How to understand the difference between accuracy, precision and tolerance?

You have bought a device that has “high resolution” in the specifications of a 3D printer, and yet the parts are not printed accurately or precisely? Then this article is for you … 

Understanding the importance of accuracy, precision and tolerance is essential in achieving satisfying 3D printing performance and quality in any application. In this article, we will analyze what these terms mean and how to think about them in the context of 3D printing. 

WHAT ARE THE DIFFERENCES BETWEEN ACCURACY, PRECISION AND TOLERANCE? 

Accuracy determines to what extent the dimensions of the printed 3D model match the actual size of the object. In real life, it can be compared to a game of darts, the closer your dart is to the predefined target, the more accurate the hit was. In the world of 3D printing, project dimensions from CAD are the reference values. Accuracy is therefore a measure of the compliance of the dimensions plotted on the computer with the print sizes. 

Precision, on the other hand, describes the repeatability of the measurement, i.e. how consistent are your projections? Your arrows can hit close to the designated point each time, but they don’t have to hit the target. In 3D printing, this can be understood as reliability; does the 3D printer provide the expected results with every print? Generally speaking, “precision” measures the repeatability of the models created and their similarity to each other. 

The tolerance defines the amount of error when printing and how accurate the item should be. This value in turn, is determined by the designer (probably you) and the subsequent use of the model. The tolerance specifies the permissible deviations from the dimensions entered in the computer and is strictly dependent on the design. For example: if a part works in the arrangement of components, it will require a smaller tolerance than a part of a plastic housing, which is not a functional part.

If you’re looking for a low span tolerance, you’ll likely need high accuracy as well, so let’s say we’re measuring precision as shooting the bull’s eye. The rightmost shots at the target are considered imprecise and imprecise.

However, if your tolerance is quite wide, the shots on the next target (second from the right) may be fine. They are not as close together as on the left-hand dials, but if the acceptable precision range is ± 2.5 rings, it is within the specification. 

Generally speaking, achieving and maintaining tighter tolerances means higher production costs, but also ensures higher quality.

HOW CAN I REFER ACCURACY, PRECISION AND TOLERANCE TO 3D PRINTING?

When thinking about the accuracy and precision of 3D printing, there are many factors to consider, but the most important is to determine the needs of a specific project. For example, a precise but inaccurate 3D printer may be the best choice for some applications. Parts produced by a cheap FDM device will surely be less accurate, but for some applications it will be enough. For example a teacher, who is teaching students about 3D printing for the first time, may not require that the model exactly matches the student’s CAD design. On the other hand, knowing that the 3D printer will produce elements that must perform certain functions, we should invest in a device that meets more stringent standards. Functional models must perform as intended and their dimensions must fit within a certain tolerance range. This is crucial for the component’s successful cooperation.

Choosing a 3D printer, you should also match the technology to a specific application. Each type of device is compatible with different materials and offers various print quality. They also differ in applications, performance and speed, operating costs and much more…

HERE ARE FOUR IMPORTANT FACTORS AFFECTING THE ACCURACY AND PRECISION OF A 3D PRINTER

3D printing technology 

Each additive manufacturing technology has its advantages and disadvantages. In FDM, models are built layer by layer. Each layer creates an opportunity for divergence and affects the level of precision or repeatability in the manufacture of components. Another thing that determines the quality of FDM prints is the way of building the model. The filament is extruded through a nozzle of a certain diameter, so there is no way to get the very small complex details that other 3D printing processes can offer.

In SLA 3D printing, liquid resin is hardened by a high-precision laser to solidify each layer. This method produces much finer details and is more reliable than the FDM method. Summing up, SLA is characterized by a much higher precision and accuracy than 3D printing from thermoplastics. The same is true of SLS technology, which uses a precision laser to merge the powder, thus forming the model.

The specifications of the 3D printer themselves do not reflect the final dimensional accuracy and quality of the prints. One of the most common errors in interpreting accuracy is describing XY resolution as dimensional accuracy. For digital light processing (DLP) printers, the XY resolution is the size of the displayed pixels. Many 3D printer manufacturers use this display pixel size or XY resolution as the overall accuracy value, interpreting the projected pixel size of 75 microns, ensure the accuracy of the machine is ± 75 microns. However, this data does not affect the accuracy of the printed part. There are many other sources of error that affect accuracy; from the complexity of the components, to the calibration of the 3D printer, to the next two, which we’ll cover: materials and post-processing.

Materials

Accuracy may also vary depending on the materials used for printing and the mechanical properties of these materials, which can also affect the likelihood of warping the 3D print.

The choice of material should depend on the application. For example, in dental applications, accuracy is critical. However, if you’re printing a conceptual model, your goal may be to get an overview of the product, and accuracy won’t matter much.

Post-processing

The recommended practice is for the elements to be cured after 3D printing in SLA and DLP technologies by placing them in a UV platesetter. Light-curing after 3D printing may lead to shrinkage. When 3D printing from resin, this factor should also be considered. How? PreForm, Formlabs’ free file preparation software, automatically compensates for this shrinkage. As a result, after curing, the prints are dimensionally consistent with the original CAD designs.

3D printing as a complete development cycle

The print quality is influenced not only by the 3D printer and the material itself, but also by each preceding and following stage of the creation process. A number of factors affect the final appearance and durability of an element, ranging from software for preparing models for printing to tools for post-processing. It is worth ensuring the highest quality of work at all stages of creating an element in order to enjoy the 3D print as expected.

Tolerance 

As opposed to machining, where parts are gradually adapted to tighter tolerances, 3D printing involves one automated production step. The tolerance of the 3D printed part is determined by a number of parameters, both design, material and 3D printer. In the case of additive manufacturing, without making use of subtractive methods, the manipulation of tolerance is impossible.

3D printing is a great option if the component is geometrically complex, has a large number of undercut and complex surfaces, and an accuracy greater than ± 0.01 mm is not required. Tolerances beyond this value must be achieved by subtractive methods, for example by hand finishing or machining.

SLA has the highest tolerance among the 3D printing technologies available on the market. Comparing machining accuracy, SLA 3D printing tolerance is somewhere between standard machining and precision machining. Flexible SLA materials will have a wider tolerance zone than rigid ones. Components working in the systems must be designed with the appropriate tolerance and fit in order to reduce the time of final processing and facilitate their assembly, as well as reduce the cost of the material.

Having a “good” 3D printer is not enough. In addition to this, we must invest in appropriate materials designed for a given application, and consider a whole range of factors affecting the printout. Nevertheless, we should always consider whether our application requires high accuracy, tolerance and precision. Understanding these three terms, we should have no problem analyzing the quality of our prints and knowing how to optimize and improve the creation process.

Source: formlabs.com