If you want to splice filament together, this tool will help you out. I printed this in ProtoPasta iron filament to give it higher heat resistance. You'll need a lighter as well.
This tool has slots for 3 mm and 1.75 mm filaments.
You can use this to splice on those short ends of filament that you find at the end of the spool. You can also use this as poor mans pallet system, so you can change colors without pausing the printer to do a color change.
Video of this project is found on https://www.youtube.com/watch?v=gvBOLAB-viE