1、 Composition of turning tool

Turning tool is composed of tool head and tool body. The cutter head is used for cutting and the cutter body is used for installation. The cutter head is generally composed of three sides, two blades and one point.

The rake face is the surface through which the chip flow passes.  

The main flank is the surface opposite to the cutting surface of the workpiece.  

The secondary flank is the surface opposite to the machined surface of the workpiece.  

The main cutting edge is the intersection line between the rake face and the main flank, which is responsible for the main cutting work.        

The auxiliary cutting edge is the intersection line between the rake face and the auxiliary flank, which takes on a small amount of cutting work and plays a certain role in polishing

The tool tip is the intersection of the main cutting edge and the auxiliary cutting edge, which is generally a small transition arc.

2、 Form and structure of turning tool

The most commonly used turning tool structures are as follows:

(1) The cutting part of the whole turning tool is obtained by grinding. The material of the whole turning tool is mostly made of high speed steel, which is generally used for low speed cutting.

(2) Welding turning tools weld cemented carbide inserts on the tool head. Different types of turning tools can use different shapes of inserts. The welded carbide turning tool can be used for high speed cutting.

3、 The main angles of turning tools and their functions are rake angle (γ 0), rake angle (α 0), main deflection angle (KR), minor deflection angle (KR) and edge inclination angle (λ s). In order to determine the angle of turning tool, three coordinate planes should be established: cutting plane, base plane and main section. For turning, if the influence of turning tool installation and cutting motion is not considered, the cutting plane can be considered as a vertical plane; the base plane is a horizontal plane; when the main cutting edge is horizontal, the section perpendicular to the main cutting edge is the main section.

(1) The rake angle, measured in the main section, is the angle between the rake face and the base face. Its function is to make the blade sharp and easy to cut. But the rake angle should not be too large, otherwise the strength of the blade will be weakened, and it will be easy to wear or even collapse. When machining plastic materials, the rake angle can be larger. For example, when cutting steel parts with cemented carbide turning tool, the rake angle can be 10-20. When machining brittle materials, the rake angle of turning tool should be larger than that of rough machining, so as to make the cutting edge sharp and the roughness of workpiece small.

(2) The back angle α 0 is measured in the main profile, which is the angle between the main back face and the cutting plane. Its function is to reduce the friction between the main rear face and the workpiece during turning. Generally, α 0 = 6 ° to 12 ° is taken as the minimum value in rough turning and the maximum value in finish turning.

(3) The main deflection angle Kr is measured in the base plane, which is the angle between the projection of the main cutting edge on the base plane and the feed direction. Its functions are as follows:

1) It can change the cutting length of the main cutting edge and affect the tool life.

2) It affects the size of radial cutting force.

Small main deflection angle can increase the cutting length of the main cutting edge, so the heat dissipation is better, which is beneficial to prolong the service life of the tool. However, when machining slender shaft, the rigidity of the workpiece is insufficient, and the small main deflection angle will increase the radial force of the cutter acting on the workpiece, which is easy to produce bending and vibration. Therefore, the main deflection angle should be larger.

The main deflection angles commonly used for turning tools are 45 °, 60 °, 75 ° and 90 ° and more than 45 ° of them.

(4) The secondary deflection angle Kr is measured in the base plane, which is the angle between the projection of the secondary cutting edge on the base plane and the feed direction. Its main function is to reduce the friction between the auxiliary cutting edge and the machined surface, so as to improve the roughness of the machined surface.

When the cutting depth AP, feed rate f and main deflection angle Kr are equal, reducing the secondary deflection angle Kr can reduce the residual area after car cutting, so as to reduce the surface roughness. Generally, Kr = 5 ° to 15 ° is selected.

The inclination angle of cutting edge measured in the cutting plane is the angle between the main cutting edge and the base plane. Its main function is to control the chip flow direction. When the main cutting edge is parallel to the base plane, λ s = 0; when the tool tip is at the lowest point of the main cutting edge, λ s is negative, the strength of the tool tip increases, and the chips flow to the machined surface for rough machining; when the tool tip is at the highest point of the main cutting edge, λ s is positive, the strength of the tool tip decreases, and the chips flow to the machined surface for finish machining. The inclination angle of turning tool is generally between – 5 ° and + 5 °.

4、 The sharpening of turning tool after the turning tool is blunt, it must be sharpened in order to restore its reasonable shape and angle. Turning tools are usually sharpened on a grinder. White alumina grinding wheel is used for grinding high speed steel turning tools and green silicon carbide grinding wheel is used for grinding cemented carbide turning tools.

When the turning tool is regrinding, the relevant tool surface can be ground according to the wear condition of the turning tool. The general order of turning tool grinding is: grinding the flank of back tool → grinding the flank of back tool → grinding the flank of front tool → grinding the arc of tool tip. After the turning tool is sharpened, the oilstone shall be used to grind each tool face. In this way, the service life of the turning tool can be effectively improved and the surface roughness of the workpiece can be reduced.

The grinding steps of turning tool are as follows:

Grind the main flank, and grind out the main deflection angle and main rake angle at the same time, as shown in figure (a);

Grind the flank of the pair, and grind out the deflection angle and back angle of the pair, as shown in (b) above;

Grind the front while grinding the front corners, as shown in figure (c);

Grind the tool faces and tips, as shown in (d) above.

The posture and method of sharpening turning tool are as follows:

People stand on the side of the grinder to prevent the fragments from flying out to hurt people when the grinding wheel is broken;

The distance between the two hands holding the knife is released, and the two elbows are clamped at the waist to reduce the shaking when grinding the knife;

When grinding the main and auxiliary flanks, the turning tool should be placed in the horizontal center of the grinding wheel, and the tool tip should be slightly tilted up about 3 ~ 8 degrees. The turning tool should move horizontally in the left and right directions after contacting the grinding wheel. When the turning tool leaves the grinding wheel, it should be lifted up to prevent the grinding edge from being damaged by the grinding wheel;

When grinding the flank, the end of the cutter bar is deflected to the left by an angle of main deflection angle; when grinding the flank, the end of the cutter bar is deflected to the right by an angle of auxiliary deflection angle;

When sharpening the arc of the tool tip, the front end of the tool is usually held by the left hand as the fulcrum, and the rear end of the tool is rotated by the right hand.

Pay attention to the following items when grinding the turning tool: (1) when grinding, hold the turning tool firmly with both hands, and place the tool bar against the bracket, so that the affected surface is gently attached to the grinding wheel. Do not use too much force to avoid crushing the grinding wheel and causing accidents.

(2) The sharpened turning tool should be moved left and right on the circumferential surface of the grinding wheel to make the grinding wheel wear evenly without grooves. Avoid rough grinding on both sides of the grinding wheel, so that the grinding wheel is forced to swing, jump, or even break.

(3) When the cutter head is ground hot, it should be cooled with water to avoid annealing and softening due to high temperature rise. When grinding cemented carbide turning tools, the tool head should not be stained with water, so as to avoid the cracks caused by the rapid cooling of the blade.

(4) Do not stand on the front of the grinding wheel to sharpen the turning tool, so as to prevent the operator from being injured when the grinding wheel is broken.

5、 The types and uses of commonly used turning tools can be divided into cylindrical turning tools, end turning tools, cutting tools, boring tools, forming turning tools and grain turning tools.

(a) 90 ° turning tool

(b) 45 ° turning tool (elbow turning tool)

(c) Cutter

(d) Boring cutter

(e) Forming turning tool

(f) Thread turning tool

(g) Carbide non regrinding turning tool