Making and using the tube makes a creative activity for the weekend - in an hour's work, you could make enough knitting pipes to keep your kids, and all their friends, constructively occupied for ages.
I used power tools and a blowtorch, but you could also use hand tools and sandpaper.
As a bonus, I'll also explain how to use the pipe. Even better, there is now a 3D printed upgrade to the idea (see step 7)!
Step 1: Materials
You could use any tube you like, as long as you have the tools to deal with it.
Step 2: Drill
I used a 3mm twist bit, which cut through the tube quite easily, but did slip a little on the smooth PVC. If you have a wood bit, with the small spike at the tip, it won't slide around as much.
To drill the pipe, I clamped it gently in my vice. If you are not using a drill-press, do not be tempted to drill right through the pipe, because the tip of the drill bit will wave around once it pierces the pipe wall, and hit the wrong spot on the other side.
Step 3: Snip
I used the same snips to roughly round off the corners of the tabs.
Step 4: Drill Again
At the base of each tab, I drilled a row of holes, this time with a 2mm bit.
The three holes proved surprisingly tricky to turn into a slot. I used the wire cutters, scissors and a sharp knife, but eventually ended up wriggling the drill bit back and forth to turn the three holed into a slot.
Step 5: Fire!
Don't get carried away, just give the tube a few "licks" with the flame. Rough edges should melt more quickly than the rest of the plastic, leaving them smooth. I also gave a blast down inside the tube to smooth off the inside of the pipe and stop the yarn catching.
You may also want to soften the tabs and curve them slightly outwards, to help the yarn stay in place, but that's a matter of personal choice.
To stop burning my fingers, I clamped the pipe in my vice. Unfortunately, when it softened, it was marked by the vice. Not shown here, a light scraping with a sharp knife took off the texture, and some of the discoloration caused by the blowtorch.
If you don't have a blowtorch, use small files and sandpaper to smooth off the jagged edges.
Step 6: Knit!
Take your yarn, and tie a small loop a few centimetres from the end. Drop the loose end down the tube (it should hang out of the end of the tube a little), and hang the loop over one of the tabs.
Wind the wool around all the tabs in turn, looping around the inside. To start off, go around the tabs twice (that is, right around twice, 1-2-3-4, 1-2-3-4, not 1-1-2-2-3-3-4-4). The second loop around should be above the first.
With your fingernails, a crochet hook or similar, lift the first, lower, loops over the top of the tabs, so that they trap the second, upper loops in place. Give the end of the wool a gentle tug to slide the loops to the bottom of the tabs.
Wrap the yarn around the tabs again, above the previous loops. Repeat the loop-lift-tug sequence until the cord you make is as long as you want.
To finish off (cast off), slide your tube down the cord, gently lifting your last four loops off the tabs. Trim the yarn if you need to, then thread the loose end through the last four loops, and pull tight.
You now have a knitted cord, to do with as you will.
If you get bored easily, or are short of time, then knit short cords and make bracelets or necklaces. If you are more patient, or have more time, you can make longer cords to use more creatively.