How is the life of CNC tool estimated?

2019-06-01 10:33:01

In the development of project work, it is often necessary to make an estimate of the tool life, which is used as a reference for budget and planning. It is usually necessary to investigate and understand the tool consumption of similar products in the same industry and the same product. Based on this, the maturity and accuracy are evaluated, and the predetermined value of the corresponding tool life of the enterprise is made. However, for various reasons, it is often desirable to obtain tool life data in a more direct form.

In the theoretical discipline of machining, the F.W. Taylor formula is often used to represent the relationship between tool durability (T) and line speed (V).

VTm=C1

Known as the T-V relationship, different workpiece materials, different tool materials, and different cutting conditions have different coefficients and indices. Different tool durability relationships can be drawn in the double-curve coordinate system, called the T-V diagram. Similarly, there are relations and graphs of T and f (feed amount) and ap (cut depth).

The Taylor formula is used in the classroom and is rarely used in the factory. Factories are accustomed to using an estimated method to obtain tool durability, or tool life.

There are generally several estimation methods:

1, according to the cutting time:

In the metal cutting tool industry, the cutting line speed is recommended with a tool life of 15 minutes. In actual use, generally take 75% of the recommended value of the tool brand manufacturer, at which time the tool life is about 60 minutes.

The number of workpieces that can be machined by one edge can be estimated as follows:

N=(19100XVXf)/(DXh)

In the formula:

N - tool life, number of workpieces that can be machined, unit: V – tool selection cutting line speed, unit: m/min f – feed amount during machining, unit: mm/rev D – workpiece diameter of the workpiece, unit: Mm h - machining length, mm

Example: Turning a workpiece with a diameter of 50 mm, length 100 mm, tool manufacturer recommended line speed 200 m / min, predetermined tool cutting time life T = 60 minutes, actual line speed 150 m / min, feed 0.1 mm / Turn to estimate tool life:

N=(19100X150X0.1)/(50X100)=57.3

That is, according to the above conditions, 57 workpieces can be machined per blade.

2. Based on the cutting distance:

The cutting distance is assuming that a cutting edge continuously cuts at a certain speed on a very large workpiece. The total length of the path that the knife travels from the beginning to the failure is called the cutting distance life. Expressed by L.

The number of workpieces that can be machined by one edge can be estimated as follows:

N=(318300XLXf)/(DXh)

In the formula:

N - tool life, number of workpieces that can be machined, unit: L - cutting distance life expectancy, unit: kilometer f - feed rate during machining, unit: mm / rpm D - diameter of the workpiece being machined, unit: mmh - Processing length, mm

Example: Turning a workpiece with a diameter of 50 mm, length 100 mm, feed rate 0.1 mm/rev, tool manufacturer's cutting distance life 10 km, estimated tool life:

N=(318300X10X0.1)/(50X100)=63.66

That is, according to the above conditions, 63 workpieces can be machined per blade.

3. Based on experience value:

Experienced practitioners have a wealth of experience in the service life of some common materials and common tools in processing certain types of workpieces, and can directly estimate the service life of the tool.

For example, coated carbide drills with a diameter between Ф25 and Ф30, the length of the drill hole is about 20~30 meters when processing ordinary carbon steel. The total length of the cast iron is generally 80 to 100 meters.

The three estimation methods mentioned above are only the general estimates and the approximate estimates. Whether it is calculated according to the cutting time or the cutting distance, it is relatively conservative. Because there are very few tool manufacturers that provide this data. Even if it is provided, it is only a special case of the provider in a specific environmental condition in the laboratory, and does not necessarily have general guiding significance.

Estimated by empirical values, there are considerable limitations, not necessarily universal commonality, and can only be roughly estimated under basically the same conditions. However, in terms of the processing of a certain type of tool in a certain type of material, this estimation method is closer to reality. Under the same or close conditions, it can be used as a reference.

In the actual estimation, the following conditions must also be considered:

(1) For the determination of the failure limit, that is, when the tool is used, it cannot be used. In addition to extreme conditions such as chipping and rupture. Mainly refers to wear, especially in finishing, it is generally considered that the razor surface of the finishing blade is normal within 0.2 mm. Titanium Machinery is the main product of the company with rotary top, screw, machine tool spindle, shaft machining, high-precision tool holder, tool holder, elastic chuck, non-standard parts processing and machine tool post. However, if it is a sizing tool, the flank wear will cause the workpiece diameter to change. Once the radial dimension changes to a dangerous situation, the tool will be changed. Another example is that there is a special requirement for the surface roughness. If the tool is slightly worn and the surface roughness is slightly lowered, that is, if the requirements are not met, the tool must be changed. Estimates must be reduced as appropriate at a certain percentage. If the radial dimension is adjusted or compensated, and the surface roughness is required to be relatively low, the estimate can be increased proportionally.

(2) The cutting speed has a considerable influence on the wear of the tool. Generally speaking, the faster the line speed, the shorter the tool life, but the line speed is too low, which affects the machining efficiency and does not necessarily affect the tool life. It is advantageous, so the choice of cutting speed must refer to the cutting parameters provided by the tool manufacturer, and then determine the most reasonable speed in combination with the site conditions.

(3) The material of the workpiece to be machined also has a considerable influence on the tool life. The seemingly identical materials have slightly different proportions of the materials contained in the interior, and the cutting performance may vary greatly. Even the exact same materials, such as different component structures, different molding methods, different heat treatment equipment or processes, and different machining tools in the previous process, can cause significant differences in tool life. Titanium Machinery is the main product of the company with rotary top, screw, machine tool spindle, shaft machining, high-precision tool holder, tool holder, elastic chuck, non-standard parts processing and machine tool post. For example, in the processing of stainless steel parts, if the roughing of the previous process is not sharp, a hardened layer is formed on the surface of the workpiece due to the cold hardening effect, resulting in the sharp wear of the finishing tool in the subsequent process, which causes serious damage to the life of the finishing tool. influences.

(4) Reasonable and accurate use of cutting fluid can significantly improve tool life. First, the cutting fluid must be accurate, clean, sufficient, and effective. Different tool materials, different workpieces, and different processing forms should be filled with different cutting fluids according to the purpose requirements, such as cooling during roughing and lubrication during finishing.

(5) Foundation, machine tool, fixture, workpiece, tool, etc., all constitute a system, and the rigidity of the whole system has a great influence on tool life. Because the slight vibration causes abnormal macrorange displacement of the tool and the workpiece, the tool unnecessarily increases the ineffective friction, and finally causes the tool to wear and the tool life decreases rapidly. Increasing the rigidity of the system is an important measure and means to improve the life of the tool. However, in order to effectively improve the rigidity of the system, detailed and complicated investigation, analysis and research work must be carried out continuously. Many people think that it takes a lot of economic cost to change a certain part of the structure. In fact, a one-time manpower and material input may result in a reduction in the cost of consumables for a few years or even a decade.

The above mentioned is the turning, boring, drilling, expansion, hinge and other processing can also refer to. Milling is quite different from it:

1. Milling is intermittent cutting. The material of the cutting edge of the tool should be resistant to impact, the toughness is better, the hardness is relatively low, and the wear resistance is relatively poor.

2. Milling is intervening processing. The actual cutting time of the blade is only 30%~50% of the total processing time, which is beneficial to the heat dissipation of the blade and can effectively extend the tool life.

3, in the processing process, different processing forms, such as face milling, circumferential milling, milling, milling; different parts such as the main cutting edge, minor cutting edge; different processing requirements, such as rough milling, finishing milling, etc. The form of failure is quite different. Tool life is different.

4. The milling cutter is a multi-blade cutter. The above formula cannot be simply applied during calculation. Generally, borrowing can only be analyzed according to actual conditions, and approximate estimation can be made.


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