My new water-cooled 1.5kw spindle takes 5/16" water lines, and my CW-3000 “cooler” has 5/8" barbed fittings. I have 5/8" vinyl hose and 5/16 PU hose, but I had no fittings to adapt them.
I’m sure Lowes or Home Depot would have been happy to provide some barbed adapter fittings for a dollar or two, but I have a lathe!
I intentionally fed somewhat fast on the theory that a rougher surface finish might improve friction fit, and at least certainly wouldn’t hurt it.
If only I had the space for a lathe 
My whole machining journey started with a used mini lathe. It… didn’t stop there! So the space I needed for a lathe looked very small at first, but that was a bit of a mirage. 
I have 3D printed what I need in the past.
I’m curious — what kind of adapter could you 3d print for 4mm ID hose? I design and print plenty of objects, but a design that I would trust with water lines over my CNC router isn’t coming to mind. I could see the flat MDF surface of my torsion box-ish table becoming suddenly un-flat with the addition of a bit of water. 
One of many found on Thingiverse
Huh, I was expecting some completely different concept. Two layers of wall thickness for 4mm. 
I’ll stick with metal I think, or machined/molded plastic. But cool to hear that these work for you. I’d think it’s be much more open to experimenting with 3d printed variants for air supply hose. 
If I was dealing with high temperature fluids I would most definitely not use 3-D printed barbs but they have held up quite well.
I was at Home Depot today for other reasons, and out of curiosity looked in the plumbing section, and they didn’t have anything to mate these two diameters. So I would have wasted my time trying to buy it there.
Finally installed these today. The PU tube was too stiff to fit over them cold. At first I tried using a heat gun to heat up the PU tube but that just made it floppy and I couldn’t get it over the barbs. Then I heated up the fitting with the PU tube over the end, and pushed it over, and that let the fitting heat up the tube it was contacting while leaving the rest of the tube cold, and I got it to fit all the way.
Installed to the vinyl tubing to the cooler:
I plan to add hose clamps to the vinyl tube, but I think that the the PU tube is on pretty securely. Even hot, it didn’t want to come off the barbs. I think that the barbs were a real success.
I ran this for real for a while. No leaks manifested in the whole system.
Lathes are magic! 
