Usually when you design a 3D-printed part, you want it to “build from the ground up”. After all, having the extruder “hang filament in mid-air” seems like a pretty difficult request. But today, I did it.

For one of the Coins-Worth pieces, I needed to add a structural piece so that one of the in-place servos could reach it. This piece normally sits in Coins-Worth rotated 180 degrees, and catches the coin so that it can be photographed. The servo already in place activates a small rod that rapidly flips the coin, and I’ve reserved a little bit of the reverse travel of the servo to lift the coin-catch enough to allow the coin to slip through.

FreeCAD view of part with bridge added (in red)

But how to get to the servo from the “feet” on the bottom of the plate? I had never created a bridge before. A bridge is essentially filament extruded in mid-air, not actually resting on anything other than supports on either end of the bridge.

Basically I dropped in a FreeCAD box, modified it into the appropriate dimensions (1 mm thick, 32 mm wide, and 10 mm long), and moved it into position with a 25 degree slant. (I may need to modify the position slightly to fine-tune the servo’s strike point.) The bottom edge of the new piece (bottom as it is printed, anyway; the top as it sits in Coins-Worth) doesn’t have anything to hang onto other than the two sides.

Watching it print for the first time was kind of nerve racking, waiting for the first layer where the bridge started to be printed. But then it happened… and it worked beautifully!

Actual photograph of the printed part.

One or two of the layer strands on the bottom of the bridge are not fully adhered to; you could pull these strands off. However, they provide just enough support for the remaining layers, which are fully adhered.

You’ll note there’s a little cut-out in the middle of the part on the top edge. This is to accommodate the flipping pin and servo clearance for actuating it. It’ll be clearer once I’ve got the whole device assembled, I promise!

At least I know I can build a bridge where I need one.