Why you should always break edges
Edge breakage is an integral part of the machining of workpieces. It ensures that sharp angles are avoided. Instead, a slight chamfer is introduced at the tip of the angle. This has the following advantages:
- Distance from the ridge
- Reducing the risk of injury
- Facilitating assembly
- Avoidance of voltage peaks.
Example of edge breakage errors
Volkswagen and Mercedes-Benz were confronted with a strange phenomenon in the 2000s. Timing chains, which were supposed to last well over a hundred thousand kilometers, became elongated and broke at far less than half their intended service life. Many major engine failures were the result. The cause was the inadmissibly long use of punching tools for the chain links. This led to the formation of burrs, which ate into the pins. As a result, the chains first elongated and then tore off. Reworking the chain links would have prevented this phenomenon.
Calculation of the edge break
Edges may not be broken arbitrarily. ISO 13715 was therefore created for the calculation of edge breakage. It specifies how edges are broken and how this breaking is to be represented.
Chamfers usually have an angle of 45° along straight edges. For bores, edge breaks of 60° are common. They are shown in the drawing as an angle with a slanted arrow. The numbers in the angle indicate the tolerance range in micrometers.
Types of edge breaks
ISO 13715 distinguishes between the following three types of edge breaks:
- Grassy
- Sharp-edged
- Burr-free
Outside edges that have a tolerance of between +0.1 and +0.5 are considered “burr”. The final dimension is therefore always above the nominal dimension and thus always produces the undesirable overhang.
Outside edges are considered “sharp-edged” if they have a tolerance of between +- 0.02 and +- 0.05. Their overhang is very small or even negative. If the negative tolerance is adhered to, burr formation is avoided in any case. In the positive direction, this effect is very small. However, there is still a risk of injury.
An edge break is “burr-free” if it maintains a tolerance of between -0.1 and -0.5. The edge is not only free of burrs, it is also easier to assemble and the risk of injury is minimized.
Production of the edge break
The edge breakage of CNC components is taken into account in the workpiece programming. The turning or milling tool currently in use can be used or a separate tool can be selected for this purpose. CNC-programmed edge breaking also enables the production of round chamfers. These are particularly convenient and safe. In manual production, the edge is broken using a file.
