How to Drill Cast Iron: 7 Step Guide

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Cast iron is one of the hardest metals in the world. Drilling and cutting cast iron is a tad bit difficult due to its hardness. Cast iron is used to make cylinder heads, pipes, cooking plates, automotive parts, etc. In this guide, you can learn how to drill into cast iron with recommended tools and methods.

When you drill into cast iron, you should know the material properties of this metal. Hardness, tensile strength, heat resistance and many other properties should be considered when drilling into cast iron. According to the material properties, we have to select the drilling tool, drill bit, and speed. So let’s see all in detail about how to drill into the cast iron step by step.

1. Clean the Cast Iron and Identify the Properties

Before starting the drilling, you should clean the area and know the properties of cast iron in detail. Cast iron can be easily rusting due to water. So when you clean, if there is any rusting, remove it.

First of all, we must identify the material properties. Typically cast iron is a mixed metal. Iron is mixed with 2-4% carbon, varying amounts of silicon, manganese and impurities such as sulfur and phosphorus. Due to this combination, cast iron is hardened.

By changing the combined materials, its hardness can be changed in various ways. The combination provides more than 4 types of cast iron such as grey cast iron, white cast iron, malleable cast iron, ductile cast iron etc. These metals are made by changing the carbon, manganese, and impurities’ proportions. So, these are used for different purposes and with different properties.

You can realize how grey cast iron properties vary with iron.

PropertiesCast Iron (grey cast)Steel
Density7824 kg/m-37200 kg/m-3
Hardness, Rockwell C11.4 – 6520
Ultimate Tensile Strength276 MPa420 MPa
Essential properties of steel and cast iron

In this table, you can realize how much cast iron is strength. When considering the hardness values, it is higher than steel. When we consider other cast-iron types, these values are higher.

But ultimate tensile strength is less. Because added carbon reduces the tensile strength of the material, these values will help you to select the drillbit, type of drill, drilling speed etc.

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2. Select the Drill Bit for Cast Iron

In this step, you have to choose the suitable drill bit for the drilling process. When you select a drill bit, you should pay attention to the hardness of the cast iron. According to the Hardness, Rockwell C values it is about 11.4-65. That means you need more hard material in order to drill this metal.

There are few drill bits that are made with more hard materials such as diamond bits, cobalt bits etc. If you use one of these, you can drill the cast iron easily. Else the bit will be overheated and melted.

Normally cobalt is with 65.5 to 67 Hardness, Rockwell C . So I would recommend you use a cobalt drill bit for the cast iron drilling process.

So when you drill cast iron, check the Hardness and Rockwell C value of the drill bit. If the drill bit is more than the cast iron value, you can use that drill bit for the drilling task.

3. Use a Suitable Drilling Machine for Cast Iron

When you select a drilling machine, you should pay attention to your comfort. You have to apply constant thrust to the drill when you are drilling due to the material’s hardness. So you had better use any bench drill. If you use a hand drill, you will not be able to provide the most efficient thrust force when drilling.

Bench drilling can provide constant force by hand, and we can do a safe, accurate drilling process using bench drilling. The hand drilling method is not recommended due to the high hardness of the cast iron metal. Even you will have to take some time to drill the punching area to place the drill without a pilot hole.

The drilling machine should be varied speed and torque. So you can drill the cast iron by applying more torque and recommended speed and feeding values.

4. Make Center Punch on Cast Iron

As a professional machinist, you are really engaged in the correct and proper work methods. Before you start the drilling, you should make the center punch in order to not move the drillbit here and there. It is really important for safety, accuracy and tool life.

You can centre punch the drilling point. Then you can drill a small drill bit if you have a more diameter hole. Else you can use the regular drill bit (cobalt, HSS) for the drilling process.

It is great if you are able to make a pilot hole using a small size diameter drill bit. It will provide you really accuracy, safety, speed, less power, and more efficient drilling until the end of the task.

Center punch
Center punch for drilling

5. Use Cutting Oil for Drilling Cast Iron

Drilling is done against friction. Due to the friction tool and material, both are heated. When the tool melting point is less than the generated temperature, it will be heated. Cast iron melting point is 1127-1204 C.

You can use cutting oil to reduce the temperature. When you drill at high speed, the tool and workpiece heat drastically. So when you drill, use the lowest speed as chips remove out efficiently.

Cutting oils act as lubricants as well as heat transfer media. When you use cutting oil, you can do a speedy drilling process.

6. Start Cast Iron Drilling Process

After you set up all, start the drilling process. The drilling process is a really careful process. For the best and the most efficient drilling process, you should follow the recommended speed, torque, feeding speed etc. let’s see what the recommended value for all types of cast iron are.

These values depend on the drill bit that we use. If you use 5% of the Cobalt drill bit, you can use these values.

Material TypeSurface Speed
(cm/Minute)
SFMFeed per
1mm-3mm
Revolutions
3mm-6mm
By drill bit Diameter
6mm-9.5mm
9.5mm-13mm
Cast Iron (soft grey)320075-1250.1016 in0.1651 in0.2032 in0.2540 in
Cast iron (grey)274350-1001.2700 in0.1905 in0.2286 in0.2794 in
Cast iron (ductile)243810-200.0762 in0.1143 in0.1651 in0.2032 in
Cast iron (malleable)167680-900.0508 in0.0762 in0.1143 in0.1524 in
Hard Speed and Feed Recommendations Source

When you start drilling, you should start slow speed and then increase the speed slowly. If you have no pilot hole or any punching point, you will not be able to drill the metal properly. Because the drill bit will be moved here and there due to the hard surface. Until the surface is damaged with the drill bit, start slowly.

Remember to apply cutting oil before you start the drilling process. Do not push much. You can push as chips come out continuously.

Feeds and Speeds – Formulas

R.P.M. = (3.8197 / Drill Diameter) x S.F.M.
S.F.M. = 0.2618 x Drill Diameter x R.P.M.
I.P.M. = I.P.R. (feed) x R.P.M. (speed)
Machine Time (seconds) = (60 x Feed minus Stroke) / I.P.M

R.P.M. = Revolutions Per Minute I.P.R. = Inches Per Revolution
S.F.M. = Surface Feet Per Minute Feed Stroke = Drill Depth + 1/3
I.P.M. = Inches Per Minute Car Reamer = 1/2 Speed of Drill

Step 07- Clean the Drilled Hole and Tool

After you have done the drilling process, you had better clean the hole and drill bit. Some chips can be inside the drilled hole, and you can remove them for further work. Some chips can be attached with the drill bit due to cutting oil. So you had better clean the drill bit before using it for another work.


Tom Mackency

Tom Mackency

Hi, I am Tom Mackency. It has been 10 years that I have been working as a professional woodworker since 2013. I am really enjoying my carrier by creating many kinds of projects in my workshop. But mostly I like for home improvement projects. Home improvement and DIY projects are the most interesting things for me. More than that, the coolest things are power tools. Those are very precious and efficient than a decade ago. So I try to introduce so many things about power tools, woodworking, DIY projects, home improvement and many more interesting topics here.


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