How Precise is a CNC Machine? Understanding the Accuracy and Repeatability

Precision of a CNC machine does not completely depend upon the machine itself but also upon the expertise of the operator operating it. Other than that, the CNC machining quality also depends on numerous other factors like tool selection, strategy of workholding, design, toolpath programming, etc. The tolerance level and accuracy of the CNC machine also depends upon the above factors and also the machine shop which is producing the part. 

This is the reason while deciding which CNC machine shop is fit for performing a complex project, it is imperative to consider two questions:  (i) how precise is the CNC machine’s output? and (ii) what is the ultimate commitment of that machine shop in fulfilling that precision-level? 

According to CNC machining experts, the CNC machine that successfully meets the desired precision is the design engineers’ choice in building parts in an appropriate machine shop. 

So, how will you know how precise a CNC machine is? Well, there are 3 parameters, namely: positional accuracy, precision in repeatability, tolerance of the end product, which help in understanding the above query. 

So let’s check them out one by one:

Positional Accuracy of CNC Machine

Positional accuracy refers to the difference between the actual distance measured between the positions on a part and the defined distance between those spots, once it is milled. If you consider a CNC mill, then its precision is based on its ability to follow its programmed path as exactly as possible. 

The mill or any other CNC machine’s precision is determined in machine shops by multiple readings/measurements and calculating the average statistical value of the deviations caused in each reading. This average value is then used to determine the machine’s precision. 

Precision in Repeatability

The CNC machine’s precision in performing repeated cuts is again tested, calculated, and defined by the machine shops only. The shop checks how perfectly the machine executes the same G-code and gives the same output. That means, the machine is fed an input where it is supposed to cut a line. 

Then the machine’s precision is measured by allowing it to repeatedly cut on the same line. Each time the reading is compared with the former one to compare the value difference (if any). Then a standard average value is determined by calculating all the repeated readings which then gives the precision value. This type of precision is basically required in the hardware manufacturing industries, where you may have to produce 10,000 or even more parts of the same design at one go. 

Finished Part’s Tolerance 

Tolerance means the allowable variation in the dimensional accuracy of the finished part. The tighter the tolerance, the better is the output. That means the end product is exactly of the dimensions you wanted it to be. Here, the machine shop tests the difference between the set value of the CNC machine and the +/- dimensional variance in the output surrounding that set value. 

This tolerance range determines the machine’s precision, in this case. The CNC machinists and programmers program the machine to produce a part maintaining the exact tolerance range as suggested by the client. The system should function accordingly in a way that the variance does not surpass the client’s specified tolerance during the material cutting and product manufacturing. 

So, basically, there is no universal value to determine a CNC machine’s precision. It mainly depends on three parameters as discussed above.  

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