Through Hole in CNC Machining

In CNC machining, a through hole is a hole that passes completely through a part. The hole starts on one surface and opens on the opposite surface. Both ends of the hole are open.

In simple words, if a hole goes all the way through the material, it is a through hole.

On an engineering drawing, a through hole is often shown with the word THRU or THROUGH.

This means the machinist must make a hole with a diameter of 10 mm, and the hole must pass completely through the material.

A Simple Through Hole Example

Imagine an aluminum plate that is 20 mm thick. The engineering drawing shows:

The machinist must make an 8 mm diameter hole through the full 20 mm thickness. The drill enters from the top surface and continues until it comes out from the bottom surface.

If the drawing shows:

this means the part has four through holes, and each hole has a diameter of 8 mm.

Through Hole vs. Blind Hole

A through hole is different from a blind hole.

A through hole passes completely through the part. A blind hole stops inside the material and does not open on the opposite side.

For example, imagine a steel block that is 30 mm thick.

How Is a Through Hole Made?

Drilling is the most common way to make a through hole. A drill enters the material and continues until it passes through the bottom surface.

However, drilling is not always the final machining process. The required process depends on the size, tolerance, surface quality, material, and function of the hole.

For example, a normal clearance hole for a bolt may only need drilling. A precision hole for a locating pin may need drilling first and then reaming to get better size accuracy and surface quality.

Larger or high-accuracy holes may also need boring or other finishing operations.

Why Does the Drill Go Past the Bottom Surface?

When machining a through hole, the drill normally travels slightly past the bottom surface of the part.

This is because the end of a normal drill has a pointed shape. If the drill stops exactly at the bottom surface, the hole may not be fully open at its complete diameter.

For example, if an aluminum plate is 10 mm thick, the drill may need to travel deeper than 10 mm to make sure the hole is completely through.

The machinist must therefore make sure there is enough space below the part. The tool must not hit the fixture, vise, or machine table.

Through Threaded Holes

A through hole can also contain an internal thread.

For example:

This means an M6 × 1 thread goes completely through the part. It is often called a through threaded hole or through tapped hole.

A through threaded hole is different from a blind threaded hole. In a blind threaded hole, the thread stops at a specified depth inside the part.

Through threaded holes can often be easier to machine because the tap or thread cutting tool can move through the hole. Chips also have more space to leave the cutting area.

Burrs on Through Holes

When a drill breaks through the bottom surface, it may leave a small sharp edge or extra material around the hole. This is called a burr.

For many CNC machined parts, these burrs need to be removed after drilling. A machinist may use a deburring tool or make a small chamfer around the hole edge.

For example, a drawing may show:

This can mean that a 0.5 mm chamfer is required on the specified edge. A drawing may also include a general note such as “Break sharp edges.”

What Should You Check on a CNC Drawing?

When you see a through hole on an engineering drawing, do not only check the hole diameter. Other information can change how the hole must be machined and inspected.

Important details can include:

• Hole diameter
• Number of holes
• Hole position
• Dimensional tolerance
• Thread size
• Chamfer
• Countersink
• Counterbore
• Surface finish requirements

For example:

This still means four 8 mm through holes, but the H7 tolerance requires much tighter control of the hole size. Simple drilling may not be enough, so reaming or boring may be required.

Common Uses of Through Holes

Through holes are very common in CNC machined parts. They can be used for:

• Bolts and screws
• Locating pins
• Part assembly
• Mounting components
• Alignment
• Air or fluid flow

For example, two metal plates may have matching through holes. A bolt can pass through both plates and connect them together.