Deviation

The Precision: Read this before you build your CNC machine.

CNC machines are intended to be precise and accurate. But if you’re new to machining or missed out on learning a few things, it’s easy to misunderstand the concept of precision and accuracy.

The first thing you need to understand is that precision and accuracy is very dependent on many variables. It is very important to decide the correct amount of it for your project.

not precise & not accurate
not precise & not accurate
precise & not accurate
precise & not accurate
precise & accurate
precise & accurate

All machines make some mistakes during positioning. There is no such machine that can find a position with absolute zero error. We can simply say that precision is the maximum amount of this error.

Sample:
We want the machine to go from X0, Y0 to X10,Y0. Then we measure the distance. If we read the result between 9,995 and 10,005, we can say that the machine has an accuracy of 0.01mm in the X axis.

As you can see, we need a measuring device with a resolution of at least 0.005mm in order to be able to measure with 0.01mm precision.

* Exaggerated representation of inaccuracy

Ideal Machine
Ideal Machine
Real Machine
Real Machine
Deviation
Deviation
Tolerance
Tolerance

Truth is another story. Precision defines the minimum distance we can rely on, while accuracy defines the overall correctness of results. Ideally, we would expect the machine to have a 100% square frame and perfectly aligned linear rails. Therefore, the results will be completely accurate. However, in reality it is very different. You can’t build a completely accurate framework and have perfectly accurate components. As a result, your work will come out with some inaccuracies. This is not a bad thing. As long as the inaccuracy is within the expected tolerance, it’s absolutely fine.

I usually use 0.15mm tolerance when 3D printing mechanical parts. This means that even if the part dimensions deviate by 0.15 mm, it is acceptable. I would call it “precise and accurate”. You can see how precision and accuracy depend on the application.

It is safe to say that precision and accuracy are defined by tolerances acceptable for the application. Therefore, you should decide on the purpose of the cnc machine and learn about the acceptable tolerances. This is the starting point for your CNC machine design.

You can see more technical explanations in this wikipedia article: https://en.wikipedia.org/wiki/Accuracy_and_precision

You can easily understand that more precision and accuracy is more expensive and requires more hard work to achieve.

Level (mm)Machine CostTool CostHard work
0.1$$🦾
0.05$$$$$🦾🦾
0.01$$$$$$$$$$$$$$🦾🦾🦾🦾🦾
0.001 $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$🦾🦾🦾🦾🦾🦾🦾🦾🦾🦾

Now we can consider the material properties. Both machine frame and workpiece materials affect precision and accuracy. If you don’t know yet, almost all materials are alive in sizes below 0.05mm. They constantly change their shape/size. Almost all materials expand and shrink with temperature changes. Woods and plastics also change their shape and mechanical properties according to the ambient humidity.here is more. Because humidity and temperature do not always affect the material equally, the material expands and shrinks in different ways each time. If the ambient temperature is 27C during machining and 21C when measuring the machined part, how can you judge the precision?

That was the question in the early days of industrial evolution. Then they invented tolerances. As long as the final work falls within the tolerance range, it’s fine.

You can read this article for more technical explanation: https://en.wikipedia.org/wiki/Engineering_tolerance

Since the final precision determines the cost of the day-to-day operation and the machine, it is better to take the lowest precision that is fit for purpose. If you’re building a cnc machine for plastics and woodworking or FDM 3D printing, you don’t need any precision below 0.05mm. For machining metals, you can consider tolerances of 0.01 to 0.05 mm.

Anything below 0.01mm tolerances requires a high level of experience and quality of machinery and equipment. You might even consider wearing appropriate gloves to make sure your body temperature doesn’t affect the measuring tool or the workpiece. Well, if you had such an experience, you probably wouldn’t be reading this article.

Frame design and material also affect the final precision. Vibration can easily ruin any sensitivity below 0.05mm. In most cases, I recommend examining the professional machines available for this purpose.

For the machine frame, aging is important. It makes the frame more stable. Most machinists have cast iron frames that have been naturally or artificially aged. So if it’s possible to use a manual machine for your purpose, consider buying it and converting it to a cnc machine. Then you can have a very rigid and stable machine.

Machine components are another thing to consider. Most precision component manufacturers have at least 3 accuracy and preload classes for their products. If a component manufacturer doesn’t provide any level of accuracy or preload, it means they don’t control their product. If the manufacturer is not sure, how can you be sure? Cheap components are not for tolerances below 0.05mm.

Preloading is also important for high accuracy. As you know, even the hardest material deflects under load. If your linear rails have very low preload, they will change deflect under load and lose some precision. But it will last longer because it can absorb more force. If the component has a high preload, it has already deflected enough. This means that it will not lose precision under load. However, it won’t absorb as much force as the lightly preloaded one and won’t last long. Lightly preloaded components can better tolerate frame errors.

This is where high precision becomes costly. The highest grade components are wildly expensive. You also need to prepare a very precise framework, which is very costly and difficult to achieve. Then you need extremely robust and precise tools that are also very expensive.

So, before start building your machine, you better find the balance between the cost and tolerances for your application. Therefore you can build a good reliable machine in a proper cost.

I hope this post was helpful to you and I hope you build a great machine!