Taps and dies are great for cutting new threads or repairing old threads on metals, plastics and other hard materials. Taps cut threads on the inside of holes and dies cut threads on the outside of rods.

Step 1: Choosing the right size of tap or die can be the trickiest part of the job. Use a thread gauge to identify threads per inch (TPI) on standard American bolts or the pitch (distance between each thread) on metric bolts. Use a micrometer to measure the diameter of a hole or thickness of rod without threads. When repairing worn-out threads it may be necessary to use a die one size smaller than the original threads or a tap one size larger.

Step 2: Standard taps are tapered to facilitate the cutting of new threads in a hole. Bottoming taps or taps with flat bottoms are designed to extend existing threads to the bottom of a hole. Bottoming taps cannot be used to start new threads in an untapped hole.

Step 3: Soft metals and plastics do not hold threads well. When tapping or threading soft materials, choose a coarse or large thread pitch.

Step 4: The force needed to cut threads corresponds directly to the hardness of the material and the size of the tap or die. Larger taps and dies require more force. Using a tap wrench or a die stock handle is the best way to ensure useable threads.

Step 5: Precision-size holes are essential for tapping. Use numbered or letter bits rather than fractional-sized bits to drill the proper-sized hole before tapping. Tap and drill size charts are readily available at local hardware stores or online.

Step 6: Secure the object to be threaded in a vise or by other means to ensure it does not move during the threading or tapping process. Apply thread cutting oil or lubricate generously to the tap or die throughout the process.

Step 7: Turn the die onto the rod or the tap into the hole slowly. Apply even pressure, keeping the object and the cutting tool perpendicular at all times.

Step 8: For every full turn forward, reverse the tap or die back half a turn to loosen the chips or shavings (swarf) created from the thread cutting process. For deep holes, remove the tap periodically and brush swarf away before continuing.