Sacrificial Bridge Avoids 3D Printed Supports

[Tommy] shares a simple 3D printing design tip that will be self-evident to some, but a bit of a revelation to others: the concept of a sacrificial bridge to avoid awkward support structures. In the picture shown, the black 3D print has small bridges and each bridge has a hole. The purpose of these bits is to hold a hex nut captive in the area under the bridge; a bolt goes in through the round hole in the top.

Readers familiar with 3D printing will see right away that printing the bridges might be a problem. When a printer gets to the first layer of the bridge, it will be trying to lay filament in empty space. By itself this is not usually a problem as long as a bridge is short, flat, and featureless. Unfortunately this bridge has a hole in it, and that hole means the printer will be trying to draw circles in mid-air, rather than simply stretching filament point-to-point across a gap. One solution would be to add a small amount of support structure, but that just moves the problem. Removing small supports from enclosed spaces can be a real hassle.

To solve this [Tommy] added what he calls a “sacrificial bridge”, shown as blue in the CAD image. He essentially gives the hole a flat bottom, so that the printer first lays down a thin but solid bridge as a foundation. Then, the portion with the round hole is printed on top of that. With this small design change, the print becomes much more reliable with no support structure required.

There is a bit of post-work involved since each hole needs to be drilled out to punch through the thin sacrificial bridge underneath, but it definitely beats digging out little bits of support structure instead.

25 thoughts on “Sacrificial Bridge Avoids 3D Printed Supports

    1. i used square nuts in my pi tablet case. they are mostly used hardpoints for addons. i friction fitted them in slots in the edges of the case. was going to friction stir weld them into place but they had a good friction fit and would stay put without much encouragement. the only overhangs in this case were the tops of the screw holes. square nuts are great for this kind of thing.

  1. I believe I first learned this trick from Nophead’s blog posts about the design of the Mendel90. Since then I’ve used it many times. Usually I just trim out the bridge layer with a craft knife, but occasionally a drill is easier.

  2. A related hack .. if. you have an overhang that is too small to persuade the slicer to generate support, but edge for the overhang needs to be precise, make a super-thin (like 0.1 mm thick) feature that juts out far enough to persuade the slicer to generate support. This generally results in a well formed edge for the overhang, and the sacrificial feature is easy to grind or sand off.

    1. That seems to be a very good solution. At least if you can purchase threaded inserts as easy as nuts. But I just thought about melting hex nuts in fitting holes with the iron. This should be of similar strength than captive nuts.

  3. I would simply put a single wall cylinder between the holes.
    Matching the hole ID.
    This supports the bridge and keeps the bottom of the hole nice and crisp.
    You simply cut it away after printing.

    1. it’s workable, but as the article says, it can be relatively difficult to remove that support, depending. the article’s recommended layer is always easy to remove with a drill bit.

      also, if you’ve got a bowden extruder, then a thin tube like that has its own potential problems *sigh*

  4. i independently invented this. *pats own back*

    did i tell you all how i invented micro-blogging (twitter) in 2004?

    but really, for nut traps i usually make walls instead of ceilings…

  5. Just leave out the nut, print a solid block with a center hole and then tap it yourself with a 3€ tool.
    Best additional investment to my printer was a set of tap drills and a wrench.
    Beware that usually those taps are made for through-hole – if you need blind holes, there are special drills for that as well where the thread climb is way sharper.

  6. Yet another elegant solution is to just capture the nuts in the print: you leave out a nut-shaped space in the model (with just enough extra space on the sides to be able to insert the nut without having to press it in) and add a stop command in the print at that layer height. When the printer stops, you manually insert the nuts and then tell the printer to continue and print over the nuts. That way, you don’t have the bridge problem in the first place and the nuts end up totally encased except for the hole on top.

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