End mill milling cutter tools are essential tools for machining. During machining, breakage, wear, and chipping may occur. How to solve these problems?
Characteristics of End Mill Tool Damage
Micro Collapse of End Mill Cutting Edge
When the workpiece material structure, hardness, and allowance are uneven, the rake angle is too large, resulting in low tool cutting edge strength, insufficient rigidity of the process system, and vibration. Or when intermittent cutting is performed and the milling cutter grinding quality is poor, the cutting edge is prone to micro-collapse. That is, small collapses, gaps, or peeling appear in the cutting edge area. After this happens, the cutting tool will lose some of its cutting ability, but it can continue to work. During continued cutting, the damaged part of the cutting edge area may expand rapidly, resulting in greater damage.
Milling Cutter Cutting Edge or Tool Tip Chipping
This type of damage often occurs under more severe cutting conditions than micro-chipping of the cutting edge, or is a further development of micro-chipping. The size and range of chipping are larger than micro-chipping, causing the milling cutter tool to completely lose its cutting ability and have to stop working. The chipping of the tool tip is often called tip drop.
Carbide Blade or Tool Breakage
When the cutting conditions are extremely bad and the cutting amount is too large. There is an impact load, and there are microcracks in the blade or tool material. When there is residual stress in the blade due to welding and grinding. Coupled with factors such as careless operation, the blade or tool may break. After this form of damage occurs, the milling tool cutter cannot be used anymore and is scrapped.
Blade Surface Peeling
For very brittle materials, such as cemented carbide, ceramics, PCBN, etc. with high TiC content. Due to defects or potential cracks in the surface structure. Or residual stress exists in the surface due to welding and grinding. When the cutting process is not stable enough or the tool surface is subjected to alternating contact stress, it is very easy to produce surface peeling. Peeling may occur on the front blade face, and the back blade face. The peeling material is flaky and the peeling area is large. Coated tools are more likely to peel. After slight peeling, the blade can continue to work, and it will lose its cutting ability after severe peeling.
Plastic Deformation of Cutting Tool Cutting Parts
Due to the low strength and hardness of steel and high-speed steel cutting tools, plastic deformation may occur in their cutting parts. When cemented carbide tools work directly under high temperature and triaxial compressive stress, plastic flow of the surface will also occur. Even the cutting edge or tip will undergo plastic deformation and cause collapse. Collapse generally occurs when the cutting amount is large and hard materials are processed. The elastic modulus of TiC-based cemented carbide is smaller than that of WC-based cemented carbide. Therefore, the former’s ability to resist plastic deformation is accelerated, or it fails quickly. PCD, PCBNqiex basically do not undergo plastic deformation.
Thermal Cracking of Carbide Inserts
When the tool is subjected to alternating mechanical loads and thermal loads, the surface of the cutting part expands and contracts repeatedly due to heat and cold. Alternating thermal stress is inevitably generated, which causes the blade to fatigue and crack. For example, when the carbide milling cutter is milling at high speed, the teeth are constantly subjected to periodic impact and alternating thermal stress, and comb-like cracks are generated on the front cutting edge.
Although some tools do not have obvious alternating loads and alternating stresses. However, thermal stress will also be generated due to inconsistent temperatures between the surface and inner layers. In addition, there are inevitably defects inside the tool material. Therefore, the blade may also crack. After the crack is formed, the tool can sometimes continue to work for a period of time. Sometimes the crack expands rapidly, causing the blade to break or the blade surface to peel off severely.
Cutting Tool Wear
Classification by the Cause of Tool Wear
Abrasive Wear
There are often some extremely hard tiny particles in the processed material, which can scratch grooves on the surface of the tool, which is abrasive wear. Abrasive wear exists on all surfaces, and the front cutting edge is the most obvious. And hemp wear can occur at all cutting speeds. But for low-speed cutting, due to the low cutting temperature. Wear caused by other reasons is not obvious, so abrasive wear is the main reason. In addition, the lower the hardness of the tool, the more serious the abrasive hemp wear.
Cold Welding Wear
During cutting, there is great pressure and strong friction between the workpiece, cutting and front and rear blades, so cold welding will occur. Due to the relative movement between the friction pairs, the cold weld will break and be carried away by one side, thus causing cold welding wear. Cold welding wear is generally more serious at medium cutting speeds.
According to experiments, brittle metals have stronger cold welding resistance than plastic metals. Multiphase metals are less resistant than unidirectional metals. Metal compounds have less tendency to cold weld than single substances. The cold welding tendency of group B elements and iron in the periodic table of chemical elements is small. Cold welding is more serious when high-speed steel and cemented carbide are cut at low speeds.
Diffusion Wear
During high-temperature cutting and workpiece-tool contact, the chemical elements of both parties diffuse with each other in the solid state, changing the composition structure of the tool. The tool surface becomes fragile, which aggravates the wear of the tool. The diffusion phenomenon always maintains the continuous diffusion of objects with high depth gradient to objects with low depth gradient.
For example, at 800℃, the cobalt in cemented carbide quickly diffuses into the chips and workpieces, and WC decomposes into tungsten and carbon and diffuses into the steel. When the PCD tool is cutting steel and iron materials, when the cutting temperature is higher than 800℃. The carbon atoms in PCD will be transferred to the surface of the workpiece with a large diffusion intensity to form a new alloy, and the tool surface will be graphitized.
Cobalt and tungsten diffuse more seriously, and titanium, tantalum, and niobium have strong anti-diffusion ability. Therefore, YT cemented carbide has better wear resistance. When cutting ceramics and PCBN, when the temperature is as high as 1000℃~1300℃, diffusion wear is not significant. Due to the same material, the workpiece, chips and tools will generate thermoelectric potential in the contact area during cutting. This thermoelectric potential has the effect of promoting diffusion and accelerating tool wear. This diffusion wear under the action of thermoelectric potential is called “thermoelectric wear”.
Oxidative Wear
When the temperature rises, the surface of the tool is oxidized to produce softer oxides, which are rubbed by chips and formed by oxidation wear. For example, at 700℃~800℃, oxygen in the air reacts with cobalt, carbides, titanium carbides, etc. in cemented carbide to form softer oxides. At 1000℃, PCBN reacts chemically with water vapor.
Differentiation by Wear Form
Rake Face Wear
When cutting plastic materials at a high speed, the part of the rake face close to the cutting force. Under the action of the chips, it will wear into a crescent shape, so it is also called crescent wear. In the early stage of wear, the rake angle of the tool increases, which improves the cutting conditions. It is also conducive to the curling and breaking of the chips. But when the crescent is further enlarged, the strength of the cutting edge is greatly weakened. Eventually, it may cause the cutting edge to break and be damaged. When cutting brittle materials, or cutting plastic materials at a lower cutting speed and a thinner cutting thickness, crescent wear is generally not produced.
Cutting Tool Nose Wear
Tool tip wear is the wear on the arc flank surface of the tool tip and the adjacent secondary flank surface. It is a continuation of the wear on the tool flank surface. Due to the poor heat dissipation conditions and stress concentration here, the wear rate is faster than the back face. Sometimes a series of small grooves with a spacing equal to the feed amount will be formed on the secondary back face, which is called groove wear. They are mainly caused by the hardened layer and cutting lines of the machined surface. Groove wear is most likely to occur when cutting difficult-to-cut materials with a strong tendency to harden. Tool tip wear has the greatest impact on the surface roughness and machining accuracy of the workpiece.
Flank Wear
When cutting plastic materials with a large cutting thickness. Due to the presence of built-up edge, the flank of the tool may not contact the workpiece. In addition, the flank usually contacts the workpiece, and a wear zone with a back angle of 0 is formed on the flank. Generally, in the middle of the working length of the cutting edge, the wear of the flank is relatively uniform. Therefore, the wear degree of the flank can be measured by the width VB of the flank wear zone of the cutting edge in this section.
Since all types of tools will almost all experience flank wear under different cutting conditions. Especially when cutting brittle materials or cutting plastic materials with a small cutting thickness, the wear of the tool is mainly flank wear. And the measurement of the width VB of the wear zone is relatively simple. Therefore, VB is usually used to indicate the wear degree of the tool. The larger the VB, the greater the cutting force and the cutting vibration. It will also affect the wear at the arc of the tool tip. Thus affecting the machining accuracy and machining surface quality.
Methods to Prevent Tool Damage
- According to the characteristics of the materials and parts being processed, reasonably select the types and grades of tool materials. On the premise of having a certain hardness and wear resistance, it is necessary to ensure that the tool material has the necessary toughness.
- Reasonably select tool geometric parameters. By adjusting the front and rear angles, the main and secondary deflection angles, the blade inclination angles and other angles. Ensure that the cutting edge and the tip have good strength. Grinding a negative chamfer on the cutting edge is an effective measure to prevent chipping.
- Ensure the quality of welding and sharpening, and avoid various defects caused by poor welding and sharpening. The tools used in key processes should be ground to improve the surface quality and check for cracks.
- Reasonably select the cutting amount, avoid excessive cutting force and excessive cutting temperature to prevent tool breakage.
- Ensure that the process system has good rigidity and reduce vibration as much as possible.
- Take the correct operating method to try to make the tool bear no or less sudden load.
