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I've always been wondering about the actual accuracy of 3D printing devices. When looking for the perfect machine to buy, I looked at the speed, price, filaments supported etc, but also accuracy. I once asked somebody who could give me some advice on what to look at.

One of the things I was told about was that many printers don't necessarily have that crazy precision of 0.05 mm (50 micron). Another person told me something different - he said most of those printers actually were capable of putting out 50 micron layer height. How is it really?

Another thing is that the official slicers for those machines also claim that this precision is real, for instance the PrusaSlicer v2.0.

There are many high-end, very expensive machines and even they sometimes claim their resolution is worse than 50 microns.

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    $\begingroup$ Ultimaker has a 20-60 micron resolution for the Z layer height depending on the used nozzle size, I've used these small layer sizes a couple of times, products come out amazing, almost looks like an injected part but takes really long to print. $\endgroup$ – 0scar Jun 24 at 8:56
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    $\begingroup$ It's worth noting the difference between precision and accuracy. Being able to move in ~50 micron increments doesn't necessarily mean that it will always end up within 50 microns of where it's supposed to be. If it advertises 50 micron precision but the steps are actually 48 microns on average, that means that trying to move 10cm will result in moving only 9.6cm—definitely not accurate. $\endgroup$ – The Guy with The Hat Jun 24 at 20:56
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There's not a simple answer to this question, or if there is, it's "no". However the situation is a lot more complicated. When printer specs cite accuracy like this, they're usually basing the claim on the nominal size of the smallest movements on each axis by one "microstep" of the stepper motors. There's a great article on Hackaday explaining the how this affects accuracy: How Accurate is Microstepping Really.

At the mechanical positioning level - putting the print head where it needs to be to extrude the material with the desired accuracy - you have at least these factors limiting your accuracy:

  • Microsteps are generally spaced roughly monotonically between whole steps, but do not necessarily divide the whole step into even portions. How well they do is a matter of the stepper drivers your printer's controller board uses. Generally, microsteps are 1/16 of a step (although there are drivers with 1/8, 1/32, or even 1/256, maybe others too), so if you see a rated accuracy of 0.05 mm, a whole step, which might be the minimum you can get reliable accuracy from, is likely 0.8 mm.

  • Stepper motors are deflected slightly - up to 2 whole steps but less than one step is more likely if they're not overloaded - under load. So are belts. How much this affects you depends on the design of the printer and how much mass each axis is moving. Direct drive extruders are much worse in this regard. Delta printers are probably best in it.

These can be mitigated somewhat, with tradeoffs, by using stepper motors with more steps per rotation, better stepper driver chips, reduction with gears, etc.

On top of that, you also have extrusion and properties of the print material limiting your accuracy:

  • The extruder motor is subject to the same accuracy issues as the positioning ones. If you extrude too much or too little material anywhere, you'll necessarily have accuracy issues. You can compute them based on the cross-sectional area of filament, size of extruder gear, extruder motor step and microstep size, etc.

  • If the filament diameter is not perfectly consistent, you'll also extrude too much or too little material.

  • If material is not cooled or kept warm appropriately as it's extruded (this varies by material), it will sag, warp, or curl, ending up in a different place from where you wanted it.

  • The more you vary the ratio between nozzle/extrusion width and layer height from an ideal ratio, the more the shape of the extruded material paths will differ from the model you're trying to print. With thick layers especially they'll become rounded rather than near-flat along the walls.

In theory, a lot of these issues probably could be mitigated a lot better than they are now just by better slicing - the logic that happens on a computer to convert the original 3D model into instructions for where to extrude material.

With all that said, you can get pretty amazing accuracy still, especially with a good or well-tuned so-so printer. On my cheap Ender 3, after dealing with some issues now and then that made glaringly obvious problems, I can get dimensional accuracy within 0.1 mm in the X and Y directions, at least for some models. So I think it's very plausible that a better, or better-tuned, printer could get 0.05 mm accuracy.

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    $\begingroup$ This only really addresses the precision of positioning, not of the printed part. $\endgroup$ – Sean Houlihane Jun 24 at 8:19
  • $\begingroup$ @SeanHoulihane The second half of the answer explicitly covers the accuracy of printing, doesn't it? And it's trivially obvious (and axiomatic from control engineering) that it is not possible to print more accurately than you can position. So the accuracy of movement is the limit for printing precision - printing can only ever be worse than that, not better. $\endgroup$ – Graham Jun 24 at 8:59
  • $\begingroup$ Sorry, yes, you covered it (and the relevant factors), but I think your 'trivially obvious' is not at all obvious to people who want to ask about precision - and with today's technology it seems positioning is one of the smaller sources of end result error - making it a bad metric. $\endgroup$ – Sean Houlihane Jun 24 at 9:09
  • $\begingroup$ Then then is the tolerance of the step angle. a 5% tolerance on a motor with 1.8° per step means that the rotor can move anywhere between 1.71° and 1.89° $\endgroup$ – user77232 Jun 24 at 14:33
  • $\begingroup$ Nominal resolution of positioning being a "bad metric" is largely what I was getting at in this answer - it doesn't tell you whether positioning is actually that precise or accurate, nor whether other parts of printing will limit the accuracy in much larger ways. $\endgroup$ – R.. Jun 24 at 14:35
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One of the things I was told about was that many printers don't necessarily have that crazy precision of 0.05 mm (50 micron). Another person told me something different - he said most of those printers actually were capable of putting out 50 micron layer height. How is it really?

Both things you've read are completely correct.

Most printers are capable of 50 micron layer heights. However, layer height does not equal "accuracy" or "precision". The layer height specification is a useless marketing term that you should ignore; layer height is to 3D printers what dynamic contrast is to monitors.

All FDM printers are inherently quite bad at producing parts with tight tolerances. The filament extrusion process introduces lots of variables that are hard to control: the diameter of the filament may vary, there is a delay between feeding filament into to the extruder and it coming out, and the gooey filament that comes out of the extruder behaves in unpredictable ways.

Nobody has figured out how to quantify "accuracy" for 3D printers in a way that correlates with the quality of the finished parts. It is impossible to tell which printer produces "better" or more accurate parts from the specification sheet of a printer.

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  • $\begingroup$ I'd hazard a guess manufacturers can't figure out how to quantify the accuracy of their 3D printers in the same way as Google can't figure out a search of ad-free apps in Google Play. Figuring that out simply doesn't serve their interests. $\endgroup$ – Dmitry Grigoryev Jun 25 at 7:25
  • $\begingroup$ @DmitryGrigoryev At least one manufacturer should be interested in figuring that out (i.e., the manufacturer that makes the most accurate printers). $\endgroup$ – Tom van der Zanden Jun 25 at 12:51
  • $\begingroup$ Even if you make the most accurate printers, listing true accuracy in your printer's specs while everyone else lists resolution labelled as "accuracy" will put you at a disadvantage. $\endgroup$ – Dmitry Grigoryev Jun 25 at 13:03
  • $\begingroup$ @DmitryGrigoryev that can be compensated for by listing both in comparison charts. With good research it's always possible to make good marketing material if you have a real measurable advantage. $\endgroup$ – Rick Jun 26 at 14:05
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A resolution (sometimes called "accuracy" for marketing purposes) of 0.05 mm means that if you produce a bunch of 10 mm dice and a bunch of 10.05 mm dice, then the 10.05 mm ones will be statistically larger. Note that dice don't have to actually be anywhere near 10 mm, nor does a random die from the 10.05 mm pile have to be larger than a random die from the 10 mm pile.

A repeatability (aka "repeat accuracy") of 0.05 mm means that in the experiment above every die from the 10.05 mm pile have to be larger than every die from the 10 mm pile. Note that your dice still don't have to be anywhere near actual 10 mm.

A precision (aka trueness) of 0.05 mm means that in the experiment above the average size of a 10 mm die should be within 9.975..10.025 mm. Note that individual dice don't have to be within that interval.

Finally, the general accuracy (as defined in ISO 5725) of 0.05 mm means that every 10 mm die should be within 9.975..10.025 mm.

To sum it up, the statement from your question is true for the "commercial accuracy", but not for the general definition of accuracy. For example here is an article comparing accuracy of 3D printers in dental applications (so we're talking high-end machines), with average accuracy ranging from 0,05 to 0,1 mm and absolute accuracy in range of 0,11 to 0,17 mm.

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    $\begingroup$ So you addressed the words in the question, instead of the mechanics of the 3d printer. Brilliant! $\endgroup$ – user77232 Jun 24 at 14:28
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Tom's answer is correct, most printers will operate properly with a layer height resolution of 50 micron, using a 0.4mm nozzle. The resulting surface finish will probably be better than at a coarser layer setting, and at finer layer heights the print quality is likely to deteriorate.

Layer height is also likely to be fairly well defined (except for the first layers, overhangs, warping, etc), but this depends on the printer geometry.

The detail which is not so well controlled (or measured) is the squish/stretch of the molten plastic as it is extruded. This can have a significant impact on the localised surface finish (as well as dimensions of things like internal diameters).

Accuracy is maybe best assessed by the results of printing various types of test part than by a simple numeric parameter.

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