Countersinking and countersinking are two of the most common drilling methods in machining. Although their main purpose is the same: to accommodate fastener heads, some subtle differences mean that they are often confused with each other by professionals.
This article aims to provide a comprehensive comparison between countersinking and countersinking. Keep reading!
What is a sinkhole?
Let's start with the simple question of what an undercut is. A countersink is a shallow cylindrical recess that is machined coaxially around an existing screw/mounting hole.
The purpose of a counterbore is to accommodate the fastener head in a convenient, non-intrusive manner. A countersunk hole is deep enough so that the head of the fastener is flush with or below the surface of the workpiece.
This is a design requirement for most parts, as fastener heads that protrude above the surface are undesirable due to issues such as assembly limitations or part aesthetics.
For example, see the image below. The screw is needed to assemble the socket parts. However, the screw head may prevent plugs from being fully inserted into the socket. To solve this problem, a countersunk hole is added.
Countersunk holes in technical drawings
Drilling is a very versatile machining operation with different types of hole geometries. Due to the large number of holes in technical designs, a countersunk hole is often marked on a drawing with a countersunk hole designation.
An example of this is shown in the following figure. The technical drawing shows a hole with a diameter of 13.5 with a countersunk hole. This hole serves as a pilot hole for the sinking process.
The depression label indicates the size of the depression as 23.79 and its depth as 7.5.
What is a Spotface hole?
The next question in this discussion of face lowering versus undercutting is what a face lowering is. The simplest definition is a shallow countersunk hole. This means that, like a undercut, it is a shallow cylindrical depression over a pilot hole. However, the depth of a countersink is much smaller than that of a conventional countersunk hole.
Undercut functions
The primary function of a face countersink is to provide a flat, even bearing surface for fasteners. If the bearing surface of a fastener is uneven and rough, this can create unwanted stress on the body of the fastener, potentially damaging it. Therefore, a countersink is useful to ensure a smooth mounting surface for the mounting head.
Another important use of a countersink is to facilitate the installation of angled fasteners. Some assemblies require fasteners to be positioned at an angle to a flat surface. In this case, a flat seat is required to install the fasteners. This flat seat is therefore obtained with a countersunk hole, as shown in the following figure.
It should be noted that plan reductions should not be confused with reductions. Countersinks are very similar to flat countersinks, but they do not have a flat bottom, but are angled to accommodate tapered screw heads.
Reaming holes in technical drawings
As with countersunk holes, countersunk holes also have a unique symbol on technical drawings. The reamer symbol is derived from the reamer designation, with an “SF” mark included in the normal reamer symbol. In this case, SF stands for “Spot Face”.
The image below shows a simple example of a countersunk label. The reamer function has a diameter of 50 and a depth of 1.
Countersunk drilling vs. front reaming: a detailed comparison
Now that the concept of countersinking and countersinking is clear, it is time to delve into the comparison between countersinking and countersinking. Although they are quite similar, as we have just seen, their uniqueness is easy to understand when you look at the differences.
The following explanation describes the main differences between sinks and flat reamers.
function
The main difference between undercuts and countersinks is the application of the design. However, before we get into the differences, let's first mention the similarities. Both countersunk and countersunk holes are designed to accommodate fasteners by providing a flat seat and recess for the fastener head.
However, countersunk holes have the additional function of completely enclosing the screw head below the surface of the part. This prevents the screw head from protruding above the surface and causing mounting restrictions.
depth
Countersink drilling and countersinking differ in drilling depth. As can be seen from the functionality explained in the section above, the countersunk holes are at least as deep as the height of the mounting head. This way, the mounting head is completely hidden under the surface after installation.
The depth of the flat surface is significantly smaller. Its size is sufficient to provide a flat, recessed seat for the mounting head.
Technical drawing symbol
Another important difference between countersink and countersink is the labeling on the technical drawing. As explained above, the reamer label includes the symbol “⌴” which represents the cross section of a reamer. It contains the diameter and depth of the reamer.
The plan subsidence symbol is a derivative of the subsidence symbol with an “SF” inserted into it. This communicates the designer's intent to the machine operator, along with precise specifications such as diameter, depth and surface finish.
Surface Finish
The surface finish is generally different when comparing undercut and undercut. Although both holes are kept very smooth to provide a flat, level mounting surface for fasteners, countersinks typically have a finer surface finish.
This is primarily because the sole purpose of a countersink is to provide a flat mounting surface to achieve proper clamping pressure, while countersinks can have rough sidewalls without compromising functionality.
A short table
| Countersunk hole | Spotface | |
| Characteristics | Provide a cylindrical recess so that fastener heads are flush with a surface. | Make sure there is a smooth surface for the mounting heads. |
| depth | At least as deep as the height of the mounting head to completely hide it. | Very shallow depths, generally less than 5 mm. |
| Size | Diameter and depth are equally important in determining the size of the undercut. | The primary dimension of a face reamer is diameter, while depth is a secondary dimension with close tolerance. |
| symbol | ⌴ | SF |
Countersinking and countersinking machining
Machining counterbores and countersinks is generally not very demanding, but the machinist must be clear about the main procedure. A pilot hole is required first for countersunk and countersunk holes. Machinists drill this pilot hole using typical drilling techniques such as drilling, boring, or milling.
In the second step, the desired holes are drilled using special countersinking/surface reaming tools of the correct size.
The most important thing is to ensure that the tool is perfectly coaxial with the pilot hole. Otherwise, the hole dimension tolerance will be violated and the fastener may not seat correctly in the recess/counterbore.
Finally, machining opens the guide hole to create the required feature. Because countersunk and countersunk holes are typically secondary geometric features, experts prefer to decide between hand machining and CNC machining based on the geometry of the primary part.
Whatever this choice is, it also applies to additional hole production processes such as: B. reduction/planning.
Countersinking and face countersinking tools
An important difference in the discussion about face sinks and countersinks is the difference in editing tools for each feature. Although there is no single tool for both, since they both have similar geometries, machinists sometimes use specialized tools.
The image below shows these different geometries. A countersink tool is like a milling tool or reamer in that it needs to reach a greater depth. A countersinking tool, on the other hand, usually has very short cutting teeth.
Additionally, some of these tools also have a central anchor that guides the tool into the guide hole. This helps maintain a coaxial and straight tool descent along the tool path.
Concluding
Countersinking and countersinking features play a critical role in the manufacturing of complex assemblies and serve a specific purpose in ensuring the success of a project. As an engineer or manufacturer, it's good to know the differences, technical drawing symbols, and machining techniques and tools.
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Common questions
Are countersinks and countersinks machined with different tools?
There are special tools for both processes. However, in some cases they can also be machined with similar tools, such as end mills, or reworked with a reamer.
What is the main difference between collectors and planned collectors?
A countersunk hole generally goes deeper into the surface than a countersunk one. Additionally, the primary purpose of a counterbore is to provide a level seat for the fastener and to enclose the fastener head, while a face counterbore is machined only to provide a level, balanced seat for the fastener.
How are plan cuts and reductions measured?
The dimensions of the countersunk holes depend on the size of the clamping head. The diameter must be large enough to accommodate the fastener head and the countersunk hole must be at least as deep as the height of the fastener head to fully penetrate the surface of the workpiece.
For countersinks, the diameter dimension is the same as for countersinks, but the decision on the depth of the hole is the responsibility of the designer. It is normally kept very small and is less than 0.5 mm.
