If you are involved with residential or commercial plumbing, HVAC, electrical, auto mechanics, maintenance, or metal fabrication, pipe threads are routinely encountered. A pipe thread tap and die set is a must-have item for the professional tradesman to be able to do the job efficiently and in a timely matter.
Sometimes a pipe thread will just need to be cleaned up or perhaps a new pipe connection will need to be made. The only way to get the job done is with a NPT tap for internal threads or a NPT die for external threads. Purchasing a different NPT thread size tap or die individually can be somewhat expensive compared to a set with the various commonly used sizes included. A Pipe tap and die set is the most efficient way to deal with the task.
As a toolmaker and machine designer I occasionally find it necessary to add a pipe thread connection to a tool or machine I’m building. Injection molds require a cooling system with NPT connections. Often times a machine will have a pneumatic or a hydraulics system that require National Pipe Thread (NPT) threaded connections.
How to Cut a NPT Thread with a Tap and Die
Whether you are using a tap or die to create a new thread or just clean up an old thread a good quality cutting oil or fluid like Tap Magic should always be used. Unlike cutting a straight SAE or Metric thread where the tap or die is only cutting the 60° included angle shape of the thread, a pipe tap actually increases the diameter of a hole and a die reduces the diameter or a pipe in addition to cutting the thread shape on a taper.
That’s why it takes more torque to cut pipe threads than straight threads
Tapping Internal Pipe Threads
I recommend that after you have drilled the correct size hole for the tap you are going to use, use a chamfer tool (countersink) to open up the hole to slightly larger than the diameter of the tap at the top of the threads. This will reduce the amount of metal the tap will have to cut. It will also eliminate that sharp burr and create a much more professional looking tapped hole.
NOTE: For most applications running the tap into the work piece (metal) until the top 2 or three threads of the tap are visible is as deep as the hole should be tapped before checking fit.
Apply a generous amount of cutting oil or tapping fluid to the tap and the hole. Start turning the tap while keeping it as perpendicular to the work piece as possible. Turn the tap about 1/2 turn and then back off a 1/4 turn approximately until you feel the chip break. Pick up where the tap stopped and turn another 1/3 to 1/2 turn and back off again to break the chip. Continue with this procedure, reducing the amount of cut each cycle until the full depth is reached.
Cutting an External Pipe Thread
Although the opposite of tapping, to cut a pipe thread its a good idea to chamfer the end of the pipe first. This will allow the die to start easier and will require less effort to turn the die. Be careful to only cut the chamfer slightly smaller than the finished thread. You can determine that by checking another piece of pipe with the same size thread. You can accomplish this with an external chamfer tool or with a file if necessary.
Turn the die until it bits the metal and continue for about 1/2 a turn. Back off about 1/4 turn to break the chip and repeat the procedure, gradually decreasing the turn as the die cuts deeper until the pipe reaches the small end of the die. Clean threads and check fit with the tapped hole.
Here are a couple of Pipe Thread Tap and Die Set recommendations for you to consider. These will work for almost any NPT pipe threading job. Either one can be used for refreshing internal or external pipe threads or for cutting new threads.
The HHIP 1/8” to 1” NPT Taper Pipe Tap and Die Set is made for the professional tradesman who cuts taper pip threads frequently.
Each piece is made from HSS (High Speed Steel) with ground threads for precise threading. The tools in this set will cut correct fitting internal and external tapered pipe threads when used correctly.
The 6 Piece Taper Pipe Thread Tap and Die Set is a good quality import product that is well suited for occasional cutting and re-threading taper pipe threads.
The tools are made from heat treated High Carbon Steel, and tempered to 60 Rc. Although high carbon steel doesn’t have as high a wear resistance as HSS it is just as hard and will cut new threads just as well.
The threads on the taps and dies are milled which creates accurate threads but not as smooth and precise as with ground threads.
When used correctly these tools will cut good-fitting pipe threads in steel, copper, aluminum, brass, or plastic.