After the tool is broken, it may be unable to continue to use in serious cases, resulting in scrapping. Then how do you know when tool breakage or tool chipping occurs? In this article, we’ll introduce the types, causes and solutions for different forms of cutting tool breakage.
1. Slight chipping of cutting edge
When the material structure, hardness, and allowance of the workpiece are uneven, and the rake angle is too large, which leads to low cutting edge strength, insufficient rigidity of the processing system and vibration, intermittent cutting, and poor grinding quality, the cutting edge is prone to micro collapse, that is, tiny collapse, notch or peeling in the edge area. In this case, the tool will lose part of its cutting capacity, but it can continue to work. During continuous CNC cutting, the damaged part of the edge area may expand rapidly, resulting in greater damage.
2. Chipping of cutting edge or tip
This breakage form is often produced under worse cutting conditions than the micro collapse of the cutting edge or the further development of micro collapse. The size and range of chipping are larger than microchipping, which makes the tool completely lose its cutting ability and have to stop working. The condition of the tip breaking is often called tip dropping.
3. Cutter is broken
When the cutting conditions are extremely bad, the cutting amount is too large, there is impact load, and there are micro cracks in the blade or tool material, the blade or tool may break due to welding, grinding, residual stress in the blade, combined with the careless operation and other factors. After this kind of damage occurs, the tool can not continue to be used, resulting in scrapping.
4. Blade surface peeling
For materials with great brittleness, such as cemented carbide, ceramics, and PCBN with high tic content, due to defects or potential cracks in the surface structure, or residual stress in the surface due to welding and grinding, it is very easy to produce surface peeling when the cutting process is not stable enough or the tool surface bears alternating contact stress. The peeling may occur on the front knife surface, and the knife may occur on the rear knife surface. The peeling object is in flakes and the peeling area is large. The coated tool is likely to peel off. The blade can continue to work after slight peeling and will lose cutting ability after serious peeling.
5. Plastic deformation of cutting part
Plastic deformation may occur in the cutting part of tool steel and high-speed steel due to low strength and low hardness. When the cemented carbide works directly under high temperature and three-dimensional compressive stress, it will also produce plastic flow on the surface, and even cause plastic deformation of the cutting edge or tooltip, resulting in the collapse of the surface. Collapse generally occurs when the cutting amount is large and hard materials are processed. The elastic modulus of TiC-based cemented carbide is less than that of WC-based cemented carbide, so the former has accelerated plastic deformation resistance or rapid failure. PCD and PCBN basically do not have plastic deformation.
6. Hot cracking of blade
When the cutting tool bears the alternating mechanical load and thermal load, the surface of the cutting part inevitably produces alternating thermal stress due to repeated thermal expansion and cold contraction, resulting in fatigue and cracking of the blade. For example, during high-speed milling with a cemented carbide milling cutter, the cutter teeth are constantly subjected to periodic impact and alternating thermal stress, resulting in comb cracks on the rake face. Although some cutters do not have obvious alternating load and stress, thermal stress will also be generated due to the inconsistent temperature of the surface layer and the inner layer. In addition, there are inevitable defects in the cutter material, so the blade may also have cracks. After the crack is formed, the tool can sometimes continue to work for a period of time. Sometimes the crack expands rapidly, resulting in blade fracture or serious peeling off of the tool surface.
1. According to the characteristics of processed materials and parts, reasonably select various types and brands of tool materials. On the premise of certain hardness and wear resistance, the necessary toughness of tool material must be ensured.
2. Reasonably select the geometric parameters of the tool. By adjusting the fore-and-aft angle, main and auxiliary deflection angle, blade angle, and other angles.
3. Ensure that the cutting edge and tip have good strength. Grinding negative chamfering on the cutting edge is an effective measure to prevent tool collapse.
4. Ensure the quality of welding and grinding, and avoid various defects caused by poor welding and grinding. The cutting tools used in key processes shall be ground to improve the surface quality and check whether there are cracks.
5. Reasonably select cutting parameters to avoid excessive cutting force and excessive cutting temperature, so as to prevent tool damage.
6. Try to ensure that the machining system has good rigidity and reduces vibration.
7. Adopt correct operation methods to make the tool bear no or less sudden load as far as possible.