CNC Machines
1. What is a Cnc Machine ?
- CNC computer and numeric control
Conventionally, an operator decides and adjusts various machines parameters like feed, depth of cut etc depending on types of job, and controls the slide movements by hand. In a cnc machine function and slide movements are controlled by motors using computer programs.For a CNC machine control unit (MCU) decides cutting speed, feed, depth of cut,tool selection, coolant on off and tool paths. The MCU issues commands in form of numeric data to motor that position slides and tool accordingly.
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| Basic Model of CNC Router Machine |
CNC Machines Advantages and Disadvantages
Advantages
- High Repeatably and Precision eg. Aircraft Parts.
- Volume of Production is Very High.
- Complex Contours/Surfaces Need To Be Machined. eg Turbines
- Flexibility In Job Change, Automatic Tool Settings, less Scrap
- More Safe, Higher Productivity, Better Quality
- Less Paper Work, Faster Prototype Production ,Reduction In Lead Times
Disadvantages
- Costly Setup, Skilled Operators
- Computers, Programming Knowledge Required
- Maintenance is difficult.
Open loop and Closed loop control
- In open loop system (a) the slide may overshoot or may not reach desired position because of inertia, wear and tear and friction,hence inaccurate machining.
- In closed loop systems (b) the position sensors are used to correct slide movements and achieve higher accuracy and repeatabillty.
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Table of cnc router machine CNC programming. |
- Important thing to know: Programming consists of a series of instruction in form of letter code
- Coodinate System
- Units, Incremental or Absolute Positioning.
- Coordinates: X,Y,Z,RX,RY,RZ
- Feed Rate and Spindle Speed.
- Coolant control: on/off,flood, mist
- Tool control: tool and tool parameters
Programming consists of a series of a series of instruction in form of letter codes
preparatory codes:- G codes- Initial machine setup and establishing aperating conditions.
- N codes- specify program line number to executed by the MUC
Axis codes: X,Y,Z.
- used to specify motion of the slide along X,Y,Z direction
Feed and Speed Codes: F and C specify feed and spindle speed
Tool Codes: T- specify tool number
Miscellaneous Codes:-M codes For coolant control and other activities.
RULES FOR PROGRAMMING
Block format
N135 G01 X1.0 Y1.0 Z0.125F5
Sample Block..
- Restrictions on CNC blocks
- Each may contain only one tool move
- Each may contain any number of non-tool-move g-codes
- Each may contain only one feed rate
- Each may contain only one specified tool or spindle speed
- The block number should be sequential
- Both the program start flag and the program number must be
- Independent of all other commands ( on separate lines )
- The data within a block should follow the sequence shown in the above sample block
Explanation of commonly used G codes
- G00-preparatory code to control final position of the tool and not concerned with the path that is followed in arriving at the final destination.
- G01-Tool is required to move in a staight line connecting current position and final position.used for tool movement without any machining point control.(linear interpolation)
- G02-Tool path followed is long an arc specified by I, J and k codes (circular interpolation)
Table of Important G and M codes
G00 Rapid TransverseG02 Circular Interpolation, CW
G03 circular Interpolation, CCW
G17 XY plane, G18 XZ plane, G19 Xz plane
G20/G70 inch units
G21/G71 Metric units
G40 cutter compensation cancel
G41 cutter compensation left
G42 cutter compensation right
G43 Total length compensation (plus)
G44 Total length Compensation (plus)
G49 Tool Length compensation (minus)
G80 cancel canned cycle
G81 Drilling cycle
G82 counter boring cycle
G83 Deep hole drilling cycle
G90 Absolute Postioning
G91 Incremental Positioning
M00 Program Stop
M01 Optional Program stop
M02 program end
M03 spindle on clockwise
M08 Coolant on
M09 Coolant off
M04 spindle on conter clockwise
M05 spindle stop
M06 Tool Change
M10 clamps on
M11 clamps off
M30 Program stop, reset to start.
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Website: www.subauae.com
post by : Irfan khan
