[IE 21] L14: Machining: Numerical Control, Chemical, Electrochemical, Electrical Discharge, Thermal

Steps in Chemical Machining

1. Cleaning • contaminants on the surface of the workpiece are removed to prepare for application of the maskant and permit uniform etching. This may include degreasing, rinsing, and/or pickling. 2. Masking • if selective etching is desired, an etch-resistant maskant is applied and selected areas of the workpiece are exposed through the maskant in preparation for etching 3. Etching • the part is either immersed in an etchant or an etchant is continuously sprayed onto the surface of the workpiece. The chemical reaction is halted by rinsing 4. Stripping • the maskant is removed from the workpiece and the surface is cleaned as necessary

Thermal Machining

1. Electron-beam Machining 2. Laser-beam Machining 3. Plasma Arc Cutting

Components of a CNC

1. Part program • a detailed set of commands to be followed by the machine tool 2. Machine control unit (MCU) • a microcomputer that stores the program and executes the commands into actions by the machine tool 3. Machine tool • the device that performs the actual operations under the control of the MCU, such as a milling machine, lathe, plasma arc cutter, etc.

Part Program (Components of a CNC)

a detailed set of commands to be followed by the machine tool

Machine Control Unit (Components of a CNC)

a microcomputer that stores the program and executes the commands into actions by the machine tool

1. Cleaning (Steps in Chemical Machining)

contaminants on the surface of the workpiece are removed to prepare for application of the maskant and permit uniform etching. This may include degreasing, rinsing, and/or pickling.

2. Masking (Steps in Chemical Machining)

if selective etching is desired, an etch-resistant maskant is applied and selected areas of the workpiece are exposed through the maskant in preparation for etching

Numerical Control Machining

programs contain information about the machine tool and cutting tool geometry, the part dimensions (from rough to finish size), and the machining parameters (speeds and feeds and depth of cut) • repeatability and quality are improved over conventional (job shop) machines. Workholding devices can be made more universal, and setup time can be reduced, along with tool-change time, thus making programmable machines economical for producing small lots or even a single piece

Machine Tool (Components of a CNC)

the device that performs the actual operations under the control of the MCU, such as a milling machine, lathe, plasma arc cutter, etc.

4. Stripping (Steps in Chemical Machining)

the maskant is removed from the workpiece and the surface is cleaned as necessary

3. Etching (Steps in Chemical Machining)

the part is either immersed in an etchant or an etchant is continuously sprayed onto the surface of the workpiece. The chemical reaction is halted by rinsing

Advantages of CNC

• high accuracy in manufacturing • short production time • greater manufacturing flexibility • simpler workholding • contour machining (2- to 5-axis machining) • reduced human error

Disadvantages of CNC

• high cost • high maintenance • requirement of skilled part programmer

Chemical Machining

• material is removed from a workpiece by selectively exposing it to a chemical reagent or etchant • the mechanism for metal removal is the chemical reaction between the etchant and the workpiece, resulting in dissolution of the workpiece

Computer Numerical Control

• most NC today is computer numerical control (CNC), in which computers play an integral part of the control

Electrical Discharge Machining

• processes remove metal by discharging electric current from a pulsating DC power supply across a thin inter-electrode gap between the tool and the workpiece • the gap is filled by a dielectric fluid, which becomes locally ionized at the point where the inter-electrode gap is the narrowest • the ionization of the dielectric fluid creates a conduction path in which a spark is produce • the spark produces a tiny crater in the workpiece by melting and vaporization, and consequently tiny, spherical "chips" are produced by re-solidification of the melted quantity of workpiece material

Electrochemical Machining

• removes material by anodic dissolution with a rapidly flowing electrolyte • basically a deplating process in which the tool is the cathode and the workpiece is the anode; both must be electrically conductive

CNC Machines

• the machining center can do drilling, boring, milling, tapping, and so on, with four- or five-axis control. It can automatically select and change 40 to 180 preset tools. The table can move left/right and in/out, and the spindle can move up/down and in/out, with positioning accuracy in the range 0.00012 in. with repeatability to ±0.00004 in. over 40 in. of travel • CNC turning center with multiple-axis capability with two spindles and a 12-tool turret (toolholder) with X, Y, and Z control as well as axis control of the spindles

Electron-Beam Machining (Thermal Machining)

• thermal process that uses a beam of high-energy electrons focused on the workpiece to melt and vaporize metal • performed in a vacuum chamber • in its simplest form, a filament (tungsten or lanthanum-hexaboride) is heated to temperatures in excess of 2000°C, where a stream of electrons (more than 1 billion per second) is emitted from the tip of the filament. • electrostatic optics are used to focus and direct the beam. The desired beam path can be programmed with a computer to produce any desired pattern in the workpiece

Plasma Arc Cutting (Thermal Machining)

• uses a superheated stream of electrically ionized gas to melt and remove material • the 20,000° to 50,000°F plasma is created inside a water-cooled nozzle by electrically ionizing a suitable gas such as nitrogen, hydrogen, argon, or mixtures of these gases • process can be used on almost any conductive metal

Laser-Beam Machining (Thermal Machining)

• uses an intensely focused, coherent stream of light (a laser) to vaporize or chemically ablate materials • the material removal mechanism in LBM is dependent on the wavelength of the laser used. At UV wavelengths (i.e., between about 200 and 400 nm), the material removal mechanism in polymers (for example) is generally thermal evaporation • most common industrial laser is the CO laser