A defect on CNC machined part.

No doubt that CNC machining is the most advanced and accurate manufacturing process. With the help of computer numerical control, it creates a wide range of geometries according to the uploaded CAD model and digital instructions. However, there are a few possible machining defects in CNC technology. These defects might arise due to several reasons. The most common causes include complexity in the technical drawing, wrong tool arrangement, inappropriate machining conditions, operator expertise, and poor handling.

This article will briefly discuss common CNC machining defects, reasons for defects, and possible solutions to avoid those defects.

 

What is meant by CNC Machining Defects?

The CNC machining defects refer to any undesirable results and failure in finished products from the machining process. It includes physical damage to the workpiece, dimensional errors, poor surface finish, material defects, tool breakage, machine malfunction, and many more.

Poor machining defects might cause several minor failures in the final result of machined parts. It can cause a dimensional error, low-quality surface finish, heat build, and many more problems.

The fundamental causes of machining defects in cutting are chips, chatter, burrs, chipping, and built-up edge. Since these defects are generated by “cutting metal with a tool“ the mechanism of defect generation does not change even if the type of machine tool or processing method changes, such as machining centers, CNC lathes, and automatic lathes.

Let’s discuss the CNC machining defects by categorizing them based on the root of these problems, chips, chatter, burrs, chipping, and built-up edge.

Related:

The Impact of Tool Wear on CNC Machining Defects

 

Defects by Chip Formation:

Chip formation mechanism

A chip is the debris or material created when a cutting tool removes material from a workpiece. Chip formation is an integral part of subtractive manufacturing because that’s how material removal occurs to convert the raw material into the desired shape. Chips need to be removed smoothly & continuously. If they cannot be adequately removed, they will lead to machining defects and momentary stoppages, so it is necessary to discharge them reliably.

There are various types of metal chips: splattery powder, continuous, discontinuous, curled, stringy, long spirals, and more. Since the shape changes depending on the machining method and the workpiece, it is no exaggeration to say that you can tell the machining quality by looking at the chips. In general machining, flow-type chips are ideal, and the type of chips can know the machining state. On the other hand, dis-continuous chips are considered problematic in CNC machining.

How to ensure the machining defects caused by chips?

The following indications suggest the possibility of machining defects caused by chips.

• Chips wrap around tools and excessive chips in the machining area.
• Chips are being caught when changing the workpiece.
• If the cutting tool breaks unexpectedly, it could indicate that chips are causing defects.
• If the machining process is frequently stopping or experiencing unexpected downtime.
• Drill breakage due to chip clogging during drilling.

Prevention

• Use sharp and adequately maintained cutting tools. It will reduce the force needed to cut the material, which helps prevent chips from forming.
• Use appropriate cutting speeds & feed rates. If you cut too slowly, the tool will rub against the workpiece, creating heat and potentially causing chips. Also, cutting too quickly can restrict the removable rate because it will not be able to remove the material fast enough.
• Use chip breakers and chip removal tools.

 

Defect by Chatter

Chatter is an intermittent vibration that occurs during machining. It can be classified into two types, “forced chattering” and “self-excited chattering,” depending on the source of the vibration.

Forced chattering refers to the vibration caused by external forces acting on the machining system, such as an Imbalance in the cutting tool & workpiece, unstable machine foundation, inaccurate work holding, poor rigidity of the machining tool, and inaccurate posting. The entire batch of manufactured workpieces could be defective by chatter (Chirag Kulkarni, 2017).

There are several factors that can contribute to forced chattering, including:

1. 1. Machine spindle: The spindle of the machine can be the source of vibrations that lead to chattering if it is not running at the correct speed or if it is not properly balanced.
   2. Tool holder: A tool holder that is not designed correctly or is worn can cause vibrations that lead to chattering.
   3. Cutting conditions: High cutting speeds, high feed rates, and high depths of cut can all contribute to chattering.
   4. Workpiece properties: Hard or brittle workpiece materials can be more prone to chattering than softer materials.

To reduce or eliminate forced chattering, the following strategies can be used:

1. 1. Balance the machine spindle
   2. Use a higher-quality tool holder
   3. Use a cutting tool with a larger radius
   4. Use a more robust cutting tool material
   5. Use a cutting fluid
   6. Use a chatter-damping tool holder
   7. Use a more stable and robust machine
   8. Use a softer workpiece material
   9. Use a chatter suppression controller

Self-excited chattering is a phenomenon that occurs in machining processes when the cutting tool vibrates uncontrollably, causing a reduction in surface finish and an increase in tool wear. It is considered to be a self-excited vibration because it is caused by the interaction between the cutting tool and the workpiece, rather than an external source.

There are several factors that can contribute to self-excited chattering, including:

1. 1. Cutting tool geometry: Certain tool geometries, such as a tool with a small radius, can make the tool more susceptible to chattering.
   2. Cutting conditions: High cutting speeds, high feed rates, and high depths of cut can all contribute to chattering.
   3. Machine conditions: Loose or worn machine components can lead to vibrations that can cause chattering.
   4. Workpiece properties: Hard or brittle workpiece materials can be more prone to chattering than softer materials.

To reduce or eliminate self-excited chattering, the following strategies can be used:

1. 1. Reduce cutting speed and feed rate
   2. Use a cutting tool with a larger radius
   3. Use a more robust cutting tool material
   4. Use a cutting fluid
   5. Use a chatter-damping tool holder
   6. Use a more stable and robust machine
   7. Use a softer workpiece material
   8. Use a chatter suppression controller

Chattering leads to poor machining but also damage to the machine. The defects of chattering in CNC machining include workpiece deformation, poor dimensional accuracy, low-quality surface finish, tool breakage, safety issues, and more. Therefore, urgent countermeasures are required to prevent this defect.

How to ensure the machining defects caused by chattering?

The following indications suggest the possibility of machining defects caused by chattering.

• • A noticeable vibration in the machining process with a loud rhythmic noise.
  • Degradation in the surface finish of machined parts
  • Damage to machine and tools
  • Reduce machining accuracy.
  • Surface defects on the workpiece, such as scratches or pitting.

Prevention

• • Apply the tool with a positive rake angle, which can prevent chattering by reducing the cutting forces.
  • Ensure that the foundation of the CNC machine is stable.
  • Ensure that there is no looseness or play in the setup.
  • Maintain the perfect tool alignment Tool with a chamfer or a rounded edge can reduce the risk of chattering by absorbing the vibrations.

Defects by Chipping

Chipping refers to the material removing the process from the workpiece in small chips or fragments that occur on a cutting tool’s cutting edge. It almost always occurs in cutting work, but early detection is important because even small chippings can lead to unexpected machine stops and several damages in the CNC machining operations.

The fundamental reasons for defects caused by chipping include insufficient tool strength, high cutting forces, inappropriate tool path & geometry, and hardness of the workpiece,

How to ensure the machining defects are caused by chipping?

The following indications suggest the possibility of machining defects caused by chipping.

• Frequently stopping or experiencing unexpected downtime of the CNC machining process.
• A rough-uneven surface finish with a dull, satin-like appearance.
• Excessive wear on cutting tools
• Poor productivity or low-machining cycle time.

Prevention:

• Consistently maintain and sharpen your cutting tools and ensure their condition so that they can produce clean chip-free cuts.
• Use a tool with a hard coating, such as a diamond or carbide.
• Utilize a tool with plenty of cutting edges. It will uniformly distribute the cutting forces and lower the possibility of chipping flaws.

 

Defect by Excessive Burrs

Excessive burrs in a CNC machined part.

Burrs refer to the tiny, sharp projections or edges left on the surface of a machined part. These are basically the protruding parts of the work that are left uncut, and chips pushed out. It is no exaggeration to say that cutting processes always generate burrs ranging from large to small burrs.

Leaving burrs as they are can cause troubles and injuries in the following process, so processing that suppresses the generation of burrs and reliable deburring is essential.

Poor surface finish is the primary defect in the formation of burrs during CNC machining. Other defects include low- product quality, increased scrap and waste, and increased production time.

How to ensure the machining defects caused by excessive burrs?

The following indications suggest the possibility of machining defects caused by excessive burrs.

• Visible burrs on the machined surface.
• Rough surface with a lot of imperfections.
• Poor fit and function
• The outflow of defective products.

Prevention

• Use the appropriate cutting tool according to the material you are machining.
• Consider the deburring tool to remove the burrs created during machining.
• A tool with a longer nose radius might prevent the burr defects.

 

Defects by the Built-up Edge

Built-up edge (BUE)

Built-up edge is a phenomenon in which chips adhere to the tip of the cutting edge due to high cutting pressure and frictional heat.

The main reason for built-up edges is Inappropriate cutting speeds & feed rates, inappropriate coolant or lubricant, and poor tool conditions & geometry. In addition, some materials are prone to built-up edge problems, such as ductile materials that are easily work hardened (mild steel, brass, stainless steel, aluminum, etc.), which tend to generate a built-up edge.

The built-up edges result in poor fit & function, rough surface finish, safety issues, and dimensional inaccuracy.

How to ensure the machining defects caused by built-up edges?

The following indications suggest the possibility of machining defects caused by built-up edges.

• Cutting-edge chipping
• The visible presence of built-up edges on the workpiece.
• Unexpected machine downtime.
• Poor fit and function

Prevention:

• Use a tool with a damping coat or a damping geometry.
• Apply appropriate lubricants and coolants.

 

Tips to Avoid All Types of CNC Machining-defects.

Since some common mistakes can lead to various defects in CNC machining operations, there are a few fundamental tips to avoid these effects.

Know your material

Always check that you are using a proper setting and lathe for the type of metal in question. Consider the several physical & mechanical properties of workpiece material before the selection of tool material & geometry. In addition, apply the appropriate cutting speed & feed rate according to the material type.

For example, with very-hard metals, your lathe may not be able to work, as is the case with cast iron. So, consider the model and specifications equipment before programming.

The care and dedication of your team are essential at this time, but you also need to have a partner to assist you in predictive, preventive, or corrective maintenance cases.

Cross-check the digital instructions

When entering the code, the most common mistake with CNC machining is a simple spelling or syntax error. Although most modern lathes have mechanisms that prevent breakdowns, a simple detail like this can stop the entire production chain, damaging your equipment (10 Common Problems with CNC Machine Tools, 2021). Therefore, cross-checking of G-code is essential.

Periodic and preventive maintenance of equipment

It is essential to have a team of highly qualified professionals for the repairs and maintenance of your CNC machine. Maintain periodic & scheduled maintenance up to date, including services that improve equipment performance, such as:

• Lubrication of moving parts
• Checking and replenishing fluid levels
• Extraction or replacement of filters.

 

Summing Up

Countermeasures against machining defects are not limited to on-site ingenuity. Acquire a wide range of knowledge, from metal materials to tool characteristics, and take countermeasures.

At Prolean, our CNC experts have many years of experience in CNC machining. And they know exactly how to avoid machining defects. This way, the lead time and the quality of parts can be guaranteed. If you are looking for any CNC machining service for your project, please feel free to contact us. We will get your production plan and quote in a few hours.

 

FAQ’s

What are the most common causes of CNC machining defects?

The most common causes of CNC machining defects include complexity in the CAD model, wrong tool arrangement, inappropriate machining conditions, operator expertise, and poor handling.

What are the most Common CNC machining defects?

There are several CNC machining defects, but they can be categorized as chips defects, chatter defects, chipping defects, burrs defects, and built-up edge defects.

How to prevent common CNC machining defects?

It can be prevented according to the specific type of defects. However, some common strategies include considering material properties, correct G-code, Periodic & preventive maintenance of equipment, and appropriate machining variables.

 

Bibliography

1. (2021). 10 Common Problems with CNC Machine Tools. Hwacheon Asia Pvt.Ltd., from https://hwacheonasia.com/.
2. Chirag Kulkarni, D. B. (2017). Defect Identification in CNC Manufactured Products and their Control Measure. International Journal of Innovative Research in Science, Engineering, and Technology.