CNC milling is a versatile technique that meets the needs of many manufacturing industries. From automotive engine components to surgical nstruments and diagnostic equipment, the use of CNC milling is endless. This process comes with high accuracy and precision. Additionally, it is repeatable, and the machined parts are of consistent quality.

However, certain defects can arise during the CNC milling operation. These defects affect the aesthetics and compromise the performance of the finished product. It is essential to understand the common CNC milling defects and their causes. This article will explain the most frequent CNC issues and how to prevent them to improve overall efficiency.

Overview of the CNC Milling Process

CNC milling is an advanced machining process that produces accurate and high-quality parts. It uses pre-programmed software to dictate the movement of tools for cutting action. This operation begins with a metal block where excess material is removed gradually to create the final part. The CNC milling process starts with making the CAD model, which is a representation of the end product. You do this using suitable software tools such as SOLIDWORKS and AutoCAD. 

After that, you convert the model to a CNC-compatible format in the form of programming languages. These languages, including G codes and M codes, are how the machine can understand and interpret the model. Finally, select the appropriate milling parameters and machine the part. The machine runs continuously till the operation is complete. 

CNC milling is versatile, and you can carry out different operations. The most common are face, angular, side, and slot milling. Furthermore, this process is fast and comes with high accuracy and precision. These benefits make CNC milling the go-to technique when manufacturing automotive brake systems, aerospace structural frames, and surgical instruments.

9 Common CNC Milling Defects and Solutions

CNC milling is a quality-driven operation. However, it can result in detrimental defects if one is not careful. The following are common CNC milling defects and their solutions 

1. Chatter Marks

This is a common defect that causes uneven and wavy patterns on the workpiece surface. It resembles a conspicuous pattern of lines near the cutting path. Chatter marks are a result of unnecessary vibrations of the workpiece and instability of the cutting process. The final parts have reduced dimensional accuracy and poor aesthetic quality.

Solutions

• Use proper workholding fixtures to ensure a rigid setup.
• Regularly maintain the CNC milling machine and tighten all loose connections.
• Optimize cutting parameters based on the material properties and workpiece thickness.

2. Burr Formation 

You can also experience burr formation during a CNC milling operation. This happens when one uses a cutting speed and feed rate higher than the required. Additionally, dull cutting tools or improper tool geometry also result in burrs. These burrs appear as visible ridges on the machined surface. They can cause quality issues for parts that need to be further assembled. Burrs need to be corrected, which can increase machining time and cost.

Solutions

• Use sharp tools with the correct geometry.
• Reduce cutting speed and feed rate to match the workpiece material.
• When burrs have occurred, deburr at slower speeds.

3. Tool Marks

Another common CNC milling defect is tool marks. They appear as uneven spacing and inconsistent depth in the marks. The effects of tool marks are mainly aesthetics, as they reduce surface quality. Additionally, tool marks can affect performance in precision and high tolerance applications. Tool misalignments, using tools with insufficient rigidity, and incorrect speed are the major causes of marks.

Solutions 

• Using high-quality tool material that works with the workpiece material.
• Properly align tools and replace worn-out ones regularly.
• Optimize cutting parameters and use slower speeds for finishing passes.

4. Dimensional Inaccuracy 

Dimensional inaccuracy is a serious CNC milling problem. It occurs when the workpiece has thermal expansion as a result of the changing temperature of the operating environment. Additionally, dimensional inaccuracies happen when there are programming errors or the milling machine is not well calibrated. This gives parts dimensions that are different from the original design. Dimensional inaccuracies lead to customer dissatisfaction and increased incidence of reworks.

Solutions

• Use the right type and amount of lubricants to prevent overheating.
• Regularly calibrate the CNC milling machine. 
• Test run the program and measure critical dimensions on the parts before production begins in full.
• Check cutting tools for signs of wear and replace them when necessary.

5. Burn Marks

You can also experience burn marks when milling a part. These marks appear as discolored patches on the workpiece surface. The marked area is a potential point of localized surface damage. They weaken the workpiece properties and increase the likelihood of cracks and warping. Furthermore, burn marks are caused when the cutting process generates excessive heat. It is also a result of inadequate use of CNC coolants and lubricants.

Solutions

• Use sharp tools that easily cut through rather than rub on the material. 
• Prioritize coolants and lubricants during CNC milling operations.
• Use the correct cutting speed and feed rate to prevent overheating.

6. Overcutting 

Overcutting, also known as gouging, happens when you make deep and unintended cuts on the workpiece surface. It can also appear as scratches or grooves, which changes the shape of the part. It is caused by programming errors and tool misalignment. Occasionally, overcutting leads to permanent damage, and you may have to rework the component. This increases production costs because of more machining time.

Solutions

• Confirm the CNC program controlling tool path before machining begins.
• Properly align the milling tools with the workpiece.
• Carry out regular machine maintenance to tighten loose connections.

7. Tool Breakage

While this does not directly affect cut quality, it can lead to downtime when noticed during an operation. Tools can break for many reasons. Tool breakage happens when the cutting force is excessive as a result of inadequate depth of cut, feed rate, and cutting speed. Additionally, tools can break when cutting a workpiece that is too hard with an incorrect tool material. All these can potentially damage the workpiece.

Solutions 

• Make sure the workpiece and tool materials match.
• Inspect end and face mills regularly and check for signs of wear.
• Take breaks between operations, and do not overuse the tools.
• Use good cooling and lubrication systems.

8. Surface Pitting or Voids

This happens when there are small voids and spaces on the machined surface. It gives the workpiece an uneven and porous texture. Surface voids happen when impurities are embedded in the workpiece or there are cases of overheating. The result is a poor surface finish and build-up of localized stress points. This reduces the structural integrity of the workpiece and increases susceptibility to wear.

Solutions

• Be sure the workpiece material is of the highest quality and impurities-free.
• Use the right type and quantity of CNC coolants.
• Optimize the milling parameters, including feed rate and depth of cut, to prevent overheating.

9. Tool Deflection 

This is another CNC milling defect you can encounter. It happens when the tools move away from where they are meant to be, leading to uneven cuts in some parts of the workpiece. This increases the potential for tool breakage and loss of dimensional accuracy. Tool deflection occurs when using excessive force to cut into hard materials. It can also be a result of vibrations as a result of improper workpiece clamping.

Solutions 

• Use the proper work holding jigs and fixtures to secure the workpiece.
• Do not use too much cutting force for tough and soft materials.
• When cutting, consider using multiple passes instead of a single one.

Conclusion 

Like many manufacturing operations, custom CNC milling is not immune to defects. This process thrives in producing quality products. However, there can be instances where different types of defects, such as burn marks, tool breakage, and dimensional inaccuracies, occur. These defects happen when using the wrong cutting settings, poor machine setup, and inadequate coolants and lubricants. It is crucial to avoid and manage these defects to ensure high-quality outputs.