Milling Operations: Techniques, Tools, and CNC
Face Milling: This is a versatile operation. The cutter (face mill) is mounted on the spindle of the milling machine. The working angle of the cutter is usually perpendicular to the work surface but can be tilted to machine tapered and curved surfaces. High-speed face milling: Achieves speeds between 20,000 and 60,000 rpm. Examples of use include cutting large aluminum components for aircraft and producing dies for metal forming. The spindles have a precision rotating accuracy of 10 μm, ensuring a very accurate work surface.
Other Milling Operations
Milling cutters combined or parallel: Two or more cutters are used for machining a shaft or two parallel surfaces. Form milling: Produces curved profiles. Grooving and cutting: Circular cutters are used.
Tool Holders
Tool holders are classified into:
- Arbor cutters (a): Used for peripheral milling, combined cutters, and planned milling.
- Front cutters (b): The tool and tool holder are integrated into the same part. Face mills are the most common examples.
Summary of Basic Milling Operations
- Square Planned Milling
- Profile Milling
- Cavity Milling
- Slot Milling
- Lathe-Milling
- Thread Milling
- High-Speed Milling
- Ramp Machining
- Axial Milling
- Helical Interpolation Milling
- Circular Interpolation Milling
- Troncoidal Milling
Milling: Spine and Elbow Type
Used in general-purpose operations. The cutter spindle can be horizontal (peripheral milling) or vertical (face and end milling, boring, drilling).
Milling Bench (Obsolete)
- The work table is mounted directly on the bed.
- The work table moves only longitudinally.
- They are less versatile but are very stiff.
- Used for high production.
CNC Milling
Milling: Introduction
Milling is one of the more versatile conventional machining processes. The rotary tool removes material while the material moves along one or more axes. The tool is called a fluted cutter.
Peripheral Milling
The axis of rotation of the tool is parallel to the work surface. Fluted tools have several cutting teeth along their circumference. Each tooth acts as a single-point cutting tool.
Types of Teeth
- Straight: Cut orthogonally.
- Oblique: Slanting. Lower cutting force, smoother operation, and reduces vibrations.
Conventional Milling vs. Climb Milling
Conventional Milling (Up Milling): Counterclockwise rotation. Maximum chip thickness at the end of the cut. This is the most common peripheral milling method, producing a finer cut. Proper material fastening is essential.
Advantages of Conventional Milling
- The tooth engagement is not affected by the surface condition of the workpiece.
- Contamination or scale on the surface of the workpiece does not affect the tool.
Disadvantages of Conventional Milling
- The workpiece tends to lift upward.
- There is a tendency for vibration to occur.
Climb Milling (Down Milling): The cut starts at the surface of the workpiece, where the chip is thickest.
Advantages of Climb Milling
- The downward force keeps the workpiece in place.
Disadvantages of Climb Milling
- Not suitable for materials with scale on the surface, such as hot-worked metals, forgings, and high-impact castings.
Face Milling
The axis of rotation of the tool is perpendicular to the work surface. The tool rotates at a rotational speed N, and the workpiece moves with linear speed V. There are two types:
- Concurrent (a): Concurrent with the feed direction.
- Conventional (b): Rotation opposite to the feed direction.
Tools for Face Milling
The advancing angle influences the undeformed chip thickness. Increasing the angle decreases the thickness and increases the undistorted contact length (width of the chip). The angle of advance influences milling forces. Decreasing the angle provides a vertical force component that is increasingly small (axial force on the spindle of the cutter).