Introduction: Print-in-Place Spring Loaded Box

Hi! My name is SunShine, and in this Instructables I will show you how to use Fusion 360 as a design tool to create Print in Place designs for FDM-3D-printers, and share some of my tricks that I use to make a working design. Specifically, in this Instructables I will show you how to design a spring-loaded box, but the main focus of this instructables will be to show you the principles that can be applied to other designs!

I made a short video on this project:

(PS. If you feel like being lazy, and just want a print in place box, I have released the STL’s for it so you can simply print it without having to put in the design work, but that’s no-fun, is it? (links at the end))

What is a print in place design?

Print in place means that an assembly is printed without the need for any further manual assembly, and one can start using the part right away. The example that I will show you today is a spring-loaded box; this box could have easily been designed with multiple parts and assembled afterwards. However, today we will learn how you can put in a little extra effort in the design stage and save on time and effort in the production stage by printing the entire assembly fully assembled and ready for use!


What you will need to follow this guide:

  • Some experience with 3D-printing
  • Some experience designing in Fusion 360 (you should be comfortable with creating parts and assemblies)
  • Fusion 360 (Home/Student-license is free)
  • InkScape (optional and free)

Step 1: Get an Idea!

This is the most difficult step if you ask me. Finding something you would like to design Print in place, but not having it be too complex is a difficult task (it’s difficult to design a Curta-mechanical-calculator to be print in place), but here are some criteria I like to consider when thinking about things to design:

Does it have 2-4 moving parts?

Could it be designed without supports?

Does it look cool?

Could one simplify existing designs?

Can you design the mechanism in a single plane? (usually the easiest to design it to lay flat on the print bed.)

What mechanisms/structures do you want to use? (Bridging, overhangs, threads, gears, springs, ect.)

Step 2: Plan Your Design!

Get a pen and paper and start trying to visualize the assembly you want to create, it does not have to be a technical drawing, just a sketch. Here you have to estimate how large you want stuff, and here you will also plan what types of mechanisms you want to use, are there any parts you can combine with the help of 3D-printing?

When I designed the spring-loaded box, this was my plan that I sketched. (see picture)

While it is not pretty, you can already see that I decided to add a half-gear to the lid-design to be able to have a square box without a big spiral spring hanging out over the corner. We can also see that I have decided to combine a gear with a spiral-spring to reduce the number of parts. (You can see that I also considered an air dampener initially, however, I decided to scrap that idea, since after the initial tests i found it not to be necessary.)

I have not seen a gear-spring combination like this before, so I decided to name it a SunShine-Gear! (sun-gear was already taken). The other gear we will call the “lid- gear” since it will be attached to the lid.

Step 3: Design Your Mechanism

It’s easy to fall into the trap of trying to design the easy part first, and then end up having to re-do everything because the mechanism you had planned needed some changes to work properly, and now it does not fit with the rest. So, design the part that is going to change the most first!

The spring-loaded box also had its mechanism designed first. Afterwards it was tested and tuned until it would print easily, and function as intended. (This functional test was later included in the released files as the test piece, so that people could try to print it before having to commit to printing the entire box.)

The most important thing when designing a mechanism is to keep the clearances consistent. If someone wants to print your design on a printer that struggles with the clearance-tolerances you have selected, then they can simply compensate for that by adjusting the "horizontal expansion compensation" in their slicer. However, if you have different tolerances at different places, they might end up with some clearances becoming too lose, while others still being stuck.

I would recommend 0.3mm as a clearance between walls, from my experience it seems to be the tightest clearance most 3D-printers can do without sticking together.

The result of the following steps should be a mechanical test piece shown in the pictures.

The following steps describe how the mechanism was designed for the Spring-loaded box:

Step 4: Designing the Gears

The first step was to design the gears is to choose how many teeth and module they should have. Luckily for us, its really easy. From our previous sketches, we can estimate how large we want the box (around 70x70x70mm) and we know that the lid gear has to be in the corner, and the SunShine-Gear around the centre, so knowing that, we just assume that we want around 40mm between our two gears. (see, starting out with the mechanism is already paying out! You don’t have to worry about exact distances yet!)

The calculation for the size of a spur gear is: Module*number of teeth=PCD. (Pitch Circle Diameter)

(see picture showing the PCD of both gears)

So, if we want 40mm between our 2 gear axles, we could either:

  • - Give both gears 20 teeth with a module of 2mm,
  • - or what I ended up with; 24 Teeth on the SunShine-Gear, and 16 on the Lid-Gear, both with a module of 2mm.

In my experience a module of 2mm is the smallest you can print easily and reliably on common household 3d-printers.

Another important parameter for print in place designs is the Backlash! We will ad 0.6mm to get 0.3 mm of clearance when printing.

Now that we know the parameters of our gears, how do we design them?

We could either look up in our engineering books how to properly design a gear with all its parameters, or we could do the easy thing and use the “spur gear script” that is included in fusion 360!

(Tools>Add-ins>Scripts and add-ins>Spur Gear>Run)

With this you can easily create spur gears very quickly! (If you would rather like to create Helical gears like I used in my design, you can download the “helical gear creation” plugin here created by community member Ross Korsky.)

Now that we have 2 separate parts with gears, we can put them in an assembly and check if everything lines up the way we wanted, if so, we are ready to design the next part of the mechanism!

Step 5: Designing the SunShine-Gear-spring

This part is also quite easy.

First hollow out the gear you want to place the spring in with a simple circular extrude-cut and then with the help of the “Coil” function under “Create” you can simply make a spiral spring! After creating a spring with a square cross section, you can use the extrude tool to give the spring the desired height, by extruding the top and or bottom face.

At this point you might wonder how to decide on the spring parameters (thickness, no. rotations, corssection, ect.). For not so critical constructions like this, using LAR- principles (Looks About Right) is the easiest and fastest way to design a spring. But if you are having issues imagining what size you want, start out with a 1.2mm cross section and 3 rotations, and see if it is too stiff, or if it has too much/too little travel and adjust accordingly.

Rule of thumb: The bigger the cross section the stiffer the spring-rate. While more rotations reduce the spring rate but give you more travel.

Step 6: Connect the Lines! Uhmm, I Mean Parts, Connect the Parts!

To make a print in place mechanism, you need to connect the parts in a way that allows them to move when the print is finished.

Let’s start with the SunShine-gear, since strictly speaking, it will be connected with the main part of the body. First, I made a wall partially around the gear (keeping the 0.3mm clearance all the way) and connected it to the “axle” of the SunShine-gear with two “bridges”. There are 2 things to take note of here. First, the “wall” around the gear has 2 functions, to build the bridges on, but also to hold the sunshine gear in place, since the spring that the “axle” of the gear is attached to, could move away from the Lid-gear. The second ting to note is that we chose to bridge to the axle and not just filled in the entire area over the SunShine-gear, this is because the way slicers and 3d printers work, it will create a lot cleaner bridges. (I will go more in depth on how to build large overhangs/bridges without supports later in this write up.)

Connecting the Lid-gear is a little trickier since we must limit its movement in all directions, except for the rotation around its axis. To solve this issue, I decided to make a double conical axle, and a double conical hole in the Lid-gear. This way the gear can spin freely, but not move around axially and radially. This is possible to 3d print since there are no overhangs over 45 degrees! (see pictures)

Notice that the lower cone is a steeper 50 Degrees, rather than the traditional 45. This, and the rather aggressive chamfer on the bottom is to avoid that the “elephant-foot” effect whcich might fuse the axle and the gear together if not countered. (see pictures)

Step 7: Print and Repeat!

Now you should have a design (that on paper) should work. It’s time to put it to the test! Failure is the best teacher, and usually the first few versions need to be revised. Every design has different challenges and by extension different solutions. Sometimes it even worth the time investment to make a model where the area with the issue is isolated. This allows for more testing and tuning with less print time.

With the box design I had the issues with the axle and the lid gear sticking together, I decided to make an isolated axle and “sleeve” to tweak the parameters until it worked the way I wanted. The aggressive chamfer at the bottom, and the 50 degrees on the lower cone got “developed” during this step. (see picture)

Step 8: Build Around Your Design

Now that you have a working mechanism, it’s time to make something useful out of it! In my case it’s a box, so the rest of the design does not have to be too complex or difficult to design. However, I wanted to show off some modelling tools that don’t see being used too much, and that could come in very useful a lot of the times.

So the next few steps shows some design features that are not straight forward:

Step 9: The Huge Suportless “roof” on the Top of the Box

If you print the box sideways, as intended, you will have a large flat overhang on the top.

This is something that usually is solved by printing supports, but in our case, if we would print this box with supports, we would risk the slicer generating supports in the gears that would be difficult to remove. Supports are also a waste of materials, so designing to avoid it would be the ideal solution.

Bridging is an important tool, that we are going to use. However, as everyone knows, the longer the bridge, the more it sags. If we would simply try to bridge the gap between the already printed walls, we would end up with something resembling a fishing net. To avoid that. We use 2 tricks.

-The first one is to not bridge across right away, but to just bridge to the wall next to it. and work our way up in 0.2mm segments at a time until the remaining distance is bridgeable (see picture). But to use this, we need to have 4 walls to build between, so we would have to bridge straight across at least once, which leads us right to my second trick:

-The second trick is to build up a bridge in a V shape, this way, you will only end up with only 2-3 sagging bridging lines. It works like this: after the initial bridges (which will sag), the bridges will start to build on top of the initial bridges and after only a few layers you will get perfectly straight bridges! (and the few sagging bridge lines are easy to cut away afterwards.)

Step 10: Using Loft’s

Using lofts is a powerful tool to connect two surfaces with each other with a solid body. It can also be used to bend corners or fill in “those gaps” that you know you can physically fill but have a hard time filling with extrudes and revolves.

It was with the help of a loft that I managed to create the upper hinge. I did this by creating a construction-plane parallel to the build plate and sketched the outer diameter of the hinge. Then simply a loft operation with tangential end caps to get that smooth, almost organic, look (Tangential end-caps are called direction in fusion 360). Afterwards its easy to adjust the construction plane height to adjust the overhang angles. (see pictures)

Step 11: Importing SVG’s

While not integral to the Print in place process, sometimes its nice to add a little flair and personality to your designs. Importing SVG’s allows you to add a simple image or graphic to your design. I usually add my logo on my designs.

Depending on the layout (and source) of your picture you want to use, creating an .svg file can have many different methods. I use Inkscape, a free Scalable Vector graphics (SVG) editor/creator to import a .jpeg file and use the “trace bitmap” function to create vector lines out of the image. (I used the Color quantization-mode, but this is very specific to the type of image you want to import.)

Once that is done, one can easily import it into fusion 360 using the “import” function, and at that point, it basically functions as a sketch witch you can extrude and add to your model.

Step 12: Done!

Congrats! You have now finished your design! Give it a test print, and if everything works as intended, release it!

Thank you for your attention, and feel free to reach out for questions and comments!

This was my first Instructables and all feedback is welcome! I included some pictures of my previous projects, let me know if something seems interesting and you would like me to write an instructables about it.

You can find the STL's for the print in place box here:

You can reach me and find my work on the following sites:

3D Printed Contest

Judges Prize in the
3D Printed Contest