This tutorial is about designing and finally making an electrical junction box cap.

A few weeks ago, I installed a lamp in the storage room of our house to make it easier to find things here. I also made sure that the whole thing can be controlled via Home Assistant and installed a SONOFF ZBMINI ZigBee DIY Smart Switch. When inspecting the existing power supply line, I noticed that the cover was missing from an electrical junction box.

Since I own at least one 3D printer (Bambu A1 + Prusa i3 Mk2.5 MMA) and a junction box without cover is quite dangerous, I simply designed and printed a new cap.

What follows are the steps I took to do so.

Modelling the electrical junction box cap in Fusion360

In summary, this is how I proceeded in Fusion360, to create the cap:

1. Measuring the junction box dimensions. I used a caliper to obtain relatively accurate measurements.
   To determine the radius of the corners I used a printed fillet gauge.

2. Create a Sketch for the cap:
   • I used a square for the inner cap dimensions.
     
     
   • The I used the fillet tool to create the inner rounded corners with the determined radius of 6mm.
     
     
   • The offset tool was utilized to create a clone of the rounded square with an offset of -2mm, to generate the caps outer walls.
     
     
3. First I extruded the wall parts of the sketch. Because I forgot to measure the depth of the cap before starting to model it, I had to estimate it at 10 mm. :-/
   
   
4. Afterwards I extruded the whole sketch (wall + inner part) for the bottom of the cap (-2 mm).
   
   
5. Furthermore I created a fillet for all bottom edges (2 mm).
   
   
6. As shown in the picture below, the junction box has these small retaining ribs on its side walls.
   
   
   Therefore, I designed corresponding counterparts for the cap model.
   • I created another sketch, which I mapped on one of the inner side walls:
     
     
   • Afterwards I extruded this sketch by 1mm to create a rib
     
     
   • Then I used the fillet tool again to round up the edges of the retaining rib
     
     
   • I repeated the previous three steps with the difference that I copied all the contents of the first sketch into each new sketch and, if necessary, moved them to the correct position.
     Afterwards all four inner walls had a retaining rib.
     
     
7. La Fin

Models of the junction box cap

Here you can find a preview of all involved 3D models. By the way, the rendering is done by babylon.js.
(Babylon Viewer).

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Print Settings

To print the cap with Bambu Studio slicer, I used the following settings:

Printer	Bambu Lab A1 + AMS Lite
Nozzle	0.6 Hardened Steel
Print Bed	Textured PEI Plate
Material	Anycubic PLA
Color	White
Profil	0.24 mm Optimal @BBL A1 0.6 nozzle

Result Gallery

Publications of the junction box cap model:

• https://makerworld.com/de/models/1135088#profileId-1136132
• https://www.thingiverse.com/thing:6957301
• https://www.printables.com/model/1201459-junction-box-cap

Downloads

• Fusion360 project file (f3d)
• Model file (stl)
• Print profile (3mf)
• Complete project (zip)
• Complete project (7z)

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