What Is Tolerance in 3D Printing?

When most people think of tolerance, what comes to mind is the ability to accept or tolerate somebody or something. However, there is another meaning when it comes to manufacturing and instrumentation, where tolerance is defined as the limit of allowable error. This understanding of tolerance is also important when we speak about 3D printing.

But what exactly does tolerance mean in the context of additive manufacturing? Well, to understand that, it is first necessary to delve into two other important characteristics of successful 3D prints: accuracy and precision.

Although these can be confused, they have very specific meanings in additive manufacturing. Accuracy, for example, is how close a measurement is to true value. A common example used to study it is a bullseye, for accuracy it would be hitting it. Or in other words, how the dimensions designed in CAD software are then equal to the final part.

Precision on the other hand measures repeatability. In other words, the consistency of shots at the target. This translates to 3D printing through parts that are repeatable or able to be made the same way consistently on the same machine. Both are necessary for good quality parts.

Tolerance meanwhile impacts both accuracy and precision for 3D printed parts. But how can a user optimize settings to take tolerances into account? What determines these variations? We took a closer look in the following guide.

Factors Influencing Tolerance

You have almost certainly seen tolerances represented in 3D printing before. When discussing resolution, for example, there will always be ± followed by a much smaller number. This is the tolerance, or the amount that the resolution (think of it as vertical, in other words the layer height) could differ from the promised value.

This affects both accuracy and precision. Accuracy because tolerance will refer to how much the 3D printed part deviates from the 3D model, and precision because tolerances can make it so that each part made will slightly differ. Essentially, it is the amount of random deviation or variation permitted for a given dimension to ensure that the part will still perform as expected.

These small variations come about in various ways, starting with the 3D printing process. Fused deposition modeling, one of the more accessible and thus popular 3D printing techniques, will have a tolerance of about ±0.5% (so about ±0.5%mm per 100mm). Stereolithography, also a popular 3D printing method, is renowned for its accuracy and its tolerance is lower at ±0.2%.

But even more industrial 3D printing technologies have a specific tolerance. HP’s multi jet fusion has a tolerance of about ±0.3%, the same as SLS which also involves polymer powders. Direct Metal Laser Sintering has a tolerance of ±0.2% (translating to around .1 to .2mm per 100mm). However, PolyJet probably has the best as it is a very fine, detailed and accurate process. Here, tolerance is around ±0.05-0.1 mm for 100 mm.

Then, of course, each 3D printer will also have its own tolerance, seen for example through things like resolution. A good rule of thumb is that more basic, cheaper 3D printers will often have tolerances that are a lot higher, whereas more industrial models will have lower ones. And user behavior also plays a role. That is because some of the biggest factors influencing tolerance are material shrinkage, layer thickness, minimum feature size and build size. All of these must be considered in a build if deviation needs to be minimized.

Material shrinkage is a factor of both the printer and the material. Whenever heat is applied, there is shrinkage and the amount will differ depending not only on the manufacturer but also on the material itself. Take filaments, for example. Depending on the material and the size of the filament, the tolerance could be between 0.03 and 0.06 (filaments of 2.85mm tend to have a higher tolerance as compared to 1.75mm).

Furthermore, rapid changes in temperatures can also lead to uneven shrinkage for issues like warping. This is why many 3D printers working with higher-performing materials often have a heated chamber, to help keep temperature as steady as possible. Uneven cooling and stresses during the build can also cause variations in tolerance.

Then there is layer thickness and minimum feature size, both important when determining resolution. Layer thickness is resolution on the z-axis and differences between the model and the final part arise from issues with calibration and poor sticking between layers. Here variations can occur based on the capabilities of the printer but also from user error.

Finally, post-processing can impact tolerances. This is because processes like smoothing, sanding, and powder or support structure removal, among others, can impact the dimensions of the part. All these factors must be considered when it comes to tolerance and ensuring that the final part does not deviate too much from the design.

How to Improve Prints

As you may expect, there is not one surefire way to make 3D prints “tolerance proof.” Tolerance will be a factor no matter what you are printing, and what instead is most important is knowing what the tolerance is and how to account for it. Thankfully, there are a few different ways to do this.

First, and what is most recommended, is printing a calibration model. You can find and download these online and they will help you to understand the tolerances specific to your 3D printer as well as a material. Once the part is printed, it is necessary to measure the dimensions of the parts precisely in order to determine the variance. At the same time, if you are looking to optimize this, you can work on testing aspects like layer height, print speed, nozzle temperature and infill percentage and fine-tune them based on your needs, helping to improve accuracy and minimize deviations.

Another important consideration is printer maintenance. Regular maintenance including print bed cleaning, tightening of screws and lubrication of moving parts will ensure that the tolerances will not greatly change from one print to another. A level 3D printer and properly extruder will also improve tolerances as well as accuracy when 3D printing.

If you take these things into account, even if it is not perfect, you can get a good idea of the tolerances that you can expect when printing. This will allow you to make the necessary adjustments in design. However, it must be stressed that one of the most important factors in all of this, especially when tight fits might be necessary, is testing to ensure that you will get the expected results.

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