Automation /Numerically Controlled Machine Tools
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 Automation /Numerically Controlled Machine Tools

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مُساهمةموضوع: Automation /Numerically Controlled Machine Tools   الجمعة 01 أكتوبر 2010, 6:20 pm

Introduction

Automatic machines are generally used to minimise the need for manual effort. The benefitsare reduced operating costs, reduced operator errors, increased reliability, minimum work reduction due tohuman fatigue, illness and labour disputes..

Automatic Machines generally possess the following characteristics.

  1. They operate with minimum human involvement
  2. They include feedback to indicate process /machining deviations from the required task
  3. They make the required corrections with minimum human involvement
The following terms apply to modern automatic machines...


  • Feedback ..The measure of the actual result of the operation compared to the desired result providing a feedback generated error
  • Output.. The actual work produced.. This could be the product machined, the movement of the vehicle or item conveyed
  • Input..The data, instructions, command specifying the operations to produce the required output
  • Sensors..The additional instrumentation required to allow the feedback to be generated
  • Actuators /Drives..The additional drive systems required to provide the necessary machine movements
  • Control Center..The system used to process the input data and feedback systems and provide the necessary controls to the drive systems. The control center also includes for human interfacing
Automatic Material Handling

The transportation of materials around the floor of a workshop can be automated using AGV's (Automatically guided Vehicles)these are generally battery powered vehicles which are controlled using wires embedded into the floor or by taking using laser beams or using inertial systems.
Material handling into and out of the machine tool can be accomplished using proprietoryrobotic arms or gantry systems for heavier items. The control systems for the handlingequipment must be integrated with the machine tool control system
NC Controlled Machine Tools

Numerical Control (N/C) is the term given to the programming control system for automaticallyoperated machine tools and other manufacturing units.. Most modern machine toolscan include N/C systems. DNC is the term used for Direct numerical control whena central computer system controls a number of machine tool work stations.. CNC is the termfor computer numerical control which is local control of a machine tool be a built in computer.

N/C programmes are coded instructions written in a standard language which is interpreted by the Machine Control Unit (MCU)which converts the instructions into electric signals which control the AC, DC or servo drives or the hydraulic or pneumaticvalves feeding the fluid actuators on the machine tools.

Typical Applications for Computer Numerical Control.

Machine tools Including.

  • Milling Machines /Machining Centres
  • Centre Lathes and Turning Centres
  • Drilling Machines
  • Precision Grinding Machines
  • EDM - Spark Erosion Machines
  • Die Sinking Machines
Sheet Metal Machines Including


  • Turret Punching Machines
  • Riveting Machines
  • Forming Machines

Fabrication Machines Including


  • Flame Cutting Machines
  • Welding Machines
  • Tube Bending Machines

Automatic Inspection Machine for tracing contours. Co-ordinate System

NC systems as used by, milling machines and lathes ,are generally based on the cartesian co-ordinate system. The Z axis being the machine tool spindle.The programming movement of a CNC machine can be described in four ways.

  1. Point-to-point..The tool is moved from point to point on the workpiece. The movementsbetween the points are controlled by the machine to take the shortest possible route. This system would be used for drilling and punching
  2. Linear Path system..The system is still moved from point to point- the programmer can however set the rate of traverse between the points. This system may be used for milling a straight slot.
  3. Parallel Path system..The system is still moved from point to point- The path between the points is always parallel to an axis..This option is used for simple turning and milling operations
  4. Continuous Path control..This allows complex contours to be machined with tool movements in 3 axes simultaneously. This allows complex shapes only limited by the Machine tool and cutting tool geometries
Control Feedback

The CNC controls fall into two general types Open Loop Control and Closed Loop Control. The open loop controloption includes for stepping motor drives with no feedback other than the internal system on the drive whichprovides for accurate descrete step movements. The closed loop system is based on feedback generallywith high powered servo drives. The second option provides more reliable accuracyfor long production cycles....
CNC programming Notes

Character.. This is a number letter or symbol which is recognised by the controller

Word..a group of characters which defines a complete item of information.. There are two types ofwords as follows.

Dimensional words.. These are words directly interpreted as dimensions. They begin with X, Y, Z (referring to dimensions parallel to the relevant axes) and I, J, K (referring to arcs of circles).

Management words..These are words not related to dimensions. Examples of management words are provided below;

  1. N4 ..Sequence number N followed by up to 4 digits identifying the sequence step
  2. G2 ..Preparation function G followed by up to 2 digits (G0- G99)
  3. F4 ..Feed rate command : The character F followed by up to 4 digits
  4. S4 ..Spindle speed command:the character S followed by up to 4 digits
  5. T2 ..Tool identifier : The character T followed by up to 2 digits
  6. M2 ..Miscellaneous command : M followed by up to 2 digits (G0- G99)
Format: Different Controls systems use different formats, the relevant manual normally explains theformat. A block of data consists of a complete line of instruction words for the controller.

Word (or Letter) Address Format: The most currently most widely used format is the word (address) format. Each word commences with a letter called an address. Each word is identified within the block by its letter and not by itsposition. Thus in each block only instructions which change have to be included.
CNC coding

Block Numbers (N)
Each block is preceded with the block number e.g. N5, N10, N15 etc the numbers are in steps of 5 to allowinsertions of late code..

Preparatory functions (G)
Note: the G numbers below are for illustrative purposes only. There are actually a number of different G number tables e.g Fanuc 0MB, 0TC,3M, 5M, 5T, 6M,6T,10M, 10T, Haas Lathe, Haas Mill, Mazak M32,Okuma OSP500 lathe et.etc

Many of these can be obtained from the CNC reference links below:
G00 Rapid Positioning-Point to Point
G01 Positioning at controlled feedrate normal dimensions
G02 Circular Interpolation-Normal Dimensions
G03 Circular Interpolation CCW -Normal Dimensions
G04 Dwell for programmed interval
G05 Hold: Cancelled by operator
G06 Reserved for future use
G07 Reserved for future use
G08 Programmed Slide accelaration
G09 Programmed Slide accelaration
G10 Linear Interpolation -Short dimensions
G11 Linear Interpolation -Long dimensions
G12 3D Interpolation
G13-16 Axis Selection
G17 XY Plane Selection
G18 ZX Plane Selection
G19 YZ Plane Selection
G20 Circular Interpolation CW:Long dimensions
G21 Circular Interpolation CW:Short dimensions
G22 Coupled Motion: Positive
G23 Coupled Motion: Negative
G25-29 Available for individual use
G30 Circular Interpolation CCW:Long dimensions
G31 Circular Interpolation CCW:Short dimensions
G32 Reserved for future Standardisation
G33 Thread Cutting: constant lead
G34 Thread Cutting: increasing lead
G35-39 Thread Cutting: reducing lead
G40 Cutter compensation:Cancel
G41 Cutter compensation:left
G42 Cutter compensation:right
G43 Cutter compensation:positive
G44 Cutter compensation:negative
G45 Cutter compensation:+/+
G46 Cutter compensation:+/-
G47 Cutter compensation:- / -
G48 Cutter compensation:- / +
G49 Cutter compensation:0 /+
G50 Cutter compensation:0 / -
G51 Cutter compensation:+ / 0
G52 Cutter compensation:- / 0
G53 Linear Shift: Cancel
G54 Linear Shift: X
G55 Linear Shift: Y
G56 Linear Shift: Z
G57 Linear Shift: XY
G58 Linear Shift: XZ
G59 Linear Shift: YZ
G60 Positioning : exact 1
G61 Positioning : exact 2
G62 Positioning:fast
G63 Tapping
G64 Change of rate
G65 Reserved for future
G66 Reserved for future
G67 Reserved for future
G68 Reserved for future
G69 Reserved for future
G70 Turning-Canned Finishing Cycle
G71 Turning-Canned Roughing Cycle
G72 Turning-Canned Facing Cycle
G73 Reserved for future
G74 Turning-Canned Peck Drilling Cycle
G75 Turning-Canned Grooving Cycle
G76 Turning-Canned Threading Cycle
G77 Reserved for future
G78 Reserved for future
G79 Reserved for future
G80 Cancel Canned Cycle.-Milling
G81 Canned Drilling Cycle.-Milling
G82 Canned C'bore Cycle.-Milling
G83 Canned Deep Hole Drilling Cycle.-Milling
G84-89 Fixed cycles
G90 Absolute Positioning.-Milling-
G91 Incremental Positioning.-Milling-
G92 Repositioning or re-setting the origin point.-Milling-
G93 Reserved for future
G94 Reserved for future
G90-99 Reserved for future
G95 Reserved for future
G96 Reserved for future
G98 Turning-Linear Feedrate Per Time
G98 Milling-Cancel G92 position set.(Part Reference Zero)
G99 Turning-Feedrate Per Revolution

Dimensional Words
A CNC control will instruct the machine to move the desired tool to a position parallel to the identified axis to the position indicated by the dimension words e.g X10.0 Y-20.0. If there is no sign it shall be assumed to bepositive. To drill a hole 50mm deep at a set position the line of code would read (say)N20 G01 X30.0, Y60.0, Z -50.0.

Feed Rate
There are a number of methods of indicating the feed rate.. i.e.
F45 may indicate 45mm/min..
F0.3 may indicate 0.3mm/rev..
F10 may indicate a feed rate number for a rate predetermined by the machine tool maker..

Tool Number
The different tools used for machining a part will be allocated a different number. The tool numberwill identify the tool offset parameters and the tool loading position amongst other information.

Miscellaneous functions
A number of miscellaneous functions are available for various housekeeping operations..
M00 Program Stop
M01 Optional Stop
M02 End Program
M03 Spindle CW
M04 Spindle CCW
M05 Spindle off
M06 Tool Change
M07 Mist coolant on
M08 Flood Coolant on
M09 Coolant off
M30 End of Tape

Data Input
One method of inputting the information into CNC machines is via Manual Date InputMDI. This can be from a keyboard or via a learning mode..
The technology for stored data input has evolved from punched tape to magnetic tape to floppydisc. Program information can be input via a PC using the G & M codes as indicated above or direct forCAD CAM software..




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مُساهمةموضوع: رد: Automation /Numerically Controlled Machine Tools   الأحد 28 يوليو 2013, 11:51 pm

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مُساهمةموضوع: رد: Automation /Numerically Controlled Machine Tools   الثلاثاء 30 يوليو 2013, 4:00 am

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