Cnc 2 unit palanivendhan cnc elements

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 MILLING

CNC LASER CUTTING

CNC Plasma cutting

AUTOMOTIVE INDUSTRY Engine Block

Different products

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




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 buttons control manual movement of the table in the X and Y axes.

Move the table in the X axis while held pressed.

Move the table in the Y-axis while held pressed.

6. JOG HEAD AXIS CONTROLS

Move the head in the Z axis while held pressed.

Pressing this button together with any of the other six axis control buttons provides rapid movement of the axes in the indicated direction.

Control Panel

7. MANUAL When lit, machine is operated from control panel (Manual mode). Pressing MAN puts light out and machine is controlled by computer (Automatic mode). Press MAN again to revert to manual mode operation.

8. AIR MIST CONTROL Switches on or off the optional air mist coolant when fitted.

9. DATUM M/C - PROGRAM ABORT After initially powering up the machine and pressing the Power Reset button (23), with the machine in Manual mode (Manual mode button lit) press to datum the machine axes.

• With the machine in Manual mode, the machine can be re-datumed at any point. This should always be done if a physical stall of the axes has occurred.

•During an Automatic Cycle with a Feed Hold (1) active, pressing will Abort the current job.

Control Panel

10. TOOL CHANGE With auto tool changer fitted, press to change to next tool automatically.

11. TOOL RELEASE Releases the tool holder from the spindle if fitted with the auto tool changer. Guard door must be open and tool holder held.

12. WORK HOLDING UN-CLAMP Controls un-clamping of workpiece in pneumatic work holder vice, when fitted.

13. WORK HOLDING CLAMP Controls clamping of workpiece in pneumatic work holder vice, when fitted.

14. DOOR OPEN Controls opening of the automatic doors when fitted.

Control Panel

15. DOOR CLOSE Controls closing of the automatic doors when fitted.

16. SPINDLE CONTROLS These control the speed and direction of rotation of the spindle.

Spindle Forward - Pressing this button starts the spindle rotating forward. Holding it down increases forward speed. Pressing and holding the 'Spindle Reverse' button reduces speed in the forward direction.

Spindle Reverse - Pressing this button starts the spindle rotating in reverse direction. Holding it down increases spindle speed in reverse direction. Pressing and holding the 'Spindle Forward' button reduces speed in the reverse direction.

Spindle Stop - Pressing this button stops spindle rotation.

Control Panel

17. EMERGENCY STOP Pressing this pushbutton stops all machine movement and switches off the electronics, and the button locks in the depressed position.

If the machine is in production, an appropriate message is displayed on the PC screen.

The button must be turned to release it and the Hard Reset button (23) pressed to enable the machine to be started. The machine must be datumed using the Datum M/C key (9).

If the machine was performing a production cycle, the cycle must be repeated from the beginning.

CNC Machine Tool Assembly

Courtesy: UTS(EMCO)

CNC Machine Tool Assembly

Cnc 2 unit palanivendhan cnc elements

Engineering

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