milling – important part of machining
Milling is one of the machining processes (subtractive machining) next to turning, drilling, grinding or cutting.
This process causes the workpiece to achieve the desired shape, size and surface quality. It is a good alternative to making holes, countersinks and threads that were once drilled, tapped or turned.
Milling is performed with a machine called a milling machine.
Milling can be divided into climb milling and up milling depending on the direction of rotation of the cutter. This is a division of the so-called kinematic. State it is desirable to find an arrangement of the cutting edge in relation to the cutting layer at the exit and entry of the blade so that it is optimal and effective. You can mill in two directions, either in line with or against the direction of rotation of the cutter. “Thick to thin” is the golden rule for milling. This means that before machining begins, it is necessary to investigate how the chips will be formed. It is best when the cutter is in contact with the machined surface when the chip thickness is the highest, and when the contact disappears – the smallest. Thanks to this, the stability of the treated surface is the greatest.
Up milling is when the workpiece moves in the opposite direction to the cutter’s rotation. In the initial stage of the process, the rotating cutter rubs the surface of the material until the pressure becomes strong enough that the main cutting process begins. The weakness of this method is chip accumulation in front of the cutter, which makes it difficult to cool the material and adversely affects the cutters, which are quickly subject to significant wear. The strong point is the possibility of milling in raw material of various geometries, and even heat-treated details, because the next blades reach the already pre-machined surface, thanks to which the durability of the blades is longer.
Down milling occurs when the cutting edge of the tool moves in the direction of the material feed. With such
Down milling occurs when the cutting edge of the tool moves in the direction of the material feed. With such machining, it is necessary that there are no play in the drive system of the table and the tool carrier. This method is not recommended for use with older machines. When the cutter plunges into the metal or the surface of plastics, we encounter high resistance, which decreases as the surface processing progresses. The chip thickness is large at first and gradually drops to zero. The machined surface is smoother than with up milling. It also has a beneficial effect that chips do not accumulate under the cutter. The weakness is the high forces acting on the cutting edges of the milling cutter distributed on the machine tool and strong vibrations. Therefore, a newer machine tool is suitable for climb milling.
Milling depending on the position of the milling axis
Another division of the milling process can be made with reference to the position of the milling axis in relation to the machined surface. we can identify here:
- face milling when the milling cutter axis is perpendicular to the surface of the solid of revolution
- peripheral milling when the milling axis is parallel; We divide peripheral milling into cylindrical and shaped
- milling when the angle between the machined surface and the milling cutter axis is between 0 and 90 degrees
This classification applies to plane milling, while in the case of shaped surfaces, the angle between the axis of the cutter and the tangents to the intersection of the workpiece surface with the plane passing through the cutter axis is different for different points of this line.
The basic movement during milling is the rotation of the cutter / tool around its axis. On the other hand, an object made of metal or plastic that is to be processed moves with a feed movement. The tool clamping parameters depend on the direction of the material advance. Feed movement – also known as auxiliary movement of the workpiece, feed per minute or feed of the machine tool, it can be curvilinear, rectilinear, and also helical.
The feed rate depends on the feed per tooth and the number of teeth in the cutter. The feed per revolution (fn) measured in mm revolution affects the choice of the finishing tool.
The blade path relative to the table is formed from the combination of the linear feed motion of the workpiece and the rotary motion of the milling cutter.
The recess in the material, i.e. the thickness of the chip
The thickness of the chip tells us about the productivity and reliability of the process. Machining is only effective if the hex value is suitably adapted to the cutter.
- when it is too low – then the efficiency of the entire process is low. It can also adversely affect tool strength and chip formation
- if it is too large, it loads the cutting edge too much and may cause it to break
Chip thickness for a straight edge insert is influenced by the feed rate of the tooth and the entering angle.
Trochoidal milling on CNC
Trochoidal milling is a solution that allows to shorten the processing time of a detail and increase the durability of tools. This type of milling is best suited for creating grooves, narrow recesses and pockets – especially in vibration-prone systems.
It requires modern machine tools and specialized software. The advantages of this solution are the stability of the process and its shorter time. The cutter has a low arc of contact, which reduces vibrations, reduces heat generation during operation and reduces thermal stress. The cutter can cool down faster, therefore its service life is significantly extended. In the finishing phase, the cutter is applied to the full length of the cutting edge, giving it an even load and longer performance. The chips that are a by-product of milling are here of little thickness.
Trochoidal milling tool
The trochoidal milling tool requires higher parameters than the average. Stable core geometry and thermal shock resistance are essential, as exposure to machining and continuous cooling causes rapid temperature fluctuations.
Due to the low arc of contact, the tools can have a large number of teeth, which allows high feed rates to be achieved while protecting the life of the cutter blades.
We divide the shavings into:
- tear-offs – are formed when the resolving strength is exceeded
- cut – formed after the shear yield point is exceeded
The chips differ in length:
- confused and shape, and here we can indicate:
- screw shorted
- conical screw
- screw open
Chips are a by-product that needs to be disposed of after the process is complete. However, their parameters affect the course of their removal from the cutting zone.
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