With the
advancement of cutting tool technology, machines
need to have ultra high speeds to make the
optimum use of the changing technology. To
match this age of high speeds, work holding
for rotating machines needs high gripping
power, even at very high cutting speeds. Therefore
it is very important to know :
The cutting forces on
the job at a particular speed and feed
The initial gripping
force of the work holding device with
the spindle in stationary position and
at a given speed, taking into account
the centrifugal forces of the jaws
The draw bar pull required
to provide the inital gripping force
Many factors act on the clamping point
during any machining operation. While
it is not possible to detail the factors
here, a few relevant points that could
be important are :
The condition of the
chuck
The condition of lubrication
Surface condition of
the job
Condition of the gripping
Surface of the jaws plain, serrated or
diamonds
Length of the gripping
Surfaceof the jaws
Weight of the jaws
Speed and accompanying
centrifugal forces
Draw bar pull
While each one of the points mentioned
above has a definite role in the ultimate
gripping force, the centrifugal force
is one factor that needs better understanding.
The manufacturers of chucking
equipment all over the world including AIRTECH
are manufacturing centrifugally compensated
chucks. But these chucks have certain limitations
which are :
The chucks are manufactured
to compensate the centrifugal forces caused
by weight of base jaw and top jaws of standard
design. Any change in the weight of the
jaws, changes the centrifugal force.
Again the centrifugal force will be reduced
if the chuck is working on its minimum gripping
capacity and will be increased if working
on the maximum gripping capacity.
However it is advisable for the user of
the chuck to keep the following formula
in mind to find out the centrifugal force.
PC = 0.102.G.r.
( .N)2
30
Where.
G = Weight of base jaws and top jaws
r = The radius of the centre of gravity
N = Operating speed
PC = Centrifugal force
After having calculated the centrifugal
force, the available chucking force at a
given speed can be found with the help of
the following equation :
Pn = PO ± PC
Where
Pn = Chucking force at a given speed
PO = Chucking force when the chuck is in
a stationary state
PC = Centrifugal force
+ for internal gripping
- for external gripping
It is advisable, that the
chucking force at a given speed should be
at least 1.5/ 2.0 times the cutting force,
depending upon the constructional rigidity/
fragility of the job.
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