You would not spring bend 22mm or 28mm around your knee - but you might with a hole through a bit of 2x4 wood - that was my old man's preferred method. Might stil have said bit of wood kicking around with a nicely rounded off pair of holes (one for 15, one for 22).
Bending springs are handy if you need to make a few degrees tweak to a machine bend to fit perfectly - either bending some more or unbending a tad. Never try without a spring - you can do about 1 degree before it kinks - been there...
Ok, so in what situation should you use those internal springs, because I have seen them used by plumbers, when bending copper pipe in a pipe bending machine.
The pipe bender used is, IMHO, much better than an internal spring. That is the style I have always used at work. Sometimes used a spring at home and extracting the spring can be difficult.
The problem is probably hardened pipe. Try annealing the pipe over all the area to be bent (heat to red hot and then allow to cool naturally before attempting to bend).
I have always found it easy enough to get bending springs out if you have the end with the ring accessible. Rotate clockwise and it gets smaller and pulls out easily. Fairly easily. Well, not too difficult. But I have not used springs very much...
For 28mm, I used dry sand fill, and bent it over a bag of builders sand. That was after carefully anealing just the area where I wanted it to bend, and that worked well as it didn't bend outside the freshly anealed area. The instructions came from a very old book on working with copper, which I found in a second-hand book stall in Luton market many years ago. The bend required was only about 20 degrees.
To clean out any sand dust afterwards (as it's a gas pipe), I used the vacuum cleaner to suck a large cotton wool ball through the pipe many times.
Certainly not with a bending machine! Bending springs can be perhaps thought of as a poor man's bending machine. They were far more popular (and worked much better) with the softer thicker walled pipe of years ago.
The old FAQ entry sums them up pretty well:
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Bending springs are still viable for fully annealed microbore pipe (although often they are external springs). The also work fine with thick walled 20mm plastic electrical conduit.
They will just about work on modern pipe, but IME its really not worth the effort, and you waste far more pipe which at today's prices makes the pipe bender a cheap option.
That does not make it "right". It could be the plumber was contending with a poor quality or badly worn machine, and that was one way of getting better quality bends with it.
I find my Hilmor GLM bender will do perfectly smooth bends in both 15 and 22mm by itself. Adding a spring would just be a waste of time.
(also worth noting its very difficult to use an internal spring into a pipe cut with a pipe slice - the slight reduction in diameter makes it hard to get the spring in and out even before bending the pipe).
That is the main problem with them - you can't really use them half way along a length of tubing - unlike with a proper pipe bender. Which in lots of cases means an extra joint.
I've read about doing this - but when I have used a spring for other than a very gentle bend it has to be 'unwound' to remove. Knowing my luck the string would simply break. So I'll stick to my pipe bender - which cost a lot of money ages ago. Now they've come down in price so much there's no excuse not to have one.
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