What You Need to Know about Tools in CNC Machining
Jan. 08, 2022
An Overview of Tool Deflection and its Impact on CNC Machining
Tool deflection is exactly what its name says. It is the deflection of the tool from its expected alignment. The CNC machining tool is gripped between the chuck in a cantilever position. This means one end of the tool is held inside the chuck and the other operating end of the tool is free or cantilevered. Since in CNC machining operations, the tool consistently exerts a force against the workpiece, the workpiece material also offers resisting force. Along with the resisting force, the CNC tool is impacted by different operational forces, therefore if the tool stiffness does not sustain external forces, the tool deflects from its set alignment.
The tool deflection can certainly cause the following errors in CNC machining.
Catastrophic failure of either tool or workpiece
Tool life-cycle depletion
Dimensional inaccuracy
Rough surface finish or potential surface damage
Strategies to Prevent or Reduce Tool Deflection
Although tool deflection is not 100% avoidable, it still can be reduced by using some strategies and tactics. The CNC machinist can adopt the following tried and tested strategies to reduce tool deflection while working.
Strategies to Reduce Tool Deflection in CNC Machining
Minimize Overhang Length:
The distance between the clutching point of a CNC tool and the operating tip of a tool is called overhang length. Since the tool is mounted as a cantilever beam, it is always subjected to bending stress and external vertical and horizontal forces. According to the load-stress theory for a cantilever beam, the shorter the overhang length, the lower is the bend stress impact. Based on this concept itself, the minimization of overhang length of the tool helps in the reduction of tool deflection.
Enhance Tool Core Strength:
The core strength of a tool depends on its flute length and reach. The difference between flute length and reach defines the rigidity of the core. Therefore, for radial consistent CNC machining operations, long flute tools perform well. The core diameter if kept thick, offers more material wall to absorb shear stress therefore, the core strength and rigidity increase ultimately.
Enhance Tool Stiffness:
Generally, CNC machining tools are made of high-speed steel (HSS). High-speed steel is good for machining operations but on exposure to higher stress load, the HSS tools are prone to deflection. Carbide tools are a considerable solution over HSS tools since carbide is 3 times more rigid than HSS. However, carbide is a brittle material, it can be used for the reduction of deflection. If carbide tools are used, the operator needs to pay attention if the tool breaks.
What are the Different Types of Tool Path Strategies?
There are different types of tool path strategies that are used to optimize CNC machining services. Here are a few of them:
1. Constant Stepover: A tool path wherein the tool follows the shape of the pocket using parallel paths that are separated by a constant stepover.
Pros:
• Produces a very consistent and regular looking finish.
• This is the simplest tool path strategy. It is a default approach, and may not even be given a name in the CAD program.
2. Constant “Z” Machining: This strategy is particularly used for finishing, where the tool path tracks at a constant Z around the profile that is being machined. It is normally used for steep walls, while in other situations, another strategy is applied. The areas that are not very steep are avoided by limiting the path to contact angles that range from 30 to 90 degrees.
Pros:
• Produces a pretty finish because the scallops are all the same height.
3. Pencil Milling: A final finishing technique primarily intended to address corners and concave areas not handled by tool path strategies used earlier in the program. Pencil milling allows a tool path where the cutter diameter is the same as the diameter of the feature to be milled. Without pencil milling, or rest machining, operators were required to specify the corners that required machining. If you have powerful rest machining, pencil milling is not needed.
Pros:
• Very high surface finish.
• Convenience and productivity
4. Plunge Roughing: A roughing technique where cutting occurs through the motion of only Z-axis much like plunging a drill repeatedly into the work piece. It takes advantage of the fact that most machines are rigid in the Z-axis, and can take a higher feed rate and/or a larger cutter when used in this way. Plunging works best if the tool path is orchestrated to ensure climb milling.
Pros:
• May result in higher performance when roughing.
By implementing dynamic tool path strategies it will not only increase the tool life, surface finish, and spindle life, but also the total cost efficiency and cycle time. You can implement any of the strategies explained above based on your requirement.
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