How to Drill Through Cast Iron

Cast iron is brittle, but as far as metals go, it isn’t very hard, so drilling a hole into it isn’t as difficult as you might expect.

Moreover, you don’t have to be overly choosy about your drill bit. Any bit intended for drilling through metal will drill through cast iron.

The key is to take your time to avoid overheating the bit and wearing it out.

You can drill a hole through cast iron with any drill bit for metal. Cast iron is much more brittle than other metals and is easy to drill into.

Start drilling slowly to prevent overheating and excess friction that can damage your drill bit.

For a more in-depth guide and answer to the question “can I drill through cast iron?”, read along.

DEWALT 20V Max Cordless Drill / Driver Kit, Compact, 1/2-Inch with Titanium Drill Bit Set, Pilot Point, 21-Piece (DCD771C2 & DW1361)

Tips on How to Drill Through Cast Iron

Lubrication

Although many machinists advise against lubricating when drilling through cast iron, they don’t completely agree on the issue.

Cast iron has a high carbon content, and since carbon acts as a lubricant it’s often acceptable to drill iron without lubricant.

Lubricant makes a mess and can prevent metal debris from falling away from the bit as you drill. 

On the other hand, a lubricant, such as oil or a water-based coolant, cools the drill bit and the metal and helps to prevent cracking or excessive bit wear

If you’re drilling one or two holes, you can rely on the material itself to lubricate your bit, but if you’re drilling many holes, your bit will probably last longer if you apply a few drops of cutting oil or spray lubricant before you drill each one.

Forney 20857 Tap Magic Industrial Pro Cutting Fluid, 4 oz

Drill Bits

You don’t have to search far and wide for a bit to drill through cast iron, but not all regular drill bits are appropriate, either.

The only drill bits you should not use to drill cast iron are those intended for wood or masonry only.

Cast iron would completely destroy the brad point and extra-wide flutes of a wood boring bit, and a masonry bit is designed to use with a hammer drill; it would take all day for the dull tip of a masonry bit to penetrate cast iron.

Spade bits, Forstner bits and auger bits are similarly unsuitable for cast iron — and metal in general.

BOSCH CO14B 14 Pc. Cobalt M42 Drill Bit Set

Ideal Drill Bit

The best drill bit to use for cast iron is a cobalt drill bit with a 135-degree point angle.

The angle is sharper than that on a conventional bit, making drilling faster and more accurate. Cobalt bits are brass-colored.

As an alternative, you can also use a gold-colored titanium nitride bit. In fact, any high speed steel bit intended for metal — including all-purpose bits — is suitable for cast iron.

When using a drill press, you may find that a bit with a 118-degree point angle drills more smoothly and creates fewer shards.

Tapping – any tap appropriate for metal works on cast iron, but you might prefer to choose one made specifically for cast iron.

You can also  reach for a black oxide drill bit. They’re durable, and you won’t have tons of metal shavings stuck in the cast iron when you’re done.

230 Pieces Titanium Twist Drill Bit Set, 135° Tip High Speed Steel, Size from 3/64" up to 1/2", Ideal Drilling in Wood/Cast Iron/Aluminum Alloy/Plastic/Fiberglass, with Hard Storage

Drill Speed

The recommended maximum drilling speed for soft cast iron is 150 surface feet per minute (SFM). 

This value is related to drill revolutions per minute (RPM) by the formula:

RPM = SFM x 3.82/drill bit diameter

If you’re drilling a 1/2-inch hole, then, the maximum drill speed should be 1,146 rpm, which is about midrange on most portable drills.

Reduce this speed by half for hard cast iron — and if you’re drilling outside in the winter, reduce it to 300 rpm.

Wiping Down

Keep an old rag nearby whenever you use a drill bit on any type of metal. You’ll have to wipe up the excess cutting fluid and the metal shavings.

Wiping it on your pants or shirt can cause injuries from the sharp debris.

Marking

Always mark your surface with a permanent marker beforehand. This indicator will give you a clear vision of where you want to start, rather than leaving you to guess where you should drill.

Check

Don’t forget to check the width of the material that you’re drilling into! If you go too far, you could damage anything behind it.

Keep a ruler and a leveler in your tool kit at all times to prevent those issues.

Stabilize

Never drill into loose cast iron. Vices are an absolute necessity if it’s not attached to a surface already.

How to Drill Through Cast Iron

Step 1: Lubricate drill bits

Before you begin using your cordless drill, you’ll need to prep all of your tools to cut through the material easily.

Start by applying lubricant to your drill bits before using them. If you’re only drilling one or two holes, you don’t need to worry about lubing them.

However, anything more than that calls for cutting fluid. If you are doing a lot of drilling into metal I recommend you to get the 1 Gallon bottle of cutting fluid. Cutting fluid not only saves your bits but also prevents metal shavings from flying.

Step 2: Indentation

Create a small hole or dimple or an indent wherever you intend to drill. It’s always a good idea to mark the location, so you don’t go off course, but the indent also helps to guide the drill bit. Use a center punch and a hammer to make your marks.

Step 3: Attach drill bit

Attach the correct drill bit to your drill. Always check the size of the hole and make sure that you’re using the corresponding bit.

If you’re following the hole with a screw, you need to use a bit that’s slightly smaller than the screw.

The difference will allow the screw to hold onto the leftover metal, creating a secure bond.

COMOWARE Cobalt Drill Bit Set- 15Pcs M35 High Speed Steel Twist Jobber Length for Hardened Metal, Stainless Steel, Cast Iron and Wood Plastic with Indexed Storage Case, 1/16"-3/8"

Step 4: Bracing

Brace the cast iron in place if it’s a loose piece. If it’s connected to an immovable object, it’ll already be braced for you.

Consider a table vice to hold both sides of the material rather than only one side.

Step 5: Drill

Holding the drill upright, begin to slowly pull the trigger and place it against the indent that you created.

Don’t be afraid to start with a smaller bit if you’re worried about making a mistake. It’s always better to do it a few times rather than mess the whole thing up.

Once you’ve made the hole successfully, hold the trigger down, and slowly pull the drill back and forth in it.

This process will deburr the hole, which means that it removes the metal debris that was left behind.

amoolo Cobalt Drill Bit Set (1/16"-1/4",13 Pcs), M35 HSS Metal Drill Bits for Steel, Stainless Steel, Hard Metal, Cast Iron, Plastic and Wood

Step 6: Clean

Wipe down the surface with a dry cloth, then use a wet cloth, and repeat once more with the dry cloth.

Don’t forget to get the inside of the hole, even if it’s thin. Metal debris can cause harm, but it also prevents screws from achieving a secure bond.

Frequently Asked Questions

What is the best bit to drill through cast iron?

As mentioned above, the best drills you can use to drill through cast iron are a cobalt bit with a 135-degree point angle, a gold-colored titanium nitride bit or a black oxide drill bit. These drill bits will make drilling faster and more accurate

Can you tap a hole in cast iron?

You can, but tapping a hole drilled in cast iron requires care to avoid breaking taps in the hole when you encounter a hard spot.

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. 

Conclusion

Drilling cast iron is easy. It is a brittle metal that doesn’t have much debris. You can drill into cast iron by using the types of drill bits we mentioned above or any type of drill bit for metal.

Don’t forget to use cutting fluid if you’re drilling many holes and Always use a clamp or vice when you’re drilling into any surface. 

Wear safety goggles, gloves, and clothing and remember to always start drilling slowly to prevent excess friction, heat, and dulling.

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