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05/23/2024|Milling

Milling workpieces: finding the right milling machine for processing

As a fast method for producing flat surfaces and grooves, milling gradually replaced planing as a manufacturing process in the 19th century. DMG MORI has also played a key role in shaping milling for around 100 years. Today, the machine tool manufacturer has over 25 series of different machine types in its portfolio. Starting with the simple 3-axis machine M1 through to the 5-axis high-tech machining center of the DMU monoBLOCK series. There are also CMX V and DMC V models for vertical machining as well as the horizontal DMC H linear and NHX series. Additionally, there are the DMP 70 compact machining center and the DMU 600 P and DMU 600 Gantry XXL machines with several meters of travel.

The process is characterized by the milling tool with geometrically defined cutting edges and a cutting movement performed by the tool. The feed movement is carried out by the workpiece. The relative movement between the tool and the workpiece resulting from the two variables ensures chip removal. In contrast to other machining processes, milling cannot be carried out manually. This makes it a relatively young discipline compared to other manufacturing processes. In the 20th century, electric motors were initially established as the drive for cnc milling machines, which were soon followed by electronic control systems. Today, technology integrations such as ultrasonic milling, laser machining and grinding are also becoming increasingly important. The machine tool manufacturer completes its versatile range with automation solutions for almost all models. It also offers a comprehensive range for the end-to-end digitization of production.

Milling cutters: From speed, cutting speed and feed rate

Themed image milling
The cutting manufacturing process is characterized by the fact that a milling tool with geometrically defined cutting edges is set in rotation in order to remove material from a workpiece.

Milling is characterized by the fact that the cutting edges of the milling cutter are not engaged over the entire circumference, but that at least one cut is interrupted with each rotation of the tool per cutting edge. The corresponding technical term is proverbial, because it is an "interrupted cut". The constant "in and out" results in continuous thermal and mechanical alternating loads on the cutting edges. 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 cost-effectiveness 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 shiny are worked out of a blank "like butter".

You should know these milling processes

The German Institute for Standardization (DIN) categorizes milling processes according to the type of workpiece surface produced, the kinematics of the cutting process and the milling tool profile:

  • Face milling: Milling with a straight feed movement to produce flat surfaces. Differentiated into face milling, peripheral milling and peripheral face milling 
  • Helical milling: A helical feed movement produces helical surfaces on the workpiece, e.g. threads and cylindrical worms 
  • Hobbing: A milling cutter with a reference profile performs a hobbing movement simultaneously with the feed movement. 
  • Profile milling: Use of a tool with a workpiece-specific shape. It is used to produce straight (straight feed movement), rotationally symmetrical (circular feed movement) and profile surfaces curved in any plane (controlled feed movement). 
  • Shape milling: Here, the feed movement is controlled in a plane or spatially, thus producing the desired shape of the workpiece.

In addition to the various milling processes, a distinction is also made between climb milling and up-cut milling, depending on the direction of tool rotation and feed.

  • Synchronized milling: The direction of rotation of the milling cutter and the workpiece movement are aligned in the area of tool engagement.
  • Up-cut milling: The direction of rotation of the milling cutter and the workpiece movement in the area of tool engagement are in opposite directions.

In direct comparison, the chip thickness decreases progressively between the entry and exit of the cutting edge during climb milling, which also reduces the cutting force and prevents so-called spring-back effects. In addition, no chatter vibrations occur. This generally results in better surface qualities, which is why the process is preferably used for finishing.

Complex geometries: How the production of components works

For decades, milling has been the method of choice in metalworking to produce geometric bodies from raw parts. For this purpose, the workpiece is clamped and machined with the required milling tools. Depending on the machine type, this is done either horizontally - the spindle with the tool is fed in from the side - or vertically. In this case, the spindle comes from above. On cnc machines with kinematics in the milling head, the tool can be fed to the workpiece in any angular position. Ultimately, the component decides which orientation is better. For example, the chip fall is significantly better with horizontal milling, which is an advantage when machining bores.

Themed image Milling of a turbine disk
Milling of a turbine disk

3, 4 or 5 axes: it all depends on the workpiece

In their simplest construction, milling machines are designed for 3-axis machining: The tool moves over the workpiece in the X, Y and Z directions. On 4-axis machines, a rotary axis is added - either as a B-axis in the headstock, in the table or in the form of an attached dividing head. This allows tools to be set at an angle without having to reclamp the tool first, as is the case with 3-axis milling. With a second rotary axis, 5-axis milling is possible. Here, the workpiece can be aligned in almost any direction. The milling tool can reach five sides and produce a component almost completely. A re-clamping process is only required for machining the sixth side at most. 5-axis simultaneous milling, in which all axes are interpolated, has evolved from 5-axis milling. If the kinematics are housed in the table, the workpiece is moved continuously in all directions while the milling tool removes material. With head kinematics, the second rotary axis is also in the milling head. This results in so-called free-form surfaces, which are commonplace in mold making, for example. Over time, milling has developed into an extremely advanced machining technology with special characteristics. High-speed milling, for example, uses significantly higher spindle speeds to produce higher-quality surfaces - a productivity gain because the amount of post-processing required is reduced.

Important questions about milling

What exactly is milling?

What exactly is milling?

Milling is a machining production process with a geometrically defined cutting edge - geometric sizes and ratios of a tool are known. It belongs to the superordinate group of manufacturing technology.

What do you do when milling?

What do you do when milling?

In milling, a tool (milling cutter) is used to remove material from a workpiece. To do this, the tool is moved vertically or at an angle to the axis of rotation in a feed motion. A characteristic feature of milling is that the so-called cutting movement is performed by the tool, while the workpiece performs the feed movement.

What milling methods are available?

What milling methods are available?

The German Institute for Standardization (DIN) divides milling processes into face milling, helical milling, hobbing, profile milling and form milling based on the type of workpiece surface produced, the kinematics of the cutting process and the milling tool profile.

How many axes can a milling machine have?

How many axes can a milling machine have?

Milling machines are available in different versions. DMG MORI has more than 25 series, ranging from the simple 3-axis machine to the 5-axis high-tech machining center.