The creation of sharp internal corners is one of the few inherent limitations of machining. Despite high-performance technologies such as 5-axis machining, sharp corners require some special problems to be solved by engineers during machining.
This article will explain this common internal machining problem and discuss the various solutions machinists have come up with to deal with sharp corners.
The Problem: Processing Sharp Inside Corners
The problem of sharp corners in machining arises when cylindrical cutting tools encounter sharp internal corners in internal features such as square pockets. The shape of the cutting tools prevents them from cutting an exact corner. Instead, the minimum cutting radius corresponds to the roundness of the tool radius.
This is an unavoidable problem because the geometry of cutting tools is simply incapable of machining sharp internal corners. At the same time, some internal features are expected to have sharp corners. Typically, this requirement applies to assembly applications where the internal feature serves as a seat/chamber for assembly of an external part with sharp external corners.
How do you process sharp inside corners?
However, these contradictory requirements do not lead to a dead end. Engineers and mechanical engineers have developed numerous design maneuvers to solve this problem.
Change corners to curves
The simplest and most obvious solution is to avoid sharp inside corners altogether. Granted, this may not seem like a “solution” to the problem at hand, but experts generally recommend it. Most designs have the flexibility to change the corner radius, and small adjustments can get the job done while maintaining the same functionality.
The main reason for this recommendation is its simplicity. All of the corner finishing techniques we will discuss later require additional effort, cost, and time. If there is a way to avoid them, you should prioritize them.
The other reason is the stability of the process. Cutting tools such as end mills are not suitable for machining very deep pockets. The generally recommended maximum cutting depth is four times the tool diameter. Beyond these limits, problems such as chatter, tool breakage and poor surface finish occur. All of this affects the cutting tool's ability to produce high-quality, sharp inside corners.
Therefore, when designers decide to convert corners into curves, they must also pay attention to the radius of the curve. Depending on the depth of the pocket, you must choose an appropriate corner radius that the production department can safely machine and that also preserves the functionality of the part.
T-bone and dogbone fillets
Another solution is to add undercuts at each sharp corner. An undercut is a machining feature in which the cut extends beyond the perimeter of the internal pocket and into the corner. In other words: excess material is removed from the corners.
This works best when you need sharp internal corners to fit an external component into the internal pocket. It does not affect the functionality or performance of the assembly and still provides space for the appropriate component. Additionally, it can lead to useful weight loss.
There are two popular solutions that machinists regularly use to machine sharp corners.
Bone-in steak
The simplest and easiest type of corner undercut is the “T-bone” cut. During this process, the cutter only moves in one direction towards the corner. Typically, the cutting length is at least half the diameter of the cutter to allow enough space for objects to be joined.
Dog bones
The other type of undercut is the “dog bone,” which gets its name from its similarity to the shape of a dog bone. It differs from T-Bones in that it extends the cut in two directions instead of just one. This type of undercut is a little more complex to work with, but it is aesthetically pleasing.
Electrical Discharge Machining (EDM)
Now let's digress a little and look at solutions that are a little outside the classic scope of editing. EDM is a popular manufacturing process that uses electrical sparks between the part and tool to remove material through melting and erosion. It is particularly used in machining internal corners. We will discuss two types of EDM processes: die sinking and wire EDM.
EDM sinking
In a penetration EDM process, the cutting tool is a specially designed die that is gradually lowered (sunk) into the workpiece. Because the matrix is the negative form of the feature geometry, it is an external component. Therefore, it can easily have sharp corners.
EDM wire
Wire EDM differs from sink EDM. The tool is a thin wire that moves along the contours of the feature to cut material. Its incredibly small tool size, less than 0.1 mm in diameter, makes it ideal for machining sharp corners. This means that wire EDM can create internal corners with a radius of just 0.05 mm, which is definitely “sharp”.
However, the EDM process also has disadvantages. In general, EDM is much slower than conventional machining and manufacturers must have good reasons to justify its use in corner machining. Furthermore, EDM can be difficult for machinists to plan due to its complexity. Additionally, it is limited to only electrically conductive materials and has a poor surface finish that may require additional processing to achieve the desired level.
Manual cutting
Finally, when machines cannot produce high-quality sharp edges, manual skills are an advantage. The last resort is to use various hand tools to cut, sand and polish the inner corner to achieve the desired shape.
Common hand tools include chisels, files, and sandpaper. Understandably, manual processes are time-consuming and certainly not as accurate as machines. However, if it is not possible to use machines, they are a good alternative.
Concluding
The issue of sharp edges in machining is an interesting one and everyone, from designers to machinists, are contributing innovative solutions to solve this problem. Designers have a variety of options to choose from when it comes to sharp interior corners, giving them significant design freedom.
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