[Accurate Analysis of Knowledge Points] Tool wear standards and influencing factors

This article introduces the last four knowledge points of the basic knowledge of tool wear in the basic theory: firstly, it introduces the dullness standard, and introduces the meaning of the commonly used dullness standard VB. The work experience is interpreted for everyone, and then the principle of determining the tool durability is introduced. Finally, the influence of the workpiece material and its properties on the tool life is introduced.

1. Dulling standard and its measurement method

The dullness standard, which has also become the wear standard, is a standard of tool failure measured numerically. In the laboratory, the width of the flank wear band (VB) at 1/2 depth of cut specified by ISO is often the standard for tool dullness.

In the production process, the tool wear size along the radial direction of the workpiece (called the radial wear amount of the tool NB) is used as the standard to measure the dullness of the tool. The international standard ISO uniformly stipulates that the wear band width VB measured on the flank is used as the tool dulling standard.

where CT is the durability coefficient. Among them, the cutting speed has the greatest influence on the tool durability, followed by the feed rate, and the cutting depth has the least influence.

2.The tool durability setting principle

In different occasions, the way or principle of using the tool is different. Here are three common usage principles:

Tp (Maximum Productivity Durability)

Determined according to the principle of minimum single-piece man-hours. This principle is more prevalent in European and American production enterprises. It is understandable that in the single-piece manufacturing cost of European and American companies, the proportion of labor costs is higher. In the specific practice, the tool life is often calculated by the cutting time. If the set target is reached, instead of extending the tool life, the cutting parameters will continue to be improved, and the tool will continue to be improved according to the tool failure mode. In short, it is to let the tool continuously challenge higher cutting parameters and pursue higher production efficiency.

[Application case] The author consulted the service life of a new European and American tool when machining ductile iron workpieces, and was told that the cutting was 75 minutes under the conditions of Vc95m/min, 0.5mm feed per tooth and 1mm depth of cut. For Brinell Ductile iron with a hardness of 280, this parameter is really high. It can also be seen that European and American tool companies have developed tool materials and coating technology.

Tc (minimum cost durability)

According to the principle of the lowest cost per workpiece process; this principle is more common in large-scale manufacturing in pursuit of the best price/performance ratio. Although improving production efficiency can outperform equipment depreciation and labor costs, in a production line composed of multiple processes, it is often limited by a bottleneck process, and production efficiency cannot be improved indefinitely. Under the premise, the pursuit of the lowest cost.

[Application case] This principle does not mean that the longer the single-tool machining life of each tool, the better, but the best match.

For example, when the author optimizes the tool life of the crankshaft outer milling connecting rod diameter station, it is found that after the roughing tool life is increased to a certain level, the finishing tool life will be severely weakened. The reason is that after the life of the roughing tool increases and the wear amount increases, not only the cutting allowance left for the finishing tool increases, but also the material to be cut is deformed more due to the blunt cutting edge, resulting in a higher cutting temperature. , Austenitize the surface of ductile iron to produce a hardened layer.

Tpr (Maximum Profit Margin Durability)

According to the principle of maximum profit obtained per unit time.

3. How to determine the tool durability

For tools that are relatively simple to manufacture and sharpen, and of low cost, such as turning tools, drills, etc., the durability can be set lower. The main factor to consider is not to sacrifice the cost of industrial waste and the cost of equipment accessories in order to save the limited tool cost.

For multi-tool machine tools, combined machine tools and automated processing tools with complex tool installation, tool change and tool adjustment, the durability should be higher. Machined with indexable turning tools and ceramic tools, the tool change time is short, and the durability is optional. The factors considered are mainly to strike a balance between tool cost and production efficiency.

For key processes that do not meet the production cycle, in order to balance the workshop production, the cutting parameters can be increased to catch up with the street shooting, and the tool durability can be sacrificed accordingly. When the overall cost shared by a certain process unit time is relatively large, the tool durability should also be lower.

When finishing large pieces, in order to avoid changing the tool in the middle of machining the same surface, the durability should be specified higher, and at least one tool change should be completed; the tool durability on the production line should be specified as one shift or two shifts, so that the tool can be changed during the shift.

4. The influence of workpiece material on tool life

In production practice, the primary factor in selecting the tool durability reasonably is the workpiece material. Below, we introduce the influencing factors of workpiece material:

1. Strength and hardness

The work consumed in overcoming the strength and hardness of the material and the heat, force and friction generated during the cutting process are the main causes of tool flank wear and rake face crater wear.

In addition to the material properties of the material itself, heat treatment can also increase the strength and hardness of the material and deteriorate the machinability of the material.

2. Toughness and plasticity

Materials with good toughness and plasticity are prone to generate built-up edge and bond wear, and the shedding of built-up edge and bond will accelerate tool wear. Materials with good toughness will also encounter chip problems during processing. Although in an open machining environment, chips do not affect the machining of the workpiece, they can also become entangled in equipment components such as centers or fixtures.

3. Work hardening

Materials with a strong tendency to work harden increase the groove wear of the tool, and cause the tool to fail by weakening the tool strength or reducing the quality of the machined surface.
[Application case] In a case of stainless steel turning, the author divided into two steps of roughing and finishing in order to ensure dimensional accuracy and surface quality, and unexpectedly found that the cutting edge of the finishing process with a small allowance was severely worn, and the roughened stainless steel Surface hardening was found during metallographic analysis of the surface, and the heat that caused the surface hardening of the material should have come from the worn roughing tool.

4. Chemical affinity

A material with a strong chemical affinity with the tool material causes adhesion and diffusion, aggravating crater wear. Among them, the main indicators are:        (1) Materials with a small Young's modulus Young's modulus have a large amount of rebound after material removal, which aggravates the wear of the tool flank.

(2) Materials with poor thermal conductivity and high temperature in the cutting zone reduce the strength and hardness of the tool, cause plastic deformation, and aggravate tool wear.

(3) Metallographic structure The hard relative tool in the metallographic structure produces strong friction and wear; spheroidizing pearlite can improve the machinability of the material.

(4) Chemical composition The more alloying elements in the chemical composition and the higher the content, the poorer the machinability of the material; the addition of free-cutting elements can improve the machinability.

(5) Material deformation characteristics When cutting materials with a large shear angle and crushed cutting materials, the contact area between the chip and the tool is small, and the cutting force is concentrated on the tip of the tool, which is easy to beat or vibrate.