Conventional milling occurs when the cutting tool rotates in the opposite direction of the workpiece. While it’s a common method for removing material, it can also result in defects in CNC milling parts. Understanding these defects, their causes, and how to address them is crucial for producing high-quality machined parts.

Whether you’re an experienced machinist or just starting with CNC, we hope this post will provide valuable insights and a CNC milling strategy guide for improving machining operations and product quality

Defect 1: Varying Surface Finish

In the CNC milling process, surface finishing is an important and frequently performed operation. However, in actual machining, sometimes the result may be a varying surface finish, which means unevenness or inconsistency in the texture and appearance of machined surfaces.

Cause:

There are a few reasons, like using the wrong settings, worn-out tools, insufficient coolant, or problems with the machine. For example, during precision machining, two fast feed rates can cause the tool to vibrate and leave uneven paths on the machined surface.

Solution:

To fix this, it’s important to figure out what’s causing the problem and make the necessary adjustments to get a better final surface finish.

To prevent this problem, it’s best to set the feed rate (f-value) for surface milling at 1000mm/min or below and set the spindle speed to 4000mm/min. Another issue that can arise during surface finishing is that adjacent tool path may leave slightly different marks due to variations in the cutting direction of the tool. To avoid this, the milling operation should use full climb milling.

Defect 2: Left Dents from Tooling

This CNC Milling defect refers to the problem of visible dents or depressions on the surface of a machined part caused by the cutting tool. The size and depth of the dents depend on factors such as the tool’s geometry, cutting parameters, machining material, and machine rigidity.

Cause:

The dents can be caused by aggressive cutting parameters, improper tool selection, machining material, and machine setup. It’s important to address these factors to reduce or eliminate dents and achieve a high-quality surface finish.

Solution:

To fix this issue, a few things can be done. First, optimizing the cutting parameters can reduce the pressure applied by the tool, which can minimize the size and depth of the dents. Second, choosing a tool with a different geometry or coating can also help reduce the dents by changing the way the tool interacts with the material. Lastly, using a finishing operation like grinding or polishing can help remove any remaining dents and improve the surface finish.

Defect 3: Taper

It’s a gradual increase or decrease in diameter along the length of a machined part.

Cause:

The main causes are worn cutting tools, machine vibrations, incorrect tool alignment, or improper cutting parameters. To prevent taper, it’s essential to use sharp cutting tools, proper tool holders, and parts-clamped techniques(clamping force) and choose the correct cutting parameters.

Solution:

The cutting parameters can be adjusted to reduce the severity if a taper happens. Specialized toolings, such as boring bars or reamers, can also achieve a consistent diameter. In some cases, modifying the machine tool may be necessary to eliminate the root cause of taper, such as aligning or replacing worn bearings.

Defect 4: Oversized or Undersized Parts

When machining parts with CNC milling, it’s common to encounter parts that are either too big or too small. This can cause problems with functionality and aesthetics, leading to increased costs and lost time.

Cause:

The main reasons for this defect include incorrect cutting parameters or issues with the CNC mills, such as improper tool selection or mechanical vibration.

Solution:

Right-cutting parameters for the specific material and machining operation. This includes adjusting the feed rate, spindle speed, and depth of cut to achieve the desired results. Regularly inspecting cutting tools and performing maintenance on the machine tool can also help prevent these defects from occurring.

If oversize or undersize happens, adjusting the machining process or reworking the part is necessary. This can include adjusting the cutting parameters, re-machining the part with proper tooling, or manually finishing the part to achieve the correct size. It’s important to address the root cause of the defect to prevent similar issues from happening again.

Defect 5: Obvious Tool Marks on the Side of Product

Visible tooling marks on the milled surface is also a common defect and should be avoided as they can negatively impact the final product’s appearance.

Cause:

The main cause of this issue is improper selection of the tooling positions and parameters and cutting at different depths. Different CNC milling software provides various methods for selecting tooling depth and parameters.

Solution:

Adjustments can be made in three areas to avoid excessive tool marks on the side surfaces. Firstly, the entry point should be at the edge rather than the middle, with the exit on the opposite side. Secondly, if cutting in the middle is necessary, we should add a 3 to 5mm overlap when the tool enters and exits. Lastly, we should use full-cut depth when performing precision side surface machining.

Defect 6: Tool Change Marks During Precision Milling

When changing tools during regular or high-speed machining, it’s important to adjust the parameters to avoid visible marks that can significantly affect the appearance of the workpiece.

Solution:

To prevent noticeable marks when milling the bottom or side surfaces, adjust the Z step-over to 0.005-0.015mm. Precision machining of internal concave corners requires small tools, which can produce marks due to tool deflection. Leave 0.01-0.02mm in the X and Y directions to solve this problem.

Defect 7: Burrs and Sharp Edges

Cause:

Burrs can be caused by various factors, including improper tool selection, incorrect cutting speed, incorrect feed rate, inadequate cooling rate, and worn or damaged tools.

Solution:

Using a file to remove burrs can affect accuracy and size, so it’s essential to mill them directly without post-processing. If the burrs and sharp edges still occur in production, it’s necessary to use specialized cutting tools and plan the tool path carefully. Increasing the number of finishing passes can also help eliminate burrs and sharp edges. Machining the surface, then the side, and then the surface again can avoid burrs and the sharp cutting edge, which is particularly useful for workpieces that cannot be polished. And manually removing the burrs will be carried out when the burrs and sharp edges are still happening after using the above milling strategies.

Defect 8: Deformation

For precision machining of some special-shaped workpieces, there may be fitting errors in the software. Sometimes, if the calculated error is too large, it will cause the workpiece to deform and affect its appearance.

Cause:

Improper work holding, excessive cutting forces, tool deflection, excessive heat generation, and inadequate machine stiffness or rigidity can cause deformation in CNC milling, leading to dimensional errors or deformation of the workpiece. Inadequate clamping force or relying solely on manual clamping forces can also result in deformation.

Solution：

More consistent and precise clamping force by pneumatic or hydraulic cylinders, appropriate cutting parameters, the proper cutting tools, machine rigidity maintenance, and close cutting process monitoring is crucial to minimize this risk, especially for easily deformed material like machining brass. Also, starting from the software and controlling the error within 0.001mm is necessary, a reasonable value that does not affect the calculation speed and cause deformation of the workpiece.

Author’s Words

Thank you for reading this article on CNC milling posted for everyone interested in CNC Machining techniques. Welcome to share with us your successful milled part case. We keep learning!

More about CNC Milling.