NC and CNC Machines · NC & DNC •NC developed in 1950’s •CNC developed in 1970s •Numerical Control: A system in which actions are controlled by direct insertion of numerical

NC and CNC Machines

References C.N.C Machines by Pabla and Adithan

Automation, production systems and computer integrated manufacturing by Mikell P. Groover

NC & DNC• NC developed in 1950’s• CNC developed in 1970s• Numerical Control: A system in which actions are

controlled by direct insertion of numerical data. The system must automatically interpret at least some part of this data. (EIA)

• Direct Numerical Control is referred to a system connecting a set of numerically controlled machines to a common memory for part program or machine program storage with provision for on-demand distribution of data to the machines. (ISO 2806:1980)

CNC Machine Features• Dedicated computer performs all basic NC

functions• Part program – Can be input through keyboard – Stored in the computer memory for repeated use.– Can be edited and optimised at the machine tool itself– May be checked without actually running the machine– Subprograms may be used for repetitive machining

sequences– Allow compensation for any change in the dimension

of the cutting tool

Advantages of CNC Machine Tools• Suitable for small lots size production or

complex part geometry or parts are expensive• Close tolerances result in reduced scrap, high

accuracy and consistent quality. Therefore reduced inspection

• Reduced lead time• Longer tool life• Elimination of special jigs and fixtures so

reduced lead times and cost• Increased productivity• Reduced non productive time

Disadvantages of CNC Machine Tools

• Higher investment cost• Higher maintenance cost• Costlier CNC personnel

Use of CNCCNC machines may be used for the following situations:• Large number of operations are needed per component• Complex operations• When batches are repetitive and their size medium• High labour cost • Component requires substantial tooling• Components requires 100% inspection• Set up and Inspection times are high• Ratio of cutting time to non cutting time is high• Large variety of components are to be produces• Components require highly skilled labour

Basic Components of NC systemsThe components of an NC system are:• Program of Instructions• Controller Unit or Machine control Unit• Machine tool or other controlled equipment

Axes Identification• Z axis – Axis of the main spindle– +z: Direction that increases the distance

between the tool & work-piece• X axis – Horizontal & parallel to the work holding

surface– If Z is vertical: +x is to the right when

looking from the tool to the supporting column

– If Z is horizontal: +x is to the right when looking from the spindle towards the workpiece

• Y axis– completes the 3D coordinate system

Coordinate Systems

• Types of coordinate systems– Absolute Coordinate system– Incremental coordinate system

• Position commands are given in terms of basic length unit

If shaft encoder gives 500 pulses / revolutionBall screw has a pitch of 1mmThen,Basic length unit = 1 pulse of shaft encoder

= 1/500 = 0.002 mm

Machine Control Unit (MCU): It has two sub-units namely:• Data Processing Unit • Interprets & encodes part program into internal machine codes. • The interpolator calculates the intermediate positions of the

motion in terms of BLU (basic length unit) • The calculated data are passed to CLU for further action.

• Control Loop Unit • Data from DPU are converted into electrical signals to control the

driving system to perform the required motions. • Also controls other functions such as machine spindle ON/OFF,

coolant ON/OFF, tool clamp ON/OFF.

Control Systems• Open Loop Control System

• Stepper motors controlled by MCU generated electrical pulses are used• Each pulse drives the motor by an angle (step angle)• Thus table position depends on the number and rate of pulses given to

the motor

• Closed Loop Control System

Feedback Systems

Accuracy & Repeatability• Control Resolution: Ability of MCU to divide a

range of axis movement into points identifiable by the controller. It depends on the storage capacity (Range / 2n)

• Accuracy is the ability of the machine to position the table at a desired location. It is given by

(CR/ 2) + 3 S.D.• Repetability = + 3 S.D.

Control Systems

• Point to point control system• Straight Line control system• Continuous Path or Contouring System

Interpolation Schemes• Linear interpolation: Straight line path• Circular Interpolation: Cutting of arcs in a

plane defined by 2 axes• Helical Interpolation: Combines circular

interpolation in two axes and linear in a third• Parabolic and Cubic Interpolation: Used to

generate free form curves

Construction Of CNC: Slide-ways•Efficient cooling & lubrication is needed to avoid

thermal distortion

•No direct metal to metal contact

•Hydrostatic Slideways:–Use air or oil

–Almost frictionless

–Need a large surface area

Construction Of CNC: Slide-ways

• Linear bearings with ball and rollers– Linear roller bearings also called tychoways– Needs hardened machine bed surface or

hardened steel guides with special guide forms may be attached

• Guiding surfaces are covered with low friction material – As a coating for e.g. with

Polytetraflouroethylene (PTFE)– As replaceable strips of low friction material

Spindle• Due to high speeds and feeds spindle should be:– Short and stiff– Drive located as close to the front bearing as possible

• Spindle Drive:– Requires large speed variations needs infinitely

variable speed system– Uses electrical or fluid motors

Drive Units: Spindle Drive

• Electrical motors– Drive may be direct from motor or through belts or

gears– Both A.C. and D.C.

• A.C Induction motors– Used to drive the main spindle directly– Easy speed variation– More reliable low maintenance and cost

• D.C motors are used for step-less speed variation by changing input voltage

• Fluid Motors: Use pressurised oil or air directed on the rotor blades gives very high speeds

Drive Units: Axis Drive• Each axis is controlled by a separate

servomotor or stepper motor• Servomotor gives accurate control of velocity

and position• Stepper motor drive – Does not need feedback systems – They are simpler and cheaper– Suitable only for light duty machines

Swarf Removal• High volume of swarf• May interfere with operations like tool

changing, loading and access to machine tool• Methods of swarf removal:– Slant or vertical beds are used in turning centres–Multiple cooling jets around the cutting tool inject

cutting fluid at high pressure– Compressed air jets– Linear or rotating conveyer belts

Tooling for CNC Machines

• Cutting time for CNC machines is 70-80% (Conventional machines approx. 25%)

• Tooling Used in CNC should be– Rigid: to withstand high MRR– Capable of being preset and reset– Accurate

Cutting Tools for CNC

• CNC’s use preset and qualified tools• Tool preset: Tools are preset a known

dimension away from the machine tool• Tools are preset with the tool held in the tool

holder• Qualified tools: position of the cutting edge is

guaranteed (to high accuracy) relative to a datum on the tool holder

Qualified Tools• Position of the cutting edge is

guaranteed (to high accuracy) relative to a datum on the tool-holder

• Hard metal inserts are suited for qualified tooling as their dimensions are known

• Semi-qualified tools can be adjusted to the required dimension and may be used on different machines

Indexable Inserts• Indexable carbide inserts are used due to their lower

tool changing time• Hard and special grade carbides give high MRR:

Tungsten carbide inserts are coated with titanium carbide or titanium nitride for better wear resistance

• Chip-breakers: grooves on the insert or tool-holders• Coolant fed tools (coolant fed through shank)are used.

Interlocks are provided to avoid cutting without coolant

• Tool overhang should be minimum

Work Holding DevicesA tool-holder for CNC machines must:• Restrict linear and rotary motion of component• Facilitate quick loading and unloading• Permit multiple operations simultaneously• Not allow the component to deflect under cutting

forces• Be adaptable to automation• Allow easy swarf removal• Be fool proof