Cutting tool failure analysis
After using the newly purchased tool or the reground tool for a period of time, you will find that the surface roughness of the machined surface increases, the color and shape of the chip are different from the initial cutting, the cutting temperature increases, the cutting force increases, and the cutting system even A vibrating or abnormal sound appears. The above phenomenon indicates that the tool has been severely worn and must be replaced with a new knife or re-sharp.
Tool wear is an unavoidable phenomenon in the cutting process. However, if the tool wears too fast or abnormal wear (also called damage), it will inevitably affect the processing quality, increase the tool consumption, reduce the production efficiency, and increase the processing cost. Therefore, by studying tool wear, a reasonable cutting program can be developed to improve production efficiency and part processing quality, and to reduce processing costs.
In order to make the tool durable and minimize wear, it is necessary to understand the influence of various cutting factors on tool wear. The main factors affecting the cutting performance of the tool are: tool geometry parameters (back angle, rake angle, lead angle, tool nose radius, etc.), tool material, cutting amount, workpiece material and mechanical properties. Among these factors, the workpiece material is uncontrollable. Changing the properties of other factors can control the wear form and wear rate of the tool. For example, by changing the heat treatment state of the workpiece material, the mechanical properties of the workpiece material can be changed, thereby affecting the wear of the tool; Tool material and tool geometry can also improve tool wear; production can generally reduce tool wear and improve tool life by properly selecting the amount of cutting.
To know how to properly select the above controllable factors, it is also necessary to analyze the form of tool failure and its mechanism. The form of tool failure (Fig. 1) can be divided into two categories: normal wear and abnormal wear: normal wear is the cutting edge, rake face and flank face of the tool, and the machined surface, chip and machined surface. Contact, in the contact zone by the coupling of cutting force and cutting heat, and strong friction, the cutting edge, rake face and flank face will wear. In normal wear, the amount of wear of the tool increases uniformly as the cutting time increases. The prior damage of the tool or the peeling during use, sudden chipping, the entire edge of the blade or the blade is called abnormal wear.
Figure 1 Tool failure form
The normal wear of the tool is mainly caused by the following reasons:
1) Abrasive wear is the wear caused by chips or hard spots on the surface of the workpiece (such as carbide particles and built-up fragments) that are grooved on the tool surface (fore and flank). At low speed cutting, the wear caused by other reasons is not obvious, so for low speed cutting tools, abrasive wear is the main cause of tool wear;
2) Adhesive wear is caused by the cutting of the chips and the workpiece material along the front and back flank of the tool during cutting, which destroys the oxide layer and other adsorption films on the surface of the tool, especially the strong adhesion between the fresh surfaces just cut from the inside of the workpiece material. Wear and tear. There is a very complicated relationship between cutting speed and bond wear. Generally, the bond wear mainly occurs in the medium cutting speed range, the affinity between the tool material and the workpiece material, the hardness ratio between the tool material and the workpiece material, and the tool material group. The part, grain thickness, tool surface state and cutting fluid type all affect the tool bonding wear rate;
3) Diffusion wear is caused by the contact between the chips, the workpiece and the front and back flank of the tool under high temperature and high pressure, and the chemical interaction between the workpiece and the surface of the tool is mutually diffused to the other side, changing the chemical composition of the two. Wear due to material structure. Diffusion wear generally occurs simultaneously with bond wear. Since the diffusion speed of each element is different, the degree of diffusion wear is greatly related to the composition of the tool material. In addition, the speed of diffusion is also related to the temperature. The higher the temperature, the faster the diffusion, so the diffusion wear Mainly occurs in the high speed cutting speed range.
4) Other wear such as dissolved wear, oxidative wear, and the like.
It can be seen that the causes of tool wear are very complicated, involving various factors such as mechanical, thermal, chemical, physical, etc. Under different workpiece materials, tool materials and cutting conditions, the causes of wear and the degree of wear are different, for certain workpieces. The combination of material and tool material, cutting temperature has a decisive influence on tool wear.
There are also many reasons for abnormal wear of the tool, mainly:
1) The toughness or hardness of the tool material is too low;
2) The structure or geometric angle of the tool is unreasonable, making the cutting edge too weak or the cutting force is too large;
3) The selection of cutting amount is unreasonable, so that the cutting force is too large or the cutting temperature is too high;
4) The tool generates too much thermal stress due to sudden thermal quenching (such as intermittent cutting, coolant, etc.) to cause cracks;
5) Improper operation, etc., causing the cutting edge to be subjected to sudden mechanical or thermal shock, resulting in chipping, hot cracking, and the like. Since the flank wear (as shown in Figure 2) can be predicted more accurately, the tool life is easier to control and therefore the desired form of failure. Increased cutting temperatures due to increased cutting forces or increased cutting speeds exacerbate flank wear. Tool life is usually expressed in terms of the width VB of the flank wear band, as shown in Figure 3. Peeling caused by mechanical wear occurs both on the rake face and on the flank face. The peeling area occurring on the rake face is generally smaller than the peeling area occurring on the flank face, as shown in FIG.
Figure 4: Peeling caused by mechanical wear
Thermal diffusion can also cause spalling of the front and back flank surfaces, as shown in Figure 5.
Figure 5: Peeling caused by thermal diffusion
Abnormal wear of the tool, ie, peeling damage or blade breakage, usually occurs during interrupted cutting, and tool breakage can occur when the machining system is poorly rigid. Increasing the toughness of the tool material (increasing the content of cobalt in the cemented carbide tool material or increasing the content of TiC and TaC) can effectively avoid the occurrence of tool breakage. In addition, increasing the strength of the tool structure and increasing the rigidity of the machining system will reduce the probability of tool breakage. Boundary wear (Fig. 6) generally occurs at the cutting position where the depth of cut is in contact with the surface of the workpiece. It is a partial peeling and crater wear of the rake face. It is hard or soft in processing stainless steel, superalloy, hardened material, surface layer. When steel is used, boundary wear is prone to occur. In order to reduce the wear of such tools, CVD coated tools can be used; increasing the content of cobalt in the cemented carbide tool material (such as cobalt-rich cemented carbide) can also reduce the wear of such tools. In short, the influencing factors, failure modes and production mechanisms of tool failure are very complicated. In production, we can start by observing the form of tool failure, analyze its failure mechanism, find out the influencing factors, and propose corresponding measures to reduce tool failure.
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Label: Tool material Cutting process Cutting depth Chip cutting force
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