IntroductionAutomatic 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.
- They operate with minimum human involvement
- They include feedback to indicate process /machining deviations from the required task
- They make the required corrections with minimum human involvement
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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
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Automatic Material HandlingThe 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 ToolsNumerical 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
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Sheet Metal Machines Including
- Turret Punching Machines
- Riveting Machines
- Forming Machines
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Fabrication Machines Including
- Flame Cutting Machines
- Welding Machines
- Tube Bending Machines
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Automatic Inspection Machine for tracing contours.
Co-ordinate SystemNC 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.
- 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
- 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.
- 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
- 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
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Control FeedbackThe 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 NotesCharacter.. 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;
- N4 ..Sequence number N followed by up to 4 digits identifying the sequence step
- G2 ..Preparation function G followed by up to 2 digits (G0- G99)
- F4 ..Feed rate command : The character F followed by up to 4 digits
- S4 ..Spindle speed command:the character S followed by up to 4 digits
- T2 ..Tool identifier : The character T followed by up to 2 digits
- M2 ..Miscellaneous command : M followed by up to 2 digits (G0- G99)
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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 WordsA 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 functionsA 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 InputOne 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..
