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Process characteristics of drilling

Machining holes in the solid part of the workpiece with a drill bit is called drilling.Drilling is rough processing, the achievable dimensional tolerance level is IT13~IT11, and the surface roughness value is Ra50~12.5µm.Due to the long length of the twist drill, the small diameter of the core and the poor rigidity, as well as the influence of the horizontal edge, the drilling has the following process characteristics:

1.The drill bit is easy to skew.Due to the influence of the horizontal edge, the centering is not accurate, and the drill bit is easy to deflect when cutting; and the rigidity and guiding effect of the drill bit are poor, and the drill bit is easy to bend when cutting.When drilling on a drilling machine, as shown in Figure 7-2a, it is easy to cause the axis of the hole to shift and not straight, but the aperture does not change significantly; when drilling on a lathe, as shown in Figure 7-2b, it is easy to cause changes in the aperture, but the axis of the hole is still straight.Therefore, the end face should be processed before drilling, and a cone pit should be pre-drilled with a drill bit or a center drill, as shown in Figure 7-3, so that the drill bit can be centered.When drilling small and deep holes, in order to avoid the axis of the hole being offset and not straight, the workpiece rotation method should be used for drilling as much as possible.

2.The aperture is easy to expand.When drilling, the unequal radial force of the two cutting edges of the drill bit will cause the aperture to expand; the deviation of the cut-in when drilling in a horizontal lathe is also an important reason for the aperture to expand; in addition, the radial runout of the drill bit is also the cause of the aperture to expand.

3.The surface quality of the hole is poor.The drilling chips are wide and are forced to roll in a spiral shape in the hole. When they flow out, they rub against the hole wall and scratch the processed surface.

4.The axial force is large when drilling.This is mainly caused by the horizontal edge of the drill bit.Tests have shown that 50% of the axial force and 15% of the torque during drilling are generated by the horizontal blade.Therefore, when the drilling diameter d﹥30mm, drilling is generally carried out twice.Drill out (0.5~0.7) d for the first time, and drill to the required aperture for the second time.Since the horizontal edge does not participate in cutting for the second time, a larger feed can be used to improve the surface quality and productivity of the hole.

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