Today, I’ll share a practical article on how to control the bouncing of the cutting tool during CNC machining.
During machining, it is common to encounter overcutting caused by bouncing of the cutting tool at corners. However, with the use of appropriate tools and machining methods, the chances of bouncing can be reduced.
As shown in the diagram below, Figure A shows the state of the tool when machining a relatively flat area. When it comes to the abrupt stop at position B and prepares for reverse machining, the tool will undergo deformation due to inertia, resulting in bouncing of the tool and overcutting at the straight section of position B.
The relationship formula for tool deformation:
- δ: Tool deformation
- D: Tool diameter
- P: Force on tool
- L: Tool protrusion length
- E: Natural constant 2.718
From the above formula, we can see that there are three main factors that affect tool deformation:
- L – Tool overhang length
- D – Tool diameter
- P – Cutting force acting on the tool.
L – Tool overhang length.
As the formula shows, the tool deformation is related to the cube of the tool overhang length. For tools with the same diameter, when the overhang length is doubled, the deformation will increase by a factor of 3.
Therefore, during machining, it is recommended to minimize the tool overhang length as much as possible to reduce the risk of bouncing.
D – Tool diameter.
As the formula shows, the tool deformation is related to the fourth power of the tool diameter. For tools with the same length, when the tool diameter is reduced by a factor of 2, the deformation will increase by a factor of 4.
Therefore, during machining, it is recommended to use larger diameter tools or reinforced tools whenever possible to reduce the risk of bouncing. (As shown in the diagram on the right below: Tool A uses a taper neck and a carbide end mill, while Tool B uses a reinforced tool shank to reduce bouncing during machining.)
P – Cutting force acting on the tool.
As the formula shows, the tool deformation is directly proportional to the cutting force acting on the tool during machining. Reducing the cutting force can help to reduce the risk of bouncing. The following methods can be used to reduce the cutting force acting on the tool during machining:
Reducing the cutting force: Cutting is a process of shear deformation, and each material has its own strength (σ). To separate the material, the external force must be greater than the material’s own strength.
σ = F / S
- σ represents the strength of the material.
- F represents the force being applied.
- S represents the contact area.
According to the above formula, it is clear that the force (F) applied to the cutting tool is directly proportional to the contact area (S) between the tool and the workpiece. To reduce the force applied to the cutting tool, the contact area between the tool and the workpiece must be reduced.
Example of reducing force 1: Using the tool path corner function or increasing the R position to reduce the load on the cutting tool at the corner position, thereby reducing the risk of tool breakage.
Example of reducing force 2: When machining at deeper positions, smaller feed rates and tools with finer R corners can be used to reduce the force applied to the cutting tool during machining and lower the risk of tool breakage. The following figure shows a comparison of the contact positions between the workpiece and the D50R6 tool and the D50R0.8 tool when machining to the same depth. It is clear that using a tool with a finer R corner can reduce cutting forces more effectively than using a tool with a larger R corner when machining deep workpieces.
Conclusion
By comprehensively considering the three factors that affect tool deformation (tool overhang length, tool diameter, and cutting force), the risk of tool breakage can be reduced, machining time can be improved, and better machining precision and surface roughness can be achieved.
