Introduction to Computer Numerical control

Types of CNC Milling Machines can divide in to three categories: By number of axes - Two , three or more By the orientation of axes- Vertical or Horizontal By presence or absence of tool changer

Types of CNC Plasma Spring forming Laser Cutting machine Flame cutting machine Electric Discharge Machine Water Jet Welding CNC Punch Press

Types of Control System Point to point Tool Movement Point to point control system causes the tool to move to a point on the part and execute an operation at the point only Continues path tool Movement Continues path controller are named because they cause the tool to maintain continues contact with the part as the tool cuts a contour shape

Machine tool Axes Numerical machines can also be classified on the number of axes, or direction of motion. Machine centers usually have two-and – half, three, four, or five axis with three or four axis are more common Lathes and turning centers generally have between two and four axes

Loop System Loop system send electrical signal to drive motor controllers and receives feedback. One of the important factor that determine the tolerances to which a part can be cut is the loop system type. Open Loop System An open loop system utilizes stepping motors to create machine movement by electrical signals. the motor connected to Acme lead screw The lead screws used in this system generates friction and backlash. Backlash can cause positioning Error

Close Loop System Special motors called servos are used for executing machine movements in closed loop system. Motor type can be AC servos, DC servos, Hydraulic servos. The speed depend on amount of current or hydraulic fluid passing through it. Servos are connected to the spindle and they are connected to the machine table through the ball lead screw

Ball Lead Screw Ball lead screw is the hart of the drive system Advantages of ball lead screw over Acme lead screw are: 1. Precise position and repeatability 2. High Speed capability, 3. Less Wear 4. longer life

Position loop is closed by a device called an ENCODER. An encoder is a disk with three rings around it 1000 slits in the first ring, 1000 slits in the second ring, Only one slit in the third There are three light receiver on the other side of the disk Encoder passes the light to the receiver when the slits line up with the light and receiver. This create pulses The CNC control knows there are 1000 pulses per revolution so by counting the pulses it knows the encoder’s position exactly

Homing moves axes to a known position where each axes is initialized Homing (ZRN) Homing moves axes to a known position where each axes is initialized

Cartesian coordinate system Any point on a machine part, such as a hole to be drilled, can be described in terms of its position. the system that allows us to do this is Cartesian coordinate

Machine axes Machine axes are established according to the industry standard report EIA RS - 267A

Axes configuration X axes moves from right to left as you face the machine Y axes move toward and away from you The Z axes is the spindle movement up and down spindle . A move toward work is Z(-Z) A move away from work is Z(+ Z)

Types of tool positioning Incremental Positioning

Absolute Positioning

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