With the development of industrial technology, CNC milling machines (machining centers) are playing an increasingly important role in modern industrial production.
Choosing the right cutting tools in CNC milling machines (machining centers) is crucial for achieving high-quality parts, efficient machining, and cost-effectiveness.
In a broader sense, the selection of cutting tools can determine the fate of a factory.
CNC milling machines possess the characteristics of high speed and high efficiency in cutting processing.
The rigidity, strength, wear resistance, and installation and adjustment methods of milling cutters will directly affect the efficiency of cutting processing.
The stability of tool clamping and dimensional accuracy will also directly impact the machining precision of workpieces and the surface quality of the machined parts.
Therefore, choosing cutting tools rationally is one of the important contents in CNC machining technology.
1. Requirements of Cutting Tools in Milling Processing:
During metal cutting, the cutting part of the tool works under high cutting pressure, high cutting temperature, and severe friction conditions. In case of uneven or intermittent cutting processing, the tool will experience significant impact and vibration.
Therefore, the material of the cutting part of the tool should possess the following properties:
High hardness: Hardness is the most basic property of the tool material, and its value should be higher than that of the workpiece material to remove excess metal from the workpiece efficiently.
High wear resistance: The wear resistance determines how well the tool can resist wear and tear during severe friction. Especially when a single tool is used for multiple operations, if it wears out too quickly, it will affect the quality and precision of the machined parts.
Sufficient strength and toughness: The tool must withstand various pressures and impacts during cutting processing. The strength and toughness of the tool material can be measured by its bending strength and impact resistance.
High heat resistance and chemical stability: Heat resistance refers to the ability of the tool to maintain its original hardness, strength, toughness, and wear resistance at high temperatures. Chemical stability refers to the ability of the tool to prevent chemical reactions with the workpiece material or surrounding media at high temperatures, including antioxidant capacity. The higher the chemical stability, the slower the tool wears out, and the better the surface quality of the machined parts.
2. Common Types of Cutting Tools:
Note: The materials for cutting tools include high-speed steel, hard alloy steel, and other types of processing materials. Ceramic, artificial diamond, and cubic boron nitride are also widely used in cutting tools.
(I) Flat Milling Cutters
It is obvious that flat milling cannot be achieved without flat milling cutters. End mills, cylindrical milling cutters, and face mills are commonly used flat milling cutters.
(1) End Mills:
End mills generally adopt an insert structure, and the teeth are made of hard alloy steel. They have high production efficiency and high surface quality and are used for rough and precision milling various large plane surfaces on vertical milling machines.
(2) Cylindrical Milling Cutters:
Cylindrical milling cutters usually adopt an integral structure, mainly made of high-speed steel. They are equipped with spiral-shaped teeth to improve the smoothness of cutting, and are used for rough milling and semi-finish milling of plane surfaces on horizontal milling machines.
(3) Face Mills:
Face mills are used for milling step surfaces and side surfaces on a vertical milling machine. In addition to milling surfaces, they can also be used for milling grooves, helical grooves, various shapes of holes on workpieces, milling various disk cams and cylindrical cams, and machining internal and external curved surfaces using contour milling.
(II) Groove Machining Milling Cutters:
Note: There are straight grooves and non-through grooves. Wide straight grooves can be machined using three-edge milling cutters, while narrow straight grooves can be machined using saw blade milling cutters or small-size face mills.
Non-through grooves are best machined with face mills. T-slots are machined using T-slot milling cutters.
(1) Three-Edge Milling Cutters: Three-edge milling cutters have several structural forms such as straight teeth, staggered teeth, and insert structures. Due to the presence of cutting edges on the circumference and two side faces of the tool, a high surface quality of the machined parts can be achieved. They are mainly used for milling various grooves, step surfaces, workpiece surfaces, and platform surfaces.
(2) Slotting Cutters: Slotting cutters are used for milling various slots, cutting plate materials, bar materials, and various profiles. However, due to the lack of secondary cutting edges that perform finishing, the side surface quality of the milled slots is relatively poor.
(3) Keyway Milling Cutters: Keyway milling cutters are mainly used for milling keyways, and they have high milling accuracy.
(4) Dovetail Milling Cutters: Dovetail milling cutters are mainly used for milling surfaces such as dovetail grooves on machine tool tailstocks.
(5) Angle Milling Cutters: Angle milling cutters are divided into three types: single-angle milling cutters, symmetric double-angle milling cutters, and asymmetric double-angle milling cutters. Single-angle milling cutters are used for opening tooth grooves on the outer circle and end face of various tools, and for milling various saw-tooth clutches and pawls. Symmetric double-angle milling cutters are used for milling various V-grooves and tooth shapes of pointed teeth, trapezoidal tooth clutches. Asymmetric double-angle milling cutters are mainly used for milling various angled grooves.
(III) Forming Surface Machining Milling Cutters:
Forming milling cutters, such as semi-circular milling cutters and special milling cutters for machining blade forming surfaces and root slots of special shapes, are often used in the milling of forming surfaces on ordinary milling machines.
The disadvantage of forming milling cutters is that they are expensive to manufacture and have poor cutting performance.
3. Correct Use of CNC Face Mills:
When milling complex workpieces on a machining center, the use of CNC face mills should pay attention to the following issues:
1. Clamping of Face Mills:
CNC face mills are mostly clamped by spring collets on machining centers and are in a cantilevered state during use. During the milling process, the face mill may gradually extend or even fall out of the collet, causing the workpiece to be scrapped.
The reason is generally due to the existence of an oil film between the inner hole of the collet and the outer diameter of the face mill shank, which causes insufficient clamping force.
Usually, face mills come from the factory with anti-rust oil coating. If non-water-soluble cutting oil is used during cutting, a misty oil film will also adhere to the inner hole of the collet.
When there is an oil film on both the shank and the collet, it is difficult for the collet to clamp the shank firmly, and the face mill is prone to loosen and fall off during machining.
Therefore, before clamping the face mill, the shank part of the face mill and the inner hole of the collet should be cleaned and dried before clamping.
When the diameter of the face mill is large, even if the shank and the collet are very clean, accidents such as tool dropping may still occur. At this time, a shank with a flat slot and a corresponding side locking method should be selected.
Another problem that may arise after clamping the face mill is the possibility of breaking the face mill at the port during processing. The reason is generally because the collet has been used for too long, and the port of the collet has been worn into a conical shape. In this case, a new collet should be replaced.
2. Vibration of Face Mills:
Due to the small gap between the face mill and the collet, tool vibration may occur during the milling process.
Vibration can cause uneven cutting depth of the circular edge of the face mill, and increase the cutting width expansion ratio larger than the original value, affecting machining accuracy and tool life.
However, when the width of the groove being machined is too narrow, it can also be intentionally used to vibrate the tool to obtain the required groove width by increasing the cutting width expansion ratio.
But in this case, the maximum amplitude of the face mill should be limited to below 0.02mm, otherwise stable cutting cannot be achieved. In normal machining, the smaller the vibration of the face mill, the better.
When tool vibration occurs, the cutting speed and feed rate should be reduced. If the vibration is still significant after both have been reduced by 40%, the cutting depth should be reduced.
If the machining system experiences resonance, the reason may be that the cutting speed is too high, the feed rate is too low, the tool system rigidity is insufficient, the workpiece clamping force is insufficient, or the workpiece shape or clamping method, etc.
In this case, measures such as adjusting the amount of cutting, increasing the rigidity of the tool system, and increasing the feed rate should be taken.
3. End-edge Cutting of Face Mills:
In the CNC milling of workpiece cavities such as molds, when the cutting point is in a concave part or deep cavity, the extension length of the face mill needs to be increased. If a long-edge face mill is used, due to the large deflection of the tool, it is prone to vibration and tool wear.
Therefore, if only the end of the tool is used for cutting, it is best to use a short-edge long-shank face mill with a longer total length of the tool. When using a large-diameter face mill on a horizontal CNC machine tool to process the workpiece, the problem of end-edge cutting should be paid attention to due to the significant deformation caused by the weight of the tool.
In cases where a long-edge face mill must be used, the cutting speed and feed rate need to be significantly reduced.
4. Selection of Cutting Parameters:
The selection of cutting speed mainly depends on the material of the workpiece being machined; the selection of feed rate mainly depends on the material of the workpiece being machined and the diameter of the face mill. Some tool manufacturers‘ tool samples are accompanied by a table of cutting parameter selection for reference.
However, the selection of cutting parameters is also affected by many factors such as the machine tool, tool system, workpiece shape, and clamping method, etc. The cutting speed and feed rate should be adjusted appropriately according to the actual situation.
When the priority consideration factor is the tool life, the cutting depth and feed rate can be appropriately reduced; when the chip removal condition is poor, the cutting speed can be appropriately increased.
5. Selection of Cutting Methods:
Using climb milling is beneficial for preventing tool damage and can improve tool life. However, two points need to be noted: 1. If using a conventional machine tool for machining, efforts should be made to eliminate the gap in the feed mechanism. 2.
When there is casting residue, oxide film formed by forging process, or other hardened layers on the workpiece surface, it is advisable to use conventional milling.
6. Use of Carbide Face Mills:
The scope and requirements for the use of high-speed steel face mills are relatively wide, and even if the selection of cutting conditions is slightly improper, major problems are unlikely to occur.
While carbide face mills have good wear resistance during high-speed cutting, their range of use is not as extensive as that of high-speed steel face mills, and cutting conditions must strictly meet the requirements for the use of the tool.