CNC Machine Tool

Question 1

CNC driving system components

Answer 1

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

Question 2

Types of servo motors

Answer 2

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

Question 3

DC motor: main characteristics

Answer 3

• 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

Question 4

Synchronous AC motor: characteristics

Answer 4

• 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

Question 5

Induction AC motor: characteristics

Answer 5

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

Question 6

Stepper motor: characteristics

Answer 6

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

Question 7

Ball screw: mechanism and characteristics

Answer 7

• 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

Question 8

Types of linear guideways

Answer 8

• 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

Question 9

Linear motor: characteristics

Answer 9

• 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

Question 10

Electro-spindle: characteristics

Answer 10

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

Question 11

Inductosyn: characteristics

Answer 11

• 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

Question 12

Types of linear displacement transducers

Answer 12

• inductosyn
• reflection scales
• linear scales

Question 13

encoder: characteristics

Answer 13

• 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)

Question 14

CNC control loop

Answer 14

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

Question 15

CNC architecture

Answer 15

• 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

Question 16

Machine tool structure: design elements

Answer 16

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

Question 17

Machine tools structure: materials

Answer 17

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

Question 18

Machine tool: characteristics

Answer 18

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

Question 19

Machine tool: classification

Answer 19

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

Question 20

Turning center: characteristics

Answer 20

• 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

Question 21

Machining center: classification

Answer 21

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

Question 22

vertical spindle machining center: characteristics

Answer 22

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

Question 23

horizontal spindle machining center: characteristics

Answer 23

• 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

Question 24

types of rotational axis

Answer 24

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

Question 25

Machining center: components

Answer 25

- Automatic tool changer - tool storage - tool monitoring and setting - automatic pallet changer - automatic chip removal

Question 26

Machining centre: functionality of the tool holder

Answer 26

- interface beween tool and machine - centering - block - torque transmission - liquid for lubrication and cooling - memory for data saving

Question 27

Machining centre: types of tool holders

Answer 27

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

Question 28

Automatic tool changer: characteristics

Answer 28

- it changes the tool from spindle with the next one - motions - travel time - tool change time

Question 29

Tool storage: characteristics

Answer 29

- 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

Question 30

tool information management

Answer 30

- data about tools may be stored inside a memory in the tool holder or inside an external database - tool data: length, type,...

Question 31

Machining center: functionalities of the pallet

Answer 31

- reference for the fixture - standard coupling with the machine - it allows positioning and blocking on the machine - automatic pallet changer is available

Question 32

Machining center: functionalities of the fixture

Answer 32

- reference for the workpiece - workpiece positioning and blocking - chip removal