Computer Aided Manufacturing M.PALANIVENDHAN Department of Automobile Engineering SRM University, kattankulathur campus
HISTORY US Air Force commissioned MIT to develop the first "numerically controlled" machine in 1949. It was demonstrated in 1952. At 1970-1972 first Computer Numeric Control machines were developed. Today, computer numerical control (CNC) machines are found almost everywhere, from small job shops in rural communities to companies in large urban areas.
Basics of CNC A numerical control, or NC, system controls many machine functions and movements which were traditionally performed by skilled machinists. Numerical control developed out of the need to meet the requirements of high production rates, uniformity and consistent part quality. Programmed instructions are converted into output signals which in turn control machine operations such as spindle speeds, tool selection, tool movement, and cutting fluid flow.
Coordinates of CNC All computer controlled machines are able to accurately and repeatedly control motion in various directions. Each of these directions of motion is called an axis. Depending on the machine type there are commonly two to five axes. Additionally, a CNC axis may be either a linear axis in which movement is in a straight line, or a rotary axis with motion following a circular path.
Basic principle of CNC Each axis consists of a mechanical component, such as a slide that moves, a servo drive motor that powers the mechanical movement, and a ball screw to transfer the power from the servo drive motor to the mechanical component. These components, along with the computer controls that govern them, are referred to as an axis drive system.
CNC SYSTEM ELEMENTS A typical CNC system consists of the following six elements 1. Part program 2. Program input device 3. Machine control unit 4. Drive system 5. Machine tool 6. Feedback system
CNC CLASSIFICATION 1.Motion control: point to point (PTP) continuous (contouring) path 2. Control loops: open loop closed loop 3. Power drives: hydraulic, electric or pneumatic 4. Positioning systems: incremental absolute positioning 5. Hardwired NC and soft wired Computer Numerical Control (CNC)
Classification of CNC system Point-to-Point . each axis driven separately and at different velocities Moves at max velocity initially then slows as gets to desired position After operation complete tool is retracted and moved rapidly to next position and repeats operation Eg:Mainly used in drilling, punching, and straight milling operations
POINT TO POINT CONTROL
Classification of CNC system Continuous Path . positioning and cutting operations both controlled along paths but at different velocities Tool is cutting as it travels Requires synchronization of velocities and movements
Continuous path control
Open and closed loop control
OPEN LOOP SYSTEM In open-loop control system step motors are used Step motors are driven by electric pulses Every pulse rotates the motor spindle through a certain amount By counting the pulses, the amount of motion can be controlled No feedback signal for error correction Lower positioning accuracy
CLOSED LOOP SYSTEM In closed-loop control systems DC or AC motors are used Position transducers are used to generate position feedback signals for error correction Better accuracy can be achieved More expensive Suitable for large size machine tools
INTERPOLATION Types of Interpolation Linear . tool moves in straight line from start to end in 2 or 3 axes Circular . needs start and end points, coordinates of center of circle, radius, and direction of tool along the arc Parabolic and Cubic . paths approximated by curves using higher order mathematical equations
TYPES OF INTERPOLATION
CNC LASER CUTTING
CNC Plasma cutting
AUTOMOTIVE INDUSTRY Engine Block
AEROSPACE INDUSTRY Aircraft Turbine Machined by 5-Axis CNC Milling Machine
Advantages of CNC Productivity Machine utilisation is increased because more time is spent cutting and less time is taken by positioning. Reduced setup time increases utilisation too. Quality Parts are more accurate. Parts are more repeatable. Less waste due to scrap.
Advantages of CNC Reduced inventory Reduced setup time permits smaller economic batch quantities. Lower lead time allows lower stock levels. Lower stock levels reduce interest charges and working capital requirements. Machining Complex shapes Slide movements under computer control. Computer controller can calculate steps. First NC machine built 1951 at MIT for aircraft skin milling.
Advantages of CNC Management Control CNC leads to CAD Process planning Production planning
Draw backs of CNC High capital cost Machine tools cost $30,000 - $1,500,000 Retraining and recruitment of staff New support facilities High maintenance requirements Not cost-effective for low-level production on simple parts As geometric complexity or volume increases CNC becomes more economical Maintenance personnel must have both mechanical and electronics expertise
Features of CNC Machining Centers
CNC Machine Components Cutting Tool / Collet Caddy Very common to have 100 or more diverse cutting tools loaded into a collet caddy They are numbered for ease of identification The operator must know the common tools by simple visual observation
Boxford Vertical Mill Cutting Tools Collet Caddy # Tool in Collet (different lengths) 5 2mm end mill, ball end 6 2mm end mill, slot drill 3 4mm end mill, slot drill 4 6mm end mill, slot drill 1 8mm end mill, slot drill 2 25 mm end mill, facing cutter
CNC Bench Vise Notice the smooth machined surfaces; be careful not to damage surfaces
CAM & X-axis Table Alignment To machine a part with CNC, the end of the tool must be known relative to the bench vise, x-axis table, or some other fixed distance
Z-axis Arbor Keep the lands clean and oiled A slight tilt will cause noticeable cut deformations Align the protrusions with slots in the cutting tool quick change adapter body
Air Nozzle Used for: Keeping cutting area free of excessive cutting debris Quick cleaning When supplied with misting, provides cutting tool cooling
End Mill / Facing Cutter Hogs off large amounts of material Radius of cut very large Provides a known surface from which to measure from Requires workpiece to be robustly supported due to high torque created
Quick Change End Mill Adapter Helps provide quick changing of cutting tools without having to use complex setup procedures
Power Panel Lockable On-Off Switch Circuit Breakers Power Cord
Control Panel 1. FEED HOLD Acts as an interrupt control to the program when pressed, preventing any further tool movement until canceled. During an Automatic cycle, after canceling CYCLE START must be pressed to resume the cycle. 2. SINGLE BLOCK With machine running in automatic mode controlled from the PC, this allows the operation of only a single block (line) of the program at a time to be toggled on/off. Press CYCLE START to proceed. 3. CYCLE START This sets the machine in automatic mode and starts the machining from the PC program. Also used to resume an Automatic cycle after a FEED HOLD is canceled and to action the next line if Single Block is active.
Control Panel 4. FEED SPEED CONTROL Provides stepless control of tool feed speed in the X, Y and Z axes from 0 to 100% in 10% increments. 5. JOG TABLE AXIS CONTROLS These four bu