Milling is the machining process of using rotary cutters to remove material[1] from a workpiece advancing (or feeding) in a direction at an angle with the axis of the tool.[2][3] It covers a wide variety of different operations and machines, on scales from small individual parts to large, heavy-duty gang milling operations. It is one of the most commonly used processes in industry and machine shops today for machining parts to precise sizes and shapes.
Milling can be done with a wide range of machine tools. The original class of machine tools for milling was the milling machine (often called a mill). After the advent of computer numerical control (CNC), milling machines evolved into machining centers (milling machines with automatic tool changers, tool magazines or carousels, CNC control, coolant systems, and enclosures), generally classified as vertical machining centers (VMCs) and horizontal machining centers (HMCs). The integration of milling into turningenvironments and of turning into milling environments, begun with live tooling for lathes and the occasional use of mills for turning operations, led to a new class of machine tools,multitasking machines (MTMs), which are purpose-built to provide for a default machining strategy of using any combination of milling and turning within the same work envelope.
Process
Milling is a cutting process that uses a milling cutter to remove material from the surface of a workpiece. The milling cutter is a rotary cutting tool, often with multiple cutting points. As opposed to drilling, where the tool is advanced along its rotation axis, the cutter in milling is usually moved perpendicular to its axis so that cutting occurs on the circumference of the cutter. As the milling cutter enters the workpiece, the cutting edges (flutes or teeth) of the tool repeatedly cut into and exit from the material, shaving off chips (swarf) from the workpiece with each pass. The cutting action is shear deformation; the metal is pushed off the workpiece in tiny clumps that hang together to more or less extent (depending on the metal type) to form chips. This makes metal cutting a bit different (in its mechanics) from slicing softer materials with a blade.
The milling process removes material by performing many separate, small cuts. This is accomplished by using a cutter with many teeth, spinning the cutter at high speed, or advancing the material through the cutter slowly; most often it is some combination of these three approaches.[2] The speeds and feeds used are varied to suit a combination of variables. The speed at which the piece advances through the cutter is called feed rate, or just feed; it is most often measured in length of material per full revolution of the cutter.
There are two major classes of milling process:
- In face milling, the cutting action occurs primarily at the end corners of the milling cutter. Face milling is used to cut flat surfaces (faces) into the workpiece, or to cut flat-bottomed cavities.
- In peripheral milling, the cutting action occurs primarily along the circumference of the cutter, so that the cross section of the milled surface ends up receiving the shape of the cutter. In this case the blades of the cutter can be seen as scooping out material from the work piece. Peripheral milling is well suited to the cutting of deep slots, threads, and gear teeth.
Surface finish
As material passes through the cutting area of a milling machine, the blades of the cutter take swarfs of material at regular intervals. Surfaces cut by the side of the cutter (as in peripheral milling) therefore always contain regular ridges. The distance between ridges and the height of the ridges depend on the feed rate, number of cutting surfaces, the cutter diameter.[4] With a narrow cutter and rapid feed rate, these revolution ridges can be significant variations in the surface height.
Trochoidal marks, characteristic of face milling.
The face milling process can in principle produce very flat surfaces, however in practice the result always shows visible trochoidal marks following the motion of points on the cutter’s end face. Theserevolution marks give the characteristic finish of a face milled surface. Revolution marks can have significant roughness depending on factors such as flatness of the cutter’s end face and the degree of perpendicularity between the cutter’s rotation axis and feed direction. Often a final pass with a slow feed rate is used to compensate for a poor milling setup, in order to reduce the roughness of revolution marks. In a precise face milling operation, the revolution marks will only be microscopic scratches due to imperfections in the cutting edge.
Stewart platform
A Stewart platform is a type of parallel robot that incorporates six prismatic actuators, commonly hydraulic jacks. These actuators are mounted in pairs to the mechanism’s base, crossing over to three mounting points on a top plate. Devices placed on the top plate can be moved in the sixdegrees of freedom in which it is possible for a freely-suspended body to move. These are the three linear movements x, y, z (lateral, longitudinal and vertical), and the three rotations pitch, roll, & yaw. The term “six-axis” platform is also used.
Applications
Stewart platforms have applications in flight simulators, machine tool technology, crane technology, underwater research, air-to-sea rescue, satellite dish positioning, telescopesand orthopedic surgery.
Flight Simulation
A Stewart Platform in use by Lufthansa
The Stewart platform design is extensively used in flight simulation, particularly in the so-called full flight simulator for which all 6 degrees of freedom are required.
In this role, the payload is a replica cockpit and a visual display system, normally of several channels, for showing the outside-world visual scene to the aircraft crew that are being trained. Payload weights in the case of a full flight simulator for a large transport aircraft can be up to about 15,000 kilogrammes.
RoboCrane
James S. Albus of the National Institute of Standards and Technology (NIST) developed the RoboCrane, where the platform hangs from six cables instead of being supported by six jacks.
Eric Gough’s Tire Testing Machine, which is a Stewart Platform with large jacks
LIDS
The Low impact docking system developed by NASA uses a Stewart platform to manipulate space vehicles during the docking process.
Taylor Spatial Frame
Dr. J. Charles Taylor utilized the Stewart platform to develop the Taylor Spatial Frame,[6] an external fixator used in orthopedic surgery for the correction of bone deformities and treatment of complex fractures.
Eric Gough – inventor of the 6-axis jack layout
Eric Gough was an automotive engineer and worked at Fort Dunlop, the Dunlop Tyres factory in Birmingham, England.[7] He developed his “Universal Tyre-Testing Machine” (also called the “Universal Rig”) in the 1950s and his platform was operational by 1954.[1] The rig was able to mechanically test tyres under combined loads. Dr. Gough died in 1972 but his testing rig continued to be used up until the late 1980s when the factory was closed down and then demolished. His rig was saved and transported to the Science Museum (London)storage facility at Wroughton near Swindon.
