When using a high-speed steel drill bit, it is unreasonable to use the cutting edge (apex angle, chisel edge, rake angle, back angle, etc.) without any sharpening, or without purpose or unnecessary honing. In order to make the best use of the material, the drill bit must be sharply ground according to the different properties of the material to be processed, so that the drill bit can truly perform its proper performance.
Before the bit is sharpened, it is necessary to understand the structure of the drill bit and the characteristics of the material to be processed and the contradiction between the two, to understand the problems caused in the drilling, so as to achieve targeted grinding and solve the problem.
First, the problems and solutions when processing different materials
1. Cast iron has a low hardness (generally 175-250HB), low strength, high brittleness, small plastic deformation, low thermal conductivity, loose tissue, and smashed chips. The wear of the drill bit is almost all on the back, and the outer corner The greatest wear and tear, hindering tool life and productivity.
For the above problems, the following methods can be taken:
a. The drill bit is easy to wear at the corner of the outer edge of the double apex angle, then it is simply ground to form a double apex angle, which can increase the life of the drill by 1 to 2 times.
b. Appropriately increase the back angle. The outer blade back angle adopts 13°~16° to reduce friction and improve the life of the drill bit.
c. When drilling, increase the feed rate and reduce the cutting speed appropriately. It is not suitable to use high speed and small feed.
d. If coolant is used, it must not be used intermittently, and the flow rate should be appropriate. Otherwise, the small amount and intermittent will cause partial hardening of the hole, and the powdered chip grinding edge will also accelerate the bit wear.
2. High-strength steel is like tool steel, carburized steel, spring steel, bearing steel, heat-resistant steel, etc. When drilling, the cutting edge of the drill bit wears faster and has a shorter life.
For the above problems, the following methods can be used:
a. Use high-performance high-speed steel or hard alloy drill bits.
b. For hard and brittle steel, reduce the rake angle of the outer edge of the drill bit to improve the edge strength; for hard and plastic steel, it is not necessary to reduce it.
c. The top angle can be selected around 118°.
d. Use phosphorus-containing cutting oil as the coolant.
3. Stainless steel - generally three types of stainless steel such as martensite, ferrite and austenite. Martensitic stainless steel can resist atmospheric corrosion and has good mechanical properties. The quenching and tempering treatment can improve the cutting performance. If the hardness is higher than 30HRC, the bit wears faster. Ferritic stainless steel has a higher chromium content than martensitic stainless steel, so heat treatment cannot be strengthened, and its cutting performance is inferior to that of martensitic stainless steel. The mechanical strength of austenitic stainless steel is similar to that of medium carbon steel, and its plasticity and toughness are high. The load is very high during cutting. The work hardening phenomenon is serious during the cutting process, the heat conduction is poor, and the cutting heat is concentrated at the cutting edge of the drill bit. It is prone to build-up edges during the cutting process, and its structure contains a small amount of titanium carbide, which causes the drill bit to wear quickly.
For the above problems, the following methods can be used:
a. Appropriate selection of larger feed rate and lower cutting speed, which is conducive to chip breaking, can effectively improve the life of the drill bit.
b. Increase the apex angle (2φ=135°~140°) to improve the life of the drill bit.
c. When sharpening the top angle of the drill bit, the two drill edges must be symmetrical.
d. The rigidity of the machine tool and the clamping chuck is better.
e. The coolant should be sufficient.
4. Titanium alloy has the characteristics of high strength, small specific gravity, corrosion resistance, low temperature resistance and high temperature strength. According to the annealed structure, it can be divided into α phase titanium alloy, β phase titanium alloy, and α + β phase titanium alloy. The α-phase titanium alloy (TA type) cannot be heat-treated and strengthened, so the room temperature performance is not high, and it has moderate plasticity, and the machinability is acceptable. The β phase titanium alloy can obtain high room temperature performance by quenching and aging treatment. When α+β phase titanium alloy (TC type) is processed, the contact length between the chip and the front is short, and the cutting force acts near the cutting edge. Because the thermal conductivity is very small, the cutting edge temperature is high, which accelerates the bit wear and is processed. The hardening phenomenon is more serious and the elastic modulus is smaller, so the shrinkage of the drill hole is larger, which also affects the life of the drill bit.
For the above problems, the following solutions can be used:
a. Because the titanium alloy has large elastic deformation and the hole is easy to shrink, the drill tip is slightly ground to increase the expansion amount of the hole.
b. Increase the relief angle of the main cutting edge and reduce the width of the land to reduce the friction between the drill bit and the hole wall.
c. Grind the transition edge of the small corner at the corner of the outer edge to improve the heat dissipation conditions.
d. At the main cutting edge, reduce the rake angle appropriately.
5. Brass A drilled part is usually cast brass, which is characterized by wear resistance and corrosion resistance, but has low hardness and strength, low cutting load, and poor plasticity. The chips are chipped (but the pressure is processed by brass). Good plasticity, the chips are not easy to break). In the process of drilling brass, the phenomenon of “scissor knives†often occurs, that is, the automatic cutting of the drill bit, especially when the hole is to be drilled, it will automatically squat down, which is very unsafe, causing the hole at the exit to be scratched and burred or The drill bit is chipped, and the bit is broken.
For the above problems, the following solutions can be taken:
a. To reduce the problem of "scissors", the cutting edge should be made less sharp, so the rake angle at the outer edge of the cutting edge is reduced by τ = 6 ° ~ 8 °. If the drilling pressure is used to machine a brass workpiece, then τ = 10°.
b. Grind the chisel edge to make it narrow, but be symmetrical so that the ideal hole can be drilled.
6. Aluminium alloy—mainly cast aluminum alloy. It is easy to produce built-up edge when drilling. The roughness of the hole is poor. When drilling deep holes, the chip removal is difficult. If it is light, the hole wall is bumped. In the tank, the drill bit is broken. The aluminum alloy has a small modulus of elasticity and a large coefficient of linear expansion, so that pore shrinkage occurs in the borehole. Silicon is cast in cast aluminum, and the silicon compound is very hard, which will increase the wear of the drill bit. However, the aluminum alloy has low strength, low hardness and a small cutting load. In addition to pure aluminum, aluminum alloys generally have low plasticity and low elongation, so chip breaking is smoother.
For the above problems, the following solutions can be taken:
a. In order to prevent the hole diameter shrinkage in the borehole, the back angle can be increased, and the drill tip can be slightly biased.
b. When drilling deep holes, the top angle can be 2φ=135°～160°.
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