How to Drill and Tap Cast Iron

by C.L. Rease

The crystalline structure of cast iron varies depending on the rate the steel was cooled. Cast iron drills differently than other types of steel due to the variations in the steel structure. When drilling rolled steel, the material drilled from the hole will work in a spiral from the tip of the drill bit. Cast iron emits numerous small chips of metal. Occasional hard spots in the cast iron can break the tip of a drill bit if you apply excessive pressure on the tip of the bit. Tapping a hole drilled in cast iron requires care to avoid breaking taps in the hole when you encounter a hard spot.

Step 1

Find the size of the fastener you need to thread into the cast iron on a drill and tap chart. Follow the line on the chart representing the fastener size to determine the drill-bit size. For example, a 1/4-20 threaded fastener on a drill and tap chart will show a #7 drill bit as an ideal size for the tap.

Step 2

Secure the drill bit from the set into the chuck of a variable-speed drill motor. Coat the drill bit with cutting fluid.

Step 3

Drill the hole into the cast iron by pumping the trigger of the drill motor to keep the rpms of the drill motor low. Coat the drill bit with additional cutting fluid if smoke appears in the drilling area.

Step 4

Install the correctly sized tap into the T-handle tap wrench. Coat the tap with cutting fluid.

Step 5

Insert the tip of the tap into the drilled hole. Pour additional cutting fluid along the tap and into the hole. Turn the tap wrench clockwise to start tapping the hole. Twist the tap wrench 1/2 turn counterclockwise if the tap binds in the hole. Continue alternating between clockwise and counterclockwise until you have a tapped hole. Remove the tap from the hole.