How to Drill Cast Iron? – The Detailed Guide

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 properties should be considered when drilling into the 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?.

Step 01 – Clean the Area 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. Because we need to select a drill bit among these 26 types of drill bits. 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 realise how grey cast iron properties varied with iron.

PropertiesCast Iron (grey cast)Steel
Density7824 kg/m-37200 kg/m-3
Hardness, Rockwell C11.4 – 65 20
Ultimate Tensile Strength276 MPa 420 MPa
Essential properties of steel and cast iron

In this table, you can realise 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.

Step 02 -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 hardness material in order to drill this metal.

There are few drill bits that are made with more hardness 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, 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.

Step 03 – Use a Suitable Drilling Machine

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 drill can provide the 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 some time to drill 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.

Read More About – How to Use a Drill? Step by Step Guide!

Step 04 – 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 centre 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.

Read More About – Drill Bit Material, Coating and Usage – In Depth Guide!

center punch for drilling

Step 05 – 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 for reducing 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 a heat transfer media. When you use cutting oil, you can do a speed drilling process.

Step 06 – Start Drilling Process

After you set up all, start the drilling process. The drilling process is a really carefully 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 in 0.2032 in 0.2540 in
Cast iron (grey)274350-1001.2700 in 0.1905 in 0.2286 in 0.2794 in
Cast iron (ductile)243810-200.0762 in 0.1143 in 0.1651 in 0.2032 in
Cast iron (malleable)167680-900.0508 in 0.0762 in 0.1143 in 0.1524 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

Read More About – Research for the Cast Iron Drilling

Step 07- Clean the 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.

And then check the sharpness of the drill bit. You can sharpen the drill bit using this guide.

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