(1) The diameter of the twist drill is limited by the hole diameter. The spiral groove makes the drill core thinner and the bit rigidity is low; there are only two ribs for guiding, the axis of the hole is easy to deviate; the chisel edge makes the centering difficult and the axial resistance increases. Large, the drill is easy to swing. Therefore, the shape and position error of the drilled hole is relatively large.
(2) The rake face and flank face of the twist drill are both curved surfaces, and the rake angle and relief angle of each point along the main cutting edge are different, and the rake angle of the chisel edge is -55°. The cutting conditions are very poor; the distribution of cutting speed along the cutting edge is unreasonable, and the cutting edge with the lowest strength has the highest cutting speed, so the wear is serious. Therefore, the precision of the processed hole is low.
(3) The full edge of the main cutting edge of the drill participates in the cutting, and the cutting speed of each point on the edge is not equal. It is easy to form spiral chips and difficult to remove chips. Therefore, the chips and the hole wall are squeezed and rubbed, which often scratches the hole wall, and the surface roughness after processing is very low.