Black iron pipe is steel pipe with a black coating that distinguishes it from other types of piping in the home. The pipe comes in various pre-cut lengths, with both ends usually threaded. However, if the pipe section has been cut through, the pipe ends on each side of the cut need to be threaded so they can then be connected together. This is done with a device called a die, which is a short cylindrical metal tube with threads on the inside.
Secure the black pipe horizontally in a pipe vice, with the pipe end 12 inches or so from the vice jaws. Clean the pipe end, using a rag.
Apply cutting oil around the last two inches of the black pipe, using a brush.
Push the die onto the end of the black pipe, so that the pipe end enters the hole in the centre of the die. Turn the die clockwise 1/2 turn, while pushing the die against the pipe. Turn the die counterclockwise 1/4 turn, to help remove steel filings from the die threads. Turn the die another clockwise 1/2 turn, before turning it counterclockwise again 1/4 turn. Repeat this procedure several times until the pipe threads are nearly the width of the die (about two inches).
Remove the die from the pipe end by turning it counterclockwise. Wipe around the threaded pipe end with a rag, to free it of cutting oil and metal shavings.
When starting to thread the pipe with the die, make doubly sure that the die is at 90 degrees to the pipe end. This will ensure that the threads are cut straight on the pipe.
Wear work gloves when using the die, and remember that metal shavings can be very sharp.