블라인드 홀 탐구: 특징, 목적, 응용 및 가공 방법

Blind holes are small design details, but they can strongly affect how a CNC part is manufactured, assembled, and inspected. For buyers and engineers, the key question is not only what a blind hole is, but also when it is worth using instead of a through hole, counterbore, countersink, or through tapped hole. A good blind hole design can protect the outside surface, create hidden fastening points, control fluid paths, and improve appearance. A poor design can increase machining time, trap chips, weaken taps, and make depth inspection more difficult. This guide explains blind hole features, purposes, CNC applications, machining methods, design rules, and common comparisons so you can choose the right hole type for functional CNC machined parts.

What Is a Blind Hole?

A blind hole is a machined hole that enters a part from one side and stops at a specified internal depth. It does not break through the opposite surface. In a CNC machined part, a blind hole can be drilled, bored, milled, reamed, or tapped depending on the required diameter, tolerance, bottom shape, and thread function. The most important feature is not only that the hole has a closed end, but also that the usable depth must be controlled accurately. A blind hole may look simple in a CAD model, but it gives the machinist less room for error because chips, coolant, and the tool tip cannot pass through the part.

In technical drawings, a blind hole should be called out with at least the hole diameter and depth. If it is threaded, the drawing should also specify thread size, thread depth, and drill depth. This matters because a drilled blind hole usually has a conical bottom, while the usable cylindrical depth stops before the drill point. If the designer only models a depth without explaining whether that depth includes the cone, the machine shop may interpret the requirement differently. Clear callouts reduce quote delays, rework, and assembly problems.

Features of Blind Hole

Closed bottom and controlled depth

The most visible feature of a blind hole is the closed bottom. This closed end protects the far surface of the part, prevents visible holes on the outside, and can help keep one side sealed. However, it also makes the operation more sensitive than a through hole. Chips collect at the bottom instead of falling out. Heat is trapped near the cutting zone. A drill, boring bar, tap, or thread mill must stop before hitting the bottom. For deep blind holes, even a small programming or tool-length error can damage the part or break a tool.

Different bottom shapes

Blind holes do not all have the same bottom geometry. A standard twist drill creates a cone-shaped drill point. This is fast and economical, but the cone reduces usable depth for bolts, dowel pins, inserts, and full thread engagement. A flat-bottom blind hole is produced with an end mill, flat-bottom drill, boring tool, or secondary finishing operation. It is more useful when a pin, shoulder screw, spring, or insert must seat on a flat surface. A blind tapped hole adds internal threads, but it also needs extra clearance below the last full thread so the tap or thread mill has a safe runout area.

• A conical bottom is usually cheaper and faster, but less suitable for full-depth seating.
• A flat bottom improves functional depth, but may require milling or a special drill.
• A threaded blind hole needs both thread depth and non-threaded clearance below the thread.
• A deep blind hole may require peck drilling, high-pressure coolant, or slower feeds to manage chips.

What is the Purpose of Blind Holes?

Why designers choose blind holes

Designers choose blind holes when the far side of a component must remain intact. This can be important for appearance, sealing, strength, safety, or assembly direction. For example, a customer-facing housing may need screws from the inside while keeping the outer face clean. A manifold block may need threaded ports that stop before intersecting another passage. A fixture plate may need precise dowel holes that locate a mating part without weakening the underside. In these cases, a through hole would be easier to machine, but it may create a visible opening, leak path, burr on the far side, or unwanted interference with another feature.

Functional reasons in CNC parts

Blind holes are also used when the fastener length, pin engagement, or insert position must be limited. A blind threaded hole can prevent a screw from protruding beyond the opposite face. A blind dowel hole can locate a part while controlling insertion depth. A blind bore made on a lathe can create a controlled internal pocket, bearing seat, or hydraulic cavity. The purpose is therefore not only to create a hole; it is to create a controlled internal feature that supports assembly without opening the whole part.

Applications of Blind Holes in CNC Parts

Mechanical assemblies and precision components

Blind holes are common in CNC machined parts because many assemblies require internal fastening or locating features. Aluminum enclosures, sensor housings, robotic brackets, fixture plates, medical device components, automotive parts, aerospace brackets, and electronic heat sinks may all include blind holes. In these products, the blind hole often supports a cap screw, shoulder screw, dowel pin, threaded insert, alignment pin, spring, or sealed plug. For CNC buyers, the key question is usually not whether blind holes can be made; they are routine in milling and turning. The key question is how accurately the shop can control the depth, thread, surface finish, and bottom geometry in the chosen material.

Milled parts, turned parts, and multi-axis parts

On CNC milling machines, blind holes are often produced normal to a flat surface, but they can also be machined at angles using 4-axis or 5-axis positioning. On CNC lathes, a blind hole may appear as a blind bore or internal pocket along the part axis. Turning a blind hole is different from drilling a shallow mounting hole because the boring bar must cut inside a cavity without crashing into the shoulder. It also needs enough clearance for chips and tool deflection. In multi-axis parts, blind holes may be located near ribs, curved surfaces, or side walls, which makes tool access and inspection more important.

• CNC enclosures: hidden screw holes, internal bosses, and mounting holes.
• Fixtures and jigs: dowel pin holes, locating pockets, and clamp screw holes.
• Fluid and pneumatic blocks: stopped ports, plug holes, and controlled passages.
• Automotive and machinery parts: blind bores, bearing pockets, and threaded mounting points.
• Electronics and heat sinks: screws that must not break through visible or thermal surfaces.

How to Drill a Blind Hole?

Basic CNC drilling workflow

The typical CNC process starts with center drilling or spotting, followed by drilling to a programmed depth. If the blind hole requires tight diameter tolerance, the shop may add boring, reaming, circular interpolation, or a finishing end mill pass. If the hole is threaded, tapping or thread milling is done after the core hole is prepared. The final result depends on three depths: the programmed tool depth, the usable cylindrical depth, and the final thread depth. These depths are not always the same, especially when a twist drill creates a conical tip at the bottom.

Chip control and coolant strategy

Chip evacuation is the main practical difficulty. In a through hole, chips can exit through the far side. In a blind hole, chips must come back out through the same entrance used by the tool. If chips pack at the bottom, they can scratch the wall, damage the cutting edge, reduce depth accuracy, or break a drill or tap. Common strategies include peck drilling, through-tool coolant, air blast, high-pressure coolant, slower feed near depth, and toolpath pauses for chip clearing. For sticky materials such as aluminum, copper, some stainless steels, and plastics, chip welding and built-up edge may require sharper tools and better lubrication.

A practical machining sequence

• Spot the hole to prevent drill walking and improve positional accuracy.
• Drill slightly deeper than the required usable depth when the design allows clearance.
• Use peck drilling for deep or small-diameter blind holes to break and evacuate chips.
• Finish the hole with boring, reaming, or milling if diameter, roundness, or flatness is critical.
• For blind tapped holes, choose spiral flute taps or thread milling when chip evacuation and tap safety are important.
• Inspect depth, diameter, thread gauge engagement, and bottom condition before final assembly.

How to Design Blind Hole?

Design rules for CNC-friendly blind holes

A good blind-hole design gives the manufacturer enough space to cut, clear chips, and inspect the feature. Avoid specifying a blind hole that is exactly the same depth as the required fastener engagement unless there is a strong reason. Extra drill depth below the useful feature often reduces cost and risk. For tapped blind holes, leave a relief or unthreaded clearance at the bottom. For small tapped holes, this clearance becomes especially important because small taps are fragile and need room to reverse before the bottom. When strength is required, increase thread engagement in a controlled way rather than simply forcing threads to the very bottom.

Drawing callouts and tolerance control

Blind-hole drawings should be explicit. A common issue is that the model shows a hole, but the print does not state whether the specified depth is drill depth, full-diameter depth, thread depth, or minimum usable depth. For a non-threaded blind hole, call out diameter, depth, depth tolerance, bottom type if relevant, and positional tolerance if the hole locates a mating part. For a blind tapped hole, call out the thread size, thread depth, drill depth, and whether the thread is class-fit critical. If the hole receives a pin or insert, state the seating requirement and any bottom flatness or minimum full-depth requirement.

Buyer-friendly checklist

• Use standard drill sizes and thread sizes when possible to reduce cost.
• Avoid very deep blind holes unless the function truly requires them.
• Do not demand a perfectly flat bottom unless a part must seat there.
• Specify thread depth and drill depth separately for blind tapped holes.
• Leave enough wall and bottom thickness to avoid breakthrough or deformation.
• Confirm surface treatment needs, because coating or anodizing may behave differently inside deep blind holes.

Blind hole vs Through hole

Structural and functional differences

A blind hole stops inside the workpiece, while a through hole passes completely through it. This simple difference changes manufacturing cost, inspection method, chip control, and design function. Through holes are usually easier and cheaper because the tool can break through, chips have a better escape path, and the depth is less critical. Blind holes are selected when the opposite face must remain sealed, clean, strong, or free from burrs. They are also used when screw length must be contained inside the part.

CNC 가공성 비교

From a CNC machining perspective, through holes generally have better machinability. They allow simpler drilling, easier tapping, and simpler deburring. Blind holes require a more controlled cycle and more careful process planning. For threaded holes, through tapping can push chips forward and out of the part, while blind tapping must pull chips upward or avoid chip formation with a suitable method. This is why spiral flute taps, form taps in suitable ductile materials, or thread milling are often preferred for blind tapped holes. The tradeoff is clear: a blind hole can solve design problems, but it usually costs more than a comparable through hole.

Blind hole vs Counterbore

Why these two features are often confused

A blind hole describes depth termination: the hole does not pass through the part. A counterbore describes shape: a larger cylindrical recess is made around a smaller hole so a socket head screw, bolt head, plug, or bushing can sit below or flush with the surface. A counterbore may be combined with a through hole or a blind hole. For example, a designer may specify a counterbored through hole for a cap screw, or a counterbored blind hole for a screw that must remain completely inside the part.

CNC machining difference

Counterbores usually require a flat-bottom recess and a controlled shoulder, so they often involve end milling, counterbore tooling, or circular interpolation. A simple blind drilled hole may be faster because it can be made with a standard drill. However, if the blind hole needs a flat bottom, its machining method becomes closer to a counterbore operation. The important design question is whether the feature is defined by its stopped depth, its larger head recess, or both. If a screw head must sit flush, the counterbore diameter and depth are critical. If a thread must stop inside the part, the blind-hole drill depth and thread depth are critical.

Blind hole vs Countersink

Geometry and fastener seating

A countersink is a conical recess at the opening of a hole. It is used mainly for flat-head screws, chamfered entrances, deburring, or lead-in geometry. A blind hole, by contrast, is defined by the fact that it stops inside the part. The two can exist together: a blind tapped hole may have a countersink or chamfer at the entrance to help screw starting and remove sharp edges. However, a countersink does not make a hole blind, and a blind hole does not automatically include a countersink.

Manufacturing and design considerations

Countersinks are usually shallower and faster to machine than functional blind holes. Their important variables are included angle, outer diameter, and surface finish under the screw head. Blind holes require more attention to depth, bottom clearance, thread depth, and chip evacuation. If the purpose is only to remove a burr or help screw entry, a light chamfer or countersink is enough. If the purpose is to hide a screw head, a counterbore may be better than a countersink for socket head screws. If the purpose is to keep the screw from breaking through the far face, a blind tapped hole is needed. Mixing these terms on a drawing can cause a supplier to quote the wrong operation.

Blind tapped hole vs Through Tapped Hole

Thread function and assembly difference

A blind tapped hole has internal threads that stop before the opposite side of the part. A through tapped hole has threads that pass through the entire thickness. Both can hold screws, but their design behavior is different. A through tapped hole is usually easier to produce and inspect because the tap can pass through and chips can leave the part. A blind tapped hole is preferred when the far side must remain sealed or visually clean, when the screw must not protrude, or when another feature lies behind the threaded area.

Tap selection and thread depth

Blind tapping requires careful tool selection. A spiral point tap pushes chips forward, which works well for through holes but can pack chips into the bottom of a blind hole. A spiral flute tap pulls chips back out of the hole and is usually more suitable for blind tapping. Thread milling can be safer for expensive parts, hard materials, large threads, or short production runs because it lowers the risk of a broken tap trapped in the part. Form tapping can work in ductile materials and avoids chips, but it requires the right hole size, material behavior, and lubrication.

When to choose each one

Use a through tapped hole when the screw path can go all the way through and the far side burr, coating, or appearance is not a problem. Use a blind tapped hole when the far face must remain closed, when the hole is part of a sealed component, or when screw protrusion is unacceptable. If the product will be anodized or coated, consider how the finish will enter the internal blind thread. Deep blind threads may not receive the same finish quality as open features, so the drawing and supplier discussion should clarify whether coating inside the hole is functional or cosmetic.

결론

A blind hole is a practical CNC feature when a part needs hidden fastening, sealed surfaces, controlled pin depth, or internal pockets without breaking through the opposite side. Compared with through holes, blind holes require more attention to depth, bottom shape, chip evacuation, thread clearance, and drawing callouts. The best design is not always the deepest or most precise hole; it is the one that gives enough functional engagement while remaining easy to machine, inspect, and assemble.

FAQ

Is a blind hole common in CNC machining?

Yes. Blind holes are very common in CNC milling and CNC turning. They appear in housings, brackets, fixtures, manifolds, inserts, and precision mechanical parts. The complexity depends on depth, diameter, material, tolerance, thread requirement, and bottom shape.

Does a blind hole always need a flat bottom?

No. A drilled blind hole usually has a conical bottom. A flat bottom is only necessary when a screw, pin, spring, insert, or shoulder must seat on a flat surface. Adding a flat-bottom requirement can increase machining time and cost.

How should a blind tapped hole be specified on a drawing?

Specify thread size, thread class if needed, required thread depth, drill depth, and any bottom clearance or flat-bottom requirement. Avoid using only the CAD model to communicate this feature, because thread depth and drill depth are often different.

Is a blind hole stronger than a through hole?

Not automatically. A blind hole can preserve the far surface and avoid a through opening, but strength depends on remaining wall thickness, material, thread engagement, load direction, and surrounding geometry. A poorly designed blind hole can be weaker or more expensive than a through hole.