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Introduction to milling cutters – How should I choose a milling cutter?
What is a milling tool?
Milling cutters have an important component of milling machines. A milling cutter is a rotating tool with one or more teeth used for milling. During the working process, each tooth intermittently cuts off the rest of the workpiece. Milling cutters are mainly used for milling planes, steps, grooves, forming surfaces, cutting off workpieces, etc.
The meaning of milling tools
During a typical milling operation, the tool in a milling machine moves perpendicular to its own axis, allowing it to remove excess material from the workpiece around the tool’s perimeter. A milling machine is a versatile machine on which a variety of machining operations can be performed. Milling machines are used to machine and manufacture parts of various shapes and sizes. Milling cutters are essential tools for performing these tasks.
Types of milling tools
In order to make milling a versatile machining process, there are a variety of milling cutters available on the market. These milling cutters are manufactured in various sizes, shapes and materials. Some milling cutters are made of high-speed steel (HSS), while others have carbide tips.
End mill:
End mills have cutting teeth on both sides; therefore, this tool can be used very successfully for a variety of drilling operations. The name “end mill” is a common term for flat-bottomed tools. The main difference between drills and end mills is that drills can only cut in the axial direction, while end mills can cut in all directions. End mills contain one or more flutes that are ultimately used in various milling operations. It is made of high speed steel or hardened material. This type of knife usually comes in two variations. One is commonly known as center cutting, which has cutting edges on both sides of the tool, and the other is off-center cutting, in which the cutting edge is only on one side.
Rough milling cutter:
Roughing end mills are also commonly known as “Pippa” tools. These end mills provide outstanding performance under the most demanding operating conditions. As the name suggests, they are used to extract large amounts of unwanted material from workpieces. Typically, this type of cutter is used with more wavy teeth. Roughing end mills produce very small chips, resulting in a rough surface finish.
Peripheral milling cutter:
When the cutting teeth on the milling cutter are present on the circumference or perimeter of the disc, then these types of milling cutters are called circumferential milling cutters or perimeter milling cutters. These cutters can only be used in horizontal milling machines.
Side milling cutter:
Side mills are another type of milling cutter in which the cutting teeth are found both on the periphery and on the face or end. Side milling cutters are generally used in stranded wire milling operations and face milling operations. They can also be used to cut grooves and make deep, narrow grooves.
Face milling cutter:
A face mill consists of a large-diameter cutting body with multiple mechanically fixed insert inserts. With the cutting stroke of a face mill, a very large amount of unwanted material can be removed through radially deep, axially narrow cuts. The diameter of a face milling cutter body usually depends on the length of the workpiece and the clearance available on both sides of the workpiece.
These face milling cutters can also be used in climb milling operations. A face mill is a very rigid cutting tool, and the surface finish it provides depends on the feed rate and the number of teeth on the tool.
Concave milling cutter:
Concave milling cutters belong to the category of forming tools. Forming tools are usually designed to create specific shapes on the workpiece. This type of milling cutter is a specially manufactured tool designed to correspond to the convex side of a circular profile. This circular outline is usually equal to or smaller than a semicircle.
Woodruff Knives:
“Woodruff” cutting tools are commonly used for cutting keyways in wood materials.
Wood grain cutting knives have slightly hollow edges and their teeth are not used for side cuts. Its tooth shapes include straight teeth and staggered teeth.
Thread milling cutter:
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Thread milling cutter is a cutting tool used to cut the external thread and internal thread tooth profile of the workpiece. The cutting process using a thread mill can produce single-pitch or variable-pitch threads from M2 to a nominal diameter of 1 mm.
Ball end milling cutter:
Ball nose mills are also commonly known as ball nose mills.
These milling cutters get their name from the hemispherical shape of their cutting ends. This type of milling cutter is often used to reduce stress concentrations during operations. It is usually suitable for processing three-dimensional curved shapes of workpieces.
Flying knife:
Throwing knives consist of a main body into which one or two blades are inserted. As the cutter head rotates, it makes a narrower or wider cut. Face milling cutters are more versatile in various situations, but they are expensive. These flying cutters can also complete the processing content of face milling cutters, and are very cheap, but are far lower than face milling cutters in terms of cutting efficiency.
Principles of milling cutter selection
Selection of milling cutter diameter
The choice of milling cutter diameter varies greatly depending on the product and production batch. The selection of tool diameter mainly depends on the specifications of the equipment and the processing size of the workpiece.
Surface milling cutter
When selecting the diameter of a face milling tool, the main consideration should be that the power required by the tool should be within the power range of the machine tool, or it can be selected based on the diameter of the machine tool spindle.
The diameter of the face milling cutter can be selected according to D=1.5d (d is the spindle diameter).
In mass production, the tool diameter can also be selected according to 1.6 times the cutting width of the workpiece.
end mill
The selection of the diameter of the end mill should mainly consider the processing size requirements of the workpiece to ensure that the power required by the tool is within the rated power range of the machine tool.
If it is a small diameter end mill, the main consideration should be whether the maximum rotation speed of the machine tool can reach the minimum cutting speed of the tool (60m/min).
Grooving knife
The diameter and width of the slot milling cutter should be selected according to the size of the workpiece to be processed, and its cutting power should be within the allowable power range of the machine tool.
- Selection of milling cutter inserts
a. For finishing. It is best to use a sharpened blade. This kind of insert has good dimensional accuracy, so milling has high positioning accuracy of the cutting edge, and good machining accuracy and surface roughness can be obtained.
b. For rough machining, it is best to use pressed blades, which can reduce processing costs.
The dimensional accuracy and sharpness of pressed blades are worse than those of grinding blades, but the edge strength of pressed blades is better, is impact-resistant during rough machining, and can withstand large depths of cut and large feeds.
c. Sharp high rake angle blades can be used for milling sticky materials (such as stainless steel). Through the cutting action of the sharp blade, the friction between the blade and the workpiece material is reduced, and the chips can leave the front end of the blade faster. - Selection of milling cutter body
a. First of all, when selecting a milling cutter, the number of teeth must be considered.
The size of the tooth pitch will determine the number of teeth participating in cutting at the same time during milling, affecting the smoothness of cutting and the requirements for the cutting speed of the machine tool.
Coarse-tooth milling cutters are mostly used for roughing because they have larger chip flutes.
At the same feed rate, the cutting load per tooth of a coarse-tooth milling cutter is greater than that of a fine-tooth milling cutter.
b. During fine milling, the cutting depth is shallow, generally 0.25-0.64mm. It is recommended to use a dense-tooth milling cutter.
c. During heavy rough milling, excessive cutting forces can cause chatter in less rigid machine tools.
This chatter can cause carbide inserts to chip, shortening tool life. Using coarse-tooth milling cutters reduces machine power requirements.
therefore. When the spindle hole size is small (such as R8, 30#, 40# taper hole), a coarse-tooth milling cutter can be used for effective milling.
Basic requirements for milling cutters to cut some materials
(1) High hardness and wear resistance: At normal temperature, the cutting material must have sufficient hardness to cut into the workpiece; with high wear resistance, the tool will not wear and extend its service life.
(2) Good heat resistance: The tool will generate a lot of heat during the cutting process, especially when the cutting speed is high, the temperature will be very high. Therefore, the tool material should have good heat resistance, both at high temperatures and It can still maintain a high hardness and has the ability to continue cutting. This high-temperature hardness is also called hot hardness or red hardness.
(3) High strength and good toughness: During the cutting process, the tool has to withstand a large impact force, so the tool material must have high strength, otherwise it will be easily broken and damaged. Since milling cutters are subject to impact and vibration, the milling cutter material should also have good toughness so that it is not prone to chipping or chipping.
- Commonly used materials for milling cutters
(1) High-speed tool steel (referred to as high-speed steel, front steel, etc.), divided into two types: general-purpose and special-purpose high-speed steel. It has the following characteristics:
a. The content of alloy elements tungsten, chromium, molybdenum and vanadium is relatively high, and the quenching hardness can reach HRC62-70. It can still maintain high hardness at a high temperature of 6000C.
b. The cutting edge has good strength and toughness and strong vibration resistance. It can be used to make tools with average cutting speed. For machine tools with poor rigidity, high-speed steel milling cutters can still cut smoothly.
c. It has good process performance, is relatively easy to forge, process and sharpen, and can also manufacture tools with more complex shapes.
d. Compared with cemented carbide materials, it still has disadvantages such as lower hardness, poor red hardness and poor wear resistance.
(2) Cemented carbide: It is made of metal carbide, tungsten carbide, titanium carbide and metal binder mainly cobalt through powder metallurgy process. Its main features are as follows:
It can withstand high temperatures and can still maintain good cutting performance at around 800-10000C. When cutting, a cutting speed 4-8 times higher than that of high-speed steel can be used. High hardness at room temperature and good wear resistance. The bending strength is low, the impact toughness is poor, and the blade is not easy to sharpen.
Commonly used cemented carbide can generally be divided into three categories:
①Tungsten-cobalt carbide (YG)
Commonly used grades are YG3, YG6, and YG8. The numbers indicate the percentage of cobalt content. The more cobalt content, the better the toughness and resistance to impact and vibration, but the hardness and wear resistance will be reduced. Therefore, the alloy is suitable for cutting cast iron and non-ferrous metals, and can also be used to cut high-impact blanks and quenched steel and stainless steel parts.
②Titanium-cobalt carbide (YT)
Commonly used grades include YT5, YT15, and YT30. The numbers indicate the percentage of titanium carbide. After cemented carbide contains titanium carbide, it can increase the bonding temperature of steel, reduce the friction coefficient, and slightly increase the hardness and wear resistance. However, it reduces the bending strength and toughness and makes the properties brittle. Therefore, the Alloy-like is suitable for cutting steel parts.
③ General purpose cemented carbide
Add appropriate amounts of rare metal carbides, such as tantalum carbide and niobium carbide, to the above two types of cemented carbide to refine the grains and improve their normal and high temperature hardness, wear resistance, bonding temperature and oxidation resistance. , can increase the toughness of the alloy. Therefore, this type of cemented carbide tool has better comprehensive cutting performance and versatility. Its grades include: YW1, YW2 and YA6, etc. Because of its relatively expensive price, it is mainly used for difficult Processing materials such as high-strength steel, heat-resistant steel, stainless steel, etc.
Types of milling cutters
(1) According to the material of the cutting part of the milling cutter:
a. High-speed steel milling cutter: This type is used for more complex tools.
b. Carbide milling cutters: Most of them are welded or mechanically clamped to the cutter body.
(2) According to the use of milling cutters:
a. Milling cutters for processing flat surfaces: cylindrical milling cutters, end mills, etc.
b. Milling cutters for processing grooves (or steps): end mills, disc milling cutters, saw blade milling cutters, etc.
c. Milling cutters for special-shaped surfaces: forming milling cutters, etc.
(3) According to the structure of milling cutter
a. Sharp tooth milling cutter: The cross-section of the tooth back is a straight line or a broken line, which is easy to manufacture and sharpen and has a sharp edge.
b. Shovel tooth milling cutter: The tectonic shape of the tooth back is an Archimedean spiral. After grinding this type of milling cutter, as long as the rake angle remains unchanged and the tooth shape remains unchanged, it is suitable for forming milling cutters.
The main geometric parameters and functions of milling cutters
(1) Names of each part of the milling cutter
① Base plane: a plane that passes through any point on the cutting tool and is perpendicular to the cutting speed at that point.
② Cutting plane: a plane that passes through the cutting edge and is perpendicular to the base surface.
③ Rake face: the plane from which chips flow out.
④ Flank face: the face opposite to the machined surface
(2) Main geometric angles and functions of cylindrical milling cutters
① Rake angle γ0: the angle between the rake surface and the base surface. The function is to make the blade sharp, reduce metal deformation during cutting, and easily discharge chips, thereby saving labor in cutting.
② Relief angle α0: the angle between the flank surface and the cutting plane. Its main function is to reduce the friction between the flank surface and the cutting plane and reduce the surface roughness of the workpiece.
③ Helix angle 0:
The angle between the tangent line on the helical tooth blade and the axis of the milling cutter. Its function is to gradually cut the cutter teeth into and away from the workpiece to improve cutting stability. At the same time, for cylindrical milling cutters, it also has the function of allowing chips to flow out smoothly from the end face.
(3) Main geometric angles and functions of end mills
The end mill has one more secondary cutting edge, so in addition to the rake angle and the relief angle, there are:
① Leading angle Kr: The angle between the main cutting edge and the machined surface. Its change affects the length of the main cutting edge participating in cutting and changes the width and thickness of the chips.
② Secondary deflection angle Krˊ: The angle between the secondary cutting edge and the machined surface. Its function is to reduce the friction between the secondary cutting edge and the machined surface, and affect the smoothing effect of the secondary cutting edge on the machined surface.
③ Edge inclination angle λs: the angle between the main cutting edge and the base surface. Mainly plays the role of bevel cutting.
Forming milling cutter
Forming milling cutter is a special milling cutter used to process formed surfaces. Its blade profile needs to be designed and calculated according to the profile of the workpiece to be processed. Surfaces with complex shapes can be processed on general-purpose milling machines, ensuring that the shape is basically consistent and efficient. , widely used in batch production and mass production.
(1) Forming milling cutters can be divided into two types: sharp teeth and shovel teeth.
Milling and regrinding of tine forming milling cutters require special masters, which are difficult to manufacture and sharpen. The tooth back of the shovel tooth forming mill is scraped and ground on a shovel tooth lathe. During regrinding, only the rake surface is ground. Because the rake surface is flat, sharpening is more convenient. Currently, forming milling cutters mainly use shovel Teeth dorsal structure. The back of the shovel teeth should meet two conditions: ① The shape of the cutting edge remains unchanged after regrinding; ② The required relief angle is obtained.
(2) Tooth back curve and equation
Make an end section perpendicular to the axis of the milling cutter through any point on the cutting edge of the milling cutter, and the intersection line with the tooth back surface is called the tooth back curve of the milling cutter.
The tooth back curve should mainly meet two conditions: one is that the relief angle of the milling cutter is basically unchanged after each regrinding; the other is that it is simple to manufacture.
The only curve that can satisfy the constant relief angle is logarithmic spiral, but it is difficult to manufacture. The Archimedean spiral can satisfy that the back angle remains basically unchanged, is simple to manufacture and easy to implement. Therefore, Archimedean spiral is widely used as the tooth back curve of forming milling cutters in production.
According to geometric knowledge, the vector radius ρ value of each point on the Archimedean spiral increases or decreases in equal proportion with the increase or decrease of the rotation angle θ value of the vector radius.
Therefore, the Archimedean spiral can be obtained by combining the constant-speed rotational motion and the constant-speed linear motion along the radius.
Expressed in polar coordinates: when θ=00, ρ=R, (R is the radius of the milling cutter), when θ>00, ρ
The general equation of milling cutter tooth back is: ρ=R-CQ
Assuming that the blade does not retract, every time the milling cutter rotates through an inter-tooth angle ε = 2π/z, the amount of cutting teeth of the blade is K. According to this, the amount of lifting of the cam should also be K. In order to enable the blade to move at a constant speed, the curve on the cam should be an Archimedean spiral, so it is easy to manufacture. In addition, the cam size is only determined by the shovel sales volume K value and has nothing to do with the diameter of the milling cutter, the number of teeth and the clearance angle. As long as the production and sales volume are equal, the cam can be used universally. This is also the reason why Archimedean spiral is widely used on the tooth back of shovel tooth forming milling cutters.
When the milling cutter radius R and the cutting amount K are known, C can be obtained:
When θ=2π/z ρ=R-K
Then R-K=R-2πC /z ∴ C= Kz/2π
Phenomenon that will occur after the milling cutter is passivated:
(1) Judging from the shape of the chips, the chips become thick and flaky. Due to the increase in chip temperature, the color of the chips turns purple and smokes.
(2) The roughness of the machined surface of the workpiece is very poor, and bright spots, gnawing marks or ripples appear on the surface of the workpiece.
(3) The milling process produces severe vibration and abnormal noise.
(4) Judging from the shape of the knife edge, there are shiny white spots on the knife edge.
(5) When using carbide milling cutters to mill steel parts, a large amount of fire mist often flies out.
(6) When milling steel parts with high-speed steel milling cutters, a large amount of smoke will be produced if oil is used for lubrication and cooling.
After the milling cutter is passivated, you should stop the machine in time to check the wear of the milling cutter. If the wear is slight, you can use a whetstone to grind the cutting edge before use; if the wear is severe, you must sharpen it to prevent excessive wear of the milling cutter. wear and tear.
Conclusion
Whether choosing a milling cutter on a CNC milling machine or an ordinary milling machine, we must comprehensively consider the material and hardness of the milling cutter, and the specifications of the milling cutter, such as: blade length, tool length, blade diameter, shank diameter, etc. High-speed steel milling cutters are usually suitable for ordinary milling machines, while CNC milling machines prefer to use carbide cutting tools.
