Be Alert: the 4 Most Common CNC Machining Mistakes

As a customer of CNC machined parts it is helpful to be aware of common machining mistakes. Do your own quality control. Firstly check if your machinist is ISO 9001:2015 certified. Then ask how they prevent the below errors.

Fundamentally, a machinist creates a CNC program that tells the machine which tools to use, how to move, and exactly which piece of material to remove in order to create the final product.

The CNC machining process may be a very fast and efficient way of creating a product, but the sequence of events written in a CNC program is created by a human. It is this human element that creates the potential for mistakes to happen.

Common CNC Machining Mistakes

There are numerous mistakes that may occur during CNC machining. Detailing each one will consume much more space than this article allows for, so we will explore 4 of the most common CNC machining mistakes:

  1. Programming Errors

  2. Lack of Maintenance

  3. Improper Tools

  4. Feed and Speed

1. Programming Errors

The CNC program controls the CNC machine’s operations. A programming error will have a knock-on effect which may be as insignificant as a simple irritation, to a catastrophic event resulting in serious machine, tool or workpiece damage.

Programming errors can be difficult to detect, especially when inexperienced employees are involved. The most common errors are spelling and syntax related. Replacing an “O” with zero, or putting a comma in the incorrect place.

Errors may be difficult to detect and so most modern CNC machines have a failsafe feature to minimise serious machine and tool damage. Employee training on the use of G-codes and M-codes is the best preventative measure as this ensures the program does not endanger equipment or work pieces. Teaching new employees to program slowly and to regularly review their programs will aid with minimising programming errors.

2. Lack of Maintenance

CNC machines run at very high operating temperatures. Even though they are designed for this, blocked filters and debris inside machines may push operating temperatures outside the design parameters, resulting in serious machine damage.

Dirty machines may lead to accuracy and precision problems as work pieces may experience unwanted movement during machining. A lack of lubrication may reduce the smoothness with which machine parts move, reducing accuracy. Low coolant levels and blocked AC’s may also result in overheating.

It is advisable to implement a fixed maintenance schedule aimed at ensuring optimal CNC machine performance. This schedule may include lubricating moving parts, checking and refilling coolant levels, and blowing out or replacing AC filters.

But maintenance does not only have to occur on fixed schedules. Daily housekeeping like wiping off cutting coolant and debris can help prevent machine problems.

3. Improper Tools

There are a number of tell-tale signs that the cutting tool being used is not optimum. Burn marks on the workpiece, scratches or edge distortion are but a few. These suggest that the tool being used may be moving too slowly, is blunt, has insufficient cooling liquid flow, or is positioned incorrectly. “Chatter” is a common CNC machining error caused by either cutting tool or CNC machine vibrations. This phenomenon can reduce tool and machine life dramatically.

There are a number of calculators available to assist programmers in deciding what the correct tool is for a particular process. Cutting tool suppliers have detailed databases and product specialists able to assist programmers with selecting the optimum tools, and programming the correct parameters for those tools.

4. Feed and Speed

A common mistake is cutting too slowly. This causes scarring as the material spends too much time under the cutter. The material is being rubbed more than it is being cut. The quality of the surface finish will be poor, the tool may become dull sooner, and a significant amount of heat will be generated, burning the surface of the workpiece.

On the flip side, cutting too fast and deep may result in excess tool deflection. Excess tool deflection results in sub-optimum tool life, surface finish and machine efficiency.

Calculating optimum feed and speed rates is a very intricate process. There are a number of variables to consider such as cut width, cut depth, cutter type and machine power. Training and sharing knowledge between experienced and new employees will contribute greatly to reducing feed and speed mistakes.


Minimising manual data entry will minimise the risk of human error. As long as an over-reliance on human intervention exists, CNC machining mistakes will continue to occur. Investing in CNC machine monitoring systems will contribute to efficiency improvements and eliminating human error.

Challenge Engineering is an Australian ISO 9001:2015 certified CNC machining company based in Sydney. To find out more go to

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