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Copyright The McGraw-Hill Companies, Inc.Permission required for reproduction or display.

PowerPoint to accompany

Krar Gill Smid

Technology of Machine Tools6th Edition

CNC Machining

CentersUnit 78


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Objectives

Describe the development of the

machining center

Identify the types and construction ofmachining centers

Explain the operation of the machining

center Understand a basic CNC program for a

machining center


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CNC Machining Centers

Industrial surveys in 1960's showed smallermachine components requiring several

operations tool long time to complete Part sent to several machines before finished

There was much "operator intervention" duringmachining process

In late 1960s and early 70s, begin to designmachine that would perform severaloperations and do 90% of machining on onemachine


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Types of Machining Centers

Three types: horizontal, vertical anduniversal

Factors to determine type and size1. Size and weight of largest piece machined

2. Maximum travel of three primary axes

3. Maximum speeds and feeds available4. Horsepower of spindle

5. Number of tools automatic tool changercan hold


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Two Types of Horizontal

Machining Centers Traveling-column

One or usually two tables where work mounted

Column and cutter move toward work on one tablewhile operator changes workpiece on other table

Fixed-column

Equipped with pallet (removable table)

After workpiece machined, pallet and workpiecemoved off receiver onto shuttle; shuttle rotated,

bringing new pallet into position for shuttle andfinished work pallet into position for unloading


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Vertical Machining Center

Saddle-type construction with sliding

bedways that use a sliding vertical head

instead of quill movement

Generally used to machine flat parts held in

vise or simple fixture

Versatility increased by addition of rotary

accessories


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Universal Machining Center

Combines features of vertical and horizontal

machining centers

Spindle can be programmed in both vertical and

horizontal positions

Allows for machining all side of a part in one setup

Useful for small and medium batch parts Has additional accessories such as indexible

pallets and rotary-tilt tables


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Advantages of Universal

Machining Centers Eliminate handling and waiting time

between machines

Reduced number of fixtures and setups Reduced programming time

Improved product quality

Less work-in-process (WIP) inventory Faster product delivery to customers

Lower manufacturing costs


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Column

Saddle

Bed

X axis

Z axis

Y axis

Main Operative Parts

Main operative parts of bothvertical and horizontal

centers basically same.

Position of machining

spindle determines whetherit is classified as vertical

or horizontal.

Copyright The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.


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Primary Components of a

Machining Center



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Machining Center Accessories

Number of accessories available

Two types

Those that improve efficiency or operation of

machine tool

Those that involve holding or machining

workpiece


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Torque Control Machining

Calculates torque from measurements atspindle drive motor

Increases productivity by preventing andsensing damage to cutting tool

Torque measured when machine turning, notcutting and value stored in memory

As cutting begins, stored value subtracted fromreading at motor giving net cutting torque

Goes higher, computer reduces feedrate, turns oncoolant or even stops cycle


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Automatic Tool Changers:

Large Capacity Horizontal-Type Hold up to 200 tools

Identified by either tool number or storage pocket

number Held in storage chain

Process: (~ 11 seconds)

When one operation being performed, tool required for

next moved to pick-up position

Tool change arm removes and holds it; exchanges when

operation complete; returns tool to storage


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Automatic Tool Changers:

Smaller-Capacity Vertical, Disk-Type Holds from 12 to 24 tools

Next tool selected upon completion of

machining operation (~ 2.5 to 6 seconds) Tool carriage mounted on shuttle that slides

carriage next to tool spindle

Tool pocket aligned, spindle orients toolholderand tool lock releases

Tool changer rotates to number called, tool lockenergized and carriage slides out of way


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Tools and Toolholders

Wide variety of cutting tools

Conventional milling machines, cutting tool

cuts 20% of time Studies show machining center time

20% milling, 10% boring, and 70% hole-makingin average machine cycle

Cutting time can be as high as 75%

Large consumption of disposable toolscaused by increased tool use


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face milling cutters

two-flute end millfour-flute end mill


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Stub Drills

high-helix drill

core drill

oil hole drill


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Taps

gun

stub flute

spiral flute

fluteless


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rose reamer

fluted reamer

carbide-tipped reamer

Single-point boring tools

are used to enlarge a holeand bring it to location.


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Combination Tools

If machining center has helicalinterpolation capability, one

tool can perform drilling,chamfering, and threadingoperations in one cycle


Solid-carbide combination drill/threadtool with drill tip on end, chamfer

located at correct length for selected

application


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Sequence of operations for

combination tool, the Thriller



1. Drill point can produce through hole or a blind

hole no deeper than two times tool diameter

2. Chamfer is cut, and tool is retracted approximately

2 thread pitches from the bottom of the hole

Tool fed radially into wall of hole to full thread depth

during of a turn (180, while moving of thread

pitch in Z axis

3. Next, thread is formed by helical interpolation

cycle during one full turn (360), while moving

one thread pitch in Z axis.

Tool is brought out radially from wall, to center of

hole during of a turn (180) while moving of a

thread pitch in the Z axis.

4. On completion of the cycle, the tool is retracted

out of the hole


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Toolholders

Must have compatibility in toolholders in

order for wide variety of cutting tools to be

inserted into machine spindle quickly andaccurately

Most common toolholder has V-flange and

self-releasing taper shank Size (range from No. 30 to 60) determined by

machine capacity and designed horsepower

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Common

Toolholder

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Variety of Toolholders



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Work-Holding Devices

Standard step clamp

Used to hold down flat, large parts

Quick-release clamp good when clamps have tobe temporarily moved to machine edge

Table plate

Flat aluminum plate bolted to machine table Dowel pin and tapped holes machined into plate

to permit fastening vises or clamps

More flexible than limit of table T-slots

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Plain-style precision vises

Keyed directly to table slots

Make positioning and clamping accurate and simple

When machining multiple identical parts,

matched set of qualified vises can be used Qualified vises used when long part requires support

on both end to maintain parallelism

When using double-station cluster vises; total ofup to 20 parts held for machining operation

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Vise jaw systems

Set of master jaws placed in vise and itemssnapped into position

Parallels, modular workstops, angle plates, V-jaws,and machinable soft jaws

Add versatility and increase flexibility of aprecision vise

Can be used in both single-station and double-station vises

CNC fixtures Used to accurately locate many similar parts and

hold them securely for machining

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Programming Procedures

Programming can vary slightly from

machine to machine so important to follow

manual supplied with machine

Two classes concentrated on in text:

Bench-top teaching model

Inexpensive and easy to operate for students

Standard machine model

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Bench-Top Teaching Machines

Simple programming example explained in

detail in text as was done in Unit 75

Program notes plus full program sequence

with explanations to help understand code

Refer to G-code and M-code charts in

Unit 75

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Machining Center Setup

Before using machining center, operatorneeds to become familiar with control panel

and operational procedures Different modes and how to use menus, how to

establish machine zero, set tool length offsetsand test run program

When machine powered up, need to zero outall axes so control know location of machinehome position

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Setting Part Zero

Each part has established part zeroNot same as machine zero

Using jog mode and edge finder or dialindicator, locate part zero position in X and Yaxes

Work offset distance (position shift offset) is

distance traveled from machine home Entered on control's work coordinate page

Distances traveled for X and Y entered, while Zaxis distance left at zero

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Setting Tool Length Offset

Start with empty automatic tool changer

Load tool #1 by indexing to proper location of

tool carriage

Tool placed directly into spindle and locked

Use jog mode to touch off tool to Z0 of part

Distance traveled is Z tool offset and listed oncontrol offset page under offset for tool #1

Process repeated with each additional tool

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Program Test Run

Never machine a part without test runningprogram first

Equipped with graphics display Allow operator to see steps on control screen

without cutting part

Without graphics display Dry run program without part in machine

Use step/single block mode and feedrateoverride

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Standard-Size Machining Center

Another full example of a new part that

introduces additional machining cycles

Circular and fixed drilling cycles

Program notes and full programming

sequence shown in text with explanation of

programming steps

Refer to G- and M-code charts in Unit 75

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