The design of the hollow drill cutting

Hollow drill bit is a more appropriate tool to use portable hole machining tool. However, due to the manufacturing process of the hollow drill bit more complex, and the blind holes can not be processed, so in the metal cutting process is not widely used, usually only in the  diamond hole saw processing of a number of large-diameter through-hole of the workpiece or a noble metal or drilling equipment used only when the power is limited . Since there is no standard hollow drill styling products, so most of the processing for special materials developed hollow drill bit is required.
We worked to develop a user for processing difficult materials special hollow drill bit. Code of material to be processed U-Mn, the main chemical components include: carbon (0.56% to 0.68%), manganese (1.35% to 1.65%), silicon (0.2% to 0.35%), etc.; tensile strength ≥ 882N/mm2, hardness and wear resistance are high. The drill is used in a 17mm thick material processing? 30 +0.5 mm hole, portable drilling rig power <1000w, requires drill life> 30min, drill material W6Mo5Gr4V2. In the development process of the hollow drill bit, drill through repeated adjustment of the design parameters and drilling tests to finalize drill geometry parameters: rake angle g = 12 °, clearance angle a = 9 °, vice posterior horn a1 = 3 ° .
Here's the design of the cutting hollow drill bit to make a brief analysis of the impact.
1, drill cutting rake angle change on the impact of
Rake angle on the cutting force of the impact
The changes will affect the anterior horn of the chip material deformation, so that the cutting force change. Chip deformation greater cutting forces greater; chip deformation is smaller, the smaller the cutting force. This angle is 0 ° ~ 15 ° range of changes, the cutting force correction coefficient ranges from 1.18 to 1.
Rake angle drill durability against impact
Increasing the rake angle drill, makes the tip strength and thermal volume decreases, while the tip will affect the force. This angle is positive, the tip tensile stress; current angle is negative, the tip compressive stress. Such as the selection of the rake angle is too large, although the increased drill sharpness, reduced cutting forces, but the carbide tipped saw blade  tip tensile stress suffered greater tip strength decreased, easily broken. In many drill cutting tests are due to excessive damage to the anterior horn. However, since the material to be machined high hardness and strength, coupled with a portable drill spindle and machine rigidity is low, such as the selection of the rake angle is too small, the increase in drilling cutting force causes the spindle vibration, vibration apparent machined surface pattern, drill durability will be reduced.
2, the posterior horn of the impact of changes on the drill cutting
After increasing flank angle can reduce the friction between the cutting material, reducing the surface of the extrusion process. However, if after the angle is too large, it will reduce the edge strength and heat dissipation.
Directly affects the size of the posterior horn drill durability. In the drilling process, the drill main wear to mechanical abrasion and wear of the phase change. Consider mechanical abrasion wear when cutting life is constant, the posterior horn of the larger cutting longer available; consider phase transition wear, after the drill angle increases cooling capacity will decrease. Bit wear, with the flank wear with a gradual widening gradually increase cutting power, heat generated by friction will gradually increase, the bit temperature, when the temperature rises to the bit phase transition temperature, the bit will appear rapid wear.
3, drill sharpening process on the impact of design
Hollow drill bit less dosage, processing small quantities, so the design should consider the drill process issues, as far as common machining equipment and tools used to achieve processing and grinding.
The rake chip out, so a direct impact on the shape of the rake face and chip performance chip shape. Used chips in the process of being squeezed and friction of the rake face, and further deformation. The maximum degree of deformation of the base metal chips and to slip along the rake face, the chips bottom length is longer, thereby forming a variety of crimp shape. When using a hollow drill bit and hope the chips into crumbs or ribbon cuttings, in order to facilitate chip removal. For ease of machining and grinding, must be designed for flat rake face, without chip breaking groove. Rake in use does not require re-grinding.
Hollow drill flank is most likely to re-grinding of the face, but also the fastest surface wear, so hollow drill flank grinding sharpening is achieved.
Deputy Vice flank divided into inner and outer flank minor flank. Heavy grinding perspective, re-grinding inner and outer flanks not easy to achieve, so minor flank shall be designed for a throwaway form.
Based on the above analysis, the hollow drill blade is designed in the form shown in Figure 1. Processing Practice has proved that the design can meet the requirements and tool regrinding requirements.
4, the cutting fluid used and the impact on the drill cutting
The main characteristics of the hollow drill processing hole of the core is not to be cut, thus cutting the hollow drill twist ratio was significantly reduced, the power required for drilling and cutting the heat generated is small.
High-speed steel drill with a hollow drill, because of the hardness of the bit processing area a great influence on the temperature, thus the drilling process must be cooled using a cooling fluid (eg without coolant, the start bit will wear the phase change and rapid wear of the ring saw blades  main wear and tear). We started using an external spray cooling, but bit-position horizontal axis machining, the coolant is not easy to enter the drill blade portion, a large consumption of cooling fluid, the cooling effect is not ideal. The redesigned drill spindle structure change, the external spray cooling becomes an internal spray cooling, the cooling fluid from the hollow drill core join, the coolant can reach the cutting part of the drill, thereby significantly reducing coolant consumption, improved cooling effect.
5, the hollow drill bit to use effects
A well-designed hollow drill bit should also meet the requirements of the following aspects: ① easy to manufacture, to use common tools and common tooling; ② easy re-sharpening, grinding machine can be used for general sharpening; ③ high efficiency, long life; ④ low prices.
We have developed a hollow drill basically reached the above requirements. In actual use, drill durability is stable up to 50 minutes, the diameter tolerance, surface roughness, all reached the design requirements. Because only regrinding flank, posterior horn drill easier to control, in an ordinary grinder can be easily implemented on sharpening.