CNC machined parts often feature a wide variety of holes. These holes come in different shapes and serve different functions. It can be said that hole machining is a very important part of the parts manufacturing process. The holes manufactured based on engineering drawings determine the final function and performance of the parts.
This guide will provide you with a detailed introduction to the types, designs, characteristics, and uses of holes, allowing you to gain a deeper understanding of this important part feature, thereby helping you better choose the suitable holes for your projects.
What Is a Hole in Engineering?
In engineering design, a hole refers to an opening or internal surface feature formed within the material of a part. Engineering holes are typically cylindrical but can also have taper, steps, threads, counterbores, or other composite structures. Holes can be used for connecting fasteners, installing locating pins, supporting bearings and bushings, and can also serve as passages for lubrication, cooling, venting, drainage, cables, or fluids.
Position, diameter, depth, and tolerance are defining variables for any type of hole. References and references (lines, surfaces, axes, etc.) are used for precise positioning. Depth and diameter are crucial for selecting the appropriate tool size and machining parameters, while tolerance ensures the accuracy of the hole.
Types of Holes in Engineering
Engineering holes can be classified according to penetration method, geometric shape, assembly function and machining state. Each hole type has its own unique functions and characteristics. I will introduce them to you one by one below.
| Hole Type | Key Characteristics | Common Applications |
| Plain or Simple Hole | No threads, counterbore, or complex entry features | General connections, passages, and weight reduction |
| Through Hole | Passes completely through the part | Fasteners, ventilation, drainage |
| Blind Hole | Terminates inside the part | Threaded connections, locating, and internal passages |
| Interrupted Hole | Discontinuous hole wall or cutting path | Cross holes, slots, and complex internal cavities |
| Clearance Hole | Diameter larger than the fastener shank | Allows bolts or screws to pass through freely |
| Threaded Hole | Internal surface features threads | Direct installation of screws and bolts |
| Counterbore Hole | Cylindrical step at the hole opening | Accommodates socket head or hex socket screws |
| Countersink Hole | Conical recess at the hole opening | Installation of flat-head fasteners |
| Spotface Hole | Shallow flat surface at the hole opening | Provides bearing surface for nuts or bolt heads |
| Tapered Hole | Diameter varies along the axis | Taper pins, tapered plugs, and pipe thread connections |
| Reamed Hole | Finished by reaming | Locating pins, bearings, and precision fits |
| Stepped Hole | Multiple diameters along the same axis | Shaft shoulders, sleeves, and composite assemblies |
| Flat-Bottom Hole | Hole bottom is nearly flat | Internal support, spring seating, or sensor mounting |
| Overlapping Hole | Two or more holes partially overlap | Special clearance and non-standard openings |
| Slotted Hole | Elongated opening where length exceeds width | Position adjustment and assembly compensation |
| Counterdrill Hole | Enlarged cylindrical section with a conical drill-point bottom | Tool clearance, stepped passages, and specialized conical-seat features |
Plain or Simple Hole
Plain hole or simple hole is a basic cylindrical hole without threads, counterbores, tapers, or other additional structures. In most engineering designs, simple holes serve as basic passages for fasteners or fluids. They can be either through holes or blind holes. On drawings, this type of hole is represented by “Ø”.
Through Hole
Through holes refer to holes that completely penetrate through a part, entering from one side and exiting from the other side. It is commonly used for bolt connections, positioning pins, fluid channels, ventilation holes, and drainage holes.
Through holes can be formed by drilling or punching, and such holes are also easier to machine.
Blind Hole
Blind holes refer to holes that do not completely penetrate the thickness of a part. Blind holes only reach a certain depth of material, and the bottom of the hole is closed. They are suitable for structural components that cannot penetrate the shell, need to maintain sealing, or are used for installing screws and positioning parts. In terms of accuracy and aesthetics, blind holes are also the best choice.
Interrupted Hole
Interrupted holes refer to holes that pass through gaps or open spaces. The cutting surface of this type of hole is discontinuous. Cross holes or intersecting holes are common forms of interrupted holes, where two or more holes intersect inside the part, commonly used for hydraulic, pneumatic, lubrication and cooling passages.
Clearance Hole
Clearance holes are non-threaded holes that allow bolts, screws, or pins to pass through freely. Their diameter is larger than the outer diameter of the fastener shank, and the clamping force is provided by the internal threads in the nut or another component.
In addition, according to assembly requirements, clearance holes can also be classified as close, normal or loose clearance.
Threaded Hole
Threaded holes are holes with internal threads that allow screws or bolts to engage securely with mating components. This type of hole is mainly manufactured through tapping, thread milling, turning and other methods. Additionally, when you need to design a blind threaded hole, remember to allow extra depth for tool lead-in, thread finish, and chips.
Counterbore Hole
Counterbore is a coaxial cylindrical groove with a larger diameter added at the entrance of the base hole. It has a flat bottom but a larger opening diameter. This type of hole not only protects fasteners but also enhances the aesthetic appearance of the part.
Countersink Hole
Unlike counterbore holes, countersink holes form a conical recess at the entrance of the base hole. It allows the screw head to be flush with or slightly below the part surface, and can also ensure a smooth surface, reducing the risk of damage.
In addition, the difference between countersink holes and ordinary hole chamfers is that chamfer is mainly used to remove sharp edges or provide guidance, while countersink needs to form a controlled conical surface that bears the fitting function of fasteners.
Spotface Hole
You can think of spotface as a counterbore with shallow depth. It is a shallow, flat-bottomed machining operation performed around a hole. This type of hole design provides a flat mounting surface for fasteners and distributes pressure evenly. Spotface is also commonly used for the installation positions of nuts, washers, and bolt heads.
Tapered Hole
Tapered hole refers to a conical hole whose diameter gradually changes from one end to the other. And it can ensure a precise and controllable fit between components. This hole type can be used for locating conical pins, sealing conical plugs, connecting conical pipe threads, and so on.
Meanwhile, tapered holes can be machined by tapered reamers, turning, boring or profile milling. During inspection, you will also need to use instruments such as taper gauges, special plug gauges or CMM.
Reamed Hole
Reamed hole is the process of using a reamer to remove a small amount of material from a pre-drilled hole to improve hole diameter consistency and surface quality. And machining reamed holes typically requires the use of appropriately sized reamers and CNC drilling machines.
Stepped Hole
Stepped hole has two or more different diameters along the same axis, connected by step surfaces. It is commonly used for installing bearings, bushings, seals, springs, etc. When designing such stepped holes, you need to specify the diameter, depth, and step position of each level.
Flat-Bottom Hole
The bottom of a flat-bottomed hole is nearly flat. It is suitable for structures that require internal bearing surfaces and fixed depth volumes. Flat-bottom holes typically require flat-bottom drills, end mills, or boring operations.
Overlapping Hole
Overlapping holes consist of holes where two or more sections intersect, ultimately forming a combined opening. It is commonly used in metal forming.
Slotted Hole
Slotted hole is an elongated opening. Its length is significantly greater than its width, and its ends are usually semicircular. Oblong Hole is a common term for oval or elliptical openings. Since it is not a regular round hole, this type of hole usually requires milling, punching, laser cutting, or other profile machining methods.
Counterdrill Holes
Counterdrill hole usually refers to a stepped hole formed by using a larger diameter drill bit to counterbore the entrance part of an existing hole to a limited depth. This type of hole ensures the correct installation of fasteners or other components.
Hole Callouts & GD&T Symbols on Engineering Drawings
Hole callouts in engineering drawings are used to define basic requirements such as hole dimensions, quantity, depth, entry structure, and threading. GD&T is used to control the position, orientation, and geometric shape of holes relative to datum references. Both must be used together.
The following examples use common ASME style expressions. Your actual projects should clearly specify the use of ASME Y14.5, or ISO 129-1 and ISO 1101 standards in the drawing title block. This is also to avoid mixing rules from different systems.
Information Included in a Hole Callout
A complete hole annotation typically includes the following information:
| Callout Element | Primary Function | Example |
| Quantity | Indicates the number of identical holes | 4X |
| Diameter | Defines the hole diameter | ⌀8 |
| Extent | Indicates whether the hole is through or blind | THRU or depth symbol |
| Depth | Defines the full-diameter hole depth | ↧ 15 |
| Thread | Defines the thread specification and tolerance class | M6 × 1-6H |
| Counterbore | Defines the major diameter and depth of the counterbore | ⌴ ⌀12 ↧ 6 |
| Countersink | Defines the major diameter and included angle of the countersink | ⌵ ⌀12 × 90° |
| Spotface | Defines the diameter and depth of the shallow bearing surface | SF ⌴ ⌀15 |
| Size Tolerance | Controls the allowable range of the hole diameter | ⌀10 +0.02/0 |
| GD&T Control | Controls the position, orientation, or form of the hole | Position, Perpendicularity, etc. |
Common Hole Callout Symbols
Below, I will introduce to you in detail the meaning and usage instructions of various symbols.
| Symbol or Term | Meaning | Usage Notes |
| ⌀ | Diameter | Indicates the diameter of a hole or cylindrical feature |
| ↧ | Depth | Indicates the depth of a blind hole, counterbore, or thread |
| THRU | Through | Indicates that the hole passes completely through the part |
| ⌴ | Counterbore | Indicates a cylindrical counterbore |
| ⌵ | Countersink | Indicates a conical countersink |
| SF | Spotface | Typically used in combination with the counterbore symbol |
| 4X | Number of Places | Indicates four identical features |
| R | Radius | Indicates a fillet or arc radius |
| M | Metric Thread | For example, M8 × 1.25 |
| UNC/UNF | Unified Inch Thread | Used for coarse or fine unified inch threads |
GD&T Controls Commonly Applied to Holes
As for the aspects of GD&T used to control holes, I’ll summarize and explain them here. If you want to understand GD&T in more detail, you can refer to the article GD&T in CNC Machining.
| GD&T Control | What It Controls | Common Applications | What It Cannot Replace |
| Position | Deviation of the hole axis from its theoretical location | Bolt hole patterns, locating holes, and assembly holes | Does not control hole diameter or surface finish |
| Perpendicularity | Orientation of the hole axis relative to a datum plane | Deep holes, guide holes, and bearing holes | Does not control the hole’s XY location |
| Cylindricity | Cylindrical form of the entire hole surface | Precision bearing bores, sealing bores | Does not control location or orientation |
| Circularity | Roundness at individual cross-sections | Rotating fits and sealing surfaces | Does not control overall hole straightness |
| Straightness | Straightness of the hole axis or surface elements | Long holes and guide holes | Does not control the hole’s location on the part |
| Runout | Rotational variation of the hole surface relative to a datum axis | Coaxial holes and rotating parts | Requires an established datum axis |
| Profile | Controls the contour of complex hole openings or non-circular features | Slotted holes, cast passages, and compound contours | Requires clearly defined datums and applicable zone |
Position Tolerance
Position is the most commonly used GD&T control for engineering holes. A typical feature control frame can be expressed as:
⌖ | ⌀0.10 Ⓜ | A | B | C
Where:
- ⌖ represents position;
- ⌀0.10 represents a cylindrical position tolerance zone with a diameter of 0.10;
- Ⓜ represents application under MMC (Maximum Material Condition);
- A, B, C are primary, secondary and tertiary datum respectively.
Note:
The theoretical position of holes should be defined through basic dimensions.
How MMC Works for a Hole
For internal size features like holes, MMC is the minimum hole diameter within the allowed range.
Assume the hole size is:
⌀10.00–10.10
Position tolerance requirement is:
⌖ | ⌀0.10 Ⓜ | A | B | C
When the actual aperture is 10.00, the allowable position tolerance is 0.10. When the actual aperture increases to 10.07, a bonus tolerance of 0.07 can be obtained:
Bonus Tolerance =Actual Hole Size- MMC Hole Size
Therefore:
0.10+0.07=0.17
At this point, the total positional tolerance is 0.17. Bonus tolerance reflects the actual situation of increased assembly clearance when the hole becomes larger.
Perpendicularity of a Hole Axis
The perpendicularity of hole axis can be marked as:
⟂ | ⌀0.05 | A
This indicates that the hole axis must be located within a cylindrical tolerance zone of diameter 0.05 and perpendicular to datum A.
How to Choose the Right Hole Type
I’m sure many of you have difficulties when designing part holes, unsure which hole type is suitable for your project. Now, I will provide some suggestions for your reference. In fact, the correct hole type should be determined by the function of the part, the structure of the fastener, the fit tolerance, and the manufacturing conditions.
Hole Type Selection Table
| Design Requirement | Recommended Hole Type | Key Design Consideration |
| Allow bolts or screws to pass through freely | Clearance Hole | Diameter should be determined based on fastener size and clearance class |
| Screw connects directly into the part | Threaded Hole | Specify thread designation, tolerance class, and effective thread depth |
| Socket head screw sits below the part surface | Counterbore Hole | Allow adequate space for the screw head and installation tool |
| Flat-head screw sits flush with the surface | Countersink Hole | The countersink angle must match the fastener head |
| Establish a bearing surface on a cast or forged surface | Spotface Hole | Machine only the depth needed to achieve a complete bearing surface |
| One side of the surface must not be penetrated | Blind Hole | Distinguish between full-diameter depth, drill point depth, and effective thread depth |
| Simple to machine and easy chip evacuation required | Through Hole | Watch for burrs and breakout at the drill exit |
| Install locating pins or guide pins | Reamed Hole | Control hole diameter, position, and fit tolerance simultaneously |
| Install bearings, bushings, or seals | Precision Bored or Reamed Hole | Control size, roundness, cylindricity, and surface finish |
| Use taper pins, tapered plugs, or special sealed fittings | Tapered Hole | Specify taper rate, orientation, and effective length |
| Multiple coaxial diameters required | Stepped Hole | Specify each diameter, depth, and coaxiality relationship |
| Hole bottom must serve a bearing/support function | Flat-Bottom Hole | Confirm whether a flat bottom is truly necessary to avoid added cost |
| Allow positional adjustment during assembly | Slotted Hole | Specify width, overall length, orientation, and end radii |
| Establish a lubrication, cooling, or fluid passage | Through or Intersecting Hole | Consider internal burrs, cleanliness, and sealing method |
Common Methods for Machining Hole
There are many ways to machine holes, so which process is more suitable? In fact, you should choose the most appropriate machining technology based on hole diameter, depth, material, positional accuracy, and surface quality.
Drilling
CNC drilling can be said to be the most common hole machining method. The machine tool forms round holes by rotating the drill bit and cutting into the material along the axial direction.
This machining method is very suitable for you to use for machining ordinary through holes, blind holes, tapping pilot holes, and so on. The commonly used drill bits include twist drill, indexable drill, step drill, spade drill, and flat-bottom drill.
CNC Hole Milling
As the most common machining method, CNC milling can process holes with complex shapes. This process can use end mills for helical interpolation or circular interpolation along circular paths. It is suitable for machining holes with diameters larger than the tool, large counterbores, stepped holes and flat-bottom holes.
Moreover, compared to dedicated drill bits, the advantage of hole milling is that the same tool can machine multiple hole diameters by adjusting the program.
Boring
Boring uses single-point or multi-point boring tools to enlarge holes in parts. It can improve the hole diameter, axis position, and straightness of part holes. This process is commonly used for bearing seats, valve bodies, precision housings, etc. However, a basic hole must be established on the part before boring.
Reaming
Reaming removes a small amount of material from pre-drilled holes to achieve precise hole diameter, tight tolerance, improved roundness, and better surface finish. It is commonly used for locating pin holes, guide pin holes, and other precision fit holes.
Tapping
Tapping uses taps with complete thread profiles to form internal threads in pre-machined pilot holes. This process is well-suited for machining parts with standard threaded holes. Tapping has high production efficiency and is very suitable for mass production.
In addition, tapping methods include cutting tap and forming tap. Cutting taps create threads by removing material, while forming taps create thread profiles through plastic deformation of the material.
EDM Hole Making
Electrical discharge machining (EDM) is a non-contact hole machining method. It removes material through electric spark discharge. This process is well-suited for machining work-hardened steel, cemented carbide, and other conductive materials.
Common methods include:
- Hole-drilling EDM: machining small diameter or deep holes;
- Sinker EDM: machining blind holes and special internal shapes;
- Wire EDM: machining precision through holes and non-circular openings, but usually requires threading holes.
Gun Drilling
When ordinary drill bits have difficulty stably machining deeper holes, you can use deep hole machining systems such as gun drill. These tools typically work in conjunction with high-pressure internal cooling and specialized chip evacuation channels.
Laser Cutting and Punching
Additionally, if through holes are used on sheet metal parts, laser cutting or punching processing will be required.
Laser cutting is suitable for various hole diameters and contours, and does not require manufacturing dedicated molds for each shape.
Punching is suitable for high-volume standard holes with fast processing speed.
Hole Design for Manufacturability and Cost
The manufacturing cost of holes must also be a topic that you are very concerned about. In fact, the manufacturing cost of the hole is not only determined by the diameter, but also affected by many factors such as depth, location, entrance surface, bottom shape, tolerance, inspection method, etc.
The main costs of holes can be summarized as:
Hole Cost = Setup + Machining Time + Tooling + Deburring + Inspection + Scrap Risk
Use Standard Hole and Thread Sizes
In part design, standard drill diameters, metric or imperial threads, and common fastener specifications should still be prioritized. This is because non-standard hole diameters may require special manufacturing methods, such as CNC circular interpolation, boring, or custom reamers, as well as special inspection gauges for quality control.
Prefer Through Holes When Function Allows
Through holes are generally easier to machine, evacuate chips, and inspect than blind holes of the same diameter and depth. You also don’t need to precisely control the position of the hole bottom. Below, I have compiled the cost information for various hole types.
| Hole Feature | Relative Manufacturing Complexity | Main Cost Driver |
| Plain Through Hole | Low | Drilling and exit deburring |
| Plain Blind Hole | Low–Medium | Depth control and chip evacuation |
| Threaded Through Hole | Medium | Thread cutting and gauge inspection |
| Threaded Blind Hole | Medium–High | Tap drill depth, chip evacuation, and thread runout |
| Reamed or Bored Hole | Medium–High | Pre-machining, finishing, and dimensional inspection |
| Deep Small-Diameter Hole | High | Tool rigidity, cooling, and chip evacuation |
| Flat-Bottom Hole | High | Special tooling or additional milling |
| Back-Side Counterbore | High | Special tooling or secondary setup |
| Intersecting Hole | High | Internal burrs and cleanliness verification |
Avoid Unnecessary Deep Holes
As hole depth increases, the risks of tool extension, deflection, and vibration also rise. For parts with deep holes, processing methods such as peck drilling, internal coolant tools, step drilling, or gun drilling are also required, which will undoubtedly increase machining time.
In addition, when designing deep holes, you also need to consider the following points:
- Whether the drill bit and tool holder have sufficient entry space;
- Whether chips can be discharged smoothly;
- Whether coolant can reach the cutting area;
- Whether the hole diameter, straightness and surface requirements of the part are reasonable;
- Whether detection tools can reach the bottom of the hole;
- Whether sufficient material is retained between the hole bottom and another surface.
Avoid Partial and Edge-Breaking Holes
When designing holes on parts, you need to be careful not to place the holes too close to the edge of the part. This is to avoid incomplete holes caused by drill bit deviation. Moreover, for thin-walled parts, deformation may occur during processing or clamping.
Avoid Flat-Bottom Holes Unless Necessary
This is also a point that many people tend to overlook. Regular drill holes have a conical bottom. However, flat-bottom holes typically require flat-bottom drills, end mills, or additional boring operations. Therefore, compared to ordinary blind holes, flat-bottom hole machining adds processing steps and may leave tool corner radii at the bottom edge of the hole.
Control Only Functional Tolerances
You also know that strict hole diameter, True Position, surface roughness, etc. will increase the cost of machining and inspection. Therefore, for ordinary bolt clearance holes, you do not need to use the same precision as bearing holes.
Summary
Well, that’s all the content about engineering holes. It can be said that holes play an important role in ensuring the safety, stability and good performance of parts. Different holes have different functions, characteristics and design solutions. Your choice of engineering holes should be comprehensively considered from multiple aspects.
Of course, as an experienced manufacturer, DSR can manufacture CNC parts including through holes, blind holes, threaded holes and more according to the drawings you provide. We can provide you with various hole machining services such as CNC milling and drilling to manufacture various high-quality parts for you.
FAQ
Q1: What are screw holes called?
A: Threaded hole or clearance hole.
Q2: What is a fancy word for hole?
A: Aperture.
Q3: What is a big hole called?
A: Cavity.
Q4: What is CAD hole marking?
A: CAD hole marking refers to a annotation system in engineering drawings that uses standardized symbols and text combinations to indicate information such as hole type, diameter, depth, machining method, and tolerances.
Q5: Which hole types are easiest to machine?
A: Standard-diameter through hole.
Q6: What are the different types of drilling holes?
A: Through hole, blind hole, counterbore hole, countersink hole, pilot hole, tapped hole, reamed hole, bored hole.
Q7: What are the different types of bolt holes?
A: Over 14 types of holes. Common hole types include clearance hole, tapped hole, counterbore hole, countersink hole, etc.
Q8: What is the engineering term for a hole feature?
A: In engineering drawing and GD&T standards, features like holes have a very precise technical term—Feature of Size (FOS).
