How to Drill Cast Iron in a Few Quick Steps


Cast iron is a brittle metal yet not very hard. It is possible to easily drill through it using any drill bit developed to be used on metals.

You need to be patient and start to drill the cast iron slowly. If you do it fast, your drill bit will most likely overheat, and it can cause an excess of friction, which will permanently damage the tool.

Drilling through a cast iron item can look challenging for those who have never done it, but don’t worry. All you need to do is have the proper tools and follow these easy steps. Remember to take your time and be careful.

Prepare the Tools and the Space

tools and space for drilling
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Don’t start drilling before preparing yourself first. Everything necessary must be ready. This includes the proper drill bit and the cast iron object or piece that you will drill through.

Step One

First, mark the location where you will drill so you puncture the correct place. Nothing is worse than going through all this work just to realize that you made a hole in the wrong spot. Also, guiding the drill during the process helps a lot. It is a good idea to use a center punch and a hammer to mark the exact place.

Step Two

Next, you will have to place your cast iron piece in a table vice or somewhere safe and stable, so it doesn’t move while you are drilling it. It is even better if it is already connected to an immovable object. In that case, you’ll just need to put the item somewhere safe and make sure that it stays still during the process.

Step Three

The most important preparation step is to connect the proper drill bit to your drill before starting the process. You need to check if you can use it for puncturing metals and if it is the right size for what you are planning to do. Just check the size of the hole and see if you have the corresponding tool. 

However, you will need to use a drill bit that is somewhat smaller than the desired size if you want to fill it with a screw. The reason is to create a secure bond once the screw holds the metal.

Step Four

Now, let’s check the necessary accessories to help you be safe. Always use eye protection and safety gloves. Don’t think that accidents can’t happen to you. Ensure that the place where you will be drilling is clean of anything that can get in your way. Make yourself comfortable, too, don’t stand in an uncomfortable position since you will have to be patient to do the job correctly. If someone is standing next to you without the drilling safety accessories, ask them to leave or put protective equipment on.

Be Aware of the Drilling Speed and the Proper Bit to Use 

the proper bit to use for drilling
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Now that you’ve organized your work space, tools, and safety accessories, get ready to start. You need to set your drill speed, so it doesn’t go too fast or slow. The pace is different depending on the cast iron you will be using.

Drilling speed

If the cast iron is soft, the recommended maximum speed is 150 surface feet per minute (SFM), which you can calculate yourself through the following formula:

RPM = SFM x 3.82/drill bit diameter

RPM means revolutions per minute. So, if you are drilling a ½ inch hole, then the maximum drill speed you can use is 1,146 RPM which is about mid-range in most portable drills.

If you are drilling through hard cast iron, reduce the speed by half. And if you are doing it outside in the winter, you will need to reduce the rate to 300 RPM.

Using lubrication

You may be asking yourself if it is necessary to use lubrication or not. That is not a consensus in the machinist community, but there are ways to determine if you need to do it. Overall, it is your choice.

Cast iron has a high carbon content that acts as a lubricant, so sometimes, you can drill without adding more. Yet, a proper lubricant helps to cool the drill and the cast iron, helping to prevent cracking and other problems related to overheating. 

So you need to observe the situation. If you think that your drill is getting too hot, add lubricant. Water-based coolants and oil are suitable, too. Usually, when drilling one or two holes, there is no need for them. But the more you drill, the hotter your equipment gets.

Safely drilling through cast iron

When you start drilling, go slowly because the faster you go, the more friction you make, and so your drill gets hotter and hotter. The speed must always be below 3000 RPM, and working at the proper pace will guarantee that you make the hole the correct size. You know how hard it can be to control a fast drill, especially when going through cast iron.

If you have any concerns about the hole size, you can always use a smaller drill bit. It is always best to go slowly but safely than rush into things and make a mess. When you finish your hole, hold the trigger down and start to pull the drill back and forth slowly. This will remove any metal debris left behind, so you will have a perfectly made hole.

Choose the best drill bit to use for cast iron drilling

You can use any drill bit made to drill through metal; however, some are more efficient than others. You need to check the size and material for maximum efficiency.

A cobalt bit with a 135° point angle is the one that you are looking for. It is excellent for many things, including cast iron. The drilling is faster and more accurate because the angle is sharper than the standard bits. A drill bit with a 118° point angle is also a good option since it creates fewer shards and drills smoothly. You can use any bit appropriate for metal, but use the one suited for cast iron if you can. It will help you go through the process faster.

Don’t ever use a drill bit intended to be used on masonry or wood. The wood will undoubtedly break, and it may cause an accident. The masonry bit won’t break, but the process will be very, very slow.

Cleaning the Cast Iron

drilling the clean cast iron
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You may be done drilling, but the process is not finished yet. You still have to clean the cast iron piece or object, and there are some techniques to help you there, so the item keeps its quality for a long time.

Remember the old rag that you put near your desk? Get it and make sure that it is dry. Wipe down the surface with it, and then do the same, but this time with a wet cloth. Repeat the process with a dry cloth. You need to wipe inside the hole as well; it doesn’t matter if it is a thin one.

Wiping is essential because it helps clean your piece from any metal debris. If you don’t clean it correctly, the metal debris can hurt you or anyone that touches the item. Also, it gets in the way of any screws.


In short, you need to start by getting the proper drill bit – one that can go through metal – and getting your safety accessories, like the safety glasses. Then, you clean a stable place and put the cast iron piece or object on it. You make sure that it is stabilized and won’t move when you start working on it. If the part is loose, attach it to a table vice. After that, mark the position of the spot where you want to drill a hole. We recommend that you use a center punch and a hammer for that.

Put your drill at the right speed, which always must be below 3000 RPM for making a hole in cast iron, and start to drill slowly, so you don’t overheat your drill. Take your time. You can start with a smaller hole. Once you finish drilling, hold the trigger down and begin pulling the drill back and forth. This will help you eliminate any metal debris left in the process. It will also help the screws to create a secure bond afterward.

After you are done drilling, use a dry cloth to wipe down the cast iron, then use a wet cloth. It is important, however, to dry the item well afterward. This process is essential to make sure that no metal debris stays.

As you see, there is nothing complicated about drilling through cast iron. All you need to have is the proper tools and patience. Put your safety first and don’t try to work quickly, but try to do it safely. Following those tips and steps, you will never break a drill bit or hurt yourself in the process. Be confident in what you are doing, and stay safe.

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