Choosing the Right Milling Direction: Forward or Reverse in CNC Machining
Content Menu
● Types and uses of milling cutters
● Down milling and reverse milling
Milling cutters are typically multi-edge tools that feature multiple teeth engaged in cutting simultaneously. They have long cutting edges and operate at high cutting speeds, resulting in high productivity. Various types of milling cutters can be used to process flat surfaces, grooves, steps, and more. Additionally, they can also be utilized to create gears, threads, spline shafts, and various shaped surfaces.
Structure of milling cutter
Take indexable milling cutter as an example:
1) Main geometric angles
The milling cutter is characterized by a main deflection angle along with two rake angles: the axial rake angle and the radial rake angle. The radial rake angle (γf) primarily influences the cutting power, while the axial rake angle (γp) affects chip formation and the direction of axial force. When the axial rake angle (γp) is positive, the chips tend to fly away from the processing surface.
Rake Angle (Rake Face Contact Surface)
Negative Rake Angle: For steel, steel alloys, stainless steel, and die cast iron.
Positive Rake Angle: For sticky materials and some high-temperature alloys.
Mid-Rake Angle: For threading, grooving, profiling, and forming cutters.
Use negative rake angles whenever possible.
2) The geometry of the milling cutter is first: positive angle – positive angle
Cutting is light, and the chip removal is smooth, but the cutting edge strength is poor. Suitable for processing soft materials and stainless steel, heat-resistant steel, ordinary steel, and cast iron. This form should be preferred when low-power machine tools and process systems are not rigid enough and a built-up edge occurs.
Advantages:
Smooth cutting
Smooth chip removal
Good surface roughness
Disadvantages:
Cutting edge strength.
Not conducive to cutting contact.
The workpiece is off the machine tool table. The second is negative angle – negative angle.
High impact resistance with negative blades makes this suitable for rough milling of cast steel, cast iron, and high-strength steel. However, it consumes significant milling power and requires excellent rigidity in the process system.
Advantages:
Cutting edge strength
Productivity
Push the workpiece to the machine table.
Disadvantages:
Higher cutting forces
Chip blocking
Finally: Positive angle – Negative angle
The cutting edge has strong impact resistance and is sharp. Suitable for machining steel, cast steel, and cast iron. It also works well when milling with large allowances.
Advantages:
Smooth chip removal
Favorable cutting force
Wide range of applications
3) Milling cutter pitch
1) Close teeth: high feed rate, high milling force, small chip space.
2) Standard teeth: conventional feed rate, milling force, and chip space.
3) Coarse teeth: Use a low feed rate and low milling force and ensure ample chip space. If the milling cutter lacks a dedicated wiper blade, the surface finish will depend on whether the feed per revolution exceeds the flat width of the blade wiper.
Example: Slotting & Contouring
Number of teeth:
• Sparse teeth or standard teeth for slot milling (safety)
• Close teeth for contour milling (productivity)
Types and uses of milling cutters
Milling cutters can be categorized based on their tooth structure into two main types: sharp-tooth cutters and shovel-tooth cutters. Additionally, they can be classified according to the relative position of the teeth to the axis of the milling cutter into several types: cylindrical milling cutters, angle milling cutters, face milling cutters, and forming milling cutters.
Another way to categorize milling cutters is by the shape of their teeth. This includes straight-tooth milling cutters, spiral-tooth milling cutters, angular-tooth milling cutters, and curved-tooth milling cutters.
Furthermore, milling cutters can be differentiated based on their tool structure, encompassing integral milling cutters, combined milling cutters, group or set milling cutters, toothed milling cutters, machine-clamped welded milling cutters, and indexable milling cutters.
However, a common approach is to classify them according to the processing form of the cutting tooth back.
Pointed tooth milling cutters can be divided into the following categories:
(1) Face milling cutters encompass integral face milling cutters, toothed face milling cutters, and machine-clamped indexable face milling cutters, which are utilized for roughing, semi-finishing, and finishing of various flat surfaces and step surfaces.
(2) End milling cutters are designed to mill step surfaces, side surfaces, grooves, holes of various shapes, and both internal and external curved surfaces on workpieces. These cutters can be classified into two categories: left-handed and right-handed. However, many people still do not fully understand the distinction between left-handed and right-handed end mills.
Right-hand milling cutter
To determine whether a tool is left-hand or right-hand, use the following method: Face the vertical milling cutter. If the blade groove rises from the lower left to the upper right, it is a right-hand cutter. Conversely, if the blade groove rises from the lower right to the upper left, it is a left-hand cutter. You can also identify right-hand rotation using the right-hand rule: curl your fingers in the direction of rotation, and your thumb will point in the direction of rise, indicating right-hand rotation. The spiral blade groove is important for chip storage and is also part of the front angle and face of the milling cutter.
Left-handed milling cutter
Left-handed milling cutter is generally a tool selected under the demand for high-precision processing. It is generally used in the processing of mobile phone buttons, membrane switch panels, LCD panels, acrylic lenses, etc. However, there are some high requirements, especially the production and processing of some mobile phone buttons or electrical panels, which require high precision and smoothness. You should choose the lower row cutting and left turn, so as to avoid the phenomenon of whitening of the blade and the edge jumping of the workpiece.
(3) Keyway milling cutter Used for milling keyways, etc.
(4) Slot milling cutter and saw blade milling cutter Used for milling various slots, sides, step surfaces sawing, etc.
(5) Special slot milling cutter Used for milling various special slot shapes, including shaped slot milling cutter, half-moon keyway milling cutter, dovetail slot milling cutter, etc.
(6) An angle milling cutter is used as a milling tool for milling straight slots, spiral slots, etc.
(7) A mold milling cutter is used for CNC milling service convex and concave forming surfaces of various molds, etc.
(8) Group Milling Cutters
Group milling cutters consist of multiple milling tools designed for machining complex shapes, features on various parts, and wide surfaces of larger components.
Shovel Tooth Milling Cutters
Certain milling cutters require that their original truncated shape be preserved even after the front face is regrinded. These cutters feature back faces that resemble shovel teeth. Examples include disc slot milling cutters, convex semicircle milling cutters, concave semicircle milling cutters, double angle milling cutters, and forming milling cutters.
Down milling and reverse milling
There are two ways relative to the feed direction of the workpiece and the rotation direction of the milling cutter:
The first type of milling is called down milling. In this process, the direction of rotation of the milling cutter is aligned with the feed direction of the cutting. When the cutting begins, the milling cutter engages with the workpiece and removes the final chips.
The second type is reverse milling, where the rotation direction of the milling cutter is opposite to the feed direction of the workpiece. In this method, the milling cutter must slide over the 5 axis CNC machining workpiece for a short distance before it begins cutting. The cutting starts with a thickness of zero and gradually increases to a maximum thickness by the end of the process.
When using a three-edge milling cutter for end milling or face milling, it’s important to recognize that the cutting forces have different directions. In face milling, the milling cutter operates just outside the workpiece, so the direction of the cutting force should be carefully monitored. During down milling, the cutting force effectively presses the workpiece against the worktable; however, when milling upwards, the cutting force causes the workpiece to lift off from the worktable.
Down milling is generally preferred because it yields the best cutting performance. It is usually chosen unless the machine tool has clearance issues related to threading or other problems that down milling cannot address. Under ideal conditions, the diameter of the milling cutter should be larger than the width of the workpiece, and the axis of the milling cutter should always be positioned slightly away from the centerline of the workpiece. Directly aligning the tool with the cutting center often results in the generation of burrs.
The direction of the radial cutting force continuously changes as the cutting edge enters and exits the material. This variation can lead to spindle vibration, potential damage to the machine tool, blade breakage, and rough machined surfaces. When the milling cutter deviates slightly from the center, the cutting force stabilizes, and the milling cutter experiences a preload, which can be likened to driving in the center of a road.
Each time the milling cutter insert engages the workpiece, the cutting edge experiences an impact load. The magnitude of this load depends on the chip’s cross-section, the workpiece material, and the type of cut being made. Ensuring that the cutting edge and the workpiece effectively interlock during cutting is crucial for optimal performance.
When the milling cutter’s axis is completely outside the width of the workpiece, the impact force during the cutting process is absorbed by the outermost tip of the insert, which is typically the most sensitive part of the tool. As the milling cutter exits the workpiece, this tip still bears the cutting force from the start of the cut until the impact force is released.
If the centerline of the milling cutter aligns precisely with the edge of the workpiece, the insert will exit the cut when the chip thickness reaches its maximum, resulting in the highest impact load during both the cutting in and out phases. Conversely, when the cutter is situated within the workpiece’s width, the initial impact load is distributed along the cutting edge, with the portion farther from the sensitive tip bearing the load. This typically allows for a smoother exit from the cut.
For each insert, the manner in which the cutting edge exits the workpiece is critical. Remaining material near the time of retraction can somewhat reduce blade clearance. When the chip leaves the workpiece, it generates an instantaneous tensile force along the blade’s rake face, which often leads to burr formation. This tensile force poses a risk to the integrity of the chip edge in challenging situations.
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