How to Prevent Tool Overcutting in CNC Machining

1. Recurring issue of tool deflection causing overcutting in CNC machining

In machining, tool deflection often leads to overcutting at corner positions. However, this can be minimized by employing appropriate tools and machining methods.

Snap-Back Position and Overcut during Machining
Snap-Back Position and Overcut during Machining

2. Problem Analysis and Countermeasures

As illustrated below, Figure A represents the tool’s state while machining a relatively flat area. When an abrupt stop occurs at Position B, preparing for reverse machining, due to inertia, the tool will deform, causing an overcut at the more vertical position of B, known as the snap-back.

Bullet knife diagram

The relationship between tool deformation:

  • δ: Tool Form Factor
  • D: Tool Diameter
  • P: Force Acting on the Tool
  • L: Protruding Length of the Tool
  • E: Natural Constant 2.718

From the above formula, we can discern that there are three primary factors influencing tool deformation:

  • L – Tool overhang length
  • D – Tool diameter
  • P – Force exerted on the tool

L – Tool Overhang Length

The formula indicates that the amount of tool deformation is cubically related to the length of the tool overhang. For tools of the same diameter, if the overhang length doubles, the deformation will increase threefold.

During machining, it’s advisable to minimize the tool overhang length whenever possible to reduce the risk of tool deflection.

D – Tool Diameter

The formula suggests that the amount of tool deformation is quartic in relation to the tool diameter. For tools of the same length, if the tool diameter is halved, the deformation will increase fourfold.

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During machining, if feasible, it’s recommended to select tools with larger diameters or use reinforced tools to minimize the risk of tool deflection. (As illustrated in the image below: A uses thermic and taper neck tools, while B employs tools with reinforced shank for machining).

P – Forces Acting on the Tool

The formula shows that the deformation of the tool is directly proportional to the forces it experiences during machining.

Reducing these forces can lessen the chances of tool deflection. Several methods can be employed to decrease the forces acting on the tool during machining.

Analysis of Force Reduction:

Cutting is a process of shear deformation. Each material has its own strength (σ). For the material to separate, the applied force must exceed the material’s inherent strength.

σ = F / S

  • σ : Strength of the material
  • F : Applied force
  • S : Contact area

From the above formula, it’s evident that the force (F) exerted on the tool is directly proportional to the contact area (S) between the tool and the workpiece.

To reduce the force on the tool, it’s necessary to decrease the contact area between the tool and the workpiece.

Reduction of Force Example 1:

Use the tool path cornering feature or increase the R position to lessen the load on the tool at the corner, thereby reducing the likelihood of tool deflection.

Reducing Force Example 2:

When machining at greater depths, it is advisable to use tools with a smaller feed rate and a finer radius to lessen the forces exerted on the tool during machining, thereby reducing the risk of tool deflection.

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The illustration below compares a D50R6 tool and a D50R0.8 tool while machining at the same depth. It is evident that using a tool with a finer radius to machine deep workpieces can more effectively reduce cutting forces than using a tool with a larger radius.

3. Summary

By strategically utilizing the three factors influencing tool deformation (tool insertion length, tool diameter, and cutting force), the likelihood of tool deflection can be reduced.

This increases machining time, resulting in superior machining accuracy and surface roughness.

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