Tubes and circles are used a lot in cardmodeling. There are two different king of joining a tube. The first is if the designer has "attached" a joining strip at the end of the part like this.
When designing the parts like this we will have a visible edge on the part as I have illustrated on the two pics below.
I have put some color to the edge so it's more easy to see.But in this case the part will not be completely round.
So what I do when I'm making a part thet I want the edge to be hidden is to cut of the flip at the end
And I glue it on the inside like this.I have colored the tab so it's more visible.
When the flip is dry I place the part in my hand and with a rounded object I roll up and down the part til it doesnt curve anymore.Thats because the rod I used was a bit big,but I start with a big rod so the paper won't be bent in a way that will show.
Here we can see the maximum curve I got with the end of the knife
So I switch to a smaller tool.Don't be tempted to beleave that you can join the part at this can easily bend it instead of curve it nicely.
So when the smaller tool are used we get the proper amount of curve.
And when we glue it together...we get a nice looking tube

Making smaller tubes

There was a "contest" at the old that sounded "How small can you roll" And as far as for can make pretty small ones with some practice.Tubes wit a diameter of 1mm is not unusual.Barrels for machinguns f.eks.
But when making small tubes the same rules goes.Use a tool to shape it well before you glue it together. This example is of one of the axels for the rear wheels on the Luchs. The diameter on the tube is 4,5mm First I cut away the flip at the end
And glue it inside as shown before.
Next is to find a rod with a proper diameter.The rod I used was a 1,5mm welding electrode.
Now after some rolling the tube is tight enough
Then it's to glue it together applying some pressure at the joining point with some tweezers.
And we have a nice round tube..

ŠJohnny Svensson 2008