Common Tool Problems and Solutions in CNC Machining

For machining centers, cutting tools are consumable tools. During the machining process, they will be damaged, worn, and chipped. These phenomena are inevitable, but there are also controllable reasons such as unscientific and irregular operation and improper maintenance. Only by finding the root cause can the problem be better solved.

Symptoms of Cutting Tool Breakage

Micro Chipping of Cutting Edge

When the workpiece material structure, hardness, and allowance are uneven, the rake angle is too large, resulting in low cutting edge strength. When the process system is not rigid enough to produce vibration, or when intermittent cutting is performed and the grinding quality is poor, the cutting edge is prone to micro-collapse. That is, small collapses, gaps, or peeling occur in the cutting area. When this happens, the tool will lose some of its cutting ability, but it can still continue to work. During continued cutting, the damaged part of the cutting edge may expand rapidly, resulting in greater damage.

Chipped Cutting Edge or Tip

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 tool to completely lose its cutting ability and have to stop working. The chipping of the tool tip is often called tip drop.

Broken Blade or Cutter

When the cutting conditions are extremely bad, the cutting amount is too large, there is an impact load, there are micro cracks in the blade or tool material, there is residual stress in the blade due to welding and grinding, and factors such as careless operation may cause the blade or tool to break. After this form of damage occurs, the tool cannot be used anymore and is scrapped.

Blade Surface Peeling off

For very brittle materials, such as cemented carbide with high TiC content, ceramics, PCBN, etc., there are 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 cause surface peeling. Peeling may occur on the front blade face, and on 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, but it will lose its cutting ability after severe peeling.

Plastic Deformation of Cutting Part

Tool steel and high-speed steel may undergo plastic deformation at the cutting part due to their low strength and hardness. When cemented carbide is working under high temperature and triaxial compressive stress, plastic flow will also occur on the surface. It may even cause the cutting edge or tip to 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, so the former’s ability to resist plastic deformation is accelerated, or it fails quickly. PCD and PCBN basically do not undergo plastic deformation.

Thermal Cracking of Blades

When the tool is subjected to alternating mechanical loads and thermal loads, the surface of the cutting part will inevitably produce alternating thermal stress due to repeated thermal expansion and contraction, which will inevitably cause fatigue and cracking of the blade. For example, when a 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 face. Although some tools do not have obvious alternating loads and alternating stresses. However, due to the inconsistent temperature of the surface and inner layers, thermal stress will also be generated. In addition, there are inevitable defects in the tool material, so the blade may also crack. After the crack is formed, the tool can sometimes continue to work for a period of time, and sometimes the crack expands rapidly, causing the blade to break or the blade surface to peel off severely.

Causes 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 is most obvious on the front cutting edge. And hemp wear can occur at all cutting speeds. However, for low-speed cutting, due to the low cutting temperature, the 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 a lot of pressure and strong friction between the workpiece, the cutting and the front and rear cutting edges, so cold welding will occur. Due to the relative movement between the friction pairs, the cold weld will break and be taken away by one side, 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 smaller than unidirectional metals. Metal compounds have less tendency to cold welding than single substances. The cold welding tendency of group B elements in the chemical periodic table and iron is small. High-speed steel and cemented carbide are more serious when cutting 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 the 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”.

Oxidation Wear

When the temperature rises, the surface of the tool is oxidized to produce softer oxides, which are rubbed by the chips and formed by oxidation wear. For example, at 700℃~800℃, the oxygen in the air reacts with the cobalt, carbide, titanium carbide, etc. in the cemented carbide to form softer oxides. at 1000℃, PCBN reacts chemically with water vapor.

Wear Form of Insert

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 and is 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, and it may eventually cause the cutting edge to break and damage. When cutting brittle materials, or cutting plastic materials at a lower cutting speed and a thinner cutting thickness, crescent wear generally does not occur.

Tool Tip Wear

Tool tip wear is the wear on the back face of the tool tip arc and the adjacent secondary back face, which is a continuation of the wear on the back face of the tool. Due to the poor heat dissipation conditions here and the stress concentration, 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 on the machined surface. When cutting difficult-to-cut materials with a strong tendency to harden, groove wear is most likely to occur. Tool tip wear has the greatest impact on workpiece surface roughness and machining accuracy.

Flank Wear

When cutting plastic materials with a large cutting thickness, the flank face of the tool may not contact the workpiece due to the presence of built-up edge. In addition, the flank face usually contacts the workpiece, and a wear band with a back angle of 0 is formed on the flank face. Generally, in the middle of the working length of the cutting edge, the wear of the flank face is relatively uniform, so the wear degree of the flank face can be measured by the width VB of the flank face wear band of the cutting edge in this section.

Since all types of tools will almost all experience flank wear under different cutting conditions. In particular, 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 band 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, thereby affecting the machining accuracy and machining surface quality.

How to Prevent Tool Breakage

• According to the characteristics of the processed materials and parts, the types and grades of tool materials should be reasonably selected. Under the premise of having a certain hardness and wear resistance, the tool material must have the necessary toughness.
• Reasonably select the tool geometry 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 grinding, and avoid various defects caused by poor welding and grinding. 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 high cutting temperature to prevent tool damage.
• 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 not bear or bear less sudden load.

Causes and Countermeasures of Tool Chips

1. Reason: Improper selection of blade brand and specification. For example, the blade thickness is too thin or a brand that is too hard and brittle is selected during rough processing.

Countermeasure: Increase the thickness of the blade or install the blade vertically, and select a brand with higher bending strength and toughness.

2.Cause: Improper selection of tool geometry parameters (such as too large front and rear angles, etc.).

• Countermeasures: The tool can be redesigned from the following aspects.
• Appropriately reduce the front and rear angles.
• Use a larger negative edge inclination angle.
• Reduce the main deflection angle.
• Use a larger negative chamfer or edge arc.
• Grind the transition cutting edge to strengthen the tool tip.

3.Reason: The welding process of insert is incorrect, resulting in excessive welding stress or welding cracks.

Countermeasures:

• Avoid using a blade slot structure that is closed on three sides.
• Select the correct solder.
• Avoid using oxyacetylene flame to heat welding, and keep warm after welding to eliminate internal stress.
• Use a mechanical clamping structure as much as possible.

4.Cause: Improper grinding method causes grinding stress and grinding cracks. The oscillation of the teeth of the PCBN milling cutter is too large after grinding, which makes individual teeth overloaded and also causes the cutter to break.

Countermeasures:

• Use intermittent grinding or diamond grinding wheel grinding.
• Choose a softer grinding wheel and dress it frequently to keep it sharp.
• Pay attention to the quality of grinding and strictly control the oscillation of the milling cutter teeth.

5.Cause: Unreasonable selection of cutting parameters. For example, if the amount is too large, the machine tool will be stuck. When intermittent cutting, the cutting speed is too high, the feed rate is too large, the blank allowance is uneven, and the cutting depth is too small. When cutting materials with a high tendency to work hardening such as high manganese steel, the feed rate is too small, etc.

Countermeasures: Reselect the cutting parameters.

6.Structural reasons such as uneven bottom surface of the tool groove of the mechanical clamping tool or excessive extension of the blade.

Countermeasures:

• Fix the bottom surface of the tool groove.
• Reasonably arrange the position of the cutting fluid nozzle.
• Hardened tool bar adds carbide gaskets under the blade.

7.Cause: Excessive tool wear.

Countermeasures: Change the tool or replace the cutting edge in time.

8.Cause: Insufficient cutting fluid flow or incorrect filling method causes the blade to heat up suddenly and crack.

Countermeasures:

• Increase the flow of cutting fluid.
• Reasonably arrange the position of the cutting fluid nozzle.
• Use effective cooling methods such as spray cooling to improve the cooling effect.
• Use cutting to reduce the impact on the blade.

9.Cause: Improper tool installation. For example, the cut-off turning tool is installed too high or too low. The end mill uses asymmetric down milling, etc.

Countermeasure: Reinstall the tool.

10.Cause: The rigidity of the process system is too poor, resulting in excessive cutting vibration.

Countermeasure:

• Increase the auxiliary support of the workpiece and improve the rigidity of the workpiece clamping.
• Reduce the overhang length of the tool.
• Appropriately reduce the back angle of the tool.
• Use other vibration elimination measures.

11.Cause: Careless operation. For example, when the tool cuts in from the middle of the workpiece, the action is too violent. Stop the machine before retracting the tool.

Countermeasure: Pay attention to the operation method.

Causes, Characteristics and Control Measures of Built-up Edge

Cause of Formation

In the part close to the cutting edge, in the tool-chip contact area, due to the large downward pressure, the metal of the chip bottom layer is embedded in the microscopic uneven peaks and valleys on the front cutting edge, forming a gapless real metal-to-metal contact and causing bonding. This part of the tool-chip contact area is called the bonding area. In the bonding area, a thin layer of metal material will be accumulated on the front cutting edge of the chip bottom layer. The metal material of this part of the chip has undergone severe deformation and is strengthened at an appropriate cutting temperature. As the chips continue to flow out, under the push of the subsequent cutting flow, this layer of accumulated material will slide relative to the upper layer of the chips and leave, becoming the basis of the built-up edge. Subsequently, a second layer of accumulated cutting material will form on it, and this continuous accumulation will form a built-up edge.

Characteristics and Influence on Cutting

• The hardness is 1.5~2.0 times higher than that of the workpiece material. It can replace the rake face for cutting, which has the effect of protecting the cutting edge and reducing the wear of the rake face. However, when the built-up edge falls off, the fragments flowing through the tool-workpiece contact area will cause wear on the tool back face.
• After the built-up edge is formed, the working rake angle of the tool increases significantly, which plays a positive role in reducing chip deformation and reducing cutting force.
• Because the built-up edge protrudes beyond the cutting edge, the actual cutting depth increases, affecting the dimensional accuracy of the workpiece.
• The built-up edge will cause a “plowing” phenomenon on the workpiece surface, affecting the surface roughness of the workpiece.
• The fragments of the built-up edge will adhere to or embed the workpiece surface to form hard spots, affecting the quality of the machined surface of the workpiece.

From the above analysis, it can be seen that the built-up edge is not conducive to cutting, especially for finishing.

Control Measures

Blocked edge formation can be avoided by preventing the underlying material of the chip from bonding or deforming and strengthening with the front cutting edge. The following measures can be taken for this purpose.

• Reduce the roughness of the front cutting edge.
• Increase the rake angle of the tool.
• Reduce the cutting thickness.
• Use low-speed cutting or high-speed cutting to avoid cutting speeds that are prone to forming built-up edge.
• Perform appropriate heat treatment on the workpiece material to increase its hardness and reduce plasticity.
• Use cutting fluid with good anti-bonding performance (such as extreme pressure cutting fluid containing sulfur and chlorine).