Tool life can be defined on the time interval for each tool works satisfactorily between into successive grindings.
These are three common ways of expressing Tool life.
1. As time period in minutes between two successive grindings.
2. In terms of no. of components machined between two successive grindings.
3. In terms of the volume of the material removed between two successive grindings.
The method of assessing tool life in terms of the volume material removed per unit of time in a practical one.
|tool life vs cutting speed|
|cutting speed vs tool life|
Factors affecting Tool Life:
1. Cutting Speed.
2. Feed and Depth of cut.
3. Tool Geometry.
4. Tool Material.
5. Work Material.
6. Nature of Cutting.
7. Rigidity Machine tool and work.
8. Use of cutting fluids.
1. Effect of cutting speed:
The tool life varies inversely on cutting speed i.e. higher the cutting speed the smaller the tool life.
2. Effect of feed rate and depth of cut:
It will appreciably effect in reduction in tool life.
If the tool life in considered on constant, the cutting speed will decrease if the feed rate and depth of cut are increased.
3. The Geometry:
Geometrical parameters (Tool angles) of a cutting tool influence its performance. The Rake angle has mixed effect. If it is increased, the amount of heat generated are reduced and help in increasing the life of cutting tool. But if it is very large the cutting edge is weakened and also its capacity to conduct the heat is reduced results in reduction in mechanical strength and lowering tool life. For effective economical tool life it is necessary to strike a balance. The optimum value of rake angle needs to be used. This value varies from –50 to +100. The minus sign indicates negative rake i.e. rake angle sloping up words from Tip. Tools carrying negative rake angle provide a stronger cutting edge and hence a stronger tool. Carbide and ceramic tools are generally provided – ve angle.
Similarly relief angle or clearance angle bn influence the tool performance. These angles are provided on cutting tools to prevent the rubbing of tool flank against the machine work surface. They thus help in lowering the amount of heat generated and therefore increasing the tool life. But very large relief angles beyond certain level results in weakening of tool resulting in reduction of tool life.
Therefore a balance needs to be struck and only optimum value should be used. The angles normally vary from 50 to 80 but in special causes as carbide tipped tools up to 100.
The two cutting edge angels also have their influence on tool performance. The front cutting edge angle/end cutting edge angle effects the tool wear. Up to a certain optimum value an increase in this angle permits the higher speeds without an adverse effect on tool life. But an increase beyond certain value will result in reduction of tool life. It generally varies from 50 to 80. If the side cutting edge angle is smaller the higher speeds can be used. However it has complex effect on Tool life. A larger end cutting edge angle increases tool life.
I. Inclination angle: Tool life increases with the increase in this angle up to an optimum value.
II. Nose radius: While it increases the abrasion, it also helps in improving surface finish and tool strength and hence tool life.
4. Tool material:
The main characteristics of good cutting tool material are its hot hardness, wear resistance, impact resistance, abrasion resistance, heat conductivity and strength etc. An ideal tool material is the one which will remove the largest volume of work material at all speeds. It is not possible to get truly ideal tool material. The tool material which can with stand max cutting temperature without loosing its principal mechanical properties (splly hardness) and geometry will ensure max tool life. The higher hot hardness and toughness in tool material, the longer the tool life.
5. Work Material:
The micro-structure of work material is significant as it directly effects the hardness of material. Higher the hardness of the work material greater will be the tool wear and shorter will be the tool life. In machining pure metals, because of their tendency to stick to the tool face. Specially at high temperatures results in more friction and high amount wear on tool and therefore shorter tool life.
6. Nature of cutting:
Tool life is affected by nature of cutting i.e. whether it is continuous or intermittent. In the intermittent cutting the tool is subjected to impact loads and may give away much earlier than expected until it is made strong and tough. In continuous cutting similar tool will have relatively longer life.
7. Rigidity of machine tool and work:
Both the machine tool and work – piece should remain rigid during the machining operation. If not vibrations will take place and the cutting tool will be subjected to intermittent cutting, instead of continuous cutting. This will result in impact loading of tool and therefore shorter life.
8. Use of cutting fluids:
Cutting fluids are used in machining work for helping the efficient performance of the operation. They are used either in liquid or gaseous form. They assist the operation by cooling the tool and work, reducing the friction, improving the surface finish, helping in breaking the chips and washing them away etc. These factors help in improving the tool life, permitting higher metal removal rate and improving the quality of surface finish.
Characteristics of Cutting Tool Materials:
The materials used for manufacture of cutting tools should possess the following characteristics:
1. Ability to retain its hardness at elevated temperatures called hot hardness.
2. Ability to resist shock, called toughness.
3. High resistance to wear to ensure longer tool life.
4. Low co-efficient of friction at the chip –tool interface, so that the surface finish good and wear in minimum.
5. Should be cheap.
6 Should be able to be fabricated and shaped easily.
7. If it is to be used in the form of brazed tips, its other physical properties like tensile strength, thermal conductivity, co efficient of thermal expansion and modulus of elasticity etc. should be as Close To The Shank Material As Possible To Avoid Cracking.