When it comes to distributing A/C refrigerant, gas, fuel-oil or very high-pressure water through soft copper or aluminum pipe, there's really only one worthy alternative to soldering: flare fittings. Where soldering with an open flame could prove difficult or dangerous (a real possibility when working with gas and oil lines), flare fittings allow for a strong mechanical joint capable of withstanding pressures in the 450-3000 psi range, depending on the pressure rating of the materials being used.
Typically brass or steel, flare fittings are composed of a threaded body with flared ends, and a nut. They utilize a corresponding flare at the end of the pipe to create their highly secure connection. Creating that flared end is accomplished by using a special tool.
Pro Tip: Flaring is typically only done with malleable metal tubing like soft/annealed copper and aluminum. In situations where only hard copper pipe is available, you can still flare the pipe ends, but they will need to be annealed first. Annealing hard copper pipe is a fairly advanced skill; if you aren't comfortable with it, leave it to a pro.
Pipe flaring tools are very simple in both design and operation. They consist of two separate parts: a die block (or "flare form") with openings for different pipe sizes, and a flared cone attached to a yoke. As the cone is pressed into the pipe end, it shapes the soft metal into the 45° flare that the fitting calls for.
Heads up! Always be certain that you're using the correct flaring tool: in addition to plumbing/HVAC applications, flare connections are used in industry and the automotive world (brake lines, etc.). The angle of those flares, however, is 37.5° - not 45° as used in "home" applications.
Cut your copper or aluminum using a tube cutter
to ensure a clean cut - hacksaws and other cutting tools can make for an uneven cut or burrs, which will compromise the flare.
Use a deburring tool
or abrasive cloth to remove any burrs and debris in and around the pipe end.
Step 3: Slide the flare nut onto the pipe, with the tapered end facing away from the end of the pipe to be flared. The nuts need to be on the pipe before both ends are flared.
Step 4: Insert the pipe into the corresponding hole in the die block. Your flare should turn out fine with the pipe end flush against the block (some users insist on leaving just a bit sticking out - about an 1/8-1/16 of an inch).
Step 5: Tighten the wingnut closest to the pipe first.
Step 6: Then tighten the far wingnut.
Step 7: Place the yoke onto the die block - it should hook or clamp into place on the underside of the block.
Step 8: Position it so that the cone is directly above the pipe to be flared. Turn the handle to move the cone toward the pipe.
Step 9: Keep turning until the cone is fully seated in the pipe and can be tightened no further (without great force).
Step 10: Back the cone out and take off the yoke.
Step 11: Remove the pipe from the die block. The newly flared pipe-end should be smooth and even.
Step 12: Check it out against the fitting body, ensuring that the flare sits fully and evenly on the fitting, without reaching the threads.
Problems? Problems usually come up as the result of the pipe slipping in the block while it's being flared. Should there be any issues, you can simply cut the pipe below the end in question, and try again.
When finally installing the new flare fitting, make sure that you do not use any joint sealing compounds or tape on the threads. You can use a drop of threading oil to help make connecting your fitting easier. Always leak test after any new connection - this is especially critical if you're dealing with a gas or oil line.
As with any new skill, you should probably do a few practice runs on any spare bits of soft copper tubing you might have lying around; if you're buying new tubing, get an extra foot to play around with. When it comes to crunch time, it will pay off!