said outer edge of said groove at said leading cutting edge;
• said further cutting blade having an inner portion toward said chisel point and an outer portion toward said cutting lip of said further cutting blade, said inner portion forming a symmetrical angle with said axis of said drill corresponding with said first cutting blade, and said outer portion of said further cutting blade forming an angle with said axis less than the angle of said inner portion of said further blade with said axis;
• and
• wherein the length of said outer portion of said further cutting blade, along said further cutting blade, is substantially equal to the distance between said outer edge of said groove and said cutting lip of said first cutting blade.