How to Cut Cast Iron Pipe With an Angle Grinder

Cast iron performs well for a long time, carrying water and sewage for up to 25 years. Unfortunately, it then begins to degrade internally, developing rust from exposure to water and air. This can cause plumbing failures, and replacing the failed section is the most common solution. 

An angle grinder makes it easy to remove a section of cast iron pipe. If you’re installing a replacement pipe, refer to my article on using an angle grinder to cut metal for step by step instructions on cutting a pipe in the workshop. 

How to Use an Angle Grinder to Cut Cast Iron Pipe

To safely and effectively cut cast iron pipe, some planning is required.  Choose an appropriate disc to attach to your angle grinder.  Before cutting, stabilize the pipe and formulate a plan for getting the cut section safely to the ground. 

Cutting Cast Iron Pipe: How to Choose the Best Cutoff Wheel

Using the correct disc is critical to ensuring your safety. Never use a grinding wheel to cut. Instead, look for a cutoff wheel, which is thinner.

Cutting Wheel Material

Angle grinder discs are made from compressed grains of abrasive material, held together with a bonding agent. Different abrasive materials are best for different purposes. Cast iron, which is a hard yet brittle metal, can be most effectively cut by a wheel made from silicon carbide grains.  

Cutting Wheel Bond

When it comes to angle grinder discs, bond is what keeps the abrasive material together.  A disc with a soft bond will wear down more quickly than one with a hard bond.

It will also cut through hard metals like cast iron more effectively. The soft bond allows dull abrasive material to be shed, allowing new, sharper pieces to emerge and cut through the material. 

Cutting Wheel Grit

Grit refers to the size of the abrasive grains. It is communicated through a number that indicates how many grains could pass through a defined area. Higher numbers indicate smaller grains, while lower numbers refer to larger grains.

High grit wheels are best for achieving a clean and smooth finish. Low grit wheels work more quickly but leave rough edges. A low grit wheel will work well for removing a section of cast iron pipe.  

Learn how to drill through your cast iron with my guide.

Disc Compatibility 

To prevent kickback and the risk of a disc failure, the wheel you choose should match the size (measured in inches or millimeters) and speed (measured in RPM) of your angle grinder. Since kickback is one of the most common causes of injury when using an angle grinder, double-check that the maximum RPM listed on the disc is equal to or greater than than the RPM of your grinder before you start working. 

How to Use an Angle Grinder to Cut Cast Iron Pipe

1. Confirm the water supply has been cut off and that the pipes are empty. Mark your cut on the pipe by wrapping a strip of masking tape all the way around the material. Some people prefer to mark with chalk, but the masking tape provides a straight edge to guide your cut.  
2. Assess the immediate area around the pipe and ensure it can be cut without damaging any surrounding material. If the space is too tight for an angle grinder, a reciprocating saw is a good alternative. 
3. If the pipe is oriented vertically, attach riser clamps at ground level. You can also attach riser clamps above any other supports for extra security.  Inspect the drop-zone. Anything between the pipe and the floor should be moved out of the way if possible. 
4. Plan your cuts. For a thin pipe, you may be able to cut through in one pass. A pipe with a larger diameter may need to be cut from several different directions. 
5. Put on protective gear such as a flame-retardant work shirt or leather apron, goggles or glasses, and steel-toed boots. Cast iron is extremely heavy, and controlling the drop of a section of pipe isn’t always possible. Steel-toed boots will protect your toes from crushing damage.  If using the angle grinder over head, you must use a full face shield to avoid penetrating facial injuries as a result of shattered discs. 
6. Connect the angle grinder to power. Turn the tool on and allow the disc to rotate for about 60 seconds, until it gets up to full speed.  Align the cutting wheel with the masking tape or chalk. Orient it at a 90 degree angle. 
7. Allowing the machine to do the work, drop the cutoff wheel through the pipe. If you can’t cut all the way through, gently remove the wheel from the pipe and make another cut from the other side. Make sure not to tilt or angle the grinder as you’re backing out of the cut.
8. If an even and straight cut is important to the job at hand, use the angle grinder to score a grove all the way around the diameter of the pipe, about ⅛ of an inch deep. Then, grab a cold chisel and drive it into the groove, working your way around the pipe.  Because cast iron is a brittle metal, it should ‘snap’ easily, releasing the section of pipe. 
9. Ideally, a friend or colleague would be on hand to stabilize the pipe and help control the drop. If this isn’t possible, it is advisable to use a cold chisel for the last step, allowing you to control the drop without a dangerous power tool to deal with. Cut through the pipe in all but one place. The metal you leave in place should be slightly narrower than your chisel.  Use a ball peen hammer to drive the chisel into the remaining material, freeing the pipe. 
10. When you’re done cutting, allow the disc to come to a complete stop. Unplug the angle grinder.

Conclusion

A cast-iron pipe can be cut using a hard bond, low grit, silicon carbide cutoff wheel attached to an angle grinder. Riser clamps will help stabilize vertically oriented pipes.  For horizontal pipes, enlist a friend to help get the pipe safely to the ground. Use proper protective equipment to avoid injury.