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11/30/2022|Fundamentals of Production Engineering

What is milling?

Within the superordinate group of manufacturing technology, milling belongs to the cutting manufacturing processes with a geometrically determined cutting edge. This means nothing other than that all geometric sizes and ratios of a tool are known. A further characteristic is that in milling, the so-called cutting movement is carried out by the tool, while the workpiece takes over the feed movement. The relative movement between the tool and the workpiece resulting from the two variables ultimately ensures chip removal.

Another characteristic feature of milling is that the cutting edges of the milling tool are not engaged over the entire stroke, but each revolution of the tool is accompanied by at least one interruption of the cut per cutting edge. The corresponding technical term is proverbial, because it is an "interrupted cut". The constant "in and out" means that the cutting edges have to endure continuous thermal and mechanical alternating loads.

The loads that have to be adequately absorbed by this dynamic system of workpiece, tool and machine tool are immense and ultimately decisive for competitiveness. This dynamic triangle of forces determines the precision of the components, the quality of their surfaces and the economy of the machining process. And this is also where the fascination of the milling process arises, when contours from simple to complex, materials from soft to ultra-hard and surfaces from coarse to glossy are worked out of a blank "like butter".

Milling process

Within milling, however, there are very real differences. For example, the German Institute for Standardization (DIN) divides milling processes according to the type of workpiece surface produced, the kinematics of the cutting process and the profile of the milling tool:

  • Face milling: Face milling is milling with straight-line feed motion to produce flat surfaces and is differentiated into face milling, circumferential face milling and circumferential face milling.
  • Screw milling: Screw milling refers to milling processes in which helical surfaces are created on the workpiece under helical feed motion (e.g. threads and cylindrical screws).
  • Gear hobbing: Gear hobbing is one of the most important manufacturing processes for producing gears. In hobbing, a cutter with a reference profile performs a hobbing motion simultaneous with the feed motion. In this process, the tool and workpiece hob against each other during the cutting process, similar to a worm in a worm gear.
  • Profile milling: Profile milling is milling using a tool with a workpiece-bound shape. It is used to create straight (straight feed motion), rotationally symmetrical (circular feed motion) and arbitrarily curved profile surfaces in one plane (controlled feed motion).
  • Shape milling: Shape milling is milling in which the feed motion is controlled in a plane or spatially, thereby creating the desired shape of the workpiece.

In addition to these basic processes, milling is differentiated into contour milling and up-cut milling depending on the direction of tool rotation and feed. In contour milling, the direction of rotation of the milling cutter and the movement of the workpiece in the area of tool engagement are in the same direction. In up-cut milling, on the other hand, the direction of rotation of the milling cutter and the movement of the workpiece in the area of tool engagement are in opposite directions. 

In direct comparison, the chip thickness in contour milling decreases progressively between the entry and exit of the cutting edge, which also reduces the cutting force and avoids so-called chatter effects. In addition, chatter vibrations do not occur. This means that better surface finishes can generally be achieved with contour milling. This is why this process is preferably used for finishing operations.

One disadvantage, however, is that the cutting edges of the milling cutter plunge into the workpiece with maximum chip thickness. With up-cut milling, on the other hand, the cutting edges of the milling cutter emerge from the workpiece with maximum chip thickness. This results in squeezing and friction processes when exiting, which causes high wear of the tool.

Milling of a turbine disk
Milling of a turbine disk

What is CNC milling

The fact that the milling process in all possible kinematics has shaped the manufacturing industry up to modern times and will certainly continue to do so is primarily due to the development of the NC control (CNC). From the mid-1950s onwards, NC and later CNC technology succeeded in gradually making the complexity and productivity of the milling process increasingly independent of the manual skills of humans. Above all, multi-axis CNC milling up to the spheres of free-form surface machining would have been inconceivable without "intelligent" support from NC control. In addition, CNC technology also opened up the wide field of automation for the process in flexible manufacturing cells and interlinked production systems, which also qualified it for mass production, for example in engine production in the automotive industry.