Blind Hole in CNC Machining

In CNC machining, a blind hole is a hole that goes into a part but does not pass completely through it.

The hole starts from one surface and stops at a certain depth inside the material.

For example, if you have a steel block that is 30 mm thick and drill a hole 15 mm deep, the hole does not come out from the other side. This is a blind hole.

A drawing may show:

This means the machinist needs to make a hole with a diameter of 10 mm and a depth of 15 mm.

Blind Hole vs. Through Hole

A blind hole is different from a through hole .

A through hole goes completely through the part and is open on both sides. A blind hole stops inside the part and is open on only one side.

For example, imagine an aluminum plate that is 20 mm thick.

A Simple Blind Hole Example

Imagine a steel block with these dimensions:

• Length: 80 mm
• Width: 50 mm
• Thickness: 30 mm

The drawing shows:

This means the part has four blind holes. Each hole has a diameter of 6 mm and a depth of 15 mm.

Because the block is 30 mm thick, the holes stop inside the material and do not reach the bottom surface.

The machinist must carefully control the drilling depth so the holes are not too shallow or too deep.

How Is a Blind Hole Made?

The most common way to make a blind hole is drilling.

The drill enters the material and stops at a specified depth. Unlike a through hole, the drill cannot simply continue through the bottom of the part.

This means depth control is important. The CNC machine must know exactly how far the drill should move into the material.

For example, if the drawing requires:

the drilling operation must stop at the correct depth.

If the hole is too shallow, a screw, pin, or other part may not fit correctly. If the hole is too deep, the wall at the bottom may become too thin, or the drill may break through the opposite surface.

For more detailed information about drilling depth, tapping, chip removal, and design considerations, see our complete guide to blind holes in machining .

Drill Point and Blind Hole Depth

A standard drill normally has a pointed tip. Because of this, the bottom of a drilled blind hole is usually not completely flat.

This is important when reading an engineering drawing.

For example, a drawing may require a blind hole with 15 mm of straight cylindrical depth. The drill may need to travel deeper than 15 mm because the drill tip extends below the full-diameter part of the hole.

Engineers and machinists should therefore understand whether the specified depth refers to:

• Total hole depth
• Full-diameter depth
• Drill-tip depth

For simple parts, this difference may not be important. For precision parts, it can affect assembly and part function.

Blind Threaded Holes

A blind hole can also contain an internal thread.

For example:

This means an M6 × 1 threaded hole is required to a specified depth. It is often called a blind threaded hole or blind tapped hole.

Blind threaded holes require more attention than through threaded holes because the tapping tool cannot continue through the bottom of the part.

The machinist must control the thread depth and provide enough extra space below the usable thread.

For example, if a screw needs 10 mm of usable thread, the drilled hole may need to be deeper than 10 mm. This gives space for the tap and helps prevent the tool from hitting the bottom.

You can learn more about thread types, tapping, and design considerations in our guide to threaded holes .

Chip Removal in Blind Holes

Chip removal is important when machining blind holes.

During drilling or tapping, metal chips can collect at the bottom of the hole. Unlike a through hole, there is no open bottom for the chips to leave through.

Too many chips inside the hole can cause problems such as:

• Tool overheating
• Drill or tap breakage
• Damage at the bottom of the hole
• Incorrect hole depth
• Poor surface quality

Machinists may use coolant, compressed air, suitable cutting tools, or special drilling cycles to help remove chips.

What Is Peck Drilling?

CNC machines often use a peck drilling cycle for deeper blind holes.

Instead of drilling to the full depth in one movement, the drill moves down in smaller steps and moves back up between cuts.

This helps break the chips and move them out of the hole. It can also reduce heat around the cutting tool.

Peck drilling is especially useful for deeper holes where chip removal becomes difficult.

Why Are Blind Holes Used?

Blind holes are commonly used when a designer does not want a hole to appear on the opposite surface of the part.

Common uses include:

• Screws and bolts
• Threaded connections
• Locating pins
• Mounting components
• Hidden fasteners
• Precision assemblies

For example, a machine housing may use blind threaded holes to mount a cover. The screws enter from one side, while the opposite surface remains closed.

This can improve the appearance of the part and can also help keep the inside of an assembly closed.

What Should You Check on a CNC Drawing?

When checking a blind hole on an engineering drawing, do not only look at its diameter.

Important information can include:

• Hole diameter
• Hole depth
• Hole position
• Number of holes
• Diameter tolerance
• Depth tolerance
• Thread size
• Thread depth
• Bottom shape
• Chamfer or countersink

For example:

This means four 8 mm blind holes are required to a depth of 20 mm. The H7 tolerance also requires tighter control of the hole diameter.

Simple drilling may not be enough, so another finishing process such as reaming or boring may be needed.

Common Problems with Blind Holes

Blind holes can be more difficult to machine than through holes because the tool and chips cannot simply pass through the bottom of the part.

Common problems include:

• Incorrect hole depth
• Chips collecting at the bottom
• Drill or tap breakage
• Poor bottom surface
• Not enough usable thread depth
• Accidentally drilling through the opposite wall

Correct tool selection, CNC programming, depth control, and chip removal can help prevent these problems.