Holes are critical features in engineering drawing & manufacturing. They facilitate assembly fastening, channel, or passage for fluid flow, positioning, weight reduction, etc. There are several types of holes in engineering to serve these purposes. They are typically distinct from each other in shape, depth, functionality, and required tooling to create them.
Through, blind, tapped, countersink, interrupted, tapered, counterbore, and reamed types are frequently used in various mechanical components and systems. Understanding these holes, their design, characteristics, and functionality is essential before including them in your part’s design.
Continue reading, this article will guide you through 14 different types of holes.
What is a Hole Feature in Engineering?
A hole in engineering is defined as a circular cavity or opening geometry, which can be through or blind from the surface. In technical drawings, you can recognize them with specific symbols and corresponding GD & T’s. For instance, the ‘Ø ‘ symbol with the corresponding diameter is used to denote simple engineering holes.
Meanwhile, CNC drilling, punching, tapping, broaching, and EDM drilling are common techniques. Which technique to use depends on the characteristics specified in the design.
Location, diameter, depth, and tolerances are the defining variables for any hole type. Datum and references (lines, planes, axes, etc.) are used for accurate positioning or location. The depth and diameter are essential for choosing the right tool size and machining parameters, whereas tolerances manintains the hole accuracy and precision.
An Overview of 14 Types of Engineering Holes
| Hole Type | Cross-Section/Shape | Purpose |
| Simple Hole | Circular, uniform diameter | Assembly and clearances. |
| Through Hole | Circular, extends fully through | Fluid passage, wiring, and fastening. |
| Blind Hole | Circular, single opening | Mounting, screwing, and weight reduction. |
| Interrupted Hole | Circular, discontinuous | Bearing seats, locating pins, and bolts. |
| Threaded Hole | Circular with internal threads | Fastening with internal helical threads. |
| Tapered Hole | Conical, reducing diameter | Press joints, fluid control, and high-performance seals. |
| Counterbore Hole | Cylindrical recess | Fastener head housing and aesthetics. |
| Countersink Hole | Conical recess | Better fastening and sealing. |
| Counterdrill Hole | Conical/tapered enlargement | Smooth finishes and better screwing |
| Spotface Hole | Shallow counterbore | Pressure distribution and smooth rest for fastener heads. |
| Screw Clearance | Circular, slightly oversized | Easy screw passage, disassembly, and reassembly. |
| Reamed Hole | Refined circular opening | High accuracy and improved surface finish. |
| Overlapping Hole | Partially/fully intersecting circles | Useful in complex assemblies |
Simple Hole
It is a straight circular hole on the surface, which can be through or blind. A circular opening extends to a certain depth or all through the thickness with a uniform diameter. Simple holes are used for the assembly or clearances. They are denoted by “Ø “ in the drawing, and you can drill with common drillbits of the desired diameter.
Through Hole
Always the opening is not on both sides (or through all thicknesses), some are drilled to a certain thickness, called through holes. They involve the format of “Ø diameter Thru” in drawing representation. For example, “ Ø 30 Thru” represents a thorough hole of 30 mm diameter.
Moreover, they are critical for fluid passage, wiring space, and fastening purposes. You can create these kinds of holes by drilling or punching the workpiece.
Blind hole
Any hole that does not completely go through all workpiece thicknesses is called a blind hole. It is drilled to a certain material depth and contains only one opening. Typically, drill presses create it and the depth depends on the length of the bit. Additionally, bottom taps are useful to cut blind threads for fastening purposes.
The blind symbol is “↓” For instance, a blind hole with a diameter of 30mm and 12mm depth is denoted as “Ø 30 ↓ 12mm”.
Drilling these types of engineering holes is more challenging as the bit can break or twist due to material buildup. Therefore, the geometry of the bit ( spiral flutes are preferred ) should be considered for upward chip evacuation.
Subsequently, blind openings are applicable for mounting, screwing, or even for the weight reduction.
Intrrupped Hole
The interrupted refers to discontinuous holes, which are intersected by other openings. It contains regular drilling at a certain depth, interrupted by other features, and again continuous after the interruption. Here, you need to remember that the hole remains on the same axis after the no-material zone. They are common for bearing seats, locating pins, and bolts below the surface.
It is not challenging to create an interrupted hole as it sounds complex. A bit drills the portion above the intersection, then it moves to another portion below the open area without any additional arrangement. In some cases, you might need to adjust the spindle RPM.
Threaded Holes
As the name suggested, threaded holes involve fastening internal threads (continuous helical structure) for the fastening. CNC drilling tools like thread inserts and mills create threaded types of holes in engineering projects.
There are two forms of threaded hole callout; A regular “Ø” or “M”. Here, M represents the metric system, and the following numeric value refers to threading length.
Tapping Holes
You might be confused with the thread and tapping, both are holes with fastening threads, but the methods to make them differ from one to another.
“ All tapping holes are thread holes but all the threading holes are not tapping holes”
So, the difference is in the method of creating. Instead of inserts and drill tolls, tapping involves either cutting or forming taps specified with desired dimensions. Typically, taps come in different sizes and selection depends on the size of the hole you want to tap.
Furthermore, the tapped hole callout is “M”, followed by the nominal diameter, pitch, and depth of the thread section. For example, “M6 1.5 10” refers to a tapping hole with 6mm nominal diameter, 1,5 mm of pitch, and 10 mm of threaded depth.
Tapered Holes
First, the term “tapering” refers to the consistent reduction of diameter from one end to another in any machining feature. Hence, tapered holes involve the change in diameter from one opening to another at a uniform rate. They are commonly found in press joints, fluid pressure control systems, and high-performance seals.
Tapered holes are measured in tapered degrees (inclination angle) defining the hole is inclined by how many degrees (°) from one to another end. Often, operators modify the drill tip angle according to the desired tapering. In the call-out symbol for tapered holes, “ A straight line passes through the triangle”.
Counterbore Holes
Counterbore holes involve an enlarged opening (cylindrical recesses) to house the fastener head-like bolts or O-rings. Meanwhile, both of the cylindrical sections are coaxial. They not only protect the fasteners but also enhance the aesthetic beauty.
The counterbore hole callout is “⌴” along with the diameter symbol (Ø) and value. E.g, ⌴Ø20 refers to a counterbore with a 20mm diameter.
Countersink Holes
You can relate countersink holes with counterbore, the only difference is that the countersink contains conical recessed at the opening. Consequently, the countersink callout is “⌵” and the theoretical edge involves after the symbol.
Like countersink, counterbore also facilitates better fastening and sealing solutions for screws, bolts, etc.
Counterdrill Holes
Counterdrill features a conical or tapered enlargement at their opening to accommodate rivets, bolts, or other fasteners. This design allows fasteners to sit flush or recessed, contributing to a smooth finish. Subsequently, the symbols are similar to countersinks.
Spotface Holes
Spotface refers to a small shallow counterbore at the top and the regular size extends below coaxially. So, you can compare it with the counterbores. This design offers a uniform distribution of pressure and smooth rest for the fastener head. Additionally, these kinds of holes are also suitable for sealing applications.
Next, the symbol of spotface holes in engineering drawings is “⌴ and SF above it”, followed by diameter and other information (i.e. Through, blind, etc.).
Screw Clearance Holes
As the name refers, screw clearance holes are drilled with slightly higher diameters than the screws, bolts, or other fasteners. So, they will easily pass through the hole (beside their head). Additionally, clearance facilitates easy disassembly and re-assembly. Moreover, spotface types have the same callout symbols as simple engineering holes.
The size calculation formula:
Clearance hole size= ( Screw Diameter + Screw Head Diameter) / 2
Reamed Holes
These are not any specialized types of holes in engineering, but they are simple holes refined with reamers to achieve dimensional accuracy and better finish quality. The CNC drill can ream them using the correct size of reamers.
Overlapping Holes
Overlapping holes refer to partial or full intersections of two simple holes, typically used in metal forming applications. Although overlapping can decrease the material strength, reinforcing can counter this. Moreover, these are found in complex assemblies and custom metal fabrication.
Common Methods for Hole Machining
Now, let’s elaborate on the common machining techniques;
CNC Milling
The CNC milling accommodates diverse tooling, including the drill bits. Additionally, regular milling tools like an end mill, fly cutters, and center drill are also suitable for creating complex hole geometries. The multi-axis spindle movement allows for non-circular (irregular) holes strict to CNC programs.
A CNC mill can perform reaming, pocketing, broaching, counterboring, straight drilling, and many hole machining operations. Moreover, milling can make other features along with holes in a single machining setup.
CNC Drilling
CNC Drilling is a specialized method for efficient and high-speed drilling. A CNC drill press or machine creates different types of circular holes of varying depths and sizes.
Like milling, it all starts with creating a design in software with proper GD&T. Then, conversions into STEP or STL format. Finally, the operation will be executed with the right tool and variable setup. The drill bit rotates and goes in a downward direction feeding the material. Moreover, their machine types are gang, radial arm, multiple spindles, microdrill, etc.
EDM Drilling
EDM drilling or Hole Drill EDM machining is a non-contact method of hole-making. It uses electrode wire as a drilling tool that creates an electric spark with a workpiece inside the dielectric solution. This spark melts the material for the workpiece and creates a cavity the same as wire-electrode size, which can be as low as 0.0025 inches (0.065mm).
You can use EDM for micro-hole drilling of simple and curved holes. Additionally, it is compatible with any hard material as long as it is electrically conductive.
CNC Tapping
CNC tapping creates threads on different types of holes using corresponding taps & dies. We have discussed its hole-creation capabilities previously in the tapping hole section.
Considerations for Choosing a Correct Hole Type in Engineering Design
An incompatible hole design can cause misalignment, weak structural interiority, nonfunctioning of the product, or even assembly failure. Therefore choosing the correct hole types in engineering is essential for accurate and quality manufacturing.
Which holes are best fit for your parts or product entirely depends on the application requirements and work material.
Let’s discuss some key considerations for the right selection;
Intended Application of Hole
Do you need holes for lubricant flow, Bolt & nut fastening, sealing, and aesthetic appeal, or does it serve some other functionality? This question will help you to short-list the holes that are fully capable of fulfilling your purpose. For example, taps are best for screw fastening, and simple blind holes are suitable for mounting applications.
Hole Size and Geometry
Consider which size you need, like diameter and depth. Next, ensure the shape needed for functionality, whether is it circular, rectangular, square, or irregular. Then, analyze which kinds of holes are feasible with that shape and size.
Work Material Type
Hardness, brittleness, thermal stability, and other various material properties affect the hole machining process. Therefore, you must consider the type of work material in which you are going to create holes. It also helps to identify the tooling material.
Power Source
It involves considering the powder source according to the desired hole type you want to machine. For example, lower power sources cause inaccuracy and material clogging. Considering the available machinery and required power source ensures the manufacturability of hole designs.
Drilling Holes in Various Materials
You can create many types of holes in engineering materials including metals, alloys, plastics, composites, and wood. Moreover, their distinct properties limit the types you can create on them.
The table below outlines the hole drilling on different materials and considerations;
| Material | Special Considerations | Tooling | Speed | Feed |
| Metals & Alloys | Cooling to prevent overheating, Choose a drill bit according to hardness- Work hardening in ferrous alloys. | High-speed steel (HSS) or cobalt drill bits, Carbide-tipped bits for hard alloys. | Low to moderate | Moderate |
| Plastics | Avoid melting due to heat, Use sharp, low-friction tools, Avoid tightening of the workpiece to prevent cracking. | Acrylic or plastic-specific drill bits, Brad-point bits for precision. | High and it reduces contact time | Low to moderate |
| Composites | Minimize delamination by using a backing plate, Avoid excessive force. | Diamond-coated or carbide drill bits, Use layered comptiable drill bits. | Low to moderate | Moderate |
| Woods | Prevent splintering by supporting the exit side, Consider grain direction, Use sharp tools to avoid burning. | Prevent splintering by supporting the exit side, Consider grain direction, and Use sharp tools to avoid burning. | Moderate to high | Moderate |
Fits, Call Outs, and Tolerances of Engineering Holes
The term “fit” is related to the hole-and-shaft mechanism, which defines physical contact or clearance between them. There are three main types of fits; Transition, interference, and clearance. Clearance fit refers to a slightly larger hole diameter than the mating shaft, whereas it is less the shaft diameter in case of interference fits. Consequently, transition fits are their combined state.
Callouts are geometrical dimensioning systems for the engineering holes. They involve the symbol and critical dimensioning information.
Tolerances are quality control parameters in hole machining. They ensure the allowable deviations from ideal dimensions, depth, diameter, angles, cylindricity, etc.
Conclusion
The distinct hole variations have their own characteristics, design practices, and functionality. Accordingly, the choice depends on the purpose of the hole-feature. Some are ideal for fastening purposes, and a few offer efficient cooling channels. Meanwhile, custom holes are best for unique functional requirements. However, the results depend on the optimal designs and capability of drilling machinery.
If you are looking to machine your parts that have hole features, RapidDirect provides all related solutions for you. Our machining factory offers 5-axis CNC machining, Wire EDM, CNC Milling, and other hole-making services. Leverage our expertise to succeed in your project!
FAQS
There are more than 14 types of holes and most of them are ideal for mechanical bolting. Some most popular bolt holes are countersink, counterbore, tapped, and clearance type.
It is especially for the punching operations, where the edge should be away from at least 1.5 times of hole diameter, i.e. Hole to hole-to-edge distance ≥ 1.5 D.
There is a simple equation to determine clearance hole diameter and tolerance; Clearance hole diameter= ( Screw Diameter + Head Diameter) / 2.
To identify a hole, you can look at the drawing and identify the symbols (Ø), call out tables, and notations of international standards.
