10/1/2011 1 NC, CNC & Robotics By S K Mondal What is NC/CNC? NC is an acronym for Numerical Control and CNC is an acronym for Computer Numerical Control. What is the difference between NC and CNC ? The difference between NC and CNC is one of age and capability. The earliest NC machines performed limited functions and movements controlled by punched tape or punch cards cards. As the technology evolved, the machines were equiped with increasingly powerful microprocessors (computers) with the addition of these computers, NC machines become CNC machines. CNC machines have far more capability than their predecessor. contd….. What is the difference between NC and CNC ? Some of the enhancements that came along with CNC include: Canned Cycles, Sub Programming, Cutter Compensation, Work coordinates, Coordinate system rotation, automatic corner rounding, chamfering, and B‐ spline interpolation. Where did CNC get started? 1940 Jhon Parson developed first machine able to drill holes at specific coordinates programmed on punch cards. 1951 MIT developed servo‐mechanism 1952 MIT developed first NC machines for milling. 1970 First CNC machines came into picture Now‐a‐day’s modified 1970’s machines are used. Is it necessary to have a CAD/CAM system to program a CNC machine? No, yes, may be. It all depends on the kind of work you are doing, your customer’s requirements and your staff’s capability. CAM system give the programmer a tool for creating set up sheets and process drawings.
Transcript
10/1/2011
1
NC, CNC & Robotics
By S K Mondal
What is NC/CNC?NC is an acronym for Numerical Control and CNC is an
acronym for Computer Numerical Control.
What is the difference between NC and CNC ?The difference between NC and CNC is one of age andcapability.The earliest NC machines performed limited functionsand movements controlled by punched tape or punchcardscards.As the technology evolved, the machines were equipedwith increasingly powerful microprocessors (computers)with the addition of these computers, NC machinesbecome CNC machines.CNC machines have far more capability than theirpredecessor. contd…..
What is the difference between NC and CNC ?
Some of the enhancements that came along with CNC
include: Canned Cycles, Sub Programming, Cutter
Compensation, Work coordinates, Coordinate system
rotation, automatic corner rounding, chamfering, and B‐
spline interpolation.
Where did CNC get started?1940 Jhon Parson developed first machine able to drill
holes at specific coordinates programmed on punch
cards.
1951 MIT developed servo‐mechanism
1952 MIT developed first NC machines for milling.
1970 First CNC machines came into picture
Now‐a‐day’s modified 1970’s machines are used.
Is it necessary to have a CAD/CAM system to program a CNC machine?
No, yes, may be. It all depends on the kind of work you
are doing, your customer’s requirements and your staff ’s
capability.
CAM system give the programmer a tool for creating set
up sheets and process drawings.
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Do all machines speak the same CNC language
No, while there is fairly standard set of G and M codes,
there is some variation in their application. For example
a G0 or G00 command is universally regarded as the
command for rapid travel. Some older machines do not
have a G00 command. On these machines, rapid travel is
commanded by using the F (feed) word address.
What is a “Conversational Control”CNC machine tool builders offer an option what is
known as the conversational control. This control lets
the operator/programmer use simple descriptive
language to program the part. The control then
displayed a graphical representation of the instructions
so the operator/programmer can verify the tool path.
Are CNC machines faster than conventional machines?
Yes, No, Sometimes. When it comes to making a single,
simple part it is hard to beat a conventional mill or lathesimple part it is hard to beat a conventional mill or lathe.
CNC machines move faster in rapid travel than
conventional machines.
Are CNC machines more accurate than conventional machines?
Yes, they can be. But like anything else it depends on
h i i th hi h ll th hi hwho is running the machine, how well the machines has
been maintained, quality of setup and so on.
NC/CNC Machines‐AdvantagesHigh Repeatability and Precision e.g. Aircraft partsVolume of production is very highComplex contours/surfaces need to be machined. E.g.TurbinesFlexibility in job change, automatic tool settings, lessscrapMore safe, higher productivity, better qualityLess paper work, faster prototype production, reductionin lead times
NC/CNC/DNCDirect Numerical Control is a system that uses acentral computer to control several machines at the sametimeDistributed Numerical Control (DNC): the central
t d l d l t t th CNC
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computer downloads complete programs to the CNCmachines, which can be workstations or PCs, and can getthe information for the machine operations.The speed of the system is increased, large files can behandled and the number of machine tools used isexpanded.
Direct numerical control
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DNC
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Basic Length Unit (BLU)In NC machine, the displacement length per one pulseoutput from machine is defined as a Basic Length Unit(BLU).In the CNC computer each bit (binary digit) represents 1BLUBLU.
Bit = BLUExample: If one pulse makes a servo motor rotate by onedegree and the servo motor moves the table by 0.0001mm, one BLU will be 0.0001 mm.The lead of a ball screw is related to the displacementunit of the machine tool table.
Stepper MotorThe stepper motor is special type of synchronous motor
which is designed to rotate through a specific angle
(Called step) for each electrical pulse received from the
control unit.
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Control Systems possible in CNC MachinePoint to point mode:
Point‐to‐point straight line mode
Co‐ordinate systemAll the machine tool use Cartesian Co‐ordinate system.The first axis to be identified is the Z – axis, This isfollowed by X and Y axes respectively.
Right‐hand coordinate systems
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5 axes CNC vertical axis machining centre configuration
Absolute and Incremental Coordinate System
Absolute Coordinate System Incremental Coordinate System
The following are the steps to be followed while developing the CNC part programs.
Process planningAxes selectionTool selectionCutting process parameters planningJob and tool setup planningMachining path planningPart program writingPart program proving
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Basic CNC Principles
For a CNC machine control unit (MCU) decides cuttingspeed, feed, depth of cut, tool selection , coolant on offand tool paths. The MCU issues commands in form ofnumeric data to motors that position slides and toolaccordingly.
Part ProgrammingFANUC CONTROLL
SIEMENS CONTROLL
CNC programmingImportant things to know:
• Coordinate 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 Key LettersO ‐ Program number (Used for program identification)N ‐ Sequence number (Used for line identification)G ‐ Preparatory functionX ‐ X axis designationY ‐ Y axis designationgZ ‐ Z axis designationR ‐ Radius designationF – Feed rate designationS ‐ Spindle speed designationH ‐ Tool length offset designationD ‐ Tool radius offset designationT ‐ Tool DesignationM ‐Miscellaneous function
Explanation of commonly used G codes
• G00 – Preparatory code to control final positionof the tool and not concerned with the path thatis followed in arriving at the final destination.
• G01 – Tool is required to move in a straight lineG01 Tool is required to move in a straight lineconnecting current position and final position.Used for tool movement without any machining‐point to point control. (linear interpolation)
• G02 – Tool path followed is along an arc specifiedby I, J and K codes.( circular interpolation)
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CNC Programming BasicsEach letter address relates to a specific machinefunction. “G” and “M” letter addresses are two of themost common. A “G” letter specifies certain machinepreparations such as inch or metric modes, orabsolutes versus incremental modes.absolutes versus incremental modes.A “M” letter specifies miscellaneous machinefunctions and work like on/off switches for coolantflow, tool changing, or spindle rotation. Other letteraddresses are used to direct a wide variety of othermachine commands.
Table of Important G codesG00 Rapid Transverse
G01 Linear Interpolation
G02 Circular Interpolation, CW
G03 Circular Interpolation, CCW
G XY Pl G XZ Pl G YZ PlG17 XY Plane,G18 XZ Plane,G19 YZ Plane
G20/G70 Inch units
G21/G71 Metric Units
G40 Cutter compensation cancel
G41 Cutter compensation left
G42 Cutter compensation right
Table of Important G codesG43 Tool length compensation (plus)
G44 Tool length compensation (minus)
G49 Tool length compensation cancel
G80 Cancel canned cyclesG80 Cancel canned cycles
G81 Drilling cycle
G82 Counter boring cycle
G83 Deep hole drilling cycle
G90 Absolute positioning
G91 Incremental positioning
Rapid traverse: G00G00:
to make the machine move at maximum speed. It is used for positioning motion.
G90 G00 X20 0 Y10 0G90 G00 X20.0 Y10.0
Start
EndG90:
absolute coordinate
s
(0,0)
(10,10)
(20,10)
Linear interpolation: G01 G01:
linear interpolation at feed speed.G91 G0l X200.0 Y100.0 F200.0
YY
100.0
XStart
End
200.0
G91: incremental coordinates
Circular interpolation: G02, G03 G02, G03:
For circular interpolation, the tool destination and the circle center are programmed in one block G02 is clockwise interpolation, G03 is counterclockwise interpolation
Annotation for Circular InterpolationI0.0, J0.0, and K0.0 can be omitted. If X,Y,Z are all omitted in the program, that means start and end of arc are same points start and end of arc are same points.
N0020 G02 I20.0 (a full circle)If I, J, K, and R all appears in circular interpolation instruction, R is valid and I, J, and K are invalid
Tool CompensationTool‐Radius Compensation
Left hand G41 Right hand G42 C l l di i GCancel tool‐radius compensation G40
Tool‐Radius CompensationTool‐radius compensations make it possible to program directly from the drawing, and thus eliminate the tool‐offset calculation
G41 (G42) H××H th di f t l t t i d i it th t H××: the radius of tool to compensate is saved in a memory unit that is named H××G41/G42 is directly related with direction of tool movement and which side of part is cut.
H××: specified memory unit used to save height compensation of tool.compensation of tool.Positive compensation (G43): real position = specified position + value saved in H××Negative compensation (G44): real position = specified position ‐ value saved in H××
Tool‐Height CompensationExample:
N0010 G91 G00 X12.0 Y80.0 N0020 G44 Z‐32.0 H02；
G91: incremental coordinates
If we put 0.5mm into H02, real position = ‐32.0 ‐ 0.5 = ‐32.5
Cancel tool‐height compensation: G49
coordinates
Table of Important M codesM00 Program stopM01 Optional program stopM02 Program endM03 Spindle on clockwiseM04 Spindle on counterclockwiseM05 Spindle stopM06 Tool changeM08 Coolant onM09 Coolant offM10 Clamps onM11 Clamps offM30 Program stop, reset to start
Rules for programmingBlock Format
N135 G01 X1.0 Y1.0 Z0.125 F5
Sample Block• Restrictions on CNC blocks• 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 numbers should be sequential• Both the program start flag and the program number must beindependent of all other commands (on separate lines)• The data within a block should follow the sequence shownin the above sample block
Example of CNC Programming
What Must Be Done To Drill A Hole On A CNC Vertical Milling Machineg
APT LanguageAPT (Automatically Programmed Tools)The APT language consists of many different types of statements made up of the following valid letters, numerals and punctuation marks.Letters: ABCDEFGHIJKLMNOPQRSTWWXYZLetters: ABCDEFGHIJKLMNOPQRSTWWXYZNumerals: 0123456789/ A slash divides a statement into two sections. eg.,
GO/PAST, , A comma is used as a separator between the elements in
a statement generally to the right of the slash.= An equals is used for assigning an entity to a symbolic
name, e.g., CI = CIRCLE/25,50,30.
WordsThe words to be used in the statements are built up from
one to six letters or numerals with the first one being a
letter. No special character is allowed in the words.
The complete APT part program consists of the following four types of statements
ADAPT (ADaptation APT) was the first attempt to adapt APT programming system for smaller computersAUTOSPOT (AUTOmatic Sytem for POsitioning Tools) was developed by IBM and first introduced in 1962EXAPT (EXtended subset of APT) was developed jointly in German in about 1964 by several universities to adapt APT for E I i ibl i h APT d h h
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European use. It is compatible with APT and thus can use the same processor as APTCOMPACT was developed by Manufacturing Data Systems, Inc. (MDSI)SPLIT (Sundstrand Processing Language Internally Translated) was developed by Sundstrand Corporation, intended for its own machine toolsMAPT (Micro‐APT) is a subset of APT, to be run on the microcomputers
C3 = CIRCLE/ CENTER, PT6, TANTO, LN4C7 = CIRCLE/ CENTER, PT8, PT5
y y
C3
y
x
LN4
PT6C7
y
x
PT8
PT5
The Machining Plan:
Contouring:
Part surface: the surface on which the end of the tool isriding.
Drive surface: the surface against which the edge of thetool rides.
Check surface: a surface at which the current tool motionis to stop.
The Machining Plan
z
y
Direction of
Drive surface Check surface
cutter
x
Direction of cutter motion
Part surface
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The Machining Plan
CS CS CS
DS
TO
DS
ON
DS
PAST
The Machining Plan
Motion commands:
GOLFT/ : Move left along the drive surface
GORGT/ : Move right along the drive surface
GOUP/ : Move up along the drive surface
GODOWN/ : Move down along the drive surface
GOFWD/ : Move forward from a tangent position
GOBACK/ : Move backward from a tangent position
Machining Specifications
Postprocessor commands for a particular machine tool are:
MACHIN/ : used to specify the machine tool and call thepostprocessor for that tool:
MACHIN/ DRILL, 3,
COOLNT/ : allows the coolant fluid to be turned on or off:
COOLNT/ MIST
COOLNT/ FLOOD
COOLNT/ OFF
Machining Specifications
FEDRAT/ : specifies the feed rate for moving the tool along thepart surface in inches per minute:
FEDRAT/ 4.5
SPINDL/ : gives the spindle rotation speed in revolutions perSPINDL/ : gives the spindle rotation speed in revolutions perminute:
SPINDL/ 850
TURRET/ : can be used to call a specific tool from an automatictool changer:
TURRET/ 11
Machining Specifications
TOLERANCE SETTING: Nonlinear motion is accomplished instraight-line segments, and INTOL/ and OUTTOL/ statementsdictate the number of straight-line segments to be generated.
INTOL/ 0.0015
OUTTOL/ 0.001
Machining Specifications
PARTNO: identifies the part program and is inserted at the start ofthe program.
CLPRINT: indicates that a cutter location printout is desired.
CUTTER: specifies a cutter diameter for offset (rough versus finishp ( gcutting). If a milling cutter is 0.5 in. in diameter and we have
CUTTER/ 0.6
then the tool will be offset from the finish cut by 0.05 in.
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