One of my rolls of cheap Ebay filament had a bunch of ~2" long brittle spots randomly throughout the roll, so I started splicing it back together by hand. I eventually decided to make a jig, but I didn't really like most of the ones I saw. I don't have any 2mm teflon tubing, and I doubt that trick would actually make a weld that fits through the heatbreak on my machine anyway. I just wanted a simple alignment tool so I didn't have to keep doing it freehand.

I liked 4315352, but wanted to orient things differently to make it easier to use. Of course that means I had to start from scratch.

Sized for 1.75mm filament and M3 cap screws and hex nuts. Three holes need to be tapped as described in the file. If you want something else, you can edit the SCAD file to suit.

Just clamp the filament ends with some space between. Lay the tool flat on the workbench and use an iron or hot thing to soften the ends. Press the ends together and let it cool. The weld still needs to be trimmed to size. I just use nippers and cratex.