The usual diameter range is 0.25 to 80 millimeters. It is mainly composed of a working part and a shank. The working part has two spiral grooves shaped like a twist, hence the name. In order to reduce the friction between the guiding part and the hole wall when drilling, the twist drill gradually reduces its diameter from the drill tip to the shank in the direction of an inverted cone.

The spiral angle of the ordinary twist drill mainly affects the size of the cutting edge on the front angle, edge strength and chip removal performance, usually 25 ° ~ 32 °. The spiral groove can be processed by milling, grinding, hot rolling or hot extrusion, etc. The front end of the drill is sharpened to form the cutting part.

The cutting part of a standard ordinary twist drill has a top angle of 118, a cross-blade bevel angle of 40° to 60°, and a back angle of 8° to 20°. Due to structural reasons, the front angle at the outer edge of the large, gradually decreasing to the center, the cross blade at the negative front angle (up to -55 ° or so), drilling play a role in extrusion. In order to improve the cutting performance of the twist drill, according to the nature of the processed material will be cutting part of the grinding into a variety of shapes (such as group drill).

Twist drill shank form has a straight shank and taper shank two kinds, the former clamped in the drill chuck when processing, the latter inserted in the machine tool spindle or tailstock taper hole. General twist drill made of high-speed steel. Welded carbide inserts or crown of the twist drill is suitable for processing cast iron, hardened steel and non-metallic materials, etc., the whole carbide twist drill for processing instrumentation parts and printed circuit boards, etc..

Extended information

Twist drill labeling reference system:

Twist drills have a more complex shape and cutting part: in order to facilitate the labeling of its geometric parameters, based on the structural characteristics of twist drills and the characteristics of the movement of the work, in addition to the base plane, the cutting plane, orthogonal planes, but also the use of the end of the plane, the column section and the middle of the plane. The definition is as follows.

End plane: perpendicular to the twist drill axis. This plane is also the back plane of any point on the cutting edge and is perpendicular to the base plane of the store.

Column profile: the column profile of any point on the main cutting edge is a cylindrical surface passing through the point and taking the radius of rotation of the point as the radius and the twist drill axis as the axis. It is tangent to the work plane at that point and perpendicular to the base plane at that store.

Medium profile: the axial profile through the twist drill axis and parallel to the two main cutting edges.

Processing impact:

(1) the diameter of the twist drill is limited by the diameter of the hole, the spiral groove makes the drill core thinner, the drill is less rigid; only two prongs of the belt guided, the axis of the hole is easy to be skewed; transverse edge to make the centering difficult, the axial resistance increases, the drill bit is prone to oscillation. Therefore, the shape and position error of the drilled hole is larger.

(2) the twist drill's front and back surfaces are curved surfaces, and the front and back angles of the points along the main cutting edge are different, and the front angle of the cross edge reaches -55°. Cutting conditions are very poor; the distribution of cutting speed along the cutting edge is unreasonable, and the tip with the lowest strength has the highest cutting speed, so it wears badly. As a result, the machined hole precision is low.

(3) The main cutting edge of the drill participates in the cutting, and the cutting speed of each point on the edge is not equal, which is easy to form spiral chips, and it is difficult to remove the chips. Therefore, the chip and the hole wall extrusion friction, often scratches the hole wall, the surface roughness after processing is very low.

Baidu Encyclopedia - Ordinary Twist Drill

Baidu Encyclopedia - Twist Drill