Drilling accuracy
The accuracy of the hole is mainly composed of aperture size, position accuracy, coaxiality, roundness, surface roughness, orifice burr and other factors.
Factors affecting the accuracy of the machinised hole during drilling:
1 The clamping accuracy and cutting conditions of the drill bit, such as the knife clip, cutting speed, feed amount, cutting fluid, etc.;
2 Drill bit size and shape, such as drill bit length, blade shape, drill core shape, etc.;
3 The shape of the artefact, such as the side shape of the hole, the shape of the hole, thickness, card loading status, etc.
Expand the hole
The hole expansion is caused by the swing of the drill bit during processing. The swing of the knife clip has a great impact on the aperture and positioning accuracy of the hole, so when the clip is seriously worn, the new clip should be replaced in time. When drilling small holes, it is difficult to measure and adjust the swing, so it is best to use a thick handle small blade diameter drill bit with a good coaxiality between the edge and the handle. When machining with heavy grinding bits, most of the reasons for the decrease in hole accuracy are caused by the asymmetry in the back shape. Controlling the height difference of the blade can effectively inhibit the shear expansion of the hole.
Roundness of the hole
Due to the vibration of the drill bit, the drilled hole shape is easy to be polygonal, and there is a parallel pattern on the wall of the hole. Common polygonal holes are mostly triangles or pentagons. The reason for triangular holes is that the drill bit has two rotary centres when drilling holes, which are exchanged at a frequency of 600 intervals. The main reason for vibration is the unbalanced cutting resistance. When the drill bit rotates, due to the poor roundness of the machining hole, the resistance is unbalanced during the second turning, and the last vibration is repeated again. However, there is a certain offset in the vibration phase, resulting in complex lines on the hole wall. When the drilling depth reaches a certain level, the friction between the edge edge of the drill bit and the hole wall increases, and the vibration attenuates, so that the double line disappears and the roundness becomes better. This kind of hole type is funnel-shaped from the longitudinal section. For the same reason, pentagons, heptagonal holes, etc. may appear in cutting. In order to eliminate this phenomenon, in addition to controlling factors such as chuck vibration, cutting edge height difference, rear and blade shape asymmetry, measures should also be taken to improve drill bit rigidity, increase per turn feed, reduce rear angle, and grind the transverse edge.