What are the different types of holes in engineering?

Types of holes

Bore, Counterbore, Drilled holes and many other types of holes are used in engineering. If you’re confused by their symbols, can’t identify them in engineering drawings and are simply unaware of what are the different types of holes in engineering? then this guide is for you!

I’ve been interpreting drawings and symbols easily with a small intuition that I’ll share with you, which will help you identify callout symbols of holes, their use and difference between them. 

What is a Hole In Engineering?

A hole in engineering is a circle cut out or an opening in an object. In engineering holes can be drilled, punched or made using a rotating part fed into a stationary cutting tool. It’s important to note here that hole features are completely dependent on the type and shape of the cutting tool. 

Holes mirror the diameter, depth and geometry of the tool. When a hole is drilled to a certain depth, which is less than the depth of the object, it’s called a blind hole and when the hole passes from both sides of the object, it’s called a through hole. 

Before we delve into the common types of holes. I’ll start with a simple hole that will help you understand all the other holes and their callout symbols. 

Common types of holes

#1 Simple Hole

A simple hole is a cutout in an object with a circular cross-section. The hole has a constant diameter.

The Callout Symbol of Simple Hole

Simple holes can be read on a drawing using the ‘Ø’ diameter symbol. 

On technical drawings, the callout symbol for simple holes is read using the symbol, diameter and depth of the hole. 

The Callout Symbol of Simple Hole

Simple holes that pass through both sides of the object are through holes whereas, holes that do not pass all the way are called blind holes. 

Simple holes size

In this drawing a 20 mm diameter hole that passes through the object is represented by Ø 20.00 Through and another hole with 30.00 mm depth. 

16.00 mm diameter whose depth is less than the thickness of the block is represented by Ø 16.00 “depth symbol” 30.00.

Flat hole

For this drawing both holes are flat holes meaning their ends are not tapered or showing a conical ending, which is more commonly seen on drawings. (Conical and flat holes will be discussed in blind holes)

Application of Simple Hole

Simple holes are commonly used for fastening. These holes are great for holding temporary plastic fasteners. In addition to this, they can also hold bolts and screws and act as assembly points. 

Simple holes are also used in certain assemblies to provide a passage for air or fluid. They are also used to align multi-part assemblies.

#2 Blind Hole

A blind hole is any cut out in a material that does not pass through the material. Blind holes are always drilled or milled to a certain depth inside the material. They are closed on one end and have an opening on the other end. 

The Callout Symbol Of Blind Hole

The callout symbol is its diameter ‘Ø’ and a depth symbol. These symbols have the corresponding measurements specified in the callout. 

The Callout Symbol of Simple Hole

On technical drawings, blind holes are shown using the diameter symbol and the depth symbol.

blind holes

In this drawing two blind holes with different depth and diameter are shown. 

First blind hole has a curved ending, which is typical for dowel holes. Most machining methods to make holes often leave a curved or conical end. You’re more likely to see holes of the first type. 

Application Of Blind Hole

  • To hide screws and fasteners 

  • For tapping and threading at a later stage

  • To allow assembly and fittings

#3 Through Hole

A through hole extends from one side of the object and passes through to the other side. A through hole is open on both ends.

The Callout Symbol Of Through Hole

Through holes have a hole callout with the diameter symbol ‘Ø’ and the words ‘THRU’

through holes

In engineering drawings, through holes are either shown by Ø THRU or Ø Through along with the diameter of the hole. 

through holes pass

In engineering drawings, through holes are either shown by Ø THRU or Ø Through along with the diameter of the hole. 

Application Of Through Hole

  • To accomodate any passage of wires in cases, housings and components
  • To allow fluid and air to pass
  • Sustain stresses inside an object.

#4  Interrupted Hole

An interrupted hole is any hole that is interrupted by another feature. This can be another hole or any other part feature like an extrusion. Interrupted holes are cut using a drill or milling tool. All holes are coaxial and drilled in features in a single sequence. Where there is no material, the tool passes straight to the next feature. 

This is best depicted in the image below. 

what is Interrupted Hole

The Callout Symbol Of Interrupted Hole

There’s no specific callout for interrupted holes and depending on the types of holes drilled, the standard callouts of other holes can be used. 

In engineering drawings, through holes are either shown by Ø THRU or Ø Through along with the diameter of the hole. 

Interrupted Hole

Application Of Interrupted Hole

  • Interrupted holes are used in complex parts and can be threaded for a bolt or screw.

#5 Counterbore Holes

Any hole with a constant diameter attached to another hole with a larger diameter above it is called a counterbore hole. The purpose of the larger hole above is to hide the screw head. 

The Callout Symbol Of Counterbore Holes

Callout symbol of a counterbored hole is ‘’.

The Callout Symbol Of Counterbore Holes

In technical drawings counterbored holes are shown using section views and corresponding hole callout.

Counterbore Holes

Top view of a counterbored hole

Counterbore Holes size

Counterbore hole with callout symbols

The counterbored hole above is a blind hole with a 30.00 mm depth and 13.50 mm diameter. 

The second line shows the counterbore hole with counterbore symbol ⌴ and counterbore diameter of 23.79 mm with a thickness or depth of 7.50 mm.

The counterbore is drilled first and then the rest of the smaller hole.

Application Of Counterbore Holes

  • As standard cutouts for screws readily available

  • For socket head screws to save space instead of hex head bolt.

#6 Countersink Holes

Any hole with a constant diameter attached to another hole with a larger diameter above it is called a counterbore hole. The purpose of the larger hole above is to hide the screw head. 

Top View of Countersink Holes
Countersink Holes

The Callout Symbol Of Countersink Holes

Countersunk holes have the callout symbol ‘’.

Countersink Holes

On a technical drawing, countersink hole is read using the ‘⌵’ callout. 17.30 is the theoretical edge for an M8 countersunk hole. 90° is the angle between the two edges of the cone shaped hole. 

Drawing Of Countersink Holes

Application Of Countersink Holes

Countersunk holes are used with countersunk bolts for a smooth and flat surface. Countersink hole making is a fairly tedious process and requires precision cnc milling machines. These holes are diverse and can be made in any material including fiberglass, wood and metal.

#7 Counterdrill Holes

Counterdrilled hole has a conical shape with a recess on top (another hole). 

Counterdrill Holes

The Callout Symbol Of Counterdrill Holes

Counterdrilled holes have the same callout symbol as a countersink hole or a counterbore. The difference is that there is a depth mentioned with counterdrilled holes.

Counterdrill hole can be read using the callout. The first line shows the hole diameter and depth. 

The second line shows the counter drill diameter with a depth of 20 mm mentioned.

drawing of Counterdrill hole

Application Of Counter Drill Holes

Counterdrilled hole provides seamless finish for bolts and fasteners. The can also allow space for a rubber seal between the fastner and object.

#7 Tapered Holes

Tapered hole has a diameter changes as the hole progresses. Tapered hole usually gets narrower down the depth of the hole

Tapered hole is made using a modified drill tip angle. Many CNC milling machines are also able to make tapered holes. 

The Callout Symbol Of Tapered Holes

Despite the different methods of tapered hole cuts, the callout symbol is a triangle with a line passing through.

Tapered Holes symbol

Application of Tapered Holes

  • Press fit assemblies. 

  • As tool holders because of the conical hole

#8 Screw Clearance Holes

Screw clearance holes allow the bolt or screw to pass through with ease. Screw clearance holes or clearance holes have slightly bigger diameter than the screw shank. The bolt passes through without making contact with the material and is usually caught on the other end by a nut and tightened. 

A screw clearance hole doesn’t allow the screw head to pass and only differs from a standard screw hole by a slightly larger diameter.

Clearance Holes

The Callout Symbol Of Clearance Holes

The callout symbol for a clearance hole is just the diameter symbol and Through. 

Application Of Clearance Holes

  • Screw clearance for a bolt

  • In part assembly where a screw clearance hole can be used to fasten two parts using a bolt.

#9 Tapped Holes

Tapped holes have threads that run along the depth of the hole. These internal threads are cut inside the holes and the process that creates a tapped hole is called tapping. These threaded holes can be identified easily by spotting internal grooves. 

The Callout Symbol Of Tapped Holes

A tapped hole can have different callouts. Tapped holes are usually made according to standard bolts and screws available in the market. As such, tapped hole callouts follow a given standard. The most commonly used standard for a tapped hole is Metric. Here, the information for a tapped hole is represented slightly differently. 

drawing of Tapped Holes

In this drawing the hole callout shows the hole and the tapping. The tapped hole is made using the Metric Standard. This is shown by ‘M’ followed by the diameter, tolerance, depth symbol and depth in mm. 

Sometimes, threads in metric can be shown as M20 x 3. Here 3 is the pitch of the thread. Pitch is the distance between successive threads. 

Tapped Holes

Application Of Tapped Holes

A tapped hole only functions as a fastening component. Threads can hold screws and bolts and are often used to join two parts using a threaded fastener. 

#10 Threaded Holes

Threaded hole as the name suggests is any hole with threads. Threaded hole and tapped hole are often used interchangeably. Threaded hole has screw threads cut into it using a threading tool called tap. Tapping is the process of threading blind holes and the threaded depth can vary from below the surface to through the material. 

Threaded hole is a more generic term used for any hole with internal threads. Threads can be made using dies, taps and thread milling cycle. 

The Callout Symbol Of Threaded Holes

Threaded holes are identified on engineering drawings using the Metric Standard. Often shown by an M where a thread is present.

The Callout Symbol Of Threaded Holes

When the threads are on the outside, such as a threaded rod, they are called external threads. 

Application Of Threaded Holes

Threaded holes are made for bolts and screw thread assemblies. There are many types of threaded fasteners that can hold threaded components by screwing into the threads tightly. Threaded holes are reusable and made for components that need to be disassembled at some point. 

#11 Spotface Holes

Spotface holes have a small counterbore hole on top of a drilled hole. This larger shallow hole is great for stress distribution and holding washers. 

Spotface Holes

The Callout Symbol Of Spotface Holes

Notice the counterbore symbol in the spotface hole callout. 

The Callout Symbol Of Spotface Holes

Notice the counterbore symbol in the spotface hole callout. 

drawing of spotface hole

Application Of Spotface Hole

  • Spotface hole provides a flat surface

  • It hides any protrusions resulting from casted components

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