14 Types of Holes in Engineering
Holes are one of the most common features in engineering and manufacturing. You see them in almost everything but not all holes are the same. Some are made for screws, some for precise alignment, and others for fluid to flow.
In this guide, you’ll learn about the different types of holes in engineering.
What is a Hole Feature in Engineering?
A hole is a cut or opening in a material, usually made by drilling, punching, or casting. Holes serve many purposes, including fastening, weight reduction, ventilation, and alignment of parts.
The size, shape, and depth of a hole depend on its function in a design. Engineers use standard terms to define holes such as diameter, depth, countersink, counterbore, and tolerance. These details ensure that holes fit properly with other components.
Fits, Call Outs, and Tolerances of Engineering Holes
When designing holes, you need to consider fits, callouts, and tolerances. Let’s discuss them further.
Fits
When you make a hole in a part, it often needs to fit with another piece, like a shaft or a bolt. The way these parts fit together is called a fit. There are 3 main types of fits:
Clearance Fit - The hole is slightly bigger than the shaft so the shaft can move freely.
Interference Fit - The hole is smaller than the shaft so the shaft must be forced in.
Transition Fit - The hole and shaft sizes are very close so the fit can be tight or loose depending on the tolerance.
Callouts
Callouts are the way engineers label holes on technical drawings. They include symbols and measurements like diameter, depth, and special features (such as threading or countersinking). These details help you create accurate holes.
Tolerances
Tolerances are small variations allowed in the hole’s size and shape. No hole is perfect, but tolerances make sure it’s close enough to work properly.
They control how wide the hole is, how deep the hole goes, the shape of the hole’s edges, and how round and straight the hole is. These are to make holes function correctly in its final use.
14 Types of Engineering Holes
Each type of hole serves a specific purpose and let’s tackle them below.
1. Simple Hole
A simple hole is a round opening that can go all the way through a material or stop at a certain depth. It has the same diameter from top to bottom.
Simple holes are for assembly or clearance. Engineers mark them with a “Ø” symbol on drawings. You can drill them with standard drill bits.
2. Blind hole
A blind hope does not go through the entire thickness of the material. It stops at a certain depth and has only one opening. Engineers mark them using “↓” on drawings such as “Ø 30 ↓ 12mm” for a 30mm hole that is 12mm deep.
Drilling blind holes can be tricky because material buildup can break the drill bit. They are useful for mounting, fastening, and reducing wait.
3. Threaded Holes
A threaded hole has internal screw threads so that a bolt or screw can be inserted without using a nut. Special tools like thread mills or inserts create these holes.
Engineers use “Ø” or “M” to mark them in drawings. “M” is used for metric threads, and the number after it shows the thread size.
4. Through Hole
A through hole goes all the way through the material. It is labeled as “Ø diameter Thru” on drawings. For example, “Ø 30 Thru” means a 30mm hole that passes completely through.
These holes are used for fluid passage, wiring, and fastening. You can create them with a drill or by punching the material.
5. Interrupted Hole
An interrupted hole is a hole that is partially blocked or cut off by another feature, like a slot or another hole. It remains aligned even after passing through the interrupted section.
These holes are common in bearing seats, locating pins, and hidden bolts. Although they seem complicated, they can be drilled in separate steps without extra setup.
6. Tapping Holes
A tapping hole is similar to a threaded hole, but it is made of using a tap tool instead of inserts or thread mills. All tapping holes are threaded holes, but not all threaded holes are tapping holes.
Engineers mark them as “M” followed by diameter, pitch, and thread depth. For example, “M6 1.5 10” means a 6mm diameter hole with a 1.5mm thread pitch and 10mm depth.
7. Tapered Holes
A tapered hole has a diameter that gradually decreases from one end to the other. These holes are used in press-fit joints, fluid systems, and high-pressure seals.
Engineers measure them by the taper angle (in degrees) and use a triangle with a line through it to mark them on drawings.
8. Counterbore Holes
A counterbore hole has a wider, flat-bottomed section at the top to allow fastener heads (like bolts) to sit flush with the surface. This makes assemblies stronger and more visually appealing.
The symbol for counterbore holes is “⌴”, followed by the hole diameter, such as “⌴Ø20” for a 20mm counterbore.
9. Countersink Holes
A countersink hole has a conical (angled) section at the top, allowing flat-head screws to sit flush with the surface. It is similar to a counterbore but with a sloped edge. The symbol for a countersink hole is “⌵” in drawings.
10. Counterdrill Holes
A counterdrill hole has a tapered or conical opening to fit fasteners like rivets or bolts. This design helps fasteners sit flush, improving the strength and appearance of an assembly. It is similar to a countersink hole, but the taper is usually less steep.
11. Spotface Holes
A spotface hole is a shallow counterbore that creates a smooth, flat surface for a bolt or washer to sit on. It ensures even pressure distribution and prevents damage to fasteners. Engineers use “⌴ SF” as a callout on drawings.
12. Screw Clearance Holes
A screw clearance hole is slightly larger than the screw it holds, allowing the screw to pass through freely. This makes assembly and disassembly easier. The hole size is calculated using:
Clearance hole size = (screw diameter + screw head diameter) / 2
13. Reamed Holes
A reamed hole is a simple hole that has been refined using a reamer to achieve a highly accurate diameter and smooth finish. Reamed holes are used when tight tolerances are required.
14. Overlapping Holes
An overlapping hole is formed when two holes partially or fully intersect. These are often found in metal fabrication and complex assemblies. Overlapping holes can reduce material strength, so reinforcement may be needed.
Common Methods for Hole Machining
There are several ways to create holes in engineering. Here are the most common methods:
CNC Milling
CNC milling machines use different cutting tools, including drill bits, end mills, and fly cutters, to make holes. These machines can create complex hole shapes, including non-circular ones, by following programmed instructions.
CNC milling is useful because it can do multiple tasks in one setup, including drilling, reaming, counterboring, and pocketing, along with other machining operations.
CNC Drilling
CNC drilling is a fast and precise way to create circular holes. The process starts with designing the hole in software, converting it into a machine-readable format (STEP or STL file), and running the program.
The drill bit spins and moves downward, cutting into the material. There are different types of CNC drilling machines, such as gang drills, radial arm drills, and micro-drills, depending on the application.
CNC Tapping
CNC tapping is used to create internal threads inside holes. It uses taps and dies to cut threads so screws or bolts can be inserted. This process is commonly used for fastening holes in mechanical assemblies.
EDM Drilling
EDM (Electrical Discharge Machining) drilling is a non-contact hole-making method. Instead of using a drill bit, it uses a wire electrode that creates an electric spark inside a dielectric fluid. The spark melts the material, creating a hole.
This method is perfect for tiny holes (as small as 0.065mm) and works well with hard materials– as long as they conduct electricity. It is useful for making micro-holes and curved holes in aerospace, medical, and precision industries.
Boring
Boring is a process used to make a hole bigger or improve its accuracy. Instead of drilling a new hole, you take an existing hole and use a single-point cutting tool to remove material. You can perform boring on lathes, milling machines, or boring mills. This method is especially useful when you need to create large holes or keep the size very precise.
Boring also ensures the hole is perfectly round and aligned, and that is important for high-precision parts.
How to Choose the Right Hole Type in Machining?
Choosing the wrong hole type can lead to misalignment, weak structure, assembly failure, or a non-functioning product. So, here are some things you need to consider when choosing the right hole type:
Purpose of the Hole
First, think about why you need the hole. If it is for fastening bolts and nuts, a tapped hole is the best choice. If it needs to allow lubricant flow or sealing, a tapered or countersunk hole might work better. For mounting or alignment, a simple blind hole is a good option.
Hole Size and Shape
Next, consider the diameter and depth of the hole. Think about whether the hole needs to be circular, square, or irregular in shape. Some hole types work only with specific shapes and sizes.
Material Type
The type of material you are working with affects how the hole can be made. If the material is hard, brittle, or heat-sensitive, you may need special tools to create the hole. Hard metals might require boring or EDM drilling, while softer materials can be drilled or milled more easily.
Power Source and Machinery
Finally, think about the machine and power source available for drilling or machining the hole. A weak power source can lead to inaccurate holes and material buildup so the hole will end up unusable. Make sure your machines and tools can handle the hole type you need to create.
Conclusion
Holes are an important part of engineering and manufacturing. They help with fastening, alignment, fluid flow, and many other functions. At DEK, we can help you create precise and high-quality holes using advanced CNC milling, drilling, EDM, and tapping. Contact us now and let us help with your machining needs.
