Summary

These are friction-welding rods for rotary tools.

In the 1980s, Mattel toys offered a plastic model building kit, including a rotary tool to weld the parts together. See Photo of the original welding rods.

This spin-welding / friction-welding technique is often used for 3D part repair and joining. Search YouTube for some videos.

I wasn't happy using regular 1.75mm filament as rod, and made these thicker, stiffer rods. They work much better.

One rod is sized to replace the Dremel collet. Remove the collet, insert the rod, and secure it with the collet retaining nut..

One fits a light duty, 12V rotary tool, $10 from Harbor Freight, in the same manner. Remove this somewhat larger collet, and insert the rod. http://www.harborfreight.com search "rotary tool". Item #63235 or Item #97626 are current valid item numbers.

The last is near 1/8 inch (3mm) diameter straight rod. This one can be inserted into any 1/8 inch collet, like any regular 1/8 in. bit.
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The Dremel is far too fast at 35,000 RPM top speed. These rods are not balanced and will break off and fly away (into your eye. Wear eye protection). The cheap-tool max is 16,000 RPM, and that's as fast as I would recommend.

I prefer the Cheap-Tool single speed On/OFF for almost everything. Little power is required.

If you weld, you already know how to prepare joints (like beveling edges). If not, a few minutes of study will explain the basics. Surprisingly, running a bead of plastic seems very similar to stick welding. Some practice is worth the effort.

The rod must have nearly the same hardness and melting temperature as what you are welding. Ideally, the printed welding rod is made of the same material as whatever you want to weld together.
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Printing notes-
Print them laying on their sides so layers are longitudinal, for strength. Otherwise they snap like spaghetti.

Thin round shapes can be hard to print. On the Afinia, they needed more than the default 30 degrees support angle, or the software had problems interpreting the STL file. 40 degrees worked. All other settings were Afinia defaults.

The files are SAE. If your Metric software sees tiny shapes, scale up (X,Y and Z) by a factor of 25.4.