a single flare is made with a standard flaring tool, block and the wedge shaped tool you describe. It simply drives down into the tube and makes a nice 45 or 37 degree flare depending on what tool you're using. The double flare folds the tube down inside of itself while making the same flare angles. It is much safer and much stronger than the single flare. That's why it is used on brake lines and fuel lines. It is much less prone to cracking. Snap-On and many auto parts stores sells a kit for double flaring tubes. I paid about 50 bucks from Snap-On for mine and it has paid for itself many times over because I make all my own brake and fuel lines.
If you are using AN aluminun or steel fittings don't make the mistake that almost everybody makes. Get the right tool to make the right angle of flare you'll need. AN and automotive SAE fittings use different flare angles and are not interchangable although many people do mix them.
You're right about the stainless. boy is that stuff hard to work with. On some aircraft thrust reversers Dee Howard uses flare fittings and stainless lines. Hard to believe in this day and age a major aircraft modification center would use flare fittings on a 3000 psi system, but they did. Cracks resulting in leaks are common with these lines. All the thrust reversers I have seen other than these used MS fittings and never leak, or crack.
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