Types of Holes - The Complete Guide | The Engineer's Bible (2024)

Simple Hole

What Is a Simple Hole?

A simple hole is a circle that has been cut out of an object.

What Is the Symbol for a Simple Hole?

The symbol used for a hole is the diameter ‘Ø’ symbol.

How Are Simple Holes Shown on Engineering Drawings?

Simple holes are shown on engineering drawings by stating the diameter and the depth of the hole.

For example, a 20 diameter hole that goes straight through the component would be represented as “Ø20 Through”.

Holes that go all the way through the component are known as through holes.

Holes that do not go completely through the material are known as blind holes.

Holes can be shown as a section view in order to show a clearer image of the depth of the holes.

The bottom view is what we would see if we cut the top view in half along the ”A’ section line.

The shape of the bottom of the hole is determined by the tool used to create the hole.

For a drilled hole, the shape of the bottom of the hole will be determined by the drill tip angle.

If the hole is created by a process called ‘boring’ or by a tool with a zero tip angle, the bottom of the hole will be flat.

These two types of blind hole drilling are shown in the drawing below:

Counterbore

What Is a Counterbored Hole?

A counterbore is a simple hole with a wider hole above it.

The 3D model below shows a counterbore with the same counterbore next to it cut in half:

What Is the Counterbore Symbol?

The symbol used for a counterbore is ‘⌴’.

How Are Counterbores Shown on Engineering Drawings?

Counterbored holes are shown on engineering drawings as follows:

The top view shows how counterbored holes are shown on drawings. The bottom view explains what the counterbore dimensions are showing.

Note for counterbored holes that the pilot hole usually goes completely through the component. That’s why there isn’t a ‘Through’ or a dimension after the pilot hole diameter, because it is assumed it goes all the way through.

A ‘pilot hole’ is a small diameter hole, usually the first hole to be drilled.

The counterbore example above was for an M20 socket-head bolt with normal clearance. You can find out what size counterbore you need by looking at a chart such as this one.

What Are Counterbored Holes Used For?

Counterbores are machined for socket-head screws. They are used for applications where the bolt or screw must sit beneath the surface.

For example, in applications where there isn’t space for a hex-head bolt to be used due to clearance issues:

A washer may also be present underneath the socket-head screw.

Spotface

What Is a Spotface?

A spotface is a shallow counterbored hole.

What Is the Spotface Symbol?

The symbol for a spotface is the counterbore symbol with the letters ‘SF’ inside it:

How Are Spotfaces Shown on Drawings?

An example of a spotface is shown below, the dimensioning is very similar to a counterbore.

What Are Spotfaces Used For?

Spotfaces are used for creating a smooth and flat surface.

For example, a component that has been made via casting may not have a smooth and flat surface like a machined component.

Therefore a spotface is machined, ensuring that a part to be connected to a component with a rough surface is located correctly.

Countersink

What Is a Countersunk Hole?

A countersink is a simple hole with a conical hole above it.

The 3D model below shows a countersink with the same countersink next to it cut in half:

What Is the Countersink Symbol?

The symbol for a countersink is ‘⌵’.

How Are Countersinks Shown on Drawings?

Countersunk holes are shown on drawings as follows – the bottom section view shows what the top dimension refers to.

If the countersink was a blind hole, the ‘Through’ above would be replaced with the depth of the pilot hole.

You may be wondering what the ‘Ø40.32′ is. This is known as the theoretical edge and this size countersink is for an M20 countersunk bolt.

You can find out the countersunk dimension required for countersunk metric bolts here.

What Are Countersunk Holes Used For?

Countersunk holes are used for applications where a countersunk bolt or screw is required.

Like counterbore holes, countersink holes ensure the bolt or screw sit below the surface.

Counterdrill

What Is a Counterdrilled Hole?

A counterdrilled hole is like a countersunk hole but there is a recess above it.

The 3D model below shows a counterdrilled hole with the same counterdrilled hole next to it cut in half:

What Is the Counterdrill Symbol?

The symbol for a counterdrill is the same as a countersink (⌵) but there will be a depth stated as well.

How Are Counterdrills Shown on Drawings?

Counterdrilled holes are shown on drawings as follows – the bottom section view shows what the top dimension refers to.

A counterdrill can also be stated as C’DRILL or CDRILL on an engineering drawing.

Taper

What Is a Tapered Hole?

A tapered hole is one where the start diameter of the hole is a different to the end diameter of the hole.

Below is a 3D model of a tapered hole next to the same hole cut in half:

What Is the Tapered Hole Symbol?

The taper symbol is:

How Are Tapered Holes Shown on Drawings?

Tapered holes can be shown in a number of different ways on engineering drawings.

An example of a drawing showing a tapered hole is below:

The taper in the above example is 1:5. The end diameter has been left on for reference (in brackets).

To work out what the end diameter use the equation:

Tapered holes can also be shown without the taper symbol when dimensioning using an angle is more important:

What Are Tapered Holes Used For?

Tapered holes have lots of different engineering applications.

A common example is using a tapered hole to secure cutting tools or other tool holders.

Screw Clearance

What Is a Screw Clearance Hole?

A screw clearance hole is exactly that.

It’s a simple hole that has a slightly bigger diameter than the screw thread so that the screw can pass through.

What Is the Screw Clearance Symbol?

Screw clearance holes are shown as simple holes, and use the ‘Ø’ diameter symbol.

How Are Screw Clearance Holes Shown on Drawings?

Screw clearance holes are shown as simple holes:

What Size Screw Clearance Hole Should I Use?

You can use the metric guide for screw clearance holes or the imperial guide for screw clearance holes.

What Are Screw Clearance Holes Used For?

Screw clearance holes are used for components that need the screw or bolt to go through them.

An example of this is a bracket that needs to be attached to a block.

The bracket has screw clearance holes so that a bolt can pass through it.

The bolt is threaded into the block which has a tapped hole.

The compression force of tightening the bolt fixes the bracket to the block.

Tapped Hole

What Is a Tapped Hole?

A tapped (or threaded) hole is a hole that has a thread.

What Is the Tapped Hole Symbol?

The symbol for a tapped hole depends on the standards used. For metric holes, the diameter symbol is replaced with an ‘M’. For example a tapped hole for an M8 bolt would be ‘M8’.

For different types of threads, for example a Unified National Fine (UNF) thread, the numerical dimension is shown first then the thread type “UNF”.

How Are Tapped Holes Shown on Drawings?

Tapped holes are shown on drawings as follows:

The “2.5” is the pitch of the thread, which is the distance in millimeters between each thread.

For a UNF thread, a through 1½” 12 hole would look like this:

The ’12’ means that there are 12 threads per inch.

What Are Tapped Holes Used For?

Tapped holes are used to hold threaded components. Examples include screws, bolts and threaded rods.

Enjoyed this post? Check out our types of welds post.

Conclusion

We hope you enjoyed our new types of holes guide.

Now we’d like to hear what you have to say:

Which type of hole did you learn today?

Or which type of hole do you use most often?

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