CAD/CAM theory-3

Conventional Numerical Control

IntroductionIntroduction

• Numerical Control

• HistoryHistory – John T. Parsons

L t 40• Late 40s

• Punch Card – Aerofile

MIT• MIT– APT

» Too large – Criticized» Too large – Criticized

HistoryHistory

• 1955 ‐ John Parsons and US Air Force define a need to develop a machine tool capable of p pmachining complex and close tolerance aircraft parts with the same quality time afteraircraft parts with the same quality time after time (repeatability). MIT is the subcontractor and builds the machine for the projectand builds the machine for the project.

History: ContinuedHistory: Continued

• First NC machining center 1950• Drilling, Tapping, boring, milling• ATC• ATC

• 1968 ‐ Kearney & Trecker machine tool builders market first machining centermarket first machining center

• 1970’s ‐ CNC machine tools & Distributed Numerical ControlControl

• 1980’s ‐ Graphics based CAM systems introduced. Unix and PC based systems availabley

History: ContinuedHistory: Continued

• 1990’s ‐ Price drop in CNC technologyp gy

• 1997 ‐ PC‐Windows/NT based

Basics Components of an NC SystemBasics Components of an NC System

• Program of Instruction– MDI

– DNC

• Controller unitController unit– Tape Reader, Data buffer, Signal output ‐ Servomotors

F db k it S t l– Feedback units, Sequence control

– Control Panel

• M/C tool or other controlled process– Worktable, Spindle, Motors, Control, p , ,

Control SystemsControl Systems

• Open‐Loop Control– Stepper motor systempp y

– Current pulses sent from control unit to motor

Each pulse results in a finite amount of revolution– Each pulse results in a finite amount of revolution of the motor001” is possible


• Open‐Loop Limitations– Control unit “assumes” desired position is pachieved

– No positioning compensationNo positioning compensation

– Typically, a lower torque motor

• Open‐Loop Advantages– Less complex, Less costly, and lower maintenance p , y,costs


• Closed‐Loop Control– Variable DC motors ‐ Servos

– Positioning sensors ‐Resolvers• Feedback to control unit

• Position information compared to target location

• Location errors corrected


• Closed‐Loop Advantages– DC motors have the ability to reverse instantly y yto adjust for position error

– Error compensation allows for greater p gpositional accuracy (.0001”)

– DC motors have higher torque ranges vs.. g q gstepper motors

• Closed‐loop limitationsClosed loop limitations– Cost

Motion Control SystemsMotion Control Systems

• Point‐to‐Point systemsl ll d– Also called position systems

– System moves to a location and performs an operation at that location (e g drilling)operation at that location (e.g., drilling)

– Also applicable in robotics

• Continuous path systems – Also called contouring systems in machiningAlso called contouring systems in machining– System performs an operation during movement (e.g., Milling and turning)

Point to Point in NC DrillingPoint to Point in NC Drilling

Continuous PathContinuous Path

Interpolation MethodsInterpolation Methods

1. Linear interpolationh l b– Straight line between two points in space

2. Circular interpolationCi l d fi d b i i d i– Circular arc defined by starting point, end point, center or radius, and direction

3 Helical interpolation3. Helical interpolation– Circular plus linear motion

4 Parabolic and cubic interpolation4. Parabolic and cubic interpolation– Free form curves using higher order equations

Circular InterpolationCircular Interpolation

CNC vs NC Machine ToolsCNC vs. NC Machine Tools

• Computer Numerical Control (CNC) ‐ A numerical control system in which the data yhandling, control sequences, and response to input is determined by an on‐board computerinput is determined by an on board computer system at the machine tool.

CNCCNC

Ad t• Advantages– Increased Program storage capability at the machine tool

P di i h hi l– Program editing at the machine tool

– Control systems upgrades possible

– Option ‐resident CAM system at machine toolOption resident CAM system at machine tool

– Tool path verification

NCNC

• Numerical Control (NC) ‐ A control system which primarily processes numeric input.which primarily processes numeric input. Limited programming capability at the machine tool Limited logic beyond directmachine tool. Limited logic beyond direct input. These types of systems are referred to

“h d i l ” d l fas “hardwire controls” and were popular from the 1950’s to 1970’s.

Machining CentersMachining Centers

• A machining center can be defined as a machine tool capable of:machine tool capable of:– Multiple operation and processes in a single set‐up utilizing multiple axis

– Typically has an automatic mechanism to change tools


– Machine motion is programmable

– Servo motors drive feed mechanisms for tool axis’s

– Positioning feedback is provided by resolvers toPositioning feedback is provided by resolvers to the control system


• Example ‐ A turning center capable of OD turning, external treading, cross‐hole drilling, g g gengraving, and milling. All in machining is accomplished in one “set‐up ” Machine mayaccomplished in one set up. Machine may have multiple spindles.


Programming MethodsProgramming Methods

• Automatically Programmed Tools (APT)– A text based system in which a programmer y p gdefines a series of lines, arcs, and points which define the overall part geometry locations. These p g yfeatures are then used to generate a cutter location (CL) file.( )

Programming Methods APTProgramming Methods‐APT

– Developed as a joint effort between the aerospace industry, MIT, and the US Airforce

– Still used today and accounts for about 5 ‐10% ofStill used today and accounts for about 5 10% of all programming in the defense and aerospace industriesdus es

Programming Methods CAMProgramming Methods‐CAM

T l & t i l lib i– Tool & material libraries

– Tool path simulation

– Tool path editing

– Tool path optimization p p

– Cut time calculations for cost estimating

Programming Methods CAMProgramming Methods‐CAM

– Import / export capabilities to other systems• Examples:

Drawing Exchange Format (DXF)– Drawing Exchange Format (DXF)

– Initial Graphics Exchange Standard (IGES)

Th P CAD t NC FilThe Process CAD to NC File

• Start with graphic representation of part– Direct inputDirect input

– Import from external system• Example DXF / IGES• Example DXF / IGES

– 2D or 3D scan• Model or Blueprint• Model or Blueprint

(At this point you have a graphics file of your geometry)geometry)

The Process CAD to NC FileThe Process CAD to NC File

• Define cutter path by selecting geometryC– Contours

– Pockets

– Hole patterns

– SurfacesSurfaces

– Volume to be removed(At this point the system knows what you want to cut)(At this point the system knows what you want to cut)

Output: NC CodeOutput: NC Code

• Numerical Control (NC) Language• Numerical Control (NC) Language– A series of commands which “direct” the cutter

d f h hmotion and support systems of the machine tool.

The NC ProcedureThe NC Procedure

l i• Process Planning– Engineering Drawing– Root Sheet

• Work Center• Machine Tool

– NC OperationsNC S» NC Sequence

• Part Programming– Manual Part Programming– Compute Assisted Part Programming

NC Coordinate SystemNC Coordinate System

• Fixed and Floating Zero

• Absolute andAbsolute and Incremental positioning

Yw

Machine Coordinate System

Ym

XwZw

Workpiece Coordinate System

XmZm

Application of NCApplication of NC

• Machine tool applications:Milling drilling turning boring grinding– Milling, drilling, turning, boring, grinding

– Machining centers, turning centers, mill‐turn centers– Punch presses, thermal cutting machines, etc.p , g ,

• Other NC applications:– Component insertion machines in electronics– Drafting machines (x‐y plotters)– Coordinate measuring machinesTape laying machines for polymer composites– Tape laying machines for polymer composites

– Filament winding machines for polymer composites

Advantages of CNC Machine ToolsAdvantages of CNC Machine Tools

• Ease of part duplication• Ease of part duplication

• Flexibility

• Repeatability

• Quality control through process control• Quality control through process control


• Accommodates simple to complex partsAccommodates simple to complex parts geometry

• Improved part aesthetics

• Increased productivityIncreased productivity

• Technology costs are decreasing


• Reduced set‐up timep

• Reduced lead times

• Reduced inventory

• Better machine utilizationBetter machine utilization

• Job advancement opportunities


• CNC machine tools are more rigid than gconventional machine tools– $$$‐ Climb milling requires about 10 ‐ 15 % less horsepower vs.$$$ Climb milling requires about 10 15 % less horsepower vs.

conventional cutting, but requires a ridged machine tool with no backlash

I d R ’ d f d– Increased Rpm’s and feeds

Disadvantages of NCDisadvantages of NC

• Higher investment costCNC machines are more expensive– CNC machines are more expensive

• Higher maintenance effort– CNC machines are more technologically sophisticatedCNC machines are more technologically sophisticated

• Part programming issues– Need for skilled programmersp g– Time investment for each new part– Repeat orders are easy because part program is already availableavailable

• Higher utilization is required

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CAD/CAM theory-3