how to select a machinig centre

8/10/2019 How to Select a Machinig Centre

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Ho w to Se lec t t h e p p r o p r ia te M ac h in in g en te r

Machining Centers with built in flexibility of

operation and versatility in tooling play an

increasingly important role in competitive

manufacturing industries. In order to choose the

most appropriate type of machine

whether vertical

or horizontal and to pick the right optional

features a thorough study of the production

parameters and product specifications is necessary.

This article outlines the choices available to user

industries.

Before selecting a machining center, we must have a

clear idea of what it can do. It is essentially a milling

machine fitted with a Computer Numerical Control

(CNC) System and having independent axes

movements along all three axes

r;x y

/Z) with infinitely

variable feed rates. The spindle has infinitely variable

speeds. These features make the milling machine aCNC

Milling Machine. When an Automatic Tool Changer is

fitted to the CNC Milling Machine, it becomes a

Machining Center.

In its simplest form a Machining Center has three

independent axes movements with infinitely variable

spindle speeds, an Automatic Tool Changer (ATC), the

capabilities for contouring on 2 axes, simultaneous

interpolation on 3 axes and rigid tapping facility.

There are two varieties of Machining Centers - Vertical

Machining Center (VMC) and Horizontal Machining

Center (HMC). When the spindle is mounted along the

vertical axis(perpendicular to the table) it is called VMC

and when the spindle is mounted along the horizontal

axis (parallel to table) it is called HMC. The HMC

.Ass embiy

oj Machining Cent ers at A ce Manufacturin g Sys tem s

M r. P . Ramadas , Managing D ir ecto r o j AM S

invariably has an index table or rotary table with pallet

changer as a standard optional feature of the machine.

The VMC is only a 3-axes machine with a 12/20/24/30

tool magazine. An index table, rotary table and pallet

changer are offered as optional features on aVMC.

In today's manufacturing industry, the over-riding factor

is cost per component at the specified quality level.

The developed countries are looking at the developing

countries to supply auto component parts due to the low

cost of manufacturing, achieved through lower

investments and lower labour cost. This has forced

component manufacturing industries to look for low-

cost machines to bring down the cost of production.

By design and construction, the VMC is much lower in

cost compared to the HMC, generally 50. In

developing countries, industries use VMCs much more

than HMCs. In India the ratio of VMC to HMC

utilization is more than 5:1, whereas in developed

counties itis 2:1.

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Mach in in g c ente r sunder va ri ous st ages o f asse m blY at AMS

The following varieties of machining centers are

available to users.

Machines with BT30 (ISO 30) taper

These machines are predominantly used in the two-

wheeler industry to produce small components of

aluminium and cast iron material with light machining

operations.

Milling cutters and boring bars of up to 63mm can be

used for milling operations (up to about 1 to 2mm depth

of cut). Drilling of holes up to 20mm in aluminium and

12mm in cast iron can be done on these machines.

These machines are much faster in operation with lower

chip-to-chip time, and use smaller tools weighing

less than 3 kgs. These machines are not generally

recommended for machining steel components.

In this category, AMS offers 3 models of BT30 spindle

VMCs - Spark, DTC-300 DART, with different rapid

traverse, chip-to-chip time and spindle speeds. In a

machining operation, the idle (non-machining) time is

invariably 50 to 60 of the total cycle time. Hence to

reduce the cycle time ( when using more number of

tools), it is always recommended to choose the machine

with lower chip-to-chip time and faster acceleration /

deceleration. The machines with lower chip-to-chip time

and higher rapid traverse are more expensive.

Finally, it is the cost per component that will decide

whether to go in for a faster machine or a normal

machine. Wherever the labour cost is higher, a faster

machine than the less expensive slower machine is

recommended. This principle is valid for all categories

of machines

Machines with BT40 (IS040) taper.

The maximum number of machines produced in the

world belong to this category, accounting for about 65

of the machining center population. About 75 of

AMS production is in this

category.

Machines with BT50

(ISO 50) taper

These machines are

generally bigger in SIze

with the spindle power

being more than 11kW

They are meant for heavy

machining of steel

forgings and big castings.

The population of these

machines is less than 10 of the total machining center

population. AMS produces such machines only against

specific orders.

Mr. H. S. Ram Prak ash,

G eneralMana ge r, Desig n and

Developmen t, AMS

These categories are valid for both VMCs and HMCs.

Som e of the t yp ic al compone n ts m ac hined on m achi ning cent ers

VMCs with BT40 Taper

The AMS production range includes various sizes of

machines in this category depending upon axes strokes

and rapid traverse. These include:

MCV-300 and DTC-300 with X/Y /Z strokes as 350 /

300/300mm

.WI ER,WI ERXLandCHALLE GERwith

X/Y /Z strokes as 500/ 350/ 350 mm

MCV-400, MCV-400X, MCV-400S and MCV-400SX

with X/Y /Z strokes as 550/415/ 460mm

MCV-500, MCV-500S and MCV-500F with X/Y /Z

strokes as 800/500/500mm

MCV-600 with X/Y /Z strokes as 1000/600/600mm

.MCV-600CX with X/Y/Z strokes as

1250/600/600mm

As the machine sizes increase, spindle power also rises.

Continue d onpag e 12

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H o w to S e lec t M ac h in in g en tr e

Continued from page

11)

Selection of spindle speed and power

This is a very important feature which has to be

considered, depending upon the application.

Generally, if the machine is meant for aluminium

machining, the spindle speed should be 80-8000 rpm

with direct drive of the spindle. The full spindle power

will be available from 100 rpm to 8000 rpm. This will also

give higher acceleration

deceleration to the spindle.

The spindle run-out will be very good since there is no

side load on the spindle due to the belt drive. For

precision machining on cast iron steel, direct drive with

higher power spindle motor is recommended. For special

applications, spindle speed of 10000 rpm and 12000 rpm

can be supplied. This will entail longer lead time.

If the machine is for general applications, where the

component material is steel, cast iron or aluminium, then

speed range of 60-6000 rpm is selected. In this range, the

full power is available from 1000 to 6000 rpm, thus

facilitating higher material removal at lower speeds for

rough milling and rough boring operation. If higher

spindle power is selected (which is optional), the material

removal will be still higher at lower spindle speeds.

For example, the use of 8 125 milling cutters and

spindle speeds of less than 1000 rpm, where the spindle

power gets reduced linearly with respect to the spindle

speed. A typical spindle powerl torque diagram is given

with normal power and higher power.

As can be seen, 1 : 1.5 speed ratio between motor and

spindle provides 50 more torque. This will be helpful

for bigger size tapping operation done at lower speeds on

tougher materials like stainless steel and hardened

materiaL Sometimes even higher power spindle motor

may have to be selected with 1 : 1.5 speed ratio to provide

still higher torques and power at lower speeds

understanding of the metal removal requirements is very

important to select the appropriate spindle power and

spindle speed range.

Selection of spindle bearing and rigidity:

For exclusively machining steel components, it is

recommended to select 70 dia bearing spindle with

medium pre-load. However, medium pre-loaded 70 dia

bearing spindle cannot run beyond 4000 rpm.

Direct drive spindles come with light pre-loaded 60 or 70

dia (optional) bearings. ormally these are not suitable

for heavy machining on steel components. Very special

executions can be done with spindle cooling and light to

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medium pre-load for such applications with 8,000 rpm.

Axes rapid traverse and chip-to-chip time

Higher the rapid traverse, lower will be the idle time, and

higher is the cost of the machine. As mentioned earlier,

where labour cost and overheads are higher, it is

preferable to select a machine with higher rapid traverse

with lower chip-to-chip time . Generally, the small-scale

industries select low-speed versions of machines to keep

the initial investment low.

Size of the spindle

This depends upon the size of the component, number

of setups for each component, number of components

to be loaded for each set-up, type of fixture-plain fixture

or cradle fixture-with index table

rotary table with

tailstock.

Increased daylight area (IDL)

Lower daylight area is better for machining. But when

index tables or rotary tables are used with cradle fixture,

increased daylight area (IDL)of 160mm is

recommended so that 200mm length of tool can be used

on a component of 150mm height mounted on the

cradle plate. In case of twin-arm tool-changer machines,

IDL of 160mm can help to reduce the tool change time

without taking the table to XY reference points, thus

saving 1see. in chip-to-chip time. This feature is

preferred whenever many tool changes are required for

producing the same component and is a major cause of

idle time. Sometimes for machining a tall component, a

special daylight area of 320mm could be an advantage.

This is generally selected on the basis of component and

fixturing.

Coolant through spindle(CTS)

This feature is given in the machine to do deep-hole

drilling (up to LID ratio of 10) and provide coolant to

the cutting point to improve cutting parameters and tool

life. There are two systems - one with 20 bar coolant

SPINDLE SPEED AND POWER

o u t p u t

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Direct Drive/Tndirect Drive (1:1)

Power torque dia for standard power

of 5.5kW

Power torque dia. for higher power

of 7.5kW

Indirect Drive (1 : 1.5)

With 1:1.5 ratio between spindle

motor and spindle, spindle torque

gets increased by 50


4/5

pressure for holes of more than 8 and L/D ratio up to 5

and the second with 50 bar coolant pressure for holes of

6 to 8 and L/D ratio of more than 5 and less than 10.

Though this feature is expensive, it will pay for itself

within 6 to 12 months, depending upon the application,

with reduced tooling cost and higher productivity.

Automatic Pallet Changer (APC)

This is done sideways since the pallets are rectangular in

size and to maintain the minimum reach to the spindle.

Hydraulic component clamping feature is not on this

machine since the hoses cannot be carried to the

machine along with the pallet. This feature overlaps the

component loading and unloading time with the

machining time and would be useful only when the

machining time is more than the load and unload time.

When the machining time is very high compared to the

load and unload time, the feature may not be very cost

effective. With this feature, the productivity of the

machine is not operator-dependent and can be pre-

determined. The operator will have enough time to clean

the rest of the pads before loading the new part. The

parts can be inspected when the machining is going on.

Another advantage with this feature would be to load the

first and second set-up fixture on one pallet, and the

third and fourth set-up on the second pallet, thus

eliminating the need to change the fixture and hold the

part inventory for the third and fourth set-up. A quarter

of the machines produced byAMS arewith APe.

Number of tools on the ATC

This is decided on the number of set-ups and number of

tools required for all the set-ups on the machine. If

empty pockets are available on the magazine, sister tools

(duplicate tools) of low tool life can be loaded in the

empty pockets, eliminating the tool set-up time. AMS

machines come with 6 / 12 / 20/ 24 and 30 tool

magazmes.

Index Table / Rotary Table

Whenever operations are to be done at intervals of 30 /

SPINDLE SPEED AND TORQUE

Torque 80 15 m i n Op erat in gZ on e

(Nm) 72

70

F I i I I - ,-- - - - - 1

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6 I \ Ooe r tin Z e

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Torque 15 min

Nm ) Operating Zone

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in

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Op rating one

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00 2000 4000 6000 8000

Sp ind l e sp eed RP M )

o

2000 4000 6000

S p in d le s p ee d RP M)

Direct Drive /Lndirecr Drive (1:1)

Power torgue dia for standard power

of 5.5kW

Power torque dia. for higher power

of 7.5kW

Indirect Drive (1 : 1.5)

With 1 :1.5 ratio between spindle

motor and spindle, spindle torque

ge ts inc rea sed by 50

13

45, then 12 / 8 position index tables are selected. There

are various types and sizes of index tables -ltll0 / 200,

BTP 80 / 100, depending upon the application. Index

tables use Hirth coupling to provide the final position,

thus giving very good repeatability of the position -

within 3 sees to arc. Whenever the positions required are

in increments of 0.001, CNC Rotary tables are used.

CNC Rotary tables of 180 / 202 / 250 are provided

depending upon the size of the machine and the inertia

of the fixture. These tables are also provided with helical

interpolation feature, using it as the fourth axis of the

machine (cam cutting).

Chip conveyor

This is a very important feature. All chip conveyors

supplied to AMS have 100 microns drum filtration

facility to keep the coolant tank cleaner and to prevent

the blocking of the coolant passage and pipe.

It is recommended to use scraper conveyor to handle

aluminium and cast iron chips and duplex conveyor(slat-

cum-scraper) to handle long chips of steel, cast iron and

aluminium chips. Screw conveyors to transfer the long

and heavy chips of steel and cast iron are recommended

when chips produced are more. This is an optional

feature on the machine.

Hydraulic power pack

Hydraulic power pack is required for machines with

APC for pallet transfer and component clamping.

Generally power packs work at 35/40 bar. In exceptional

cases, power packs with 70/100 bar pressure are

supplied. The power packs are provided with drain

cooling systems to control oil temperatures. Standard

AMS machines are not supplied with hydraulic power

packs.

Coolant systems

Coolant through flexinox is supplied as a standard

feature. To provide more coolant to flush chips on the

fixture, flood coolant system can be selected. A separate

pump facilitates chip flushing on the base. The coolant

gun can be provided to clean the rest pad or component

on the machine. Whenever chip conveyor is selected,

there is a continuous back washing of the drum filter to

remove chips from the periphery of the drum filter.

Voltage stabilzer

These are supplied with various capacities of 10 kVA to

40 kVA, depending upon the size of the machine. It can

either be directly procured by the customer or supplied

byAMS.

False plate

This is provided over the table/pallet. Whenever

frequent fixture change is envisaged on the machine, this

feature is selected to prevent any damage to the machine

table.

Conti nu ed onpag e 14


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H o w to S e le c t M ac h in in g en t r e

Continu ed from page 13)

Choice of VMC versus HMC

By the virtue of the design, HMCs are more than double

the cost of VMCs. In the Indian manufacturing industry,

VMCs using index / rotary tables are more cost-effective

in the manufacture of components for both the

domestic and export market. For precision applications,

HMCs are generally selected. The APC design in HMC

provides the feature to clamp the component

hydraulically. But this feature is very expensive. Most of

the features relevant to VMCs are applicable' for HMCs

also.

G rin d in g p p l i c a t i o n s

External CNC Cylindrical

Grinder

eGRIND

Component: Spring seat needle lift sensor

Material

Application

Hardness

Work holding

Specials

Accuracies

16MnCr55

OD

Face Grinding

700Hv

Diaphragm Chuck

In process gauging

Dia. 3.932 + 0.004.

Cpk achieved 3.27

Length 46.8 + 0.015mm.

Cpk achieved 2.77

48 Seconds

ycle time

14

It is the experience of using machining centers that will

finally help the customer to select the right type of

machine and features. The machining centers can be

used either for cell manufacturing systems or as stand-

alone machines. HMCs are provided with multi-pallet

systems to integrate them with FMS. However, these are

not very common in Indian industry.

Of 'l~te, automatic loading and unloading of

components on the machine through gantry robots or

stand-alone robots and interconnection of the machine

with a conveyor system is picking up. This may become a

common requirement within the next 5 years time, as the

trend is towards unmanned machining.

Component: Gear fuel pump

Application

Hardness

Work holding

: OD Face Grinding

: 34-56HRC

: Expanding Mandrel

Setup 1 for OD Setup

2 for face grinding

: In process gauge for

OD and face

: Dia. 29.616 + 0.0064.

Cpk achieved 3.27

Length 31.714 - 0.008mm.

Cpk achieved 2.77

: 56 Seconds

Specials

Accuracies

Cycle time

how to select a machinig centre

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