Cutting and threading steel RMC and IMC
NOTE: Although coupling threads are straight tapped, conduit threads are tapered.
Field threading is to be performed in accordance with the following procedures unless manufacturer’s instructions differ. The operating and safety instructions should be read and understood prior to operating the equipment.
- Use a standard 3/4 inch per foot (1 in 16) taper National Pipe Thread (NPT) die. The threads shall be cut full and clean using sharp dies. (See ANSI / ASME B.1.20.1-1983 (R2001) Standard for Pipe Threads, General Purpose (Inch)).
- Do not use worn dies. Although ragged and torn threads or threads which are not cut deep enough can be caused by poor threading practices, they can also indicate worn dies. If inspection shows this to be true, see Annex A for procedure to change dies.
- To adjust the dies, loosen the screws or locking collar that hold the cutting dies in the head. When the screws or collar are loosened, the dies should move freely away from the head.
- Screw the die head onto the threaded portion of a factory-threaded nipple or factory-threaded conduit until the die fits the factory thread. If the die head has an adjusting lever, set the head to cut a slightly oversized thread.
NOTE: This will ordinarily be one thread short of being flush with the face of a thread gauge when the gauge is hand tight. This is within the tolerance limits that allow the thread to be one thread short or long of being flush with the gauge face.
- Tighten the screws or locking collar so that the dies are tightly held in the head.
- Remove the set up piece of threaded conduit. The die is ready for use.
- After adjusting the dies as outlined above, proceed as follows:
- Cut the conduit with a saw or roll cutter. Be careful to make a straight cut.
NOTE: If the die is not started on the pipe squarely, crooked threads will result. When using the wheel and roll cutter to cut pipe, the cutter must be revolved completely around the pipe. Tighten the handle about one quarter turn after each rotation and repeat this procedure until the pipe is cut through.
- After cutting and prior to threading, ream the interior and remove sharp edges from the exterior.
NOTE: Reaming the conduit after threading will stretch or flare the end of the conduit.
- To start a universal die head, press it against the conduit end with one hand and turn the stock with the other. With a drop head die, the stock remains stationary and the head rotates. After the dies have engaged for a thread or two, they will feed along without pressure.
- Stop the cutting as soon as the die has taken hold, and apply thread cutting oil freely to the dies and the area to be threaded.
NOTE: Frequent flooding of the dies with a good grade of cutting oil will further safeguard against poor threads. The oil keeps the material lubricated and ensures a smoother cut by reducing friction and heat. Insufficient cutting oil will also cause ragged threads. The flow of the cutting fluid to the die head should be such that the cutting surfaces of the die segments are flooded. As a general rule, there is no such thing as too much oil at the die head.
- Thread one thread short of the end of the chaser.
NOTE: It is a good practice to thread one thread short to prevent butting of conduit in a coupling and allow the coupling to cover all of the threads on the conduit when wrench tight.
- Back the die head off and clean the chips from the thread.
Importance of thread length
The length of the thread is important, and the applicable UL requirements specify the manufactured length of the thread and the tolerance. A ring gauge is used to determine the correct thread length at the factory. Good practice is to thread the conduit one thread short. This is to prevent conduit from butting inside the coupling. This practice will permit a good electrical connection between the conduits and couplings.
To ensure that the threads are properly engaged, the coupling should be made up hand-tight, then wrench tightened. Generally, wrench-tightening should not exceed three additional threads. It should never be necessary to use an extension handle on a wrench to make up a tight joint. The only time an extension handle should be used is to dismantle a stubborn joint in an existing line.
A simple rule regarding the use of tools is to select the right type and the right size. The proper size wrench for a given conduit trade size is indicated in Table 2.
Protection of field cut threads
NEC® Section 300.6 (A) requires that where corrosion protection is necessary and the conduit is threaded in the field, the thread shall be coated with an approved electrically conductive, corrosion-resistant compound. Coatings for this purpose, listed under UL category “FOIZ,” are available. Zinc-rich paint or other coatings acceptable to the AHJ may be used.
NOTE: Corrosion protection is provided on factory-cut threads at time of manufacturing. Conduit, elbows or nipples that are threaded anywhere other than at the factory where the product was listed are considered field cut.
Cut the EMT square using a hack saw or band saw. Do not use roll-type tubing cutters.
NOTE: Roll-type cutters require reaming, which flares the wall of EMT, making fittings difficult to install.
A tool designed for the purpose is best for reaming the inside of EMT. Where side cutter pliers or other general tools are used, take special care not to flare the ends.