Difficult tasks in machine repair can include the manufacturing and tempering of springs and the re-tempering of worn ones. A procedure that works well for re-tempering springs (especially flat leaf springs) that have taken a set from use and normal wear and tear follows:
Remove the spring from the machine using a spring-vise which compresses the spring as it is removed. Otherwise you may break the spring do to sudden release if using pliers. Use a torch to heat it to a dull red to remove the temper and let it cool slowly. Re-bend it to match whatever model spring you may have (or study the spring to determine how it should be properly bent) and lightly file and polish the spring a bit to remove any spots that could be stress points during the tempering. Add strength back into the spring by heating again with the torch to a dull red plunging it into a container full of linseed oil (available at hardware stores or art supply stores) or 30 weight non-detergent motor oil (as an inferior substitute). Do not use water or the spring will be brittle. The spring has strength now, but needs to be tempered so now fill a small shallow container (like the metal lid of an old mayonnaise jar or mason jar or any other metal container) with the oil until the spring is covered and set the oil on fire and let it burn down completely and let the spring air cool completely. The spring is now ready for use. The spring will be dark colored so if needed you can lightly polish the spring for appearance with a fine grit polishing compound, but do not cut into the spring’s surface. In most cases a dark colored spring is fine.
Making new springs is rather similar. You first soften the steel with a torch by heating it until it’s blue and letting it air cool slowly. You shape the spring with saws, files, and grinders or you wind the spring taking into account the spring will rebound and be of a larger diameter especially if not dead soft. You can wind a spring by cutting a line into a mandrel and taping it for a screw to hold down the start of the spring for the wind or you can drill a hole in the mandrel and cut the spring when done. To temper the spring heat until a dull red and plunge into linseed oil and and remove from the oil and then with that oil still on the part hold it in a very low temperature flame which is just hot enough to light the oil. Let it mostly burn out and quench again and repeat the process two more times. And the spring will be tempered for use.