As the most common tool in the hole processing, drill is widely used in mechanical manufacturing, especially for the hole processing of cooling device, tube sheet of power generation equipment and steam generator.
Ⅰ. Characteristics of drilling
A drill usually has two main cutting edges. During machining, the drill cuts while it rotates. The rake angle of the bit is larger and larger from the central axis to the outer edge. And the closer it is to the outer circle, the higher the cutting speed of the bit is and the cutting speed decreases to the center, and the cutting speed of the rotary center of the bit is zero. The cross edge of the bit is located near the axis of the rotation center, and its secondary rake angle is large, so there is no chip space, and the cutting speed is low, so it will produce large axial resistance. If the transverse cutting edge is grinded to type A or type C in DIN1414 and the cutting edge near the central axis is positive rake angle, the cutting resistance can be reduced and the cutting performance can be improved significantly.
According to the different shapes, materials, structures and functions of work pieces, drills can be divided into many types, such as high-speed steel drills (twist drills, group drills and flat drills), solid carbide drills, indexable shallow hole drills, deep hole drills, nesting drills and head changing drills.
Ⅱ. Chip breaking and chip removal
The cutting of the bit is carried out in the narrow hole, and the chip must be discharged through the edge groove of the bit, so the chip shape has a great influence on the cutting performance of the bit. The common chip shapes are flake chip, tubular chip, needle chip, conical spiral chip, belt chip, fan chip, powder chip and so on.
1) The key of drilling-chip control
When the chip shape is not suitable, the following problems will occur:
①Fine chips block the edge groove, affect the drilling accuracy, reduce the life of the bit, and even break the bit (such as powder chips, fan chips, etc.);
②Long chips entangle the bit and hinder the operation, causing bit damage or preventing cutting fluid from entering the hole (such as spiral chips, banded chips, etc.).
2) How to solve the problem of improper chip shape?
① The methods of increasing feed rate, intermittent feed, grinding cross edge and installing chip breaker can be used separately or jointly to improve the chip breaking and chip removal effect and eliminate the problems caused by chip cutting.
② Professional chip breaking bit can be used for CNC drilling. For example, a chip breaking edge is added to the groove of the drill bit to break the chips into more easily removed chips. The debris can be removed smoothly along the groove, and the phenomenon of blockage in the groove will not occur. Therefore, the cutting effect of the new chip breaker is much smoother than that of the traditional bit.
At the same time, the short broken iron chips make it easier for the coolant to flow to the drill point, which further improves the heat dissipation effect and cutting performance in the machining process. Moreover, because the new chip breaking edge penetrates the whole groove of the drill bit, it can still maintain its shape and function after many times of grinding. In addition to the above function improvement, it is worth mentioning that the design strengthens the rigidity of the drill body and significantly increases the number of boreholes before a single grinding.
Ⅲ. Drilling accuracy
The accuracy of hole is mainly composed of aperture size, position accuracy, coaxiality, roundness, surface roughness and burr.
(1) When drilling, the factors that affect the accuracy of the machined hole are as follows:
① The clamping accuracy and cutting conditions of the bit, such as tool holder, cutting speed, feed rate, cutting fluid etc.
② Bit size and shape, such as bit length, edge shape, core shape etc.
③ Work piece shape, such as hole side shape, hole shape, thickness, clamping status etc.
Reaming is caused by the swing of the drill bit in machining. The swing of the tool holder has a great influence on the hole diameter and the positioning accuracy of the hole, so when the tool holder is seriously worn, it should be replaced in time. When drilling small holes, it is difficult to measure and adjust the swing, so it is better to use the bit with coarse shank and small blade diameter, which has good coaxiality between the blade and the shank. When the regrinding bit is used for machining, the reason for the decline of hole accuracy is mostly due to the asymmetry of the back shape. Controlling the height difference of the cutting edge can effectively restrain the cutting expansion of the hole.
(3) Roundness of holes
Due to the vibration of the drill bit, the drilled hole is easy to be polygonal, and there are lines like a double line on the hole wall. The common polygon hole is triangle or pentagon. The reason of triangular hole is that the drill bit has two rotation centers when CNC drilling, which vibrate once every 600 intervals. The main reason of vibration is that the cutting resistance is not balanced. When the drill bit rotates for one revolution, the resistance is not balanced in the second revolution due to the poor roundness of the machined hole. The last vibration is repeated again, but the vibration phase has a certain offset, resulting in the hole being damaged the wall is lined with rifled lines. When the depth of CNC drilling reaches a certain degree, the friction between the edge surface of bit edge and the hole wall increases, the vibration attenuates, the double line disappears, and the roundness becomes better. From the longitudinal section, the hole is funnel-shaped. For the same reason, pentagonal and heptagonal holes may appear in cutting. In order to eliminate this phenomenon, in addition to controlling the vibration of the chuck, the height difference of the cutting edge, and the asymmetry of the back and blade shape, measures should be taken to improve the rigidity of the bit, increase the feed rate per revolution, reduce the back angle, and sharpen the cross edge.
(4) Drilling holes on inclined and curved surfaces
When the bit's cutting surface or penetration surface is inclined, curved or stepped, the positioning accuracy is poor. Because the bit is radial single side cutting, the tool life is reduced.
(5) In order to improve the positioning accuracy, the following measures can be taken:
① Firstly, drill the center hole;
② Mill the hole seat with end mill;
③ Select the drill with good penetration and rigidity;
④ Reduce the feed speed.
(6) Burr treatment
During CNC drilling, burr will appear at the entrance and exit of the hole, especially when machining tough materials and thin plates. The reason is that when the bit is about to drill through, the processed material will appear plastic deformation. At this time, the triangular part which should be cut by the edge of the bit near the outer edge will be deformed and bent outward under the action of axial cutting force, and will be further curled under the action of the chamfer of the outer edge of the bit and the edge of the edge belt to form curl or burr.