A milling machine (colloquially also milling machine ) is a cutting machine tool . The milling machine uses rotating cutting tools to remove material from a workpiece in order to bring it into the desired shape. The milling is derived from drilling off, but the milling are at least three feed directions are available, which also complex spatial bodies can be produced. Complex parts such as an engine block or gear wheels can be manufactured on milling machines , but simple work such as drilling or reaming precise holes is also possible.
Milling machines are characterized by three or more axes of movement that are assigned to the tool or workpiece carrier. The mostly multi-edged milling tool moves through the workpiece and removes material by machining. Simple milling machines for the workshop consist of a manually horizontally and vertically movable machine table as well as a horizontally movable milling head, the milling cutter of which can also be manually extended with the quill . However, the axes of movement are assigned quite differently to modern machines and they often also have rotatable and swiveling tool or workpiece holders.
The first milling tool is said to be Jacques Vaucanson , the oldest surviving milling machine comes from the American Eli Whitney from Connecticut, who used it in weapons production. The first automatic universal milling machine was designed by the American Joseph Brown in 1860. By around 1900 machines for milling gears were also technically mature.
Although the milling machine has been adapted to a wide range of processing cases and the models differ greatly as a result, a large number of similarities can be summarized.
Frame and machine stand
As the lowest assembly, the frame carries all other components; it must be set up and adjusted on a special foundation depending on the weight of the machine. In most milling machines, the frame and machine frame form a unit to which the guides are attached. This type of machine is also known as monoblock machines and is only suitable for small to medium sizes. The monoblock construction offers the advantages of high dynamic load capacity, small machine footprint and ergonomic operability, but the manufacturing or transport costs can no longer be justified economically above a certain size. Larger machines consist of welded steel structures or are modularly built on site. In the case of manually operated machines, all the motors and gears for the work spindle and feed are integrated in the frame, while in newer machines the motors with gears are built as close as possible to the driven element. On many machining centers, the machine frame can no longer be viewed as a separate part or as a component of the frame, as the frame often takes up the entire external dimensions of the machine and only has an opening at the front for loading. In large machines, the machine frames move along the workpiece and are sometimes connected to a portal by a cross-beam.
Especially for heavy workpieces, the gantry design is increasingly being considered, in which the tool carries out all the necessary movements, which means that the machine is subjected to less stress. For CNC milling centers , more and more manufacturers are relying on modified gantry designs with swiveling and rotating tables, as a large work area is available here with small machine dimensions.
Machine table and bed
Depending on the size of the machines, the workpieces are clamped on a machine table or a bed . On conventionally constructed milling machines, the machine table, designed as an angled table, hangs on the cross slide and bracket in the guides of the machine stand. The console moves in a vertical direction and the cross slide in a horizontal direction. A cross table can also be mounted to give the table movement in a third axis . This variant is found more often on horizontal milling machines. Most machining centers today have angle-controlled rotary tables for all-round machining of workpieces, which in rare cases even suffice for demanding turning operations.
Heavy and bulky workpieces are machined on bed milling machines, as the bed usually rests in every position at all corner points and thus counteracts bending. Consoles, on the other hand, tilt down sideways in the end positions and thus cause geometric inaccuracies.
Milling head with main spindle
The main spindle with sometimes an integrated quill is mounted in the milling head. Conventional milling cutters usually have a vertical work spindle with a manually extendable quill and a steep taper mount in a milling head that can be swiveled by 90 degrees. To clamp the tools, a screw must be screwed into the milling arbor with a crank in order to pull it into the steep taper. The quill is mainly used for drilling work, since an exact depth is usually not required here and, due to the lack of rigidity, it is unsuitable for milling work in the radial direction. In contrast, CNC milling machines have milling heads with hydraulic tool clamping systems and hollow or steep shank taper for quick, safe and automatic tool changing. Motors with motor spindles integrated in the milling head can also be found in the CNC machines. It is not uncommon for machining centers with five or more axes to swivel the milling heads in two axes by 180 degrees.
The feed movement of the individual axes is either derived from the main drive or individually generated by servomotors and transferred to the slide with screw drives, which is standard with numerically controlled machines. With CNC machines i. d. Usually three-phase synchronous motors, torque motors or linear motors are used as feed drives. Above all, it is important to ensure that the drives run free of play so that the workpiece cannot be pulled into the tool during down-cut milling. On older models, in addition to the mechanical drive, each axis can also be operated manually using cranks. In order to dampen vibrations and reduce non-productive time, hydrostatic screw drives and linear drives with roller-bearing guides are also becoming increasingly popular .
- Milling machines are divided into console milling machines, bed milling machines and special milling machines according to their design and intended use.
- Depending on the position of the milling spindle, a distinction is made between horizontal and vertical milling machines.
- Depending on the type of control, a distinction is made between mechanically and numerically controlled milling machines.
Console milling machines
The decisive feature of a console milling machine is the machine table attached to a stand or bed, which can be moved in different directions. They are mainly suitable for processing small to medium-sized workpieces.
Bed milling machines
Bed milling machines have in common that the bed, the "machine table", is fully supported in every position and therefore there are no tipping moments. Along the bed, several machine frames equipped with different tools can simultaneously process the preferably heavy or bulky workpieces.
Universal tool milling and drilling machine
The universal tool milling and drilling machine is a universal console milling machine and among the console milling machines it is probably the most widely used design in individual production and the workshop, as it has a vertical and horizontal milling spindle and can be expanded in many ways. The console hangs at the front of the machine frame in a vertical guide whose axis of movement is referred to as the Z-axis. It also carries the cross slide with which the machine table is moved in a horizontal axis, the so-called X-axis. With the universal milling machine, the workpiece executes the movements in the X and Z axes (i.e. sideways and vertically). The tool executes the movement in the Y-axis. At the top of the frame there is a horizontal dovetail guide on which the headstock with the horizontal spindle, the gear unit and in some cases the spindle drive motor and the milling head sit. When moving on the Y-axis, this entire assembly moves. On some milling machines, a cross table is also mounted on the console , which means that the workpiece can also move on the Y-axis. The vertical movement can also be done manually using a quill in the milling head; however, the travel is limited to about 200 mm and is only suitable for drilling, as the rigidity decreases due to the far extended quill.
The main motor located in the frame drives the feed . Various feed speeds can be set via a change gear . Furthermore, there is the possibility of moving each axis manually using cranks, which is a must for small movements on mechanically controlled milling machines, since no mechanical end point can be set for the automatic feeds. The exception is the end point of the axis, which is triggered by a safety coupling. Experienced milling cutters can also mill curves by manually operating two axes at the same time.
Various machine tables fit on the console, whereby the angle table is most often mounted on the simple universal milling machine. If the milling machine has a CNC control , swiveling rotary tables are usually installed, as this means that an additional axis is available. In order to mill inclined surfaces, either the milling head can be swiveled in several directions by up to 90 degrees, or a swiveling machine table is mounted on the console, which means that large areas can also be machined.
Portal milling machine
The portal milling machine has proven itself for large workpieces and at the same time the most demanding work results , as the workpiece table lies flat here and the machine has a high level of rigidity. Depending on the design, the table moves in the longitudinal axis, sometimes also in the vertical axis on small machines, and the tool in the remaining axes. In the gantry design, all three movement axes are assigned to the machine portal . If the machine has a fixed column with a movable crossbeam on top, it is called a bridge gantry construction.
Single and double column bed milling machines
One and two-column bed milling machines are essentially to be understood as machines on whose bed one or two machine columns are movably arranged. As a rule, a tool unit is attached to each machine frame, which can also reach the entire clamping area of the bed. Large workpieces can be machined more cost-effectively on two-column machines. The only extra effort is to prevent the stands from colliding by being careful when programming. CNC-controlled variants usually do this independently during programming with a warning and during operation by switching off the axis drives. The length of many machines of this type requires a rack and pinion drive for the machine frame. Shaft drives would bend and make them vibrate.
Copy milling machines
Copy milling machines (or postform milling machines) use various methods to scan models or templates and transfer the movement mechanically, hydraulically or electromechanically to the milling tool (automatic pendulum or contour milling). With a round post-forming milling device, any contours, including circumferential ones, can be milled by turning the workpiece and model. Complicated workpieces can thus be produced without numerical control, but the production of a template is only worthwhile for large quantities.
Machining centers (BAZ) are CNC-controlled machines for the complete machining of a workpiece in order to save repeated reclamping. On the one hand, an improvement in accuracy can be expected, on the other hand, unnecessary non-productive times are eliminated. Automatic chip removal, tool change and, in some cases, workpiece supply and disposal are characteristic. The machine construction can be very different, but the form shown in the picture with vertical main spindle and swiveling table is the most common.
In order to expand the processing spectrum even further, some manufacturers rely on the use of several manufacturing processes in one machine. Integrated grinding spindles, lasers for low surface removal or treatment or rotating tables suitable for turning work are now in use in practice. By linking several machining centers, flexible production can be easily implemented.
The growing popularity of the machining center can be seen in the area reserved for it at well-known machine tool fairs, which has been increasing continuously for years.
Table milling machine
The table milling machine is used in woodworking for trimming edges with straight milling cutters or for attaching shapes to strips, boards and panels with milling cutters. It consists of a rectangular cast plate as a work surface with a circular opening in the middle of the stop attached to the side of the table, from which the milling spindle protrudes. The arbor holder is conical in that the morse taper of the pin is secured by means of a differential nut.
The shank of the milling arbor is used to hold the milling tool. The milling tool is clamped with spacer rings and a milling mandrel nut so that the nut is flush with the end of the thread. As a rule, today's milling arbor nuts have an anti-rotation lock in clockwise and counter-clockwise rotation to ensure safety.
The feed is usually done by hand along the set guide devices on the milling stop line. The guide devices include the feed device (also belongs to the manual feed), the pressure and protection device (can be folded back), the pressure or bow pressure spring, the pressure shoe or deflector bracket, the pressure roller and the pressure comb. All devices serve to cover the tool as much as possible and to guarantee the guidance of the workpiece to be milled.
There are different types of stop bridges to choose from depending on the type of cutter. The most common are stops with swiveling bridging bars and stops with insertable stop bridges. These are only used for guidance and not to cover tools. There are table extensions that can be attached to both sides of the machine with a quick-release fastener to extend the table and to attach cross stops during insert milling. The cross stops serve as kickback protection.
As a variant, table milling machines with swiveling milling spindles are also offered under the trade name "Swiveling spindle milling machine", with which individual bevels can be produced as trimmings without special form milling cutters.
Underground milling machine
The underfloor milling machine for woodworking is similar to the table milling machine, but there are one or more vertically standing milling spindles in the middle of the work surface instead of being positioned on the side stop. The drive motors are accordingly attached below the work surface.
With stationary routers, shapes are mainly copied and fittings are embedded. They are particularly suitable for the production of mass-produced articles made of wood, plastic and similar materials. The machines made of gray cast iron for vibration damping are similar to a pillar drilling machine or a simple console milling machine. The milling head or the machine table can be swiveled by up to 90 degrees in both directions on some makes. The milling spindle usually has a support in the axial direction, the actual feed is carried out by the workpiece. With copy milling, the workpiece is clamped on a negative form, which is guided during milling by a copying pen set into the table centrically to the milling cutter. The speeds reach up to 20,000 / min.
Complete processing machine
The complete processing machine can also perform other processing steps in addition to milling. A wide variety of technologies, such as milling, drilling, deep hole drilling, gear cutting, measuring or hobbing are implemented in complete processing machines. Such machines have been built since around 1984. The original complete processing machine was the Millturn WNC 500S MT, which was developed and produced at the time by Voest Alpine Steinel (now WFL Millturn Technologies). The structure of a complete machining center is a combination between a lathe and machining center. The machine bed, the main drive, the workpiece clamping and support, for example by steady rests or tailstock, are similar to a lathe. The tool carrier and the tool changing system are designed similar to a machining center. With the complete processing machine, special attention must be paid to the tool system. The tool interface must fulfill the tasks of the static turning tools as well as the rotating milling and drilling tools. Short cones with a flat surface have proven themselves as tool interfaces.
More milling machines
- In addition, within wood and plastic processing, there are the hand-held milling machines commonly used in the trade (e.g. routers , groove milling machines ) and stationary milling machines (with a table milling machine , chain milling machine ).
- An attachment for chainsaws was developed as a hand-held milling machine for removing bark . The so-called "bark strip knife" was tested with scientific support from the Bavarian Forest National Park and is used to combat bark beetles in an environmentally friendly manner .
- In electronics, circuit board milling machines , so-called milling plotters , are used to produce printed circuit boards, which work out the required conductor paths from an initially continuous copper layer. The application is mostly limited to the production of prototypes and individual pieces.
- see brochure of the Ruwi company ( Memento of October 23, 2014 in the Internet Archive ) (PDF; 1.1 MB).
- Klaus Erkelenz, Bernd Wittchen, Edgar Zeiß: wood specialist for joiners, wood mechanics and window makers. 3rd edition, Teubner, Stuttgart / Leipzig 1998, ISBN 978-3-519-25911-4 , doi : 10.1007 / 978-3-322-94115-2 , p. 174.
- Niederbayerische Zeitung, Grafenauer Anzeiger, April 10, 2019, National Park develops bark strip knives as "bark beetle killer" ( Memento from April 10, 2019 in the Internet Archive )
- MuW-Zeitschriftenverlag, April 10, 2019, The attachment for chainsaws developed by the National Park enables bark beetle control and preservation of biodiversity ( Memento from April 10, 2019 in the Internet Archive )
- Christoph Selig: Milling blanks. In: Design, construction and operation of CNC machines for hobby technicians. From Einfach-CNC.de, accessed on January 27, 2017.
- Stephan Mischnick: Milling plotter. ( Memento of the original from April 6, 2018 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. A do-it-yourself project. On Strippenstrolch.de, accessed on April 6, 2018.