Machining center tools wearing and chipping badly — what’s the root cause
In machining centers, cutting tools are consumables that can break, wear, or chip during machining. These phenomena are unavoidable, but there are also controllable causes such as unscientific or non-standard operation and improper maintenance. Finding the root cause is crucial for solving the problem.
01 Manifestations of Tool Breakage
(1) Micro-chipping of the cutting edge
When the workpiece material has an uneven structure, hardness, and allowance, a large rake angle leading to low cutting edge strength, insufficient rigidity of the machining system causing vibration, or intermittent cutting, or poor grinding quality, the cutting edge is prone to micro-chipping, i.e., small chips, gaps, or peeling appear in the cutting edge area. After this occurs, the tool will lose some cutting ability but can continue to work. During continued cutting, the damaged part of the cutting edge area may rapidly expand, leading to greater breakage.
(2) Chipping of the cutting edge or tool tip
This type of breakage often occurs under cutting conditions more severe than those causing micro-chipping, or is a further development of micro-chipping. The size and range of the chipping are larger than micro-chipping, causing the tool to completely lose its cutting ability and forcing it to stop working. The breaking of the cutting tip is often called tip chipping.
(3) Cutting Tool or Tool Breakage
When cutting conditions are incredibly harsh, cutting parameters are excessive, impact loads are present, micro-cracks are present in the cutting tool or tool material, and residual stress in the cutting tool due to welding or sharpening, coupled with improper operation, the cutting tool or tool may break. After this type of damage occurs, the tool cannot be used and must be scrapped.
(4) Cutting Tool Surface Peeling
For highly brittle materials, such as cemented carbide with high TiC content, ceramics, PCBN, etc., surface defects or potential cracks in the surface structure, or residual stress in the surface due to welding or sharpening, can easily lead to surface peeling when the cutting process is unstable, or the tool surface is subjected to alternating contact stress. Peeling may occur on the rake or flank face, and the peeled material is flaky, with a large peeling area. Coated tools are more likely to peel. After slight peeling, the cutting tool can continue to work, but severe peeling will result in the loss of cutting ability.
(5) Plastic Deformation of Cutting Parts
Due to their low strength and hardness, tool steel and high-speed steel may undergo plastic deformation during cutting. When cemented carbide operates at high temperature and under triaxial compressive stress, surface plastic flow may occur, leading to plastic deformation and collapse of the cutting edge or tool tip. Collapse generally occurs when the cutting depth is large and complex materials are being machined. TiC-based cemented carbide has a lower elastic modulus than WC-based cemented carbide; thus, its resistance to plastic deformation is lower, and it fails more rapidly. PCD and PCBN generally do not experience plastic deformation.
(6) Thermal Cracking of Inserts
When a tool is subjected to alternating mechanical and thermal loads, the cutting part’s surface inevitably experiences alternating thermal stress from repeated thermal expansion and contraction, leading to fatigue and cracking of the insert. For example, when a cemented carbide end mill is used for high-speed milling, the cutting teeth are continuously subjected to periodic impacts and alternating thermal stress, resulting in comb-like cracks on the rake face. Although some cutting tools do not exhibit obvious alternating loads and stresses, the inconsistent temperatures between the surface and inner layers can still generate thermal stress. Combined with the unavoidable defects within the tool material, cracks may also form in the insert. Sometimes the tool continues to operate after crack formation, while at other times cracks propagate rapidly, leading to insert breakage or severe chipping of the tool face.
02 Causes of Tool Wear
(1) Abrasive Wear
The workpiece often contains extremely hard microparticles that can scratch grooves on the tool surface; this is abrasive wear. Abrasive wear occurs on all surfaces, but is most pronounced on the rake face. Abrasive wear can occur at various cutting speeds. Still, at low cutting speeds, due to the lower cutting temperature, wear from other factors is less noticeable, making abrasive wear the primary cause. Furthermore, the lower the tool hardness, the more severe the abrasive wear.
(2) Cold Weld Wear
During cutting, significant pressure and intense friction between the workpiece, the cutting surface, and the rake and rake faces result in cold welding. Due to relative movement between the friction pairs, the cold weld will break and be carried away by one side, leading to cold weld wear. Cold-welding wear is generally more severe at medium cutting speeds. Experiments show that brittle metals have stronger resistance to cold welding than ductile metals; multiphase metals have weaker resistance than uniphase metals; metal compounds have a lower tendency to cold weld than elemental metals; and Group B elements in the periodic table have a lower tendency to cold weld with iron. Cold welding is more severe when high-speed steel and cemented carbide are cut at low speeds.
(3) Diffusion Wea
During high-temperature cutting and contact between the workpiece and the tool, the chemical elements of both diffuse into each other in the solid state, altering the tool’s composition and structure, making the tool surface more fragile, and exacerbating tool wear. Diffusion always proceeds continuously from objects with a high depth gradient to those with a low depth gradient.
For example, at 800℃, cobalt in cemented carbide rapidly diffuses into the chips and workpiece, and WC decomposes into tungsten and carbon, which diffuse into the steel; when PCD tools cut steel and iron materials at cutting temperatures above 800℃, carbon atoms in the PCD will transfer to the workpiece surface with a high diffusion intensity to form a new alloy, resulting in graphitization of the tool surface. Cobalt and tungsten exhibit significant diffusion, while titanium, tantalum, and niobium show more substantial resistance to diffusion. Therefore, YT-type cemented carbides exhibit good wear resistance. During ceramic and PCBN cutting, diffusion wear is not significant at temperatures reaching 1000℃~1300℃. Because the workpiece, chips, and tool are made of the same material, a thermoelectric potential is generated in the contact area during cutting. This thermoelectric potential promotes diffusion and accelerates tool wear. This type of diffusion wear under the influence of thermoelectric potential is called “thermoelectric wear.”
(4) Oxidative Wear
When the temperature rises, the tool surface oxidizes, forming softer oxides that are rubbed by chips, leading to oxidative wear. For example, at 700℃~800℃, oxygen in the air reacts with cobalt, carbides, and titanium carbide in cemented carbides to form softer oxides; at 1000℃, PCBN reacts chemically with water vapour.
03. Wear Patterns of Cutting Tools
(1) Rake Face Wear
When cutting ductile materials at high speeds, the area on the rake face closest to the cutting force will wear into a crescent shape due to chip action, hence the name crescent wear. In the early stages of wear, the rake angle increases, improving cutting conditions and facilitating chip curling and breakage. However, as the crescent rises further, the cutting edge strength is significantly weakened, potentially leading to chipping and damage. Crescent wear generally does not occur when cutting brittle materials or cutting ductile materials at lower cutting speeds and thinner cutting thicknesses.
(2) Tip Wear
Tilt wear refers to the wear on the flank face of the tip arc and the adjacent secondary flank face. It is a continuation of the wear on the tool’s flank face. Due to poor heat dissipation and stress concentration at this location, the wear rate is faster than on the flank face. Sometimes, a series of small grooves with a spacing equal to the feed rate will form on the secondary flank face, called groove wear. These are mainly caused by the hardened layer and cutting lines on the machined surface. Groove wear is most likely to occur when cutting difficult-to-machine materials with a high tendency for work hardening. Tool tip wear has the most significant impact on workpiece surface roughness and machining accuracy.
(3) Rake Face Wear
When cutting ductile materials with a large cutting thickness, the tool flank may not contact the workpiece due to a built-up edge. Otherwise, the flank face usually contacts the workpiece, forming a wear band. Generally, flank face wear is more uniform in the middle of the working length of the cutting edge; therefore, the degree of flank face wear can be measured by the width VB of the flank face wear band in that section of the cutting edge.
Since flank face wear occurs in almost all types of tools under different cutting conditions, especially when cutting brittle materials or ductile materials with a small cutting thickness, tool wear is mainly flank face wear. Furthermore, measuring the wear band width VB is relatively simple; therefore, VB is often used to represent the degree of tool wear. A larger VB not only increases cutting force and induces cutting vibration but also increases wear at the tool tip arc, thereby affecting machining accuracy and surface quality.
04. Methods to Prevent Tool Breakage
(1) Select appropriate tool materials and grades based on the characteristics of the workpiece and the material being machined. While ensuring sufficient hardness and wear resistance, the tool material must also exhibit enough toughness.
(2) Select appropriate tool geometry parameters. Adjust the rake angle, primary and secondary rake angles, and inclination angle to ensure good strength of the cutting edge and tool tip. Grinding a negative chamfer on the cutting edge is an effective measure to prevent tool breakage.
(3) Ensure the quality of welding and sharpening to avoid defects caused by poor welding or sharpening. Tools used in key processes should have their blades ground to improve surface quality and be inspected for cracks.
(4) Select appropriate cutting parameters to avoid excessive cutting force and temperature, thus preventing tool breakage.
(5) Ensure the process system has as much rigidity as possible to reduce vibration.
(6) Adopt correct operating methods to minimize or avoid sudden loads on the tool.
05. Causes and Countermeasures for Tool Chipping
(1) Inappropriate selection of insert grade and specifications, such as inserts that are too thin or inserts that are too hard and brittle for roughing.
Countermeasures: Increase insert thickness or mount inserts vertically, and select inserts with higher bending strength and toughness.
(2) Inappropriate selection of tool geometry parameters (such as huge rake and clearance angles).
Countermeasures: Redesign the tool from the following aspects:
1) Appropriately reduce the rake and clearance angles;
2) Use a larger negative rake angle;
3) Reduce the principal cutting edge angle;
4) Use a larger negative chamfer or cutting edge radius;
5) Grind the transition cutting edge to strengthen the tool tip.
(3) Incorrect welding process of the insert, resulting in excessive welding stress or welding cracks.
Countermeasures:
1) Avoid using a three-sided closed insert groove structure;
2) Select the correct solder.
3) Avoid using oxy-acetylene flame welding, and maintain heat after welding to eliminate internal stress;
4) Use mechanical clamping structures whenever possible.
(4) Improper grinding methods cause grinding stress and grinding cracks; excessive vibration of the cutter teeth after grinding PCBN milling cutters can overload individual teeth and cause tool breakage.
Countermeasures:
1) Use intermittent grinding or diamond wheel grinding;
2) Select a softer grinding wheel and frequently dress it to keep it sharp;
3) Pay attention to grinding quality and strictly control the vibration of the milling cutter teeth.
(5) Inappropriate selection of cutting parameters, such as excessive parameters causing machine tool stalling; excessively high cutting speed, excessive feed rate, uneven blank allowance, and insufficient depth of cut during intermittent cutting; insufficient feed rate when cutting materials with high work hardening tendency, such as high manganese steel.
Countermeasures: Reselect cutting parameters.
(6) Structural issues such as uneven tool groove bottom or excessively long insert extension in mechanically clamped tools.
Countermeasures:
1) Repair the tool groove bottom.
2) Properly position the coolant nozzles;
3) Add a carbide shim under the insert on a hardened tool holder.
(7) Excessive tool wear.
Countermeasures: Replace the tool or cutting edge promptly.
(8) Insufficient coolant flow or incorrect filling method causes the insert to overheat and crack.
Countermeasures:
1) Increase the coolant flow rate;
2) Properly position the coolant nozzles;
3) Use effective cooling methods such as spray cooling to improve the cooling effect.
4) Reduce impact on the insert.
(9) Incorrect tool installation, such as: cutting-off tools installed too high or too low; asymmetrical climb milling used on face mills.
Countermeasures: Reinstall the tool.
(10) Poor rigidity of the machining system leads to excessive cutting vibration.
Countermeasures:
1) Increase auxiliary support for the workpiece to improve workpiece clamping rigidity.
2) Reduce the overhang length of the tool;
3) Appropriately reduce the clearance angle of the tool;
4) Adopt other vibration-damping measures.
(11) Careless operation, such as: the tool cuts into the middle of the workpiece too abruptly, or stopping the machine before retracting the tool.
Countermeasures: Pay attention to the operating method.
06. Causes, characteristics, and control measures of built-up edge
(1) Causes of formation
Near the cutting edge, in the tool-chip contact area, under the significant downward pressure, the bottom layer of the chip metal embeds into the microscopic uneven peaks and valleys on the rake face, forming a gapless, accurate metal-to-metal contact and producing adhesion. This part of the tool-chip contact area is called the adhesion zone. In the adhesion zone, a thin layer of metal material will accumulate and remain on the rake face. This part of the chip metal material has undergone severe deformation and is strengthened at an appropriate cutting temperature. As chips continuously flow out and are subsequently cut, this layer of accumulated material slides away from the upper chips, forming the basis of the built-up edge. Subsequently, a second layer of accumulated cutting material forms on top of it, and this continuous layering forms the built-up edge.
(2) Characteristics and Impact on Machining
1) Its hardness is 1.5 to 2.0 times higher than the workpiece material, allowing it to replace the rake face for cutting. It protects the cutting edge and reduces rake face wear, but fragments from the built-up edge flowing through the tool-workpiece contact area cause flank face wear.
2) After the built-up edge forms, the working rake angle of the tool increases significantly, positively impacting chip deformation and cutting force.
3) Because the built-up edge protrudes beyond the cutting edge, the actual cutting depth increases, affecting the dimensional accuracy of the workpiece.
4) The built-up edge creates a “ploughing” effect on the workpiece surface, affecting surface roughness.
5) Fragments of the built-up edge can adhere to or embed in the workpiece surface, creating hard spots that affect the quality of the machined surface.
As the above analysis shows, built-up edge (BUE) is detrimental to machining, especially finishing.
(3) Control Measures
To prevent the bottom layer of the chip from adhering to or deforming the rake face and thereby weakening it, BUE formation can be avoided. Therefore, the following measures can be taken:
1) Reduce the roughness of the rake face.
2) Increase the rake angle of the tool;
3) Reduce the cutting thickness;
4) Use low-speed or high-speed cutting to avoid cutting speeds that easily form BUE;
5) Perform appropriate heat treatment on the workpiece material to increase its hardness and reduce its plasticity;
6) Use a cutting fluid with good anti-adhesion properties (such as extreme pressure cutting fluids containing sulphur and chlorine).