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Selection of milling cutters under complex machining conditions

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Selection of milling cutters under complex machining conditions

In machining, in order to maximize the machining quality and repeatability, the right tool must be selected and determined correctly, which is especially important for some challenging and difficult machining. This paper aims at some difficult machining conditions (such as high-speed tool and high-speed tool path)

Today's CAD/CAM software systems can precisely control the arc length of the bite in a high-speed trochoidal toolpath (Note: a trochoidal toolpath is a curved path formed by a fixed point on a circle rolling along a straight line), while Get extremely high cutting accuracy. Even when the cutter cuts into corners or other complex geometries, its engagement does not increase. To take advantage of this technological advancement, tool manufacturers have designed and developed advanced small diameter milling cutters. Small-diameter cutters are less expensive than larger-diameter cutters and, by using high-speed toolpaths, tend to remove more workpiece material per unit time. This is because the larger diameter cutter has a larger contact surface with the workpiece, thus requiring lower feed rates and more traditional small feed rates. Therefore, small diameter milling cutters can achieve higher metal removal rates instead.

However, tool designers still need to ensure that these small-diameter cutters are not only suitable for trochoidal cutting, but also match the workpiece material being cut. Today, the geometry of many high-efficiency tools is tailored to the specific material being machined and the cutting technique employed. For example, with an optimized toolpath, a full groove can be milled in H13 steel with a hardness of HRC54 with a 6-flute cutter. A slot with a width of 25.4mm can be cut with a milling cutter with a diameter of 12.7mm. If a 12.7mm diameter cutter was used to machine a 12.7mm width slot, the tool would have too much surface contact with the workpiece and cause the tool to fail quickly. A useful rule of thumb is to use a cutter with a diameter 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, so the maximum diameter of the 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 angle of engagement. This means that the milling cutter can use more cutting edges and higher feed rates.

Machine rigidity 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 usually be <12.7mm. Larger diameter milling cutters generate higher cutting forces that may exceed the machine's capacity, resulting in chatter, deformation, poor surface finish and reduced tool life.

In addition, when using a milling cutter with a diameter of 1/2 the size of the narrowest part of the workpiece, the angle of engagement can be kept small and does not increase when the tool is turned. For example, if the workpiece machining program adopts a 10% tool pass, the angle of engagement is 37°. With the old, traditional toolpaths, each time the cutter changed direction, its engagement angle would increase to 127°. With the newer high-speed toolpaths, the sound of the cutter around the corners is no different than when cutting straight. If a milling cutter makes the same sound during all cuts, it is not subject to large thermal and mechanical shocks. If the cutter squeaks every time it turns or cuts into a corner, it may be a sign that the cutter diameter may need to be downsized to reduce the angle of engagement. If the sound of the cutting remains the same, it means that the cutting pressure on the milling cutter is uniform and does not fluctuate up and down with the change of the workpiece geometry, because the angle of engagement remains constant.

Milling small parts

Ring cutters are the best choice for milling tight spots such as helical holes and rib milling, or when the diameter of the cutter is close to the radius of the workpiece. The robust annular shape of this cutter creates a chip-thinning effect, allowing it to be milled at higher feed rates. In addition, the cutter has a smaller radius than conventional ball end mills, which allows for larger passes while still maintaining the flatness of the machined surface without the typical ball end mill machining problems. Large knife marks.

Ring cutters are ideal for helical hole milling and rib milling, where more contact between the tool and the machined surface is unavoidable, while a double-edged ring cutter can minimize surface contact with the workpiece. This reduces cutting heat and tool deformation. In both types of machining, the ring cutter is usually closed when cutting, so the maximum radial pass should be 25% of the cutter diameter, and the maximum Z depth of cut per pass should be the cutter 2% of the diameter. In helical milling, when the milling cutter cuts into the workpiece with a helical tool path, the helical cut-in angle is 2°-3° until it reaches a Z-direction depth of cut that is 2% of the cutter diameter.

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

Bullnose cutters work best when milling open areas with flat ribs or straight walls. Bullnose cutters with 4-6 flutes are particularly good at profiling external shapes with straight walls or very open areas. The more flutes the milling cutter has, the higher the feed rate that can be used. However, machining programmers still need to minimize tool-to-work surface contact and use a small radial cut width. When machining on a less rigid machine tool, it is advantageous to use a smaller diameter cutter because the smaller diameter cutter reduces surface contact with the workpiece.

The use of the multi-blade bullnose cutter (including the pass and depth of cut) is the same as that of the annular cutter. They can use trochoidal toolpaths (or new toolpaths that control the angle of engagement of the tool) for grooving hardened materials. As mentioned before, the most important thing is to ensure that the cutter diameter is about 50% of the slot width, that the cutter has enough room to move, and that the angle of engagement does not increase and generate excessive cutting heat.

Milling graphite material

When cutting graphite materials, its high abrasiveness causes standard carbide tools to wear quickly, and worn tools will not be able to precisely cut the complex geometries required. When milling graphite, the toolpath and milling method are not the most critical factors, and the type of milling cutter used usually depends on the shape of the graphite electrode. Due to their excellent wear resistance, diamond-coated milling cutters are widely used in graphite milling. Diamond grown on carbide tool substrates creates extremely hard wear-resistant coatings that significantly extend tool life. Diamond-coated tools last 10-30 times longer than uncoated carbide tools.

For example, when machining a complex graphite electrode of 152.4mm square with a 12.7mm diameter uncoated carbide ball end mill, the sharp edge shape and detailed features of the milling cutter cutting edge are usually reduced after about 4 hours of milling. start to peel. A diamond-coated cutter can last for more than 98 hours without spalling at the cutting edge.

When machining certain graphite workpiece shapes (such as thin ribbed plates), sharp geometries and small workpieces, the sharpness of the cutting edge of the milling cutter is particularly high. In this type of machining, a 2-3 μm thick diamond coating can prolong tool life and keep the cutting edge sharp. Because of the lower cost of this thinner diamond coating, it is ideal for low-end machining where tool life is not critical. The typical thickness of diamond coating of 18μm is mainly used for high-end machining with high tool life requirements.

The use of thinner diamond coatings allows moldmakers who are producing smaller batches and wish to reduce tool costs without sacrificing tool life for cost reduction. They can still take advantage of the performance benefits of true diamond-coated carbide tools, while taking advantage of thinner diamond coatings to meet their specific machining needs. Today's diamond coating thicknesses range roughly from 2 to 25 μm.

The best tool for a particular job should depend not only on the material being cut, but also on the type of cut 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.

Pub Time : 2022-05-20 10:43:32 >> News list
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