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◆Names of drill bit parts
●Twist drill bit (Helical)

S-profile chisel edge regrinding

Combination of the second flank edge grinding and
cross chisel edge regrinding

 
 
Gun reamer
◆ Structural composition and cutting characteristics
Helix angle
The inclination angle against the flute in the drill bit axis direction, equivalent to rake angle of lathe tooling; rake angle of drill bit is different depending on position
of cutting edge. The circumference is the largest, and the center part is the smallest. It forms negative rake angle in chisel edge part to crush material of work pieces.
Flute length
Can determine according to machining depth of hole,
guide bushing length, regrinding size etc. Because it has great impact on lifetime of drill bit, the flute length must be as short as possible.
Tip angle
Generally it is 118°，and it can be set according to purposes.
Soft and easy-cut materials small←apex angle→large used in cutting hard materials or high efficiency machining
Core bit diameter
It is the important factor affecting rigidity and cutting of drill bit, can be set according to purposes.
Margin

The front dimension determines drill bit diameter.
When machining, it plays a role of guiding drill bit. When determining margin width, the friction between hole and it need to be considered.
Back taper
To reduce the friction between the drill bit and the wall of processed hole, the part from the front to Holder shall be with little taper, generally comparing with flute length of 100mm, expressed in diameter reduction. It is generally between 0.40mm-0.1mm.
When drill bits for high efficiency machining are blocked in drilling holes of materials of some work pieces, back taper shall be larger.