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Milling Techniques for Effective Machining

When selecting a milling techniques cutter suitable for a machining task, various issues such as the geometry, size, and workpiece material to be processed must be considered. With new high-speed milling, the cutting tool design and composition also play an important factor due to both tool life span and efficiencies in tool changes. There are universal tools that may offer milling techniques for effective machining without requiring frequent tool changes, but specific tooling may be best based on the variables of your machine capabilities, workpiece design and material, and budget. Consider these factors to be sure you have the right milling cutters for your machining tasks.

1. Tool Durability

High process speeds with large swarf chunks cause both heat and vibration on the workpiece and the tool. This tool wear causes loss of material on the tool and change in the tool shape which will both affect your workpiece quality. Other factors affecting the intensity of tool wear include the workpiece material, the parameters of cutting, and means of cooling. Cooling options, like cutting oil are necessary to help control the heat buildup, but can not prevent it. Heat causes premature wear that can be managed with a range of titanium coatings. Tool coating is as important as the composite the tool is made of concerning oxidization and heat control.

Heat and vibration, both, can also be managed with the cutting tool material composition. Ranging from carbide steel to ceramic and diamond tips, varying degrees in toughness and hardness are available to best suit the material you are machining. Tool hardness decreases at higher temperatures and is directly proportional to tool wear. Toughness is proportional to impact loads. They are inversely proportional, so each cutting process must be analyzed for the selection of appropriate tool composition.

Tool edge preparation is also an important consideration. Mechanical wear in the form of abrasion or adhesive wear causes chips and scratches on the tool. A rounded tool edge prevents the growth of cracks and chips and offers a slower, even abrasion process.

 

2. Milling Cutter Main Angle

When the tool moves into action, the main declination angle has a great influence on the radial cutting force and cutting depth. The main declination angle is the angle between the cutting edge and the cutting plane. The magnitude of the radial cutting force directly affects the cutting power and the vibration resistance of the tool. The smaller the main declination angle of the milling cutter, the smaller the radial cutting force and the better the vibration resistance, but the cutting depth also decreases. Several standard angles are available providing various milling options for a workpiece.

When milling the plane with square shoulders, select a 90° lead angle. This kind of tool has good versatility and is used in single-piece and small-batch processing. Because the radial cutting force of this type of tool is equal to the cutting force, the feed resistance is large and it is easy to vibrate, so the machine tool is required to have greater power and sufficient rigidity.

The 90° angle can also be used when machining a flat surface with a square shoulder. A milling cutter with an 88° main angle, however, provides an improved cutting performance. Face milling with 90° square shoulder milling cutters is also very common for certain milling needs, like when the shape of the milled workpiece is irregular, or the surface of the casting will cause the depth of cut to change.

The standard 45° face milling cutter may benefit more in other cutting applications. In face milling, a face milling cutter with an angle of 45° will make the chips thinner. As the cutting angle decreases from 90°, the chip thickness will be less than the feed per tooth, which in turn can increase the feed rate to 1.4 times the original. The radial cutting force of the 45° main declination milling cutter is greatly reduced, which is approximately equal to the axial cutting force. The cutting load is distributed on the longer cutting edge. It has good vibration resistance and is suitable for the overhang of the spindle of the boring and milling machine. When machining flat surfaces with this type of tool, the blade breakage rate is low and the durability is high; when machining cast iron parts, the edges of the workpiece are not prone to chipping. It is well suited to longer processing occasions.

 

In this example of how we machined Darth Vader, took well over 80 hours if it wasn’t for the efficiencies of using our DMG MORI 5 Axis machine we wouldn’t have been able to finish this machine is built to read ahead complex five-axis programs and pre-process the movements so that it doesn’t stop while calculating the next move, which would then create tool marks on the workpiece and also longer machining.

3. Milling Cutter Size Selection

The diameter of a standard face milling cutter can range from Φ16mm to Φ630mm. The diameter of the milling cutter should be selected according to the milling width and depth. Generally, the larger the depth and width before milling, the larger the diameter of the milling cutter. During rough milling, the diameter of the milling cutter of the milling machine is smaller; when finishing milling, the diameter of the milling cutter is larger to accommodate the entire processing width of the workpiece and to reduce the traces of tool connection between two adjacent feeds.

When face milling large parts, cutters with smaller diameters are used, which leaves much room for improving productivity. In an ideal situation, a milling cutter should have 70% of the cutting edges involved in cutting. When milling holes with a milling cutter, the tool size becomes particularly important. Compared with the hole diameter, the diameter of the milling cutter is too small, then a core may be formed in the center of the hole during processing. When the core falls, it may damage the workpiece or tool. If the diameter of the milling cutter is too large, it will damage the tool itself and the workpiece, because the milling cutter is not cutting in the center and may collide at the bottom of the tool.

 

4. Selection of Milling Method

Another way to improve the milling process is to optimize the milling strategy of the face milling cutter. When programming surface milling, the user must first consider the way the tool cuts into the workpiece. Usually, the milling cutter simply cuts directly into the workpiece. This cutting method is accompanied by a large impact noise because when the insert is withdrawn, the milling cutter produces the thickest chips. Because the blade forms a large impact on the workpiece material, it often causes vibration and produces tensile stress that will shorten the life of the tool.

A better way to feed is to use the rolling cutting method. As the name implies, the milling cutter rolls into the workpiece without reducing the feed rate and cutting speed. This means that the milling cutter must rotate clockwise to ensure that it is processed in a milling manner. The chips formed in this way are from thick to thin, which can reduce vibration and tensile stress on the tool, and transfer more cutting heat to the chips. By changing the way, the milling cutter cuts into the workpiece each time, the tool life can be doubled. To achieve this in-feed method, the programming radius of the tool path should be half of the diameter of the milling cutter, and increase the offset distance from the tool to the workpiece.

Although the rolling cutting method is mainly used to improve the way the tool cuts into the workpiece, the same machining principle can also be applied to other stages of milling. For large-area plane milling, the commonly used programming method is to let the tool pass through the entire length of the workpiece one after another and complete the next cut in the opposite direction. To maintain a constant radial tool intake and eliminate vibrations, the use of a combination of helical lower knife and rolling milling workpiece corners usually results in better results.

Mechanics are familiar with the cutting noise caused by vibration. It usually occurs when the tool cuts into the workpiece, or when the tool makes a sharp 90° turn while eating. Roll milling of workpiece corners can eliminate this noise and extend tool life. In general, program the corner radius of the workpiece to be 75%-100% of the diameter of the milling cutter. This can shorten the arc length of the milling cutter, allowing higher feed rates and reduced vibration.

To prolong the life of the tool, in the face milling process, the tool should be programmed to avoid the hole or interrupted part of the workpiece (if possible). When the face milling cutter passes through the middle of a hole in the workpiece, the cutter is milled along one side of the hole and the reverse milling is performed on the other side of the hole, which will cause a great impact on the insert. This can be avoided by bypassing holes and pockets when programming the tool path.

 

5. Vertical Milling

More and more manufacturers use milling cutters to machine holes in helical or circular interpolation. Although the processing speed of this method is slightly slower than that of drilling, it is more advantageous for many processes. When drilling holes on irregular surfaces, it may be difficult for the drill bit to drill into the workpiece along the centerline, causing the drill bit to drift on the workpiece surface. Drill bits also require about 10 horsepower for each 25mm hole diameter, which means that when drilling on a small power tool, the optimal power value may not be achieved. Using a milling cutter is also optimal for parts requiring many holes, even of different sizes. If the tool magazine’s capacity is limited, the use of milling holes can avoid a frequent shutdown of the machine for tool replacement.

By selecting the appropriate milling cutter angle, size and feed method, the tool can cut into the workpiece material with minimum vibration and tensile stress. Taking a different approach to drilling can also better utilize milling equipment with less downtime. Coupled with choosing the right tool design and composition, these milling techniques for effective machining can create highly efficient, low-cost processing of workpiece blanks into exquisite parts.

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