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 CNC
CNC 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 systems
The 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 / revolution Ball screw has a pitch of 1mm
Then,
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 Devices
A 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
