Tool geometric parameters are the general term of the inclination angle of each surface of the cutting part and the shape of the tip part of the tool, including rack angle, back angle, edge inclination, etc.
They have a great impact on machining.
Choosing appropriate tool geometric parameters can improve productivity, machining quality and tool durability.
（1） Function and selection of rack angle
1. Function of rack angle
Rake angle is one of the important geometric angles of cutting tools.
Its value, positive and negative have a great impact on cutting deformation, cutting force, cutting temperature, tool wear and machined surface quality.
(1) Affect cutting deformation.
Increasing the rake angle can reduce the cutting deformation, cutting force, cutting heat and cutting power.
(2) Affect cutting edge strength and heat dissipation.
Increasing the rake angle will reduce the wedge angle, the cutting edge strength and the heat dissipation volume;
Excessive rake angle may lead to bending stress at the cutting edge, resulting in edge collapse.
(3) Affect chip shape and chip breaking effect.
The degree of cutting deformation is different, and the degree of chip breaking is different (see Fig. 1-49).
(4) Affect the quality of machined surface.
Fig. 1-49 effect of rake angle on chip shape
a) Frontal angle
b) Negative front angle
2. Selection principle
The reasonable rake angle of the tool mainly depends on the performance of the tool material and the workpiece material, namely:
(1) When the bending strength and impact toughness of tool material are high, large rake angle can be selected.
(2) When the strength and hardness of the workpiece material are large, the smaller rake angle should be selected to ensure the strength of the cutting edge;
On the contrary, a larger front angle should be selected.
(3) Other specific processing conditions.
3. Rack angle selection catchphrase
- The rack angle plays an important role, so it should be selected reasonably;
- The hardness of the workpiece is high, and the rack angle should be small;
- The plasticity of the workpiece is large, and the rack angle should be large;
- Cemented carbide knife, the rack angle should be small;
- For high-speed steel cutting tools, the rake angle should be large;
- The rack angle of rough machining is small and that of finish machining is large.
4. Use negative chamfering to strengthen the cutting edge
The increase of tool rake angle can reduce the cutting deformation and cutting force, but it is often limited by the edge strength.
Grinding chamfering on the rake face of the positive rake angle is a better solution (see Fig. 1-50).
Chamfering surface can be negative rake angle, zero rake angle or small positive rake angle. Negative chamfering is widely used in practical application.
Fig. 1-50 chamfering on rake face
a) Negative rake chamfering
b) Zero rake chamfering
The main function of chamfering is to enhance the cutting edge and reduce tool damage.
When rough machining or intermittent cutting brittle and hard materials, negative chamfering can reduce edge collapse and improve the service life of the tool.
When milling quenched steel with ceramic cutter, the edge without chamfering cannot be machined.
In addition, the wedge angle at the chamfering of the tool is large, which can improve the heat dissipation conditions.
（2） Function and selection of back angle
1. Function of back angle
The main function of the back angle is to reduce the friction between the back cutter surface and the machined surface, which affects the machined surface quality and tool service life.
(1) Increasing the back angle can reduce the contact length between the elastic recovery layer of the machined surface and the back knife surface, and reduce the friction and wear of the back knife surface.
(2) Increase the back angle, so that the wedge angle decreases, the blunt circle radius rn of the edge decreases, and the edge is sharp.
(3) When the flank grinding is the same as the standard VB, when regrinding, the tool with large back angle grinds off the metal and has a large volume.
2. Selection principle
(1) When the plasticity and toughness of the workpiece material are large, the larger back angle shall be taken.
For example, a large back angle should be taken for processing titanium alloy.
(2) When finishing, the cutting thickness is small, and a larger back angle should be selected.
(3) For sizing tools (such as round hole broach and reamer), it is advisable to select a small back angle to prolong the service life of the tool.
(4) When the rigidity of the process system is poor and vibration is easy to occur, the smaller back angle shall be selected.
（3） Function and selection of main (auxiliary) deflection angle
1. Function of main deflection angle
(1) Affect the quality of machined surface. Increase the main deflection angle and auxiliary deflection angle to increase the machined surface roughness value.
(2) It affects the cutting layer size, tool tip strength and chip breaking effect.
(3) Affect the ratio of cutting force components.
2. Selection principle of main deflection angle
(1) When the rigidity of the process system is good, a small main deflection angle should be selected to improve the service life of the tool and the quality of the machined surface.
(2) When machining very hard workpiece materials (such as chilled cast iron and quenched steel), it is appropriate to take a small main deflection angle to reduce the load on the cutting edge per unit length, improve the heat dissipation conditions of the tool tip and improve the service life of the tool.
(3) The shape and specific conditions of the workpiece shall also be considered.
3. Selection principle of secondary deflection angle
(1) Under the condition of good rigidity of the process system and no vibration, a small secondary deflection angle should be taken to reduce the roughness of the machined surface.
(2) When finishing, the secondary deflection angle is smaller than that of rough machining.
(3) When machining workpiece materials with high strength and hardness or intermittent cutting, in order to improve the strength of the tool tip, it is appropriate to take a small pair deflection angle value (4 ° ~ 6 °).
(4) Due to the limitation of structural strength or machining dimensional accuracy, only a small auxiliary deflection angle (1 ° ~ 2 °) can be taken for cutting (slot) cutter, saw blade milling cutter, drill bit and reamer.
（4） Function and selection of blade inclination
1. Function of blade inclination
(1) Affect the outflow direction of chips.
When λs is negative, the chip flows to the machined surface and is easy to scratch the machined surface;
When λs is positive, the chip flows to the surface to be machined, as shown in fig. 1-51.
Therefore, the inclination of the positive edge is often taken in finishing.
Fig. 1-51 effect of positive and negative edge inclination on chip outflow direction
- a) Zero edge inclination
- b) negative edge inclination
- C) positive edge inclination
(2) It affects the strength of the tool tip and the impact position of the cutting edge during intermittent cutting.
(3) Affect the sharpness of the cutting edge.
(4) The ratio that affects the cutting force.
(5) Affect the actual working length of the cutting edge.
2. Selection of blade inclination
(1) It is mainly selected according to the processing nature.
Fine car often take λs=0°～5°；
Pick up during rough turning λs = 0 ° ~ – 5 ° to improve the cutting edge strength of the tool;
When there is impact load, in order to protect the tool tip, λs = – 5 ° ~ – 15 ° is often taken.
(2) Select according to the stiffness of the process system.
When the rigidity of the process system is insufficient, negative blade inclination should not be selected.
(3) Select according to the tool material.
For the tool material with high brittleness, in order to ensure the strength of the cutting edge, the positive edge inclination should not be selected.
(4) Select according to the workpiece material.
When machining high hardness workpiece materials, negative edge inclination should be taken.
3. Blade inclination selection catchphrase
- The blade angle is large, and the chip flow depends on it;
- Positive value chip moves forward, negative value chip moves backward;
- The strength and hardness of the workpiece are large, and the negative value can deal with it;
- The machining accuracy is high, and the chips must run forward;
- Intermittent cutting has impact, and the tool tip can be avoided with low sag.