The fundamental requirements for a cutting tool to effectively cut metal are that the tool must be harder than the material being cut and the material must have enough strength to hold the tool in place, allowing it to penetrate the workpiece. Once these requirements are met, the tool must also possess sufficient power to overcome the resistance of the workpiece material. Additionally, the geometric shape of the tool can significantly impact the actual cutting performance and results. Selecting the appropriate tool geometry can increase tool life, maintain machining precision, and reduce cutting forces, among other benefits. Common tool geometries include:
Tool edge angle
The rake angle can range from positive to negative, as shown in the diagram below. A positive rake angle forms a smaller cutting edge angle, allowing the tool to easily enter the workpiece and facilitate smooth chip flow, reducing cutting pressure and increasing cutting efficiency. However, a large positive rake angle forms a sharp cutting edge, making the tool susceptible to wear or breakage. On the other hand, a negative rake angle provides a strong cutting edge, suitable for cutting high-strength materials.
Also known as the clearance angle, it is always a positive value. Its purpose is to avoid interference or physical rubbing between the tool flank and the workpiece surface during cutting, as shown below. A smaller relief angle provides greater support to the cutting edge and is generally used for workpieces with high mechanical properties. A larger relief angle sharpens the cutting edge, but reduces the strength of the cutting edge, making it more susceptible to wear or breakage and is suitable for softer or lower strength workpiece materials.
The helix angle refers to the spiral shape of the milling cutter slot and can be either left-handed or right-handed, as shown below. During cutting, the cutting force F increases to its maximum when the edge enters the workpiece (as seen in the bottom right diagram) and decreases rapidly when the edge leaves the workpiece. This causes vibration during cutting. The helix angle disperses the cutting force in two other directions, horizontal force FH and vertical force FV, avoiding excessive concentration of forces. A larger helix angle increases horizontal force FH, leading to tool oscillation during cutting, while a smaller helix angle increases vertical force FV, leading to the risk of tool detachment from the shank during high-speed rotation.
In the CNC machining center, the ideal chip handling condition is for the chips to break naturally into small segments and be smoothly expelled without interfering with or damaging the workpiece surface or impacting the tool. To achieve this, a chip breaker or chip breaker groove is designed on the top surface of the tool, automatically limiting the chip length and forcing it to break. The design considerations for a chip breaker groove include the groove width W, groove depth H, and groove shoulder radius R.
Over-Center and Non-Over-Center Tools
Disposable bullnose cutters are often larger in diameter (D) than the blade angle (R), leading to a region at the bottom center of the tool where the blade is non-over-center, as shown in the bottom left diagram. When machining holes or grooves on workpieces, this can result in the problem shown in the bottom right diagram. Over-center tools have their cutting edges over the center, eliminating this issue and making them suitable for drilling.
Number of Cutting Edges
The relationship between the number of cutting edges in milling cutters and cutting performance can vary based on the workpiece material, cutter shape, and surface finish. A larger number of cutting edges provide a smoother finish since more cutting edges are involved in the cutting action. However, there may not be enough chip space to accommodate the chips, leading to interference, and the cutting edges may be weaker. Therefore, for rough cutting and high feed rates, especially for softer materials, a larger chip space is required. Increasing the size of the cutting edges and reducing the number of cutting edges can provide the best chip space, increase cutting edge strength, and extend the tool life. For fine and precision machining, a larger number of cutting edges and finer teeth milling cutters are preferred.
Recommended reading: Common tools for CNC machining