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Computer Aided Manufacturing > CNC Machine Tool > Flashcards

CNC Machine Tool Flashcards

1
Q

CNC driving system components

A
  • servo driving mechanism
  • servo motor
  • transmission (ball screw + nut)
  • spindle driving mechanism
  • spindle motor
  • transmission (belt)
  • electro-spindles have no transmission
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2
Q

Types of servo motors

A
  1. DC servo motor
  2. synchronous AC servo motor
  3. Induction AC servo motor
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3
Q

DC motor: main characteristics

A
  • 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
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4
Q

Synchronous AC motor: characteristics

A
  • 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
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5
Q

Induction AC motor: characteristics

A
  • stator with rotating magnedic field
  • rotor with a second rotating magnetic field
  • robust
  • low cost
  • high initial current
  • use: spindle
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6
Q

Stepper motor: characteristics

A
  • open-loop control
  • low cost
  • high torque at low speed
  • use: axis
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7
Q

Ball screw: mechanism and characteristics

A
  • 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
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8
Q

Types of linear guideways

A
  • 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
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9
Q

Linear motor: characteristics

A
  • 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
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10
Q

Electro-spindle: characteristics

A
  • direct coupling spindle-motor
  • compact and simple
  • high accuracy
  • high rotational speed
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11
Q

Inductosyn: characteristics

A
  • 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
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12
Q

Types of linear displacement transducers

A
  • inductosyn
  • reflection scales
  • linear scales
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13
Q

encoder: characteristics

A
  • 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)

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14
Q

CNC control loop

A

The objective is the control of the position of the table; three loops: current, speed, position

  1. semi-closed loop
    - measure of position is given by a sensor in the motor
    - the performance depends on the quality of the transmission
  2. 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
  3. 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
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15
Q

CNC architecture

A
  • MMI, NCK, PLC
  1. Numerical Control Kernel
    - interpreter
    - interpolator
    - acceleration profile generation
    - position controller
    - axis controller
  2. PLC
    - logic sequential control of auxiliary functions
  3. Man-Machine Interface
    - user interface
    - machine status
    - editing
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16
Q

Machine tool structure: design elements

A
  1. stiffness
    - main requirement
    - no deformation during operations
    - depends on material and joints
  2. damping
    - to minimize vibrations
    - increased using joints
  3. Thermal distorsion
    - low coefficient is needed to avoid errors
  4. assembly techniques
  5. Guideways
  6. linear motor drives
  7. machine fundations
    - to be defined to reduce vibrations
17
Q

Machine tools structure: materials

A
  1. Gray cast iron
    - traditional choice
    - good damping capacity
    - very heavy
  2. welded steel
    - very light
    - good mechanical properties
    - low cost
    - low damping capacity
  3. polymer concrete
    - great damping capability
    - low stiffness
    - it can be inserted into cast-iron or steel structures to increase their damping capability
18
Q

Machine tool: characteristics

A
  1. working envelope
    - space inside which the piece can be machined
    - approximated with a cube
    - it defines the size of the machine
  2. spindle
    - number, type, orientation, power, torque, speed
  3. axis
    - number, type, orientation
  4. 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
19
Q

Machine tool: classification

A
  1. single purpose machines
    - evolution of traditional machine tools
    - numerical control
    - specific machining processes
  2. multi-purpose machines
    - numerical control
    - several machining processes (flexible)
    - turning center and machining center
20
Q

Turning center: characteristics

A
  • 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
21
Q

Machining center: classification

A
  1. spindle orientation:
    - horizontal
    - vertical
  2. motion
    - tool motion
    - part motion
    - typically both tool and part move
  3. number of axis
22
Q

vertical spindle machining center: characteristics

A
  • forces mainly in the vertical direction
  • structure of the machine support the load
  • problems with chip removal since the working plane is horizontal
23
Q

horizontal spindle machining center: characteristics

A
  • 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
24
Q

types of rotational axis

A
  1. continuous
    - possible interpolation by the CNC with linear axis
  2. indexed
    - orientation fixed before the machining
    - less flexible
25
Q

Machining center: components

A
  • Automatic tool changer
  • tool storage
  • tool monitoring and setting
  • automatic pallet changer
  • automatic chip removal
26
Q

Machining centre: functionality of the tool holder

A
  • interface beween tool and machine
  • centering
  • block
  • torque transmission
  • liquid for lubrication and cooling
  • memory for data saving
27
Q

Machining centre: types of tool holders

A
  1. 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
  2. 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
28
Q

Automatic tool changer: characteristics

A
  • it changes the tool from spindle with the next one
  • motions
  • travel time
  • tool change time
29
Q

Tool storage: characteristics

A
  • 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
30
Q

tool information management

A
  • data about tools may be stored inside a memory in the tool holder or inside an external database
  • tool data: length, type,…
31
Q

Machining center: functionalities of the pallet

A
  • reference for the fixture
  • standard coupling with the machine
  • it allows positioning and blocking on the machine
  • automatic pallet changer is available
32
Q

Machining center: functionalities of the fixture

A
  • reference for the workpiece
  • workpiece positioning and blocking
  • chip removal

Computer Aided Manufacturing (15 decks)

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