Adafruit will not be shipping USPS orders Monday February 18, 2019 for the federal holiday, Presidents Day.
Expedited USPS orders placed after 11am ET Friday February 15 will go out Tuesday February 19.
Since you also have to buy machine screws, they are a little expensive at $0.15 each, in quantities of 100. These brass inserts perform better than expected. The soldering iron tip makes them incredibly easy to install and the holding strength is more than sufficient for anything you would be 3D printing in plastic.
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These work best if you put some material between the threaded insert and the part your bolting up to. So, rather than inserting your bolt like the one in the picture, insert it from the opposite side of the block. This reduces the chances that the insert will simply pull out. Instead, it forces the bolt to pull the insert through the part, which is much more unlikely. It’s a little more work up front (drill the insert hole, and a through hole) but well worth it.