Comp ter N merical ControlComputer Numerical Control

Introduction

Machining Centers A machine tool capable of:

Multiple operation and processes in a single set-up utilizing multiple axes Typically has an automatic mechanism to change tools

Three Axis Machining Center

The most basic type of machining center, and they start at about $30,000.T i ll i th ti l fi ti Typically, in the vertical configuration

How many surfaces of a cubicHow many surfaces of a cubic workpiece can be machined?

VerticalVerticalMachining Centers

VMC

C-Frame Style

C-frame style

Bed

C-frame style

Y axis Ways

Bed

C-frame style

Column

C-frame style

Column

Z axis ways

C-frame style

Headstock

C-frame style

Headstock

SpindleSpindle

C-frame style

C-frame style

C-frame style

Saddle

C-frame style

X axis waysX axis ways

Saddle

C-frame style

Table

C-frame style

Splash Guard

C-frame style

Automatic tool changer

This style of machine may have certain accessories

P ll t ChPallet Changer

C-frame style

IndexerIndexer

C-frame style

IndexerIndexer

C-frame style

IndexerIndexer

C-frame style

IndexerIndexer

C-frame style

IndexerIndexer

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

C-frame style

Rotary tableRotary table

VerticalMachining CentersMachining Centers

Gantry Style

Gantry Style

Gantry Style 10 - 100 feet or more(3 - 33 meters)(3 33 meters)

Gantry Style

Horizontal Three Axis MachineNotice any difference in axes

This type of machine starts at

Notice any difference in axes orientation compared to Vertical Machining Centre?

This type of machine starts at about $90,000.

How many surfaces of a cubicHow many surfaces of a cubic workpiece can be machined?

HorizontalMachining Centers

HMCHMC

Horizontal Machining Center

Bed

Horizontal Machining Center

Z axis ways

Bed

Horizontal Machining Center

Column

Horizontal Machining Center

ColumnY axis ways

Horizontal Machining Center

Headstock

Horizontal Machining Center

SpindleHeadstock

Horizontal Machining Center

S ddlSaddle

Horizontal Machining Center

X axis ways

S ddlSaddle

Horizontal Machining Center

Table

Horizontal Machining Center

AutomaticAutomatic tool changer

Four Axis Machining Center Four axis machining centers g

are generally horizontal, and the table rotates to create the forth axis.

True four axis machines start around $100,000

The fourth axis can also be i l t d b ddi CNCimplemented by adding a CNC controlled rotary table to a three axis vertical machine,

How many surfaces of a cubic k i b hi d?workpiece can be machined?

Five Axis Machining CenterWhat is the difference between

Starting from around $250,000 Application:

4 and 5 axis machine?

Application: complex parts and molds in the

aerospace and automotive industriesindustries.

How many surfaces of a cubic workpiece can be machined?

Can often machine a part in one setup

Why is this important?

Machining Center with Pallet Changer

With a pallet changer, the operator unloads and reloads one pallet while the machine pworks on the other.

What are the main benefitsof the pallet changers?

Other Applications of CNC Technology

Electrical wire wrap machines Component insertion machines

C di t i hi Coordinate measuring machines Tape or filament winding machines Laser cutting,

Wi EDM Wire EDM.

Advantages of CNC

Greater accuracy and repeatability (+/- 0.0003 Typical) Resolution Or Smallest Movement Possible is Typically 0.0001 Tooling costs are reduced Tooling costs are reduced

How?

More operations can be performed with each setup Less lead time Design changes are facilitated Programs can be prepared rapidly and can be recalled at any

time utilizing microprocessors. Required operator skill is less than that for a qualified machinist,

and the operator has more time to attend to other tasks in the work area.

Limitations of CNC

Relatively high initial cost of the equipment. The need and cost for programming and computer

time. Higher maintenance effort;

Special maintenance with trained personnel Special maintenance with trained personnel. High preventative maintenance since breakdowns

are costly.

How CNC Machines Work There are three basic components in an CNC system:p y

program of instruction machine control unit (MCU) the processing equipment p g q p

Three Basic Categories of Motion Systems Point to Point

No contouring capability Used For Drilling, Punching And Straight Milling

Straight cut control one axis motion at a time is controlled for machining

Contouringg multiple axiss controlled simultaneously Used For Lathes, Milling, Grinding, Welding And

Machining Centers. g

Motion Control Systems1) Open-loop system using a stepper motor) p p y g pp

A motion command is sent as an electrical pulse trainA motion command is sent as an electrical pulse train Each pulse causes the motor to rotate a fraction of one revolution Leadscrew translates the rotary motion to a linear table motion.

Why is this system called open-loop?

Recirculating Ball Screws

R f i b ll ?Reasons for using ball screws?

Backlash could be a problem. Recirculating Ball Screws are generally preloaded to give zero

b kl hbacklash.

More on Recirculating Ball Screws

Recirculating Ball Screws(Rotational to Linear Velocity Conversion)(Rotational to Linear Velocity Conversion)

Recirculating Ball Screws(Positioning Resolution)

360/

360/ 360/l

Example

A ball screw has a lead of 0.125 in/rev.What is the distance that the nut will travel if the screw is turned 15 degrees?

Stepping Motors provides open-loop, digital control of the p ov des ope oop, d g ta co t o o t e

position of a workpiece in a numerical control machine.

For each pulse it receives, the drive unit p ,manipulates the motor voltage and current, causing the motor shaft to rotate by a fixed angle (one step).

Stepping Motors(Full Step Operation)

200 step hybrid motor cross200 step hybrid motor cross section

The 1.8 degree stepping motor is the predominant standard for industrial automation.

Recirculating Ball Screws(Lead Screw Resolution)( e d Sc ew eso u o )

The positioning resolution of a ball screw is directly proportional to the smallest angle that the motor can turn.

Stepping Motors(Lead Screw Positioning Resolution)

Example

How can you improve the Lead Screw Positioning Resolution ?Positioning Resolution ?

Stepping Motors(Microstepping Operation)

The rotor can be positioned in partial steps by simultaneously controlling the currents supplied to the stator phase windings. Microstep sizes of 1/10 1/16 1/32 and 1/125 of a full step areMicrostep sizes of 1/10, 1/16, 1/32, and 1/125 of a full step are most commonly used.

Example

Open-loop system Table Speed

The speed of travel depends on the frequency of the pulse train, fp (Hz, pulse/sec)

The table feed speed, vf (in/min) depends on the pitch and the rotational p , f ( ) p pspeed (rev/min).

: rotational speed of lead screw ,

screw pitch (l) , table velocity (vf) N: steps/rev

Review Question 1Prove the previous equations.

Step angle (degrees/pulse) Total angle (degrees) traveled assuming one-to-one gear ratio

between the motor and lead screw:

between the motor and lead screw:

Movement of table (in) for l pitch of lead screw (in)

pn

360u l

By combining the above two the number of pulses required to take the table to position u could be obtained from:

n pu360

Rotational speed of leadscrew (rev/min) will depend on pulse train frequency (pulses/sec) and will be

ln pn

pff60and will be

Therefore travel speed (in/min) will be The required pulse train to run the table at a specified feed rate will

Nf

n p60

fv nlv f The required pulse train to run the table at a specified feed rate will

be obtained by combining two previous equations:

lNv

f fp 60

Review Question 2 A CNC mill has 0.125 inch lead recirculating ball screws that

are driven by a 200 step hybrid step motor with 125 microsteps per step. What is the position resolution of the machine? How many microsteps will the motor have to undergo in order to

th ill t bl tt h d t th b ll 0 1 i ?move the mill table attached to the ball screw 0.1 in?

l = 0.125 in/rev, N = 200 steps, m = 125 microsteps/step

epin/microst 000005.0125200

125.0

mNlu

microsteps 000,200000050

1.0required microsteps ofnumber

125200.

mN

000005.0