In order to tap a hole for a Heli-Coil insert in cast iron, you must make sure you have the right type of milling machine. Cast iron is very hard and when you drill and tap a hole, you must run the machine at a very low speed and feed the tools at a very slow rate to avoid breaking them. A Heli-Coil insert is often used when an existing thread has been stripped or there is a need for an extra-strong thread, so tapping them per the instructions is mandatory.
Clean the work table of the milling machine you will use to tap the cast iron. Make sure that all metal chips and any debris is removed from the top surface of the work table. Chips and debris can cause the vise to sit incorrectly resulting in a crooked tap for the Heli-Coil.
Clean the bottom of the vise and set in on the work table of the milling machine. Slide a T- bolt in the slot on either side of the vise. The T-bolts will hold the vise down during the tapping. Tighten the T-bolts down with a wrench to prevent movement while the tapping process is performed.
Place a center drill in the drill chuck and tighten it with a chuck key. Turn the milling machine on, turning clockwise at a low rpm speed around 300. Slowly lower the center drill into the cast iron about 1/4 inch. This will create a divot for the drill you will use to tap. The divot will prevent the drill from walking and breaking.
Replace the center drill with the drill that is sized specifically for the Heli-Coil you will need. Consult the Heli-Coil manual for the correct drill bit size for the Heli-Coil tap. Turn the spindle on, leaving it at about 300 rpm and drill down in the same spot. Drill as deep as necessary to accommodate the Heli-Coil insert as well as the screw or bolt that will be used in this location.
Place the correct Heli-Coil in a tap handle. Start the tap in the drilled hole by applying pressure and turning the tap handle clockwise simultaneously. Apply liberal amounts of tapping fluid and only turn about one-half a turn at a time to prevent breaking the tap. Pull the tap out on occasion and blow out the hole with compressed air. Chips on the hole being tapped can jam the tap leading to breakage.