Effect of hollow bit on cutting

Hollow bit is a kind of hole machining tool suitable for portable tools. But because the hollow bit manufacturing process is more complex, and can not process blind hole, so it is not widely used in metal cutting processing, usually only in the processing of some large diameter or precious metal workpiece through hole or drilling equipment power is limited. Since there is no standard product for hollow bits, most hollow bits used for special material processing need to be developed by themselves.

The influence of the posterior Angle

The influence of rake Angle on cutting force: the change of rake Angle will affect the deformation degree of chip material, so as to change the cutting force. The larger the chip deformation, the greater the cutting force; The smaller the chip deformation, the smaller the cutting force. When the current Angle varies from 0° to 15°, the cutting force correction coefficient varies from 1.18 to 1.

Influence of rake Angle on bit durability: when the rake Angle increases, the strength and heat dissipation volume of the tool tip will decrease, and the force on the tool tip will be affected. When the current Angle is positive, the tip is subjected to tensile stress. When the current Angle is negative, the compression stress of the tool tip. If the front Angle is too large, although it can increase the edge of the drill bit and reduce the cutting force, the tensile stress of the tip is large, the strength of the tip is reduced, and it is easy to break. Many drill bits are damaged due to excessive rake Angle in cutting tests. However, due to the high hardness and strength of processed materials, coupled with the portable drill spindle and machine rigidity is low, such as the selection of the front Angle is too small, drilling when the increase of cutting force will make the spindle vibration, processing surface appears obvious vibration, drill durability will also be reduced.

Influence of cutting performance

Increasing the back Angle can reduce the friction between the back tool surface and the cutting material and reduce the extrusion deformation of the machined surface. However, if the back Angle is too large, the blade strength and heat dissipation capacity will be reduced.

The size of the back Angle directly affects the bit durability. In drilling process, the main wear forms of drill bits are mechanical abrasion and phase change wear. Considering mechanical abrasion, when the cutting life is constant, the larger the back Angle, the longer the available cutting time; Considering phase change wear, the increase of back Angle will reduce the bit heat dissipation capacity. After bit wear, with the gradual broadening of the wear zone on the back tool surface, the cutting power gradually increases, and the heat generated by friction gradually increases, which increases the bit temperature. When the temperature rises to the bit phase transformation temperature, the bit will appear rapid wear.

Influence of grinding process

Hollow drill bit consumption is less, processing batch is small, so the design of drill bit should consider its processing technology, as far as possible with common machining equipment and common tools to achieve processing and grinding. The chip flows out through the front cutter surface, so the shape of the front cutter surface directly affects the chip shape and chip removal performance. The chip is extruded and rubbed by the front cutter surface in the process of outflow, further deforming. The chip bottom metal deformation degree is the largest, and slippage along the front knife surface, so that the chip bottom length is longer, thus forming a variety of crimped shapes. When drilling with a hollow bit, it is hoped that the chips will become debris or ribbon chips to facilitate the removal of chips. In order to facilitate machining and grinding, the front cutter face must be designed as a plane, and chip groove is not broken. The front face does not need regrinding in use. The back cutting surface of hollow bit is the most easily reground surface, and also the fastest wear surface, so the grinding of hollow bit is realized by the back cutting surface. The auxiliary rear tool surface is divided into the internal auxiliary rear tool surface and the external auxiliary rear tool surface. From the point of view of regrinding, it is not easy to regrind the inner and outer rear surfaces, so the rear surfaces should be designed in the form of non-regrinding.

Cutting fluid and drill bit

The main characteristic of the hollow bit is that the inner core of the hole is not cut when machining, so the cutting amount of the hollow bit is significantly reduced than the twist drill, and the required rig power and the heat generated in the cutting is also smaller. When drilling with HSS hollow bit, because the temperature of the processing zone has a great influence on the hardness of the bit, it is necessary to use coolant to cool down the drilling process (if no coolant is used, the bit wear will be mainly phase change wear and rapid wear at the beginning). At the beginning, we adopted the external spray cooling method, but because the drill position was processed in the horizontal axis direction, the coolant was not easy to enter the blade part of the drill, so the coolant consumption was large, and the cooling effect was not ideal. After the redesign, the main shaft structure of the drilling machine was changed from external spray cooling to internal spray cooling, and the coolant was added by the core of the hollow bit, so that the coolant could reach the cutting part of the bit smoothly, so that the coolant consumption was significantly reduced and the cooling effect was improved.