Choice of Milling Cutters under Complex Machining Conditions

In cutting processing, in order to maximize the processing quality and repeat accuracy, it is necessary to correctly select and determine the appropriate tool, which is especially important for some challenging and difficult processing. This article provides some guidelines on how to choose a milling cutter reasonably under some difficult processing conditions (such as high-speed tool paths, when milling narrow parts, straight walls, and graphite workpieces).

High-speed tool path

Today's CAD/CAM software system can precisely control the arc length of the tool in the high-speed cycloid tool path (Note: The cycloid tool path is a curved path formed by a fixed point on a circle rolling along a straight line), and Obtain extremely high cutting accuracy. Even when the milling cutter cuts into corners or other complex geometric shapes, the amount of tooling will not increase. In order to take full advantage of this technological advancement, tool manufacturers have designed and developed advanced small-diameter milling cutters. Compared with larger diameter milling cutters, small diameter milling cutters are cheaper, and by adopting high-speed tool paths, more workpiece material can be removed per unit time. This is because the contact surface of the large-diameter milling cutter and the workpiece is larger, so the feed rate needs to be reduced and the more traditional small feed rate is used. Therefore, small diameter milling cutters can achieve a higher metal removal rate.

However, tool designers still need to ensure that these small diameter milling cutters are not only suitable for trochoidal cutting, but also match the material of the workpiece being cut. Nowadays, the geometric edge shapes of many high-efficiency tools are specially designed for the specific material to be processed and the cutting technology used. For example, with an optimized tool path, a 6-edge milling cutter can be used to mill a full groove in H13 steel with a hardness of HRC54. A milling cutter with a diameter of 12.7mm can cut a slot with a width of 25.4mm. If a 12.7mm diameter milling cutter is used to machine a groove with a width of 12.7mm, the tool will have too much surface contact with the workpiece and cause the tool to quickly fail. A useful rule of thumb is to use a milling cutter with a diameter of about 1/2 the size of the narrowest part of the workpiece. In this example, the narrowest part of the workpiece is a slot with a width of 25.4mm. Therefore, the maximum diameter of the milling cutter used should not exceed 12.7mm. When the radius of the milling cutter is smaller than the size of the narrowest part of the workpiece, the cutter has room to move left and right, and can obtain the smallest cutting angle. This means that the milling cutter can use more cutting edges and higher feed rates.

The stiffness of the machine tool also helps determine the size of the tool that can be used. For example, when cutting on a 40-taper machine tool, the diameter of the milling cutter should usually be less than 12.7mm. Milling cutters with larger diameters can generate larger cutting forces that may exceed the capacity of the machine tool, resulting in chatter, deformation, poor surface finish, and shortened tool life.

In addition, when a milling cutter with a diameter of 1/2 of the size of the narrowest part of the workpiece is used, it can maintain a small cutting angle, and it will not increase when the tool is turned. For example, if the cutting step used by the workpiece processing program is 10%, the cutting angle is 37°. If an old, traditional tool path is used, every time the milling cutter changes its direction, its tool angle will increase to 127°. When the newer high-speed tool path is used, the sound of the milling cutter at the corner is the same as when cutting in a straight line. If a milling cutter makes the same sound during all cutting processes, it indicates that it has not been subjected to major thermal shocks and mechanical shocks. If the milling cutter makes a squeaking noise every time it turns or cuts into the corner, it indicates that the diameter of the milling cutter may need to be reduced to reduce the angle of engagement. If the cutting sound remains the same, it indicates that the cutting pressure of the milling cutter is uniform, and it does not fluctuate up and down with the change of the workpiece geometry, because the angle of the tool is always kept constant.

Milling narrow parts

The ring milling cutter is the best choice for milling narrow parts (such as spiral milling holes and milling ribs, or when the diameter of the milling cutter is close to the radius of the workpiece). The robust annular shape of this milling cutter can produce a chip thinning effect, enabling it to mill at a higher feed rate. In addition, the radius of the milling cutter is smaller than that of the traditional ball end milling cutter, so the cutting step can be increased while still maintaining the flatness of the machined surface, without the usual production of ball end milling cutters. Larger knife marks.

The ring milling cutter is very suitable for spiral milling and milling ribs, because in these processing, the tool will inevitably have more contact with the machined surface, and the double-edged ring milling cutter can minimize the surface contact with the workpiece. Thereby reducing cutting heat and tool deformation. In these two types of processing, the ring milling cutter is usually closed when cutting. Therefore, the maximum radial cutting step should be 25% of the milling cutter diameter, and the maximum Z-direction cutting depth for each pass should be the milling cutter 2% of the diameter. In spiral milling, when the milling cutter cuts into the workpiece with a spiral tool path, the spiral cut-in angle is 2°-3° until it reaches the Z-direction cutting depth of 2% of the milling cutter's diameter.

If the ring milling cutter is in an open state when cutting (such as milling the corner of the workpiece or cleaning the features of the workpiece), its radial step depends on the hardness of the workpiece material. When milling workpiece materials with a hardness of HRC30-50, the maximum radial cutting step should be 5% of the diameter of the milling cutter; when the material hardness is higher than HRC50, the maximum radial cutting step and the maximum Z of each pass The depth of cut is 2% of the diameter of the milling cutter.

Milling straight walls

When milling open areas with flat ribs or straight walls, the best effect is to use a bull nose cutter. The 4-6 flute bull nose milling cutter is especially good at profiling milling of external shapes with straight walls or very open areas. The greater the number of edges of the milling cutter, the greater the feed rate that can be used. However, machining programmers still need to minimize the contact between the tool and the workpiece surface and use a smaller radial cutting width. When machining on a machine with poor rigidity, it is more advantageous to use a milling cutter with a smaller diameter, because the small diameter milling cutter can reduce the surface contact with the workpiece.

The use method of multi-edge bull nose milling cutter (including cutting step and cutting depth) is the same as that of ring milling cutter. They can use the cycloid tool path (or a new tool path that can control the cutting angle of the tool) for grooving hardened materials. As mentioned earlier, the most important thing is to ensure that the diameter of the milling cutter is about 50% of the groove width, so that the milling cutter has enough space to move, and that the angle of the cutter does not increase and generate excessive cutting heat.

Milling graphite material

When cutting graphite materials, its high abrasiveness will quickly wear standard cemented carbide tools, and the worn tools will not be able to accurately cut the required complex geometric shapes. When milling graphite, the tool path and milling method are not the most critical factors. The type of milling cutter used usually depends on the shape of the graphite electrode. Because diamond-coated milling cutters have excellent wear resistance, they are widely used in graphite milling. The diamond grown on the carbide tool matrix will form a wear-resistant coating with extremely high hardness and significantly prolong tool life. The life of diamond-coated tools is 10-30 times longer than that of uncoated carbide tools.

For example, when using an uncoated carbide ball end mill with a diameter of 12.7mm to machine a 152.4mm square complex graphite electrode, usually about 4 hours after milling, the sharp edge shape and detailed features of the cutting edge of the milling cutter Began to peel off. A diamond-coated milling cutter can continue milling for more than 98 hours without peeling off the cutting edge.

When processing certain graphite workpiece shapes (such as thin ribs), sharp geometric profiles and small-size workpieces, the sharpness of the cutting edge of the milling cutter is particularly high. In this type of processing, the use of a 2-3μm thick diamond coating can prolong the tool life and keep the cutting edge sharp. Due to the low cost of this relatively thin diamond coating, it is very suitable for low-end machining that does not require high tool life. The diamond coating with a typical thickness of 18μm is mainly used for high-end machining that requires high tool life.

The use of a thinner diamond coating allows moldmakers who have smaller production batches and want to reduce tool costs without sacrificing tool life in order to reduce costs. They can still take advantage of the performance advantages of real diamond-coated carbide tools, and at the same time can use a thinner diamond coating to meet their specific processing needs. The thickness of today's diamond coating is roughly 2-25μm.

The best tool for a particular machining should not only depend on the material being cut, but also on the type of cutting and milling method used. By optimizing tools, cutting speeds, feed rates and machining programming skills, parts can be produced faster and better at lower machining costs.