CNC Machine Tool Flashcards
CNC driving system components
- servo driving mechanism
- servo motor
- transmission (ball screw + nut)
- spindle driving mechanism
- spindle motor
- transmission (belt)
- electro-spindles have no transmission
Types of servo motors
- DC servo motor
- synchronous AC servo motor
- Induction AC servo motor
DC motor: main characteristics
- stator with a magnet
- rotor with a metal armature, siupplied by the outside via brushes and commutator
- easy speed and torque control
- high maintenance cost (brushes)
- use: axis and spindle
Synchronous AC motor: characteristics
- stator with a permanent magnet
- rotor supplied to create a rotating magnetic field
- high and constant torque
- low maintenance cost (brushless)
- high cost
- torque ripple and vibrations
- use: axis and spindle
Induction AC motor: characteristics
- stator with rotating magnedic field
- rotor with a second rotating magnetic field
- robust
- low cost
- high initial current
- use: spindle
Stepper motor: characteristics
- open-loop control
- low cost
- high torque at low speed
- use: axis
Ball screw: mechanism and characteristics
- lead screw operated by a ball bearing
- a nut is moved by the contact with the balll
- the nut allows the balls to rotate in a closed loop
- low friction
- no backslash with preload
- high accuracy
Types of linear guideways
- sliding guideways
- rolling guideways
- low friction
- high accuracy
- low transmission force
- low maintenance cost
- hydrostatic guideways: pressurized fluid injected inside a cavity to move the table
Linear motor: characteristics
- like a rotary electric motor that has been rolled out flat
- very low friction
- rapid feeds and accelerations
- no transmission
- low maintenance
- high accuracy
- high cost
- heating
- complex control
Electro-spindle: characteristics
- direct coupling spindle-motor
- compact and simple
- high accuracy
- high rotational speed
Inductosyn: characteristics
- scale with a sequence of rectangles with period p
- slider with two circuits separated by p/4
- for the induction principle, in the circuits of the moving slider different voltages can be measured, which give the cyclic incremental position of the slider
Types of linear displacement transducers
- inductosyn
- reflection scales
- linear scales
encoder: characteristics
- incremental or absolute
1) incremental
- light emitted from a led
- rotating disk with slits
- 3 slits in a fixed panel: A,B,Z
- A and B slits generate two digital signals with 90° phase difference
- Z slit gives a signal when one revolution is completed
- incremental measure of pulses gives the position
2) absolute
- slits on the rotating disk are arranged on concentric circles
- a single fixed slit provides a binary word that indicates the position
- the noise is not accumulated (not incremental sensor)
CNC control loop
The objective is the control of the position of the table; three loops: current, speed, position
- semi-closed loop
- measure of position is given by a sensor in the motor
- the performance depends on the quality of the transmission - closed loop
- measure of position given by a sensor on the table (linear scale)
- the transmission is inside the control loop
- the value for the gain of the position controller is limited, since a too high value gives unstability, resulting in hunting vibrations - Hybrid loop
- position control based on a sensor on the motor, compensated by a measure of position of the table
- no limit to the gain
- best performances
CNC architecture
- MMI, NCK, PLC
- Numerical Control Kernel
- interpreter
- interpolator
- acceleration profile generation
- position controller
- axis controller - PLC
- logic sequential control of auxiliary functions - Man-Machine Interface
- user interface
- machine status
- editing
Machine tool structure: design elements
- stiffness
- main requirement
- no deformation during operations
- depends on material and joints - damping
- to minimize vibrations
- increased using joints - Thermal distorsion
- low coefficient is needed to avoid errors - assembly techniques
- Guideways
- linear motor drives
- machine fundations
- to be defined to reduce vibrations
Machine tools structure: materials
- Gray cast iron
- traditional choice
- good damping capacity
- very heavy - welded steel
- very light
- good mechanical properties
- low cost
- low damping capacity - polymer concrete
- great damping capability
- low stiffness
- it can be inserted into cast-iron or steel structures to increase their damping capability
Machine tool: characteristics
- working envelope
- space inside which the piece can be machined
- approximated with a cube
- it defines the size of the machine - spindle
- number, type, orientation, power, torque, speed - axis
- number, type, orientation - technical characteristics
- velocities, accelerations, idle times
- accuracy (average error), repeatability (variability of the erro), resolution (smallest displacement possible) in positioning
- reliability: machine can be reused many times with the same results
Machine tool: classification
- single purpose machines
- evolution of traditional machine tools
- numerical control
- specific machining processes - multi-purpose machines
- numerical control
- several machining processes (flexible)
- turning center and machining center
Turning center: characteristics
- equipped with loader and unloader system to change the finished part
- one or more turrets: each turret has different tools, and multiple operations can be performed at the same time
- one or more spindles with different position or orientation
- automated chip removal
- tool storage
- tool monitoring and setting
Machining center: classification
- spindle orientation:
- horizontal
- vertical - motion
- tool motion
- part motion
- typically both tool and part move - number of axis
vertical spindle machining center: characteristics
- forces mainly in the vertical direction
- structure of the machine support the load
- problems with chip removal since the working plane is horizontal
horizontal spindle machining center: characteristics
- forces are supported by the fixture
- rotation and tilting of the table improve the accessibility
- possible rotational degree of freedom of the spindle to imrove accessibility
types of rotational axis
- continuous
- possible interpolation by the CNC with linear axis - indexed
- orientation fixed before the machining
- less flexible
Machining center: components
- Automatic tool changer
- tool storage
- tool monitoring and setting
- automatic pallet changer
- automatic chip removal
Machining centre: functionality of the tool holder
- interface beween tool and machine
- centering
- block
- torque transmission
- liquid for lubrication and cooling
- memory for data saving
Machining centre: types of tool holders
- ISO
- conical cavity for alignment
- interface for manipulation using robotic arms
- torque transmission (friction alone is not enough)
- the tool is pulled and blocked inside the spindle
- problems:
- centrifugal forces during rotation opens the cavity, producing vibrations
- less blocking force
- the position of the tool changes, producing errors - Hollow shaft HSK
- de facto standard
- the blocking system is designed to improve the blocking force during rotation
- reference plane does not change during rotation
Automatic tool changer: characteristics
- it changes the tool from spindle with the next one
- motions
- travel time
- tool change time
Tool storage: characteristics
- tool flow is from tool storage to tool changer to the spindle
- possible configurations: disk, turret, chain
- it may be shared with different machines, to reduce cost
tool information management
- data about tools may be stored inside a memory in the tool holder or inside an external database
- tool data: length, type,…
Machining center: functionalities of the pallet
- reference for the fixture
- standard coupling with the machine
- it allows positioning and blocking on the machine
- automatic pallet changer is available
Machining center: functionalities of the fixture
- reference for the workpiece
- workpiece positioning and blocking
- chip removal
