What Does Milling Mean?
Milling is a machining process that involves the use of cutting tools that are rotated at a set speed and then brought into contact with a work piece. The work piece is typically held in place by some sort of clamping device. The cutting tools begin to remove material when they touch the work piece. Some materials may require milling to be performed on their surfaces to ensure that there are no crevices that could be subject to corrosion. Milling may also be performed to properly prepare surfaces for corrosion prevention coatings.
Milling has become one of the most common forms of machining, which is a material removal process that could create a variety of features needed in a part by cutting away any unwanted material. This process requires tools such as a milling machine, fixture, workpiece and cutter. On surfaces that have been milled, there is a distinctive pattern of stress corrosion cracking that will be evident with cracks that nucleate along the milling path with shorter cracks nucleating perpendicularly. As the surface tensile stress diminishes, however, perpendicularly to the milling direction, the nucleation of primary cracks will be parallel to the machining. These are driven by the surface profile that occurs after the machining process.
Corrosionpedia Explains Milling
Milling can be performed either manually or automatically. Vertical and horizontal milling machines are commonly used manually for maintenance and small job shop purposes. When complex parts with large volumes need to be milled, they are typically milled by a CNC machine.
There are several different types of milling cutting tools available:
- End Mill: This is possibly the most common type. An end mill can be used to cut laterally. Some end mills can cut both laterally and axially.
- Face Mill: This type of mill is used to make lateral cuts similar to an end mill, but can also be used to cut the entire surface of a material instead of just a portion.
- Thread Mill
Milling can be performed on many different materials. Commonly available machinability ratings are a good place to start to determine if a material can be easily milled. Special milling tools and processes may be required for materials that are difficult to machine.
Mechanical surface finishing may lead to the formation of residual stresses, and even cases of stress corrosion crack formation after exposure to boiling magnesium chloride in austenitic stainless steel. The stresses received by the machinery prior to milling are biaxial and compressive. The abrasive grinding that is produced significantly reduces compressive stresses in the machining, inducing much lower perpendicular stresses into the system. Milling produces high biaxial tensile stresses that may be relatively insensitive to cut depth, although this may be a factor that varies as a function of feed rate. Milled samples have an interesting crack pattern, which is a very significant observation to be made about them. Stress corrosion cracks in milled samples tend to initiate perpendicular to the dominant stress in the mode crack opening.
New Milling Challenges
Two main factors driving the development of new milling tool materials and coatings are:
- New emerging workpiece materials and milling processes.
- Technologies designed to address them.
New workpiece materials are being developed to address many factors. These include ever-increasing global demand for bigger, faster and more affordable modes of transportation (e.g., automobiles and airplanes), combined with growing concerns for green and renewable energy sources, high performance components, and higher safety standards.
Some of the key properties required for milling tools to machine these high-performance work materials include:
- High hot hardness, i.e., retention of the cutting edge at elevated temperatures near the tool/workpiece interface.
- Ability to withstand high cutting forces during machining.
- Low thermal conductivity to resist edge degradation such as depth-of-cut notching, plastic deformation and oxidation caused by high temperatures at the cutting edge.
- Chemical inertness to minimize formation of built-up edge (BUE) and the possibility of coating delamination.
- High wear resistance to reduce abrasive wear at the cutting edge due to hard intermetallic compounds in the microstructure.
- Geometry that provides efficient cutting, good chip-breaking and minimizes heat generation during machining to reduce subsurface defects on the workpiece.