Introduction
When developing projects , tool life estimates are often necessary as a reference . This is typically done by investigating and understanding tool consumption data for similar processes within the same industry and for the same product.
This data is then used as a foundation for maturity and accuracy assessments to determine the appropriate tool life estimates for the company. However, for various reasons, a more direct method of obtaining tool life data is often desirable.
Taylor’s formula
In the theoretical discipline of machining, the Taylor equation (FWT Taylor) is commonly used to express the relationship between tool life (T) and linear speed (V). VTm = C1, known as the TV relationship, has different coefficients and exponents for different workpiece materials, tool materials, and cutting conditions.
Tool life relationship charts can be plotted within a hyperbolic coordinate system, known as TV diagrams. Similarly, equations and charts exist for the relationship between T and f (feed rate) and ap (depth of cut).
Taylor’s formula is used in classrooms and laboratories, but rarely in factories.
Factories usually use estimation methods to obtain tool durability, or tool life. There are generally several estimation methods:
1, According to cutting time:
In the metal cutting tool industry, the recommended cutting speed is based on a tool life of 15 minutes. In actual use, the tool brand manufacturer’s recommended value is generally 75% of the value, which results in a tool life of approximately 60 minutes.
The number of workpieces that can be processed by one blade can be estimated by the following formula:
N=(19100XVXf)/(DXh)
N – tool life, number of workpieces that can be processed, unit: piece
V โ Cutting speed of the tool, unit: m/min
f โ feed rate during machining, unit: mm/rev
D โ Diameter of the workpiece being processed, unit: mm
h – processing length, mm
Example: When turning a workpiece with a diameter of 50 mm and a length of 100 mm, the tool manufacturer recommends a linear speed of 200 m/min, and the predetermined tool cutting time life T = 60 minutes. The actual linear speed used is 150 m/min, and the feed rate is 0.1 mm/rev. Estimated tool life:
N=(19100X150X0.1)/(50X100)=57.3
That is, according to the above conditions, each cutting edge can process 57 workpieces.
2, Measured by cutting distance:
Cutting distance refers to the total distance a cutting edge travels from the start of cutting to failure, assuming it is continuously cutting a very large workpiece at a certain speed. This is called cutting distance life and is represented by L.
The number of workpieces that can be processed by one blade can be estimated by the following formula:
N=(318300XLXf)/(DXh)
N – tool life, number of workpieces that can be processed, unit: piece
L – expected life of cutting distance, unit: km
f โ feed rate during machining, unit: mm/rev
D โ Diameter of the workpiece being processed, unit: mm
h – processing length, mm
Example: When turning a workpiece with a diameter of 50 mm and a length of 100 mm, with a feed rate of 0.1 mm/rev, and the tool manufacturer specifies a cutting distance life of 10 km, estimate the tool life:
N=(318300X10X0.1)/(50X100)=63.66
That is, according to the above conditions, each cutting edge can process 63 workpieces.
3, In terms of experience:
Experienced practitioners have accumulated rich experience in the service life of some common materials and common tools in processing a certain type of workpiece made of specific materials, and can directly estimate the service life of the tools.
For example, a coated carbide drill with a diameter between ะค25 and ะค30 can drill holes of approximately 20 to 30 meters in length when machining ordinary carbon steel, and 80 to 100 meters in length when machining cast iron.
The three estimation methods mentioned above are generally rough estimates. Whether based on cutting time or cutting distance, they are all conservative. This is because few tool manufacturers provide these figures. Even if they do, they are based on specific laboratory conditions and are not necessarily universally applicable.
Estimating based on empirical data has significant limitations and may not always be universally applicable. It can only provide a rough estimate under similar conditions. However, for a particular brand of tool, this method is relatively realistic when processing a particular material. Under similar or similar conditions, it can serve as a reference.
When making actual estimates, the following conditions must also be considered:
1) Determination of the failure limit , that is, under what circumstances the tool cannot be used. Except for extreme conditions such as chipping and cracking. Mainly refers to wear, especially in fine processing.
It is generally believed that the wear of the flank of the finishing blade within 0.2 mm is normal. However, if it is a sizing tool, the wear of the flank will cause the diameter of the workpiece to change.
Once the radial dimension changes to a dangerous situation, the tool must be replaced. For example, if there are special requirements for surface roughness, the tool is slightly worn, and the surface roughness decreases slightly, that is, it cannot meet the requirements, and the tool must be replaced. When estimating, the estimated value must be reduced according to a certain proportion.
If the radial dimension can be adjusted or compensated, and the surface roughness requirements are relatively low, the estimated value can be increased proportionally.
2) Cutting speed has a considerable impact on tool wear. Generally speaking, the faster the linear speed, the shorter the tool life. However, if the linear speed is too low, it will affect the processing efficiency on the one hand, and it may not be beneficial to the tool life on the other hand.
Therefore, the selection of cutting speed must refer to the cutting parameters provided by the tool manufacturer, and then determine the most reasonable speed based on the on-site conditions.
3) The material of the workpiece also has a significant impact on tool life. Even seemingly identical materials, with slightly different internal composition ratios, can exhibit significant differences in cutting performance.
Even with identical materials, differences in component structure, forming methods, heat treatment equipment or processes, and pre-processing tooling can lead to significant differences in tool life.
For example, in stainless steel machining, if the roughing tool used in the pre-process is not sharp, a hardened layer will form on the workpiece surface due to the cold work hardening effect.
This can cause rapid wear of the finishing tool in the subsequent process, severely impacting the finishing tool life.
4) Proper and accurate use of cutting fluid can significantly extend tool life. First, the cutting fluid must be accurate, clean, sufficient, and effective.
Different cutting fluids should be used based on the intended purpose, such as cooling during roughing and lubrication during finishing, depending on the tool material, workpiece, and machining method.
5) The foundation, machine tools, fixtures, workpieces, cutting tools, and everything else constitute a system, and the rigidity of the entire system significantly impacts tool life.
Tiny vibrations can cause abnormal micro-displacement between the tool and the workpiece, unnecessarily increasing friction, ultimately leading to tool wear and a rapid decrease in tool life. Improving system rigidity is an important measure and means to increase tool life, but effective improvement requires continuous, detailed, and complex investigation, analysis, and research.
Many believe that modifying a specific structure is costly, but this is not the case. A one-time investment of manpower and material resources can yield long-term savings in consumables costs for years, or even decades.
The above mentioned are turning and boring, and drilling, reaming, reaming and other processing can also be referred to. Milling is quite different from them:
- Milling is intermittent cutting. The tool blade material should be impact-resistant, have good toughness, relatively low hardness, and relatively poor wear resistance.
- Milling is an intermittent processing. The actual cutting time of the blade is only 30% to 50% of the total processing time, which is conducive to the heat dissipation of the blade and can effectively extend the life of the tool.
- During the machining process, different machining modes, such as face milling, peripheral milling, face milling, slot milling; different parts, such as main cutting edge, secondary cutting edge; different machining requirements, such as rough milling, fine milling, etc., will lead to different failure modes. The tool life will also be different.
- Milling cutters are multi-edge tools, so the above formula cannot be simply applied when calculating. Generally, you can only make an approximate estimate based on actual conditions, analysis and borrowing.
