One advantage of 3D printing is that it doesn't matter how complex the 3D printed object is. Elaborate and sometimes fully-assembled mechanical components can be printed in one piece. Beyond just being incredibly satisfying to pull a fully-assembled and movable part off of the 3D print bed, there are also plenty of advantages to designing the mechanical components into the print. This article focuses on hinges and pins and is meant to help with design considerations and inspire new design ideas.

Hinges:

Hinges are used almost everywhere moving parts are present. They are helpful for creating basic movable components, whether they are part of a robotic arm, rotating gears, or a closing lid. There are many types of hinge designs, but for the purpose of this article, I'll describe linear, rotational and combination hinges

1. Linear Hinges

Linear hinges are the most common type of hinge. You've probably seen them on doors, lids, the knees/elbows of robots, and so on. They are used in applications that require opening and closing or forward and backward movement.

Their design consists of pins that sit in slots and various shapes that pivot around those pins. The pins themselves need to be large enough to handle the forces on the hinge, yet loose enough to enable movement. There are a few different types of basic linear pin designs:

  • Pin through the object: Difficult to 3D print because the pin goes through the entire object. For more strength, use a metal pin:
  • Short nub pins: Recommended for 3D printing. Use 45-degree pins to avoid overhangs.
  • Living (flexible) Hinge: Orientation should print with layers that are perpendicular to bend angle. Not for ideal for hinges are used often (unless using a flexible material).  Strong materials like nylon, or certain patterns that enable more movement in the flexing can extend the life of this type of hinge.
  • Additional Hardware: Depending on the application, sometimes using basic purchased hardware is the best. Combinations of screws and nuts can be used to create pins, metal hinges can also be used to connect 3D printed parts, etc.  

Pre-Assembled vs Assembled After:

Hinges can be printed in separate pieces and assembled after printing or pre-assembled. In each case, they require different design considerations--particularly in the pin design.

Assembled after printing
:
In the case of design applications where the hinges need to be designed as separate pieces, it's best to design a guide track to help the pin slide into place. The track should be designed to be tight enough so the pin doesn't slip out of the slot, yet have enough of a gap for the pin to slide in once (e.g. a tolerance of ~0.3mm). The pin should fit in the slot with enough space to freely rotate (e.g. a tolerance of ~0.5-0.7mm - see more about tolerances in this article).  

In the above case, the hinge has cylindrical nubs to enable it to slide across the track, which has narrow walls that flex slightly. However, in certain printing orientations, cylinders are printed with overhang, and/or bulges which can cause the pin to fit incorrectly or to break. So in most cases, it's better to design the pins with 45-degree angles (see more about overhang and print orientation here).

Pre-Assembled:
When designing pre-assembled hinges, it's important to include 45-degree angles to avoid surface quality issues, as well as a large enough gap for smooth motion (~0.5mm tolerance should be fine for most printers). 

2. Rotational Hinges:

Rotational hinges are helpful when two parts or shafts need to rotate relative to each other, for example in gears, transmission shafts, bearings, wheels, spinning widgets, and many others. Like linear hinges, they can be designed as single parts or separate pieces. 

Assembled after printing:
Snap-in pins (advanced design) can be used as a way to snap in rotational components, like gears, wheels, or spinners into a 3D printed assembly. In this case, a pin is inserted into holes that act as pivot points.

The pin itself should be designed with an internal slot to give it some flexibility and enable a snap fit, and angled walls to hold parts in place. As usual, it should have 45-degree pins on each end. 

Keeping the top and bottom flat ensures a higher-quality print without the need for supports and enables the option for part of the assembly to remain static. 

To explore this assembly further in the Orchard interface, click here, or on the picture:

Pre-Assembled:
Like linear hinges, they key is to design them with 45-degree angles and a ~0.5mm gap. The angles help hold the shaft in place while it rotates:

3. Two-Way Hinges (Rotational+Linear):

Two-way hinges are a combination of rotational and linear hinges (i.e. they have two degrees of motion). They are used in applications where more free motion is required, centered around the hinge, like robotic shoulders, or pivoting filament guides. They generally require a sphere shape as the main pivot point. Because the top and bottom of a sphere has angles between 0-45 degrees, they will print with slight overhangs. To avoid the overhangs, you can cut the top and bottom parts off in the design:

All of these basic hinges can be utilized and combined to create interesting designs, like a bracelet made from the basic linear hinges (click the picture to see the model):

In summary:
For pre-assembled hinges and pins, avoid any angles below 45 degrees.
For assembled pins, incorporate guide slots/tracks. 

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