Blind Hole vs. Through Hole CNC Machining: How to Choose the Right Hole Type

When designing a CNC-machined part, should you use a blind hole or a through hole?

The answer depends on how the part will be assembled, whether the opposite surface must remain closed, and how the hole will be machined and inspected.

Understanding blind hole and through hole CNC machining helps engineers select the right hole design, avoid unnecessary manufacturing costs, and reduce assembly problems.

A blind hole starts at one surface and stops inside the material. It does not pass completely through the part.

A through hole passes entirely through the material and opens on both sides.

For example, a hole for a screw that must not appear on the opposite side of an aluminum housing is often designed as a blind hole. A hole for a bolt that passes through a mounting plate and is secured with a nut is usually a through hole.

Neither type is automatically better. The correct choice depends on the function of the part.

Blind holes are useful when the hole must stop at a defined depth.

Common applications include:

1.Threaded holes in aluminum housings

2.Mounting holes where the opposite surface must remain intact

3.Dowel-pin holes for positioning

4.Holes for springs, sensors, or locating components

5.Enclosures where a screw must not protrude through the exterior surface

Imagine a housing that needs an M6 screw to secure a cover.

If the screw only needs to enter from the top, a blind tapped hole can provide the required thread while keeping the opposite surface closed.

However, the drawing must specify the required usable thread depth. The total drilled depth may need to be greater because a conventional drill produces a conical tip at the bottom, and the tap needs additional clearance.

A blind hole that is too shallow may not provide sufficient thread engagement. One that is too deep may leave insufficient material beneath the hole.

Through holes are suitable when a fastener, pin, shaft, or other component must pass completely through the workpiece.

Common applications include:

1.Bolt-and-nut assemblies

2.Mounting plates

3.Positioning brackets

4.Clearance holes

5.Cable and component access

6.Alignment features

Suppose an aluminum plate is 8 mm thick and needs a clearance hole for a bolt that will be secured with a nut on the opposite side.

A through hole is normally the straightforward choice because the bolt passes through the plate and the nut provides the fastening force.

The hole does not need to hold internal threads, and chips can generally exit through the opposite face during drilling.

For reference, our published CNC-machined aluminum mounting plate project includes a central through hole, multiple mounting holes, and stepped hole features. It demonstrates how different hole geometries can serve different assembly purposes on the same component.

Through holes are often simpler to drill because the tool can pass through the material and chips can exit the opposite side.

Blind holes require closer control of the final depth. Depending on the geometry, they may also require peck drilling, special tapping tools, or additional depth inspection.

However, the complete design matters more than the hole type alone.

For example:

1.A deep blind hole with a tight depth tolerance may require additional machining and inspection.

2.A through hole may need deburring on the exit side.

3.A through hole in a finished enclosure may require a plug or another operation to close the opening.

4.A blind hole may preserve a finished exterior surface but require more careful depth control.

For this reason, blind holes are not always more expensive overall. The best design minimizes total manufacturing and assembly work while meeting the functional requirements.

In a through hole, chips can generally leave through the opposite side.

In a blind hole, chips must be removed through the opening. Deep blind holes therefore require more attention to chip evacuation, coolant access, and tool selection.

A standard twist drill produces a conical bottom rather than a perfectly flat floor.

If a blind hole requires a specific usable depth, the drill-point depth must be considered separately.

If the design requires a flat bottom, another machining operation may be needed.

For blind tapped holes, distinguish between:

1.Required full thread depth

2.Total drilled depth

3.Remaining material below the hole

These are different dimensions.

Through holes may produce exit-side burrs that need to be removed. Blind holes avoid breakthrough on the opposite face but still require control of the hole entrance, internal surface, and bottom condition.

The inspection method should match the dimensions and features that actually affect assembly.

A clear drawing helps the manufacturer select the correct process and avoid unnecessary clarification.

For a blind hole, specify the diameter, required depth, thread specification where applicable, and any important bottom or remaining-wall requirements.

For a through hole, specify the diameter, tolerance, location, and any countersink, counterbore, chamfer, or deburring requirements.

For both types, identify the critical datum references and hole-position tolerances.

Do not apply unnecessarily tight tolerances to every hole. Focus on the dimensions that control fit and function.

Our Custom CNC Machining Services support drawing-based components requiring CNC milling, drilling, and threading.

The difference between a blind hole and a through hole is simple: a blind hole stops inside the material, while a through hole passes completely through it.

The design decision should be based on assembly, surface requirements, thread engagement, chip evacuation, inspection, and total manufacturing cost.

Choose the hole type that solves the assembly problem with the least unnecessary manufacturing work.

Have a CNC-machined component with blind holes, through holes, threaded features, or precision mounting holes?

Send your 2D drawing, 3D CAD file, material, quantity, and critical tolerance requirements.

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