A milling cutter is a rotary cutter with one or more teeth for milling processing. When working, each cutter tooth intermittently cuts off the margin of the workpiece. Milling cutters are mainly used for milling upper planes, steps, grooves, forming surface processing and cutting off workpieces. Several common forms of milling cutter products are shown in Figure 4-1.
Figure 4-1 Type of milling cutter
a) Cylindrical face milling cutter b) Face milling cutter c) Slot milling cutter d) Double face milling cutter e) Three face milling cutter f) Staggered tooth face milling cutter g) End mill h) Keyway milling cutter i) Single angle Milling cutter j) Double angle milling cutter k) Forming milling cutter
1. Classification of milling cutters
(1) Classified by function
1. Cylindrical milling cutter is used for processing planes on horizontal milling machines, and the cutter teeth are distributed on the circumference of the milling cutter. According to the tooth shape, it is divided into two types: straight tooth and helical tooth. According to the number of teeth, it is divided into sparse teeth and dense teeth. The helical tooth and sparse tooth milling cutters have a small number of teeth, high tooth strength and large chip holding space, which are suitable for rough machining; while dense tooth milling cutters are suitable for fine machining.
2. Face milling cutter is used to process plane on vertical milling machine, horizontal milling machine or gantry milling machine. There are knife teeth on the end face and the circumference. Face milling cutters are also divided into coarse teeth and fine teeth. There are three types of structures: integral type, insert type and indexable type.
3. End mills are used for processing grooves and step surfaces. The cutter teeth are on the circumference and end surfaces, and generally cannot be fed along the axial direction during work. When the end mill has a tooth passing through the center, it can feed axially.
4. Three-sided edge milling cutter is used to process various grooves and stepped surfaces, with teeth on both sides and circumference.
5. Angle milling cutters are used for milling grooves at a certain angle. There are two types of single-angle milling cutters and double-angle milling cutters.
6. Saw blade milling cutter is used to process deep grooves and cut off the workpiece, and there are more teeth on the circumference. In order to reduce the friction during milling, there are 15'~1° secondary deflection angles on both sides of the cutter teeth.
7. Die milling cutters. Die milling cutters are used to process mold cavities or convex mold forming surfaces. Die milling cutters are evolved from end mills. According to the shape of the working part, they can be divided into three types: conical flat head, cylindrical ball head, and conical ball head. Cemented carbide mold milling cutters are very versatile. In addition to milling various mold cavities, they can also replace hand files and grinding wheels to clean the flash of casting, forging, and welding workpieces, and to smooth some forming surfaces. Processing etc. The milling cutter can be used on pneumatic or electric tools, and its productivity and life span are dozens of times higher than those of grinding wheels and files.
8. Gear cutters Gear cutters that work according to the profiling method or the non-instant envelope method are divided into disc gear milling cutters and finger gear milling cutters according to different shapes.
9. Thread milling cutter A tool for milling threads through a three-axis or more than three-axis linkage machining center. In addition, there are keyway milling cutters, dovetail milling cutters, T-slot milling cutters and various forming milling cutters.
(2) Classified by product structure
1. Integral type: The cutter body and the cutter teeth are made into one body.
2. Integral welding tooth type The cutter teeth are made of cemented carbide or other wear-resistant tool materials and brazed on the cutter body.
3. Insert tooth type The tooth is fastened to the tool body by mechanical clamping. This replaceable cutter tooth can be a cutter head of integral cutter material, or a cutter head of welding cutter material. A milling cutter with a cutter head mounted on a cutter body for sharpening is called an internally sharpened milling cutter; a cutter head sharpened separately on a fixture is called an externally sharpened milling cutter.
Second, the geometric angle of the milling cutter
Although there are many types and shapes of milling cutters, they can be summarized into two basic forms: cylindrical milling cutters and face milling cutters. Each tooth can be regarded as a simple turning tool. The difference is the rotation of the milling cutter and the cutter. More teeth. Therefore, the geometric angle of the entire milling cutter can be understood by only analyzing one tooth. Take the face milling cutter as an example to analyze the geometric angle of the milling cutter. The marking angle of the face milling cutter is shown in Figure 4-2. One tooth of a face milling cutter is equivalent to a small turning tool. Its geometric angle is basically similar to that of an external turning tool. The difference is that there is only one base surface for each tooth of the milling cutter, which is determined by the tool tip and the milling cutter axis. The plane is the base plane. Therefore, each tooth of the face milling cutter has four basic angles: rake angle, relief angle, entering angle and edge inclination angle.
(1) Rake angle γο: The included angle between the front surface and the base surface, measured in an orthogonal plane.
(2) Relief angle αo: The included angle between the back and the cutting plane, measured in an orthogonal plane.
(3) Leading deflection angle κr: The angle between the main cutting plane and the assumed working plane, measured in the base plane.
(4) Blade inclination λs: The included angle between the main cutting edge and the base surface.
The relevant angles of the face milling cutter in the main profile system are shown in Figure 4-2. When designing, manufacturing, and sharpening, the relevant angles in the profile system of the feed, back-cutting amount and the radial front are also required. Angle γf and axial rake angle γp.
Figure 4-2 Geometric angle of face milling cutter
Three, milling method
(1) Milling method of end milling
When machining a plane with a face milling cutter, there are three milling methods according to the relative position of the milling cutter and the working surface of the workpiece (or the relationship between the tool): symmetrical milling, asymmetrical up-milling and asymmetrical down-milling, as shown in Figure 4. -3 shown.
Figure 4-3 Three milling methods of end milling
a) Symmetrical milling b) Asymmetrical up milling c) Asymmetrical down milling
1. Symmetrical milling The axis of the milling cutter is located at the center of symmetry of the milling arc length, that is, symmetrical milling when the cutting thickness is the same when cutting in and cutting out. This kind of milling method has a large average cutting thickness. When milling hardened steel with a small feed per tooth, symmetrical milling should be used in order to make the cutter teeth cut into the workpiece beyond the chilled layer.
2. Asymmetrical up-milling. When cutting in, the cutting thickness is less than that when cutting out. It is asymmetrical up-milling. When milling carbon steel and general alloy steel, this type of milling method can reduce the impact during cutting, and more than double the service life of the carbide face milling cutter.
3. Asymmetric down milling The cutting thickness when cutting in is greater than the cutting thickness when cutting out is asymmetric down milling. Practice has proved that when asymmetric down milling is used to process stainless steel and heat-resistant alloys, it can reduce the spalling wear of cemented carbide and increase the cutting speed by 40% to 60%.
(2) The milling method of circumferential milling
According to the changing law of cutting layer parameters during milling, circumferential milling has two forms: up milling and down milling. When up-milling milling (see Figure 4-4), the direction of the cutting speed when the milling cutter cuts into the workpiece is opposite to the feed direction of the workpiece. This kind of milling method is called up-milling. When up-milling, the cutting thickness of the cutter teeth gradually increases from zero. When the cutter tooth starts to cut, due to the influence of the blunt radius of the cutting edge, the cutter tooth slips on the working surface, produces squeezing and friction, which causes a serious chilled layer on this section of the surface. When it slides to a certain extent (that is, the cutting thickness ≥ the blunt radius of the cutting edge), the teeth can cut into the workpiece. When the next cutter tooth cuts in, it squeezes and slides on the chilled layer, which makes the cutter tooth easy to wear and at the same time increases the surface roughness value of the workpiece. In addition, during up-milling, when the contact angle is greater than a certain value, the vertical milling component will increase, which will easily loosen the clamping of the workpiece and cause vibration.
Figure 4-4 Up-milling of circumferential milling
As shown in Figure 4-5 during down milling, the cutting speed direction when the milling cutter cuts into the workpiece is the same as the feed direction of the workpiece. This type of milling is called down milling. During down milling, the cutting thickness of the cutter teeth is the largest when cutting in, and then gradually decreases, avoiding the extrusion, sliding and gnawing phenomenon when cutting in by up-cut milling, and the cutting distance of the cutter teeth is shorter. The milling cutter wears less, the service life can be 2 to 3 times longer than that of up milling, and the quality of the machined surface is also better. In particular, the effect of hard-to-machine materials with a strong tendency of milling hardening is more obvious. The vertical component force acting on the cutting layer in the front is always downward, so the vertical component force of the whole milling cutter acting on the workpiece is large, and the workpiece is always pressed on the fixture, avoiding the vibration of the workpiece, and is safe and reliable.
Figure 4-5 Down milling for circumferential milling
Fourth, the general principle of milling cutter selection
1. The selection process of milling cutters generally considers the following aspects for selection (see Figure 4-6):
(1) Part shape (considering the machining profile): The machining profile can generally be flat, deep, cavity, thread, etc. The tools used for different machining profiles are different. For example, a fillet milling cutter can mill convex surfaces, but not Milling concave surfaces.
(2) Material: Consider its machinability, chip forming, hardness and alloying elements. Tool manufacturers generally divide materials into steel, stainless steel, cast iron, non-ferrous metals, superalloys, titanium alloys and hard materials.
(3) Machining conditions: Machining conditions include the stability of the workpiece system of the machine tool fixture, the clamping situation of the tool holder and so on.