How to Fix 3D Printing Parts That Are Not Fitting Together
Reading Time: 12 minutes 3D printed parts that are not fitting together can be frustrating, especially when they are required for some kind of functional design or aesthetic. This article will help you fix 3D printed parts that don’t fit together properly.
To fix 3D printed parts that are not fitting together, you want to fix any under or over extrusion by calibrating your e-steps setting, as well as your flow rate. After this, you can use some tolerance tests to see how well your 3D printer performs. Adjusting your horizontal expansion setting in Cura can help.
This is the basic answer but keep on reading for more detailed information to finally fix this issue.
How to Fix 3D Printing Parts Not Fitting Together
There are a few solutions to try if your 3D printing parts don’t fit together, and I will list them below:
- Calibrate the e-step per mm setting
- Calibrate your flow rate/extrusion multiplier
- Adjust your horizontal expansion setting
- Calibrate your tolerance & dimensional accuracy
- Increase the number of polygons on your model (resolution)
- Re-scale your model
- Use a lower layer height & higher line width
- Lower your printing temperature
- Change your slicing tolerance setting to “inclusive”
Proper calibration ensures that the printer extrudes the right amount of material to avoid gaps or lumps in your prints. If the parts don’t fit in, then most likely the printer is over extruding, whereas if you notice gaps between the parts, it means that the printer is under extruding.
I will explain how to resolve these issues in different ways.
1. Calibrate Your E-Steps Per mm Setting
E-Steps/mm is the setting that determines how many steps the extruder motor moves in order to move 1mm of filament.
Sometimes there can be a noticeable discrepancy between the amount of filament the printer says it will extrude and the amount of filament it actually extrudes. As discussed above, this can cause unwanted lumps or dips in your print, so it is something that should be corrected.
To calibrate the E-Steps/mm, you will need to first make sure that your filament is properly heated and flowing through the nozzle, then measure and mark 100mm of filament. Go to Move Axis > Extruder > Extrude 90mm.
That way, you will be able to measure how much of these 90mm the printer actually extrudes. Now you can calculate the E-Steps/mm needed and input the new value using the following formula:
Requested Extrude Amount / Measured Extrude Amount * Current E-Steps/mm = New E-Steps value
For example, if you requested to extrude 90mm & it actually extruded 80mm, and your current E-Steps value is 93, then you’d do the following:
90 / 80 * 93 = 104.63
Remember to “store settings” and redo the calibration to see if it extrudes the amount you actually tell the printer to.
Here is a video that offers a more in-depth explanation of the calibration process for the E-Steps/mm and flow rate (discussed below).
2. Calibrate Your Flow Rate/Extrusion Multiplier
Calibrating the flow rate is an effective way to fix inaccurate prints, as well as parts that stick together or don’t fit in with each other.
However, lowering the flow rate often results in gaps between the extruded lines in the model and, although it prioritizes overall accuracy, it compromises on structural integrity.
In case you do need accuracy, and you don’t think a slightly weaker print would be a problem, you can use this Part Fitting Calibration model or Part Calibration Collection, which have S-Plugs to test for dimensional accuracy and part fitting, as well as wall calibration models for multiple nozzle sizes.
For a standard 0.4mm nozzle, download the 0.4mm Thin Wall Calibration STL file from the description and try 3D printing that. You can then measure the wall with a pair of digital calipers and see how close it is to the 0.4mm value.
Something like the NEIKO Electronic Digital Calipers from Amazon is a good choice for this. It gives you precise readings with a resolution of 0.01mm and an accuracy of 0.02mm, being constructed out of finely polished stainless steel.
Then, you can calculate the necessary flow rate using the following formula:
Requested Line Width / Measured Line Width * Current Flow Rate
3. Adjust Your Horizontal Expansion Setting
Next, let’s discuss adjusting the horizontal expansion in the slicer. This refers to the amount of offset applied to all polygons in each layer, and adjusting the value of this feature could compensate for holes that are either too big or too small.
Here is a video that discusses horizontal expansion in more detail.
Sometimes, adjusting the horizontal expansion alone might not be enough to solve inaccurate 3D prints and generally Cura seems to be a better slicer to use this feature in.
One user mentioned that their print was fixed only after both calibrating their extrusion multiplier and adjusting the horizontal expansion, this is more of an additional step to take in case the previous calibrations did not solve the issue.
A usual horizontal expansion setting is around 0.2mm for making holes smaller, while a negative value of something like -0.2mm would make holes or similar features bigger.
4. Calibrate Your Tolerance & Dimensional Accuracy
Tolerance is particularly important regarding sliding parts.
If after calibrating your printer according to the guide above your parts still don’t fit together or cannot slide, you should consider leaving a small gap between the parts to compensate for any extra material or imperfections that may occur during printing.
A good example for when tolerance is important is when printing a screw and the part it is supposed to fit in. In this case, the socket should generally be 0.1mm wider than the screw to allow for a good fit, although this can depend on both the material and the printer you are using.
There are some explanatory videos and tolerance tests you can download and print from model libraries such as Thingiverse, here are a few:
Tolerance Test Allen Key
This test consists of 6 cylinders within their respective holes, each with a different clearance. This is to find the lowest clearance that your printer can print at without the parts getting stuck together; this test requires an Allen key.
Printer Tolerance Test
This one uses one test cylinder and one test cube for tolerance calibration; it has 11 different clearances, so it is able to provide a more accurate testing. The test geometries go in the printed slots and choosing the tolerance means choosing the slot in which the geometry can slide with some ease.
Customizable 3D Tolerance Test
This test uses cylinders as well and tests 7 tolerances. You will need a screwdriver to test whether the parts are free moving by inserting it in the screwdriver slot underneath.
Here is a video that discusses tolerance test prints (associated with the last print on the list):
Sometimes, when calibration doesn’t work and adjusting the tolerance is needed, it could mean that the models were originally designed for ABS and not PLA filament (that is, if you downloaded the models and didn’t make them yourself).
ABS tends to shrink when cooling down, so if the models were designed with this in mind, printing them using PLA might not result in a fit.
Check out my article about PLA, ABS & PETG Shrinkage Compensation in 3D Printing, teaching you how to deal with shrinkage.
5. Increase the Number of Polygons on Your Model (Resolution)
Another possibility, which depends on the type of CAD software you are using, is that your model is not detailed enough.
That means that in a program which uses a polygon-based modelling system, sometimes curves or spheres may be printed with some edges if there are too few polygons in your model. When you need curved sliding or fitting parts, this might be an issue.
Usually, the insufficient polygons are a result of a low resolution in the CAD software, which is then exported into the STL file. To change this, most CAD software allow you to adjust chord height and angular tolerance.
For a smooth surface and an accurate model, the chord height should be 1/20 of the layer height, and the tolerance should be 150.
Different software have different ways of numbering these, so sometimes the angular tolerance could be between 0 and 1 (case in which you would need 0). And sometimes, you may have an overall resolution setting rather than two separate layer height and tolerance ones.
There are different things to account for when modelling for part fitting or sliding, and you will often find that models online have them mentioned in the description.
For example, if you want to cover a hole in your print, consider adding a teardrop shape rather than a flat layer at the top, as due to material sag the latter will create a drop in the model instead of a flat surface.
These are also reasons why some models online might prove to be less accurate when used with a different material or a different printer than the one they were designed for.
6. Re-Scale your Model
Apart from accuracy of the shapes, make sure to always check the accuracy of the modelling dimensions. Make sure you are working on the same scale for all parts and try to resize the models in the CAD software or slicer if they don’t fit together.
Scaling your models could also be a quick solution to make them fit together, for example in the case of a simple geometry that has to fit inside a larger one.
However, this might not work for more complex shapes, in which case working with tolerance and offsetting the parts is a better solution.
This is an issue that people who download ready-made models face, and it is often caused by material shrinkage or expansion that is not taken into account when changing the filament type.
7. Use a Lower Layer Height & Higher Line Width
Another potential fix for parts that don’t fit together could be adjusting the line width and layer height.
Although slicers generally have a set value for this that should work with the respective 3D printer they are using, it is good to keep in mind that your line width can be adjusted above the nozzle’s diameter (but not too high) and the layer height lowered.
For example, for a 0.4mm nozzle, a standard layer height would 0.2mm. If you want to get better dimensional accuracy and good part fitting, it could be worth reducing your layer height to something like 0.16mm.
You can usually have a layer height anywhere between 25-75% of the nozzle diameter.
The line width is another setting that people have used to get better part fitting on their 3D prints. The default line width is usually the same as your nozzle diameter, so for a 0.4mm nozzle, you’d have a 0.4mm line width.
Some people have had better luck with a 0.5mm line width. The general suggestion amongst 3D printer users is that the line width should be between 100% & 120% of your nozzle diameter.
Both line width and layer height are important parameters that affect not only the accuracy of a print, but also its speed and strength, so based on the recommendations above try to find the settings that work best for your specific printer and model.
8. Lower Your Printing Temperature
As mentioned previously, getting parts to fit nicely is down to how accurate and precise your model is 3D printed compared to the CAD model, and temperature can play a big part in that.
One thing that people should try to do is to lower their printing temperature or calibrate it properly.
Using temperature that are higher than optimal can lead to the material sagging in some sections, especially in the case of overhangs or bridges, which can prevent the parts from fitting together.
Many people found this to be a good solution.
One user explained that this is because a certain combination of lowering the layer height, lowering the printing temperature and increasing the cooling results in the filament solidifying the exact way it is laid, leading to better dimensional accuracy and a better part fit.
I’d recommend calibrating your temperature by 3D printing a temperature tower in Cura. Check out the video below to learn how to do this the right way.
9. Change Your Slicing Tolerance Setting to “Inclusive”
This is an experimental setting present in Cura that some users have pointed out. It supposedly defines the slicing of sloped areas to allow for more printing accuracy. Here is some official information about it from the Ultimaker website.
This feature has three types of slicing (inclusive, middle and exclusive) and it is recommended to use the inclusive setting, since this will slice the layer at its outermost part and result in a model that is closest to the actual slice file.
How to Get 3D Printing Clearance for Sliding Parts
To get 3D printing clearance for sliding parts, ensure your 3D printer is calibrated with temperature, overhangs/bridges, and dimensional accuracy. Control vibrations by having your 3D printer on a stable surface, tightening up any loose parts, and even adding weight to the 3D printer.
It is especially important to ensure you have a proper calibration, an accurate model and sufficient tolerance when it comes to sliding parts. Tolerance is also referred to as clearance, and there are various ways to test this to find out what the necessary tolerance for your model is.
Generally, your clearance should be twice your layer height for a smooth slide. However, this should be adjusted according to your specific model. It can also be a good idea to use oils such as PTFE oil or even olive oil to help sliding parts move more freely.
I mentioned some tolerance tests earlier, but there are also different ones made more specifically for testing sliding tolerance that you can try out, such as these:
- Two Way Screw – an intricate and interesting two-way screw model; this will help you find proper clearance between the parts, as well as adjust the scale and a number of other settings; some users printed the screw at 95% of its size, while others scaled the nut up, at 105%.
If your extrusion level is good, the model should be able to slide and screw into the other one fairly smoothly. Once you get a good calibration on this by adjusting various settings, you should be able to 3D print good sliding prints.
- Dragon Egg Case – another fun design to try out and find the right printer settings with; you can check the strength of the screw as well, since this one is meant to function like a container.
How to Make 3D Printed Parts That Snap Together
To make 3D printed parts that snap together, you can download snap-fit models from Thingiverse and 3D print them on your machine. There are many types of snap fit designs that you can 3D print such as cantilever and annular. You can learn how to create your own in a CAD software.
Snap-fit parts can be very useful, although they do require printing accuracy to function properly. Here is an extensive guide to types of snap-fits and how to choose and improve them to suit the needs of your model.
On a basic level, the key to modelling accurate parts that fit with each other (depending on the type of snap-fit you want as well) is creating offset negatives, which can be done in various ways, depending on the software you are using.
Here is a video explaining the modelling process of a box and a lid using Fusion 360.
While Fusion 360 has an affinity for designing snap-fit parts, with a “negatives” approach you can use Onshape, Rhinoceros 3D or even TinkerCAD, as well as other software to design your desired models.
Not to forget there are a number of interesting ready-made snap-fit models on Thingiverse that you can download and use:
- Three Cube Gears – a puzzle-like snap-fit model, great to test your printer’s handling of more complex shapes; it requires some care when assembling due to some weaker pins, but it is a good starting point if you want to explore the possibilities of snap-fit prints.
- Malolo’s Screwless/Snap Fit Raspberry Pi 4 Case functional and well documented design to attach to your 3D printer; it is used as a case for the Raspberry Pi controller.
