My take on this problem. And a good exercise in designing threads.

Why buy travel-sized toothpaste at 10x the cost of a large tube when you can just refill the travel tube??

Just screw the big and small tubes together using this part and squeeze the big tube. After the refill, I blow the toothpaste remaining in the coupler part onto my toothbrush and brush my teeth, then I wipe out the insides with a Q-tip.

As designed, the thread fit is tight. Consider printing at 102% for a slightly looser fit.

2023-07-24

• Added versions for all three couplers that are 20mm in length (shorter than the originals). When tubes are fully threaded into the part, the tube ends should "kiss" thereby eliminating the need to clean out any toothpaste from the coupler.
  • Crest2Crest-short20mm.stl
  • Crest2Colgate-short20mm.stl
  • Colgate2Colgate-short20mm.stl

2019-08-30

• Added a version that couples a Crest tube to a Colgate tube. Why? Maybe you like Colgate but have a Crest travel tube (or vice versa) -- Crest2Colgate.stl
• Added a version that couples two Colgate tubes - Colgate2Colgate.stl

2019-08-29

• Added a version that's 8mm shorter than the original -- Crest2Crest.stl

2019-05-24

• Under redesign to accommodate thread differences across manufacturer brands. Current model only fits Crest brand tubes.