A blind hole is a hole that goes only partway through a material, creating a cavity with an opening on one side and a closed bottom on the other. Think of a cup: open at the top, sealed at the bottom. This distinguishes it from a through hole, which passes completely through a workpiece like a straw through a sheet of paper. Blind holes are everywhere in manufacturing, from the threaded holes in your phone’s case to hydraulic valve blocks in heavy machinery.
How Blind Holes Differ From Through Holes
The core difference is geometry. A through hole creates an open channel from one side of a part to the other. A blind hole stops at a specific depth, leaving the opposite surface untouched. This simple distinction drives most of the design decisions around when to use each type.
Through holes are the go-to choice for bolt-and-nut assemblies, where a fastener needs to pass completely through one or more parts. They’re also used as conduits for wires, fluids, or cooling lines, and in aerospace and automotive applications, arrays of through holes reduce a component’s weight without sacrificing too much structural strength.
Blind holes serve a different purpose. They fasten and contain rather than connect and pass through. The most common reason to choose a blind hole is surface integrity: on a device enclosure or panel, every threaded hole for a screw must be blind so the outer surface stays clean and smooth. Blind holes also work as sealed receptacles in hydraulic and pneumatic systems, where the closed bottom acts as a natural seal at the end of a fluid channel. Dowel pins pressed into blind holes provide high-precision alignment between parts, and set screws or springs that need a specific insertion depth rely on blind holes to define that limit.
Common Applications
You’ll find blind holes in nearly every category of manufactured goods. Consumer electronics use them extensively: the screws holding together a laptop shell, a game controller, or a kitchen appliance thread into blind holes so no fastener tips poke through the visible exterior. In automotive and industrial equipment, blind holes house sensors, retain springs, and serve as pressure ports in valve bodies.
In printed circuit board (PCB) design, blind holes appear as “blind vias,” tiny plated holes that connect an outer layer of a multi-layer board to one or more inner layers without passing all the way through. These are common in high-density interconnect (HDI) boards, where space is tight. Blind vias free up routing room on the board surface, make it possible to connect fine-pitch components, and reduce signal interference by terminating the via at the last connected layer rather than running it through unused layers.
Why Blind Holes Are Harder to Machine
Drilling a through hole is relatively straightforward: chips (the tiny curls of material cut away by the drill) fall out the exit side, and coolant flows freely. A blind hole has no exit. The tool operates inside a confined cavity, and every chip it generates has to come back out the way it went in.
As chips accumulate in the hole, heat builds rapidly. In tough materials like stainless steel or titanium, that heat accelerates tool wear and can harden the surrounding material, making the cut even more difficult. To manage this, machinists use peck drilling cycles, where the drill periodically retracts from the hole to clear chips and allow coolant to reach the cutting edge. Titanium often requires through-tool coolant systems that deliver fluid directly to the tip of the drill under pressure.
Depth control adds another layer of complexity. A standard twist drill has a pointed tip, typically ground to a 118° or 135° angle, which means the bottom of a drilled blind hole isn’t flat. It tapers into a cone. If a flat bottom is required, a secondary operation with an end mill or flat-bottom drill is needed. Engineering drawings for blind holes usually specify a minimum depth callout with a note not to break through, since extreme precision at the bottom is less critical than ensuring the hole is deep enough for its intended purpose.
Threading a Blind Hole
Tapping (cutting threads inside) a blind hole is one of the more technically sensitive operations in machining. In a through hole, chips simply exit the far side as the tap advances. In a blind hole, they’re trapped at the bottom. If those chips compact, they can snap the tap, which is a serious problem since a broken tap lodged in a finished part is often impossible to remove without scrapping the part entirely.
The solution is a spiral-flute tap, designed specifically for blind hole work. Its helical flutes pull chips upward and out of the hole as it cuts, preventing compaction at the bottom. For getting the maximum number of usable threads close to the bottom of the hole, bottoming taps with a very short chamfer (the tapered lead-in at the tip) are used after an initial pass with a standard tap.
Thread Engagement Depth
Because a blind hole has a finite depth, engineers need to ensure enough thread engagement to hold the fastener securely. The general rule is that the minimum engagement depth depends on the material the hole is tapped into. Harder materials grip threads more effectively, so they need less depth. Softer materials need more.
- Hardened steel: 1.0 times the bolt diameter
- Low carbon steel: 1.2 times the bolt diameter
- Cast iron: 1.5 times the bolt diameter
- Aluminum alloys (like 6061-T6): 1.5 times the bolt diameter
- Soft or cast aluminum: 2.0 times the bolt diameter
- Brass or copper alloys: 1.6 times the bolt diameter
- Magnesium alloys: 2.2 times the bolt diameter
- Reinforced plastics: 2.5 times the bolt diameter
- Soft plastics (nylon, PVC): 3.0 times the bolt diameter
So a 12mm bolt threaded into an aluminum part needs at least 18mm of full thread contact. There’s an important catch: the first one or two threads at the opening of the hole and at the tip of the bolt are typically incomplete and don’t provide real holding power. Engineers subtract about two full thread pitches from the measured depth to account for this. Fine threads, which pack more threads per inch, can also be misleading. A fine thread may need eight to ten threads to match the strength that a coarse thread achieves with six.
How Blind Hole Depth Is Measured
Most blind holes on engineering drawings specify a minimum depth rather than an exact target. The practical reason is that the conical bottom left by a standard drill bit makes “depth” somewhat ambiguous: do you measure to the shoulder where the full-diameter portion ends, or to the very tip of the cone? Convention typically calls for measuring to the full-diameter depth, ignoring the tapered portion.
For shop-floor inspection, depth calipers or a height gauge with a cylindrical probe are the most common tools. The probe drops into the hole and catches the shoulder at the transition from the cylindrical wall to the tapered bottom. For higher-precision work, coordinate measuring machines can probe the hole and calculate depth mathematically. In most cases, though, the tolerance is generous enough that a simple mechanical measurement is sufficient.