how to select a machinig centre
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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
12
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
kW )
O ut p u t
kW )
1S m in
O p er a ti ng Z on e
15
m in
O pe ra t i ng Z on e
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.......6.
6
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nuou s
O p e r a
ngZ on
o t
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23kW
35kW
o
1333 3000 6000
S p in d le s p e ed R P M
o 2000 4000 6000 BOOO
S p i n d le sp e ed R P M
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
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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
o \ 60 in
6 I \ Ooe r tin Z e
t
Torque 15 min
Nm ) Operating Zone
60
/ 60
in
- - n
Op r at in g o ne
-,\
Co tinuou
t
Op rating one
: I
~
,
-. . : : : : : :
1500
50
40
:~
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I (I\nti, uo u s
\ Opera in g Zon
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30
20
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5.2
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
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