Metal Cutting in Detail
Metal cutting is the collection of processes where metal is formed into a desired shape by removing excess material.
This uses various tools and results in two products, the waste or excess material and the finished part.
There are three major categories; chip producing processes (machining), burning where metal is cut into two parts by oxidizing a kerf, and specialty processes that do not fall into the other two categories.
Examples of these processes are: Drilling a hole for chip producing processes, using a cutting torch to separate a plate of steel is an example of burning, and chemical milling is a specialty process that removes excess material by using etching chemicals and masking chemicals.
Metal cutting can also be grouped by the physical phenomenon used for the cut
- Chip forming – sawing, drilling, milling, turning, etc.
- Shearing – punching, stamping, scissoring
- Abrading – grinding, lapping, polishing, water-jet
- Heat – flame cutting, plasma cutting, laser cutting
- Electrochemical – etching, electrical discharge machining (EDM)
Overview of Methods
Every method has limitations in accuracy, cost, and effects on the material.
Heat, for example, may damage the quality of heat treated alloys, while laser cutting highly reflective materials like aluminium is more difficult.
Cutting fluid or coolant is often used whenever there is significant friction and heat at the cutting interface between a cutter, such as a drill or an end mill, and the workpiece.
It is generally applied as a spray across the face of the tool and workpiece.
Coolant is used to prevent excessive tool wear, and to prevent deformations in the workpiece that may be caused by extreme heat.
Specific Methods
Some methods use basic hand tools like hacksaws, chisels, or shears.
Milling, turning, threading, grinding, filing, and other methods are all used to create cuts in metal.
Milling is a type of machining using rotary cutters to remove material by advancing a cutter into a workpiece.
It requires a milling machine, workpiece, fixture, and cutter.
The workpiece is a pre-formed block of material that the milling process will turn into the finished part.
The workpiece is fitted to a fixture in the milling machine, to lock it in place and allow the cutter to cut cleanly and precisely.
Cutters rotate at high speed while secured to the movable parts of the milling machine.
Moving the workpiece into the cutter shaves small chips of material away and creates the desired part shape.
The finished asymmetrical parts have many features, like slots, holes, shells, pockets, and 3D surface contours.
Parts created completely through milling are often used for limited quantity needs, for example prototypes.
Tooling for other processes is also able to be made, like 3D molds.
Refining or adding features to parts from a different process is also a common milling function.
Milling also offers high tolerance and clean surface finishes.
Turning uses a lathe to cut metal into cylindrical surfaces with single point tools.
Lathes produce objects like candlestick holders, crankshafts, camshafts, and bearing mounts.
They use abrasive, cutting, or deformation tools to produce objects with rotational symmetry on an axis of rotation.
Many Metal Cutting Methods use CNC for Greater Part Accuracy
Threading includes cutting threads with a tap or die, thread milling, single-point thread cutting, thread rolling, cold root rolling and forming, and thread grinding.
Taps cut a female thread on the inside surface of a pre-drilled hole, while a die cuts a male thread on a preformed cylindrical rod.
Grinding is a cutting method that uses an abrasive wheel as the cutting device.
Grinding machines can produce fine finishes, very light cuts, or high precision forms.
The abrasive wheel can be made up of various sizes and types of stones, diamonds and inorganic materials.
Depending on the size and type of material these wheels can vary greatly in their effects, removing large amounts of material or removing tiny amounts of the surface of the workpiece; polishing it.
Modern grinding wheels can even be controlled by CNC, allowing them to have high material removal rates with high precision.
High volume production runs of precision components and aerospace applications are common for computer controlled wheels.
Filing uses a file to combine grinding and saw tooth cutting.
It is rarely used in modern production techniques, usually only to provide a relatively accurate means for the production of small parts.
Files vary in size, shape, coarseness, and whether the teeth single cut or double cut depending on the application the file is used for.
Deburring remains a common use for files in modern machine shops.
Broaching is used to cut keyways into shafts.
EBM or electron beam machining directs high-velocity electrons towards a workpiece heating and vaporizing the material.
Ultrasonic machining uses ultrasonic vibrations to machine very hard or brittle materials.
Technologies for metal cutting include
- Manual technologies: saw, chisel, shear or snips
- Machine technologies: turning, milling, drilling, grinding, sawing
- Welding/burning technologies: burning by laser, oxy-fuel burning, plasma
- Erosion technologies: water jet, electric discharge, abrasive flow machining
- Chemical technologies: photochemical machining.
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