BIG Structures Assembled From SMALL 3D Printed Connectors

In this Instructable I’m going to share my experience on how to design and manufacture structures that largely exceed 3D printers build size limitations by one or two orders of magnitude. Despite their fairly SMALL build volume, I would like to demonstrate with this project that a household 3D printers can take a central role when manufacturing BIG structures.

3D printer prices have significantly dropped during past decade, so 3D printing is much more common nowadays and should not limit creativity to just the size of objects you can build out of it. As an alternative to welding steel tubes the approach described here consists in fitting together some long and fairly simple parts with multiple custom-made connectors.

This fairly easy and inexpensive light frame building technique is exemplified in this Instructable through the design and manufacture of a custom outdoor garden feature. I made this for my own house, to create a shaded green walk where we could hang out during the summer season. The plan was to grow climbing plants onto the structure and not to glaze with greenhouse film which would have required quite a different structural capability when it comes to wind and snow loads. I printed a bunch of SMALL plastic connectors and achieved a 9m long and 2.7m wide (30 x 9 ft) structure which is a thousand times BIGGER than my 3D printer footprint (diameter 150mm or 5.9 in) !

• Extruded profiles: depending on the size of your project, you may have to source a hundred meters of aluminium tubing, which usually makes this an expensive option but luckily I managed to get hold of a bunch of scrap extruded tubes.
• Access to a FDM 3D printer
• 1-2 Kg ABS filament

When designing an arch you essentially need to set a radius and compute the corresponding angle between vertices. Making the maths require a little bit of trigonometry skills but you can refer to the diagram above.

In my case the width between walls couldn’t be changed and all I could play with was the overhead distance referenced t.

Once you have obtained the strut length, it is pretty straightforward to compute the total length of your structure. At this point it could also be interesting to define the cut and strut length between adjacent sectors in a way that minimize scrap.

In total, four designs are required to build the metal frame I have chosen, see the attachments below. The different variants are obtained by duplicating in rotation a master-fitting that slides into the chosen tube. The angle of rotation is a combination of (0,90,180°) and (alpha/2, omega/2).

Tinkercad can be used for the CAD work as long as the tubes profile is not too complicated. For this project I preferred using OpenSCAD to design the connectors mainly because of its awesome hull() function that connects the different struts together very nicely without hard edges. And with a little ingenuity, hull() can provide a nearly complete connector modelling strategy all on its own. Hull() takes any number of 3D objects and builds their convex hull; the action is like wrapping cling film tightly around the shapes and solidifying the result. If you use OpenSCAD for your creation all you need is figuring out the angles between the connectors branches and using hull() to connect up straight cylinders carefully oriented in 3D space. In Fusion 360 you would use the Loft function to achieve a similar result.

The simple script below is all you need to perfectly connect the struts with each other. In this example two opposite struts are positionned at flat and the other two seat at a 17 degree angle.

//Hull demo for tube connectors - openSCAD script
hull()
for (i=[0:1:3]){ //loop on 4 struts
   rotate([0,i%2==1?17:0,i*90]) //tilt by an angle (alpha/2)=17° when loop counter is even 
   translate([19,0,0]) rotate([0,90,0]) //tubes extremities positioned 20mm from the center
   cylinder(d=24,h=2,center=true,$fn=120); // precursor cylinder in contact with each strut
}

In a similar way, you then have to make a loop to position the extension that fits into the tube. I decided to leave almost no-gap for the tubes to being firmly secured without risking damaging the plastic connector. It is good practice to add a chamfer to help inserting the tube.

Next, open your favorite slicer software that often comes with a 3D printer. The main settings you would need to specify are the following: 0.2 mm layer height, four perimeters, and 30% honeycomb infill. Supports will have to be generated to keep the connector well bonded to the heated bed.

After the usual trial and error phase, I was able to fine tune the settings to make the print successful and get the desired strength. It took me a dozen of iterations (mostly on supports settings and contour speed) to get to an acceptable result that passed a small load bend test in a vice. At that point I started series production to get the required number of the different elements. You will have to print spares as some might break during assembly.

Having a second person on hand for assemnbling the structure was critically important so that one person could hold the piece(s) in position on one side while the other slide in the other side. We started preassembling the few pieces of each sector at a time and lifting them up together so that we didn't have to do so much work up in the air.

The forces that are exerted on the connectors during assembly should not be underestimated and there is a high risk of breaking parts. If you try to bend one end of a tube while the other end is already connected to a connector, you've essentially created a giant lever. A lever that can easily break the arms of the connecting pieces as shown below. This approach was used to stress test some key elements of the structure instead of going through rigorous static modelling / sizing. However when the arch is firmly attached to both ends, bending efforts are limited as long as nobody takes this structure as an overhead fitness ladder!

All in all I'm very happy with the end result though, I received a lot of compliments about it and it serve its purpose perfectly since almost a year now, having experienced large temperature variations, winds and heavy rain. Eventually the structure will be filled with loads of leaves.

If you like the idea of building BIG structures with a SMALL printer, I hope this Instructable gave you some fresh ideas for you to improve upon my work in your own projects.