Defining Precision and Accuracy in CNC Machining

There are a few terms and phrases very commonly used in reference to CNC Machining. The industry is well known for its precision and speed. Terms like tolerance, accuracy, and precision can be confusing. The development of smart technology and the robust machines now available make machining seem like a simple task for any application. There are still many factors that influence the outcome of a finished part. Tolerances are often quoted as a level of performance and are always specified on machine part drawings or programs. Tolerances define the parameters of variability in size and shape of the finished part and are often critical to its end use. That tolerance depends on several factors. Defining precision and accuracy can help us better understand how to approach the machining task for maximized performance.

Accuracy

This term refers to the difference between the actual measurement and the measurement perceived by the CNC machine process. The Miriam Webster dictionary defines it as: freedom from mistake or error:

CORRECTNESS AND conformity to truth or to a standard or model, or degree of conformity of a measure to a standard or a true value:

EXACTNESS Exactness suits the definition best in CNC manufacturing. Factors that affect accuracy beyond programming include vibration, thermal expansion, cutting tool, and workholding. As programming becomes more complex it allows for higher accuracy by compensating for these factors.

Precision

A perfect example of why these terms can be difficult to understand without specific reference to CNC machining is another reference to Miriam Webster. Precision is referred to as exactness and accuracy but it is the defined adjective that aligns with CNC machining.

1: adapted for extremely accurate measurement or operation

2: held to low tolerance in manufacture

3: marked by precision of execution

Precision refers to the repeatability of a set process delivering the same result. Both of these terms in CNC machining are partnered with tolerance. Tolerance allows for the precision and accuracy calculations based on a variable guide from zero tolerance, or perfect.

Maximizing Performance

A CNC machine will include specifications for accuracy and precision that are based on static measure. Under load, when in motion and working with different tools and raw materials, those specifications will change. These variables can not be accounted for until the machine is on location and hard at work but the machine manufacturer should be able to assess your intended work and recommend a machine that will perform under load and in your shop environment.

Once the machine is on the shop floor, the end user is responsible for maximizing accuracy and repeatability for the life of the machine. An overview would include:

Calibration – The factory settings that provided the benchmark measurements for accuracy and precision need to be maintained. Regular use will move a machine from level affecting the original calibration so it needs to be checked and reset regularly.

Tool & Spindle Wear – Two of the largest contributing factors to both accuracy and repeatability are the tool and spindle as they come in the closest contact with the end product. Both can be affected by thermal expansion, friction, and cutting path speed and feed rate. Dull tools will contribute to friction and will change tolerances due to the change in their shape.

Maintenance – Regular cleaning between jobs and larger maintenance scheduling keeps equipment performing its best and extends life. Parts must be checked for wear, fluids replaced, and xx. See more on maintenance here.

User Education – The expert machinists born from manual machining were almost extensions of the machine, itself, with their knowledge and understanding. New technology can replace only some of that experience but does help new operators and machinists with basic guidelines. Regular steps like cleaning before use, setting machine tolerance to Zero, and understanding raw material properties and tolerances will increase machine precision.

 These simple descriptions defining precision and accuracy will contribute to your knowledge when speaking to a manufacturer about your next project. At Rapid Enterprises we take the time to educate the client for accurate decision-making. Contact us to ask about your next project.