High-speed steel straight shank twist drill is adopted to machine miniature holes(1mmand below), breakage is the main form of drill failure. Since drill breakage occurs suddenly and is difficult to predict in advance, and the broken drill often gets stuck in the incomplete hole, it is difficult to take out, which causes a lot of trouble in production, and also makes it difficult to realize automated drilling of miniature holes. During the drilling process, the drill is subjected to a combination of three kinds of loads, namely, torsion, bending and compression. The study on the breakage morphology and stress state of the drill can clarify the causes of drill breakage and the role of each load in drill breakage, so as to take necessary measures to prevent the drill from breaking suddenly. For example, the main load causing drill breakage is detected online by using drilling force meter, and an alarm signal is issued when this load increases to a certain value, so that the drill is retracted to avoid drill breakage and drilling quality decline.

1 Drill Bit Fracture Observation and Analysis

Microscopic observation of the fracture surface of the micro-drill revealed that the majority of the cracks occurred between the middle and the root of the spiral groove, away from the cutting edge. When viewed along the direction of the spiral groove, the angle between the outer normal of the fracture section and the axis of the drill bit was approximately 40°～50°. Figure 1 shows the SEM photo of the fracture surface of a 0.34mm high-speed steel straight shank twist drill when drilling _18Cr2_Ni4WA low carbon alloy steel.

According to the theory of elasticity and the strength theory of metal materials, for a straight bar with non-circular cross section subjected to free torsion, the shear stress on the cross section is zero at the sharp angle and maximum at the groove(which is the critical point), and the cross section of the bar is the section where the maximum shear stress occurs; the section where the maximum normal stress occurs at the point of maximum shear stress is the critical section, and the included angle between this section and the section where the maximum shear stress occurs is45°. Therefore, the maximum shear stress of the drill bit occurs on the cross section at the bottom of the spiral groove under the action of torque, and the included angle between the section where the maximum normal stress occurs and the cross section of the drill bit is45°. Since the direction of the section where the measured drill bit breaks is basically consistent with the section where the drill bit is subjected to a torsional load, it can be concluded that the excessive torsional load is the main reason for the breakage of the drill bit.

2.Experimental and Analytical Study of the Fracture of Drills Subjected to Torque, Bending, and axial Loads

To verify the correctness of the above analysis, the high-speed steel straight shank twist drill with a diameter of 0.34mm was loaded in an ideal torsion, bending, and compression manner until the drill bit failed, and its failure section was compared with the drilling failure section.

The maximum tensile stress occurs on the cross section of the drill bit under pure bending load, and the direction of the drill bit's fracture should be consistent with the direction of the drill bit's cross section, which is consistent with the experimental results(see Figure2).

The drill bit fails by shearing when subjected to a compressive load, and the section at failure should be the section at which the maximum shear stress occurs, which is at an angle of approximately45°to the cross section of the drill bit. Unlike the torsional load, the orientation of the failure section is random and independent of the position of the spiral groove, and can occur at any position at an45°angle to the cross section. The shear stress is equal on this series of oblique sections. The measured compressive failure section is at an45°angle to the cross section(see Figure3), which is consistent with the above analysis.

The failure section of the drill under pure torsional loading is shown inFigure 4, which is basically consistent with the failure section of the drill shown inFigure 1that broke during drilling. The failure section of the torsional destruction has a prominent feature, that is, when viewed towards the spiral groove of the drill, the section of the failure section is at a45°angle to the cross section, which is also the maximum positive stress at the point of maximum shear stress.

3 Analysis of the causes of micro-drill breakage

The dispersion of the life of micro-drills is very large, still taking the high-speed steel twist drill with a diameter of 0.34mm as an example, when drilling 18Cr2Ni4WA low-carbon alloy steel, the life of the drill bit is about 10 ～ 80mm when the hole depth is 2mm. Observations with a microscope on the cutting edge of the drill bit show that some drill bits with higher life have obvious wear marks on the back tool face before breaking, while some drill bits break without obvious wear marks on the cutting edge, and a small number of drill bits have a life of less than 10mm due to defects in the material of the drill bit or a large error in the grinding angle of the cutting edge. It can be seen that some random factors in the drilling process have a great influence on the life of the drill bit.

During normal drilling, all loads are within the bearing capacity of the drill bit, and the drill bit breaks when the drilling torque suddenly fluctuates and exceeds its bearing capacity. The core thickness of the micro-drilled core is relatively large, and the ratio of the core thickness to the diameter of the drill bit is about 0.3～0.4 ( ordinary diameter drill bit below 0.2), which makes the spiral groove shallow and the chip removal and chip discharge capacity low. In addition, the liquid has surface tension, and it is difficult for the cutting fluid to enter the micro-hole, which can easily cause chip blockage, making the drilling torque suddenly increase and the drill bit break.

It can be seen that chip blockage, wear of cutting edge and manufacturing defects of the drill bit are the main reasons for the breakage of micro twist drill. Therefore, before the micro twist drill is used for drilling, the grinding angle of the cutting edge should be checked to eliminate the drill bits with large deviation; when the drilling force is online detected by the drilling force tester, the drilling torque should be used as the detection and control index.