Milling cutter selection under complex machining conditions

In the machining process, in order to maximize the machining quality and repeatability, it is necessary to correctly select and determine the right tool. This is especially important for challenging and difficult machining. This article provides some guidelines for how to properly select a milling cutter under difficult machining conditions, such as high-speed toolpaths, milling of narrow sections, straight-wall and graphite workpieces.

High speed tool path

Today’s CAD/CAM software system precisely controls the arc length of the knife in the high-speed cycloidal tool path (note: the cycloidal tool path is a curved path formed by a fixed point on a circle that rolls along a straight line) Get extremely high cutting accuracy. Even when the cutter cuts into a corner or other complex geometry, the amount of knife it does not increase. To take full advantage of this technological advancement, tool manufacturers have designed advanced small diameter milling cutters. Smaller diameter milling cutters are less expensive than larger diameter milling cutters, and by using high speed toolpaths, more workpiece material can be cut per unit time. This is because the large-diameter milling cutter has a larger contact surface with the workpiece, so it is necessary to reduce the feed rate and adopt a more conventional small feed rate. Therefore, a small diameter milling cutter 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 cycloidal cutting, but also for the material of the workpiece being cut. Today, the geometry of many high-efficiency tools is specifically designed for the specific material being machined and the cutting technology used. For example, with an optimized tool path, a 6-blade cutter can be used to mill a full groove on H13 steel with a hardness of HRC54. A groove with a width of 25.4 mm can be cut with a 12.7 mm diameter milling cutter. If a 12.7 mm wide groove is used to machine a groove with a width of 12.7 mm, the tool will have excessive surface contact with the workpiece and cause the tool to fail quickly. A useful rule of thumb is to use a milling cutter with a diameter of approximately 1/2 the narrowest part of the workpiece. In this example, the narrowest part of the workpiece is a groove with a width of 25.4 mm. Therefore, the maximum diameter of the milling cutter used should not exceed 12.7 mm. When the cutter radius is smaller than the narrowest part of the workpiece, the tool has a space to move left and right, and the minimum angle of the knife can be obtained. This means that the milling cutter can use more cutting edges and higher feed rates.

Machine stiffness also helps determine the size of the tool that can be used. For example, when cutting on a 40-taper machine, the cutter diameter should normally be <12.7mm. Larger diameter milling cutters produce large cutting forces that may exceed the machine’s ability to withstand, resulting in chatter, deformation, poor surface finish, and reduced tool life.

In addition, when a milling cutter having a diameter of 1/2 the narrowest part of the workpiece is used, a small knife angle can be maintained and the tool does not increase when it is turned. For example, if the workpiece machining program uses a 10% step, the angle of the knife is 37°. If an old, traditional tool path is used, the angle of the knife will increase to 127° each time the cutter changes direction. With the newer high-speed tool path, the cutter produces the same sound at the corners as the straight line. If a milling cutter produces the same sound during all cutting processes, it indicates that it is not subject to large thermal shocks and mechanical shocks. If the milling cutter makes a whistling sound every time it turns or cuts into a corner, it may indicate that it may be necessary to reduce the diameter of the milling cutter to reduce the angle of the knife. If the sound from the cutting remains the same, it indicates that the cutting pressure of the milling cutter is uniform and does not fluctuate up and down with the geometry of the workpiece, because the angle of the knife is always constant.

Milling narrow parts

Ring milling cutters are the best choice for milling narrow parts such as spiral milling and milling ribs, or when the diameter of the milling cutter is close to the radius of the workpiece. The robust, toroidal shape of this milling cutter produces a chip reduction effect that allows it to be milled at higher feed rates. In addition, the radius of the milling cutter is smaller than that of a conventional ball-end milling cutter, so that the walking step can be increased while still maintaining the flatness of the machined surface without the usual occurrence of ball-end milling. Large knife marks.

Ring milling cutters are ideal for spiral milling and milling ribs because in these processes, the tool inevitably comes into contact with the machined surface, and the double-edged ring milling cutter minimizes surface contact with the workpiece. Thereby reducing cutting heat and tool deformation. In these two types of machining, the ring milling cutter is usually closed when cutting. Therefore, the maximum radial travel step should be 25% of the diameter of the milling cutter, and the maximum Z-cut depth of each pass should be the milling cutter. 2% of the diameter. In the spiral milling hole, when the milling cutter cuts into the workpiece with a spiral cutter rail, the spiral cut-in angle is 2° – 3° until the Z-cut depth of the milling cutter diameter is 2%.

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

Milling straight wall

The use of a bull nose cutter works best when milling open areas with flat ribs or straight walls. The 4-6 bladed nose nose cutter is especially good at copying the outer shape with a straight wall or a very open part. The more the number of edges of the milling cutter, the greater the feed rate that can be used. However, the machining programmer still needs to minimize the surface contact of the tool with the workpiece and use a smaller radial cut width. When machining on less rigid machine tools, it is advantageous to use a smaller diameter milling cutter because the small diameter milling cutter reduces surface contact with the workpiece.

The use of multi-blade beef nose milling cutters (including the stepping distance and depth of cut) is the same as that of the ring milling cutter. They can use a cycloidal tool path (or a new tool path that controls the tool’s angle of the tool) to grooving hardened materials. As mentioned before, 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 moving space and ensures that the angle of the knife does not increase and excessive cutting heat is generated.

Milling graphite material

When cutting graphite materials, the high abrasiveness causes the standard carbide tool to wear quickly, and the worn tool will not be able to accurately cut the required complex geometry. Tool path and milling are not the most critical factors when milling graphite. The type of milling cutter typically depends on the shape of the graphite electrode. Diamond coated milling cutters are widely used in graphite milling because of their excellent wear resistance. Diamonds grown on the carbide tool base form a wear-resistant coating that is extremely hard and significantly extends tool life. Diamond coated tools have a lifespan 10-30 times longer than uncoated carbide tools.

For example, when machining a 152.4 mm square complex graphite electrode with a 12.7 mm diameter uncoated carbide ball end mill, the sharp edge and detail of the cutting edge of the milling cutter is usually about 4 hours after milling. Start peeling off. A diamond-coated milling cutter can continue milling for more than 98 hours, and its cutting edges will not peel off.

When machining certain graphite workpiece shapes (such as thin ribs), sharp geometric profiles, and small-sized workpieces, the sharpness of the cutting edge of the milling cutter is particularly high. In this type of processing, a 2-3 μm thick diamond coating extends tool life and keeps the edge sharp. Due to the relatively low cost of this relatively thin diamond coating, it is ideal for low-end machining where tool life is not critical. A typical diamond coating with a thickness of 18 μm is mainly used for high-end machining that requires high tool life.

With a thinner diamond coating, mold makers who produce smaller batch sizes and want to reduce tooling costs do not have to sacrifice tool life to reduce costs. They can still take advantage of the performance of true diamond-coated carbide tools while using thinner diamond coatings to meet their specific processing needs. Today’s diamond coatings range in thickness from approximately 2 to 25 μm.

The optimum tool for a particular machine depends not only on the material being machined, but also on the type of cutting used and the method of milling. By optimizing tooling, cutting speed, feedrate and machining programming skills, parts can be produced faster and better at lower processing costs.

Post time: Aug-07-2018
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