4 mistakes to avoid when CNC machining

What mistakes should you avoid when CNC machining?

A CNC machine can be the single best investment any machine shop will make. When used correctly, they can enhance your productivity and help you handle a wide range of machining operations safely. But as many machinists will tell you, it can be easy to make mistakes when operating a CNC machine. 

cnc machining mistakes

Fortunately, the worst machining offences can be avoided by building good habits and learning from best practices. But not all mistakes are as easy to avoid. Here are a few to watch out for when handling a CNC machine. 

1. Using the wrong cutting tools

Every cutting tool in a machinist’s toolkit is there for a specific purpose and machining operation. But sometimes, even the pros can slip and accidentally use the wrong cutting tool. 

Some classic signs of a cutting tool being used for the wrong application include:

  • Degraded surface finishes
  • Vibrations or chatter
  • Sizing/tolerance issues
  • Burn marks on the workpiece.

These issues show that the cutting tool being used is blunt, moving too slowly,  positioned incorrectly, or has poor cooling liquid flow. Do this often enough and you could reduce the life of your machine.

2. Ignoring maintenance

CNC machines are high-performance machines designed to run at high speeds. To do this safely, they require regular maintenance, including cleaning filters and removing debris from inside the machine. 

Dirty machines can cause chatter and vibrations, leading to problems with accuracy and precision during cutting operations. Likewise, poor lubrication can reduce the smoothness of moving parts, leading to issues like inaccurate measurements and poor surface finishes. Low coolant levels can also cause the machine to overheat.

A good rule of thumb is to follow a fixed maintenance schedule to keep your CNC machine in top shape. This schedule should include:

  • Lubricating all moving parts
  • Checking and refilling coolant when necessary
  • Cleaning or replacing filters when necessary.

You should also have a list of daily housekeeping chores, such as wiping off cutting fluids and consistently segregating metal chips for later recycling. 

3. Improper feed rate and speed

Your feed rate and speed depend on the type of cutting operation and material of your workpiece. 

For example, a softer metal like aluminium (and other non-ferrous metals) requires less cutting force but a higher spindle speed than, say, cast iron. Cutting aluminium slowly can scar the material as its softer and more malleable composition cannot spend too much time under the cutter. 

When this happens, the aluminium workpiece ends up being rubbed more than cut. This leads to the workpiece having a poor finish and possible burn marks, as well as premature wear on the cutting tool.

Conversely, milling a workpiece made of a harder metal like cast iron at too high of a speed can lead to tool deflection. This can lead to flank wear and thermal cracking in the tool. “Hard” metals like cast iron need a slower spindle speed to reduce chatter. 

4. Having insufficient knowledge of M-codes

M-codes or machine codes are part of the language used to input instructions into a CNC machine. An M-code is required to command a CNC machine to activate on/off switches for components and accessories like the:

  • Spindle
  • Coolant
  • Door
  • Rotary table
  • Chip conveyor

These M-code functions and their applications will be found in the machine’s owner’s manual. Machinists must take the time to understand each M-code to prevent errors and issues that affect the machine’s lifespan. 

For example, when using a rotary axis for roughing cycles with a large tool, it’s recommended to activate the clamping system — a step that requires an M-code. If the machinist is unaware of this code, the rotary axis won’t be clamped, and the machine could sustain unnecessary wear. 


Gary Clegg has been in the industry for over 35 years, having worked his way up from an apprentice to production manager. Gary is a time-served CNC machine engineer and has experience in all fields of engineering, including milling, drilling, turning and threading. After leaving the shop floor to join the tool supply industry, he gained additional knowledge and experience in the tool and workholding category, making him an expert in static and driven tools, with contacts with all leading CNC manufacturers.