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3.2
Static gripping force.
Chucks convert the axial stroke of the draw-tube to a radial stroke of the
jaws by means of an inclined plane (wedge) system.
The wedge changes the "draw-pull" into a much greater "gripping force".
This gripping force is applied to the workpiece, providing the necessary
force to counter the torque created by the cutting tools during the machi-
ning cycle. The "max gripping force" (Fsmax) and the "max draw force"
(Ftmax) are contained in the technical features and inscribed on the front
of each power chuck. To calculate the "static gripping force" (Fso) for
each "draw force" (Ft) use coefficient "K" typical of every power chuck.
The value of "K" can be easily calculated with the technical features in
catalogues or on the front of the power chuck:
K =
So, at each value of Ft, corresponds a value of Fso according to the
formula:
Example: for a 210 BHD-3 jaw chuck, we determine Fso for Ft=30kN
Fsmax
110 kN
K = –––––––– = –––––––––––– =
Ftmax
38 kN
Coefficient "K" has been determined by experiment on a new power chuck,
clean and properly greased with SMW-AUTOBLOK type K 05 grease.
IMPORTANT: Always keep the chuck well lubricated with SMW
AUTOBLOK type K 05 or K 67 grease .
It's important to use an SMW-AUTOBLOK gripping force dynamometer
type 339H to check the static gripping force.
3.3
Dynamic gripping force and centrifugal force.
Power chucks are used on modern CNC lathes at high rotation speeds.
When the power chuck rotates, all the parts which are not anchored
radially i.e. the master jaws, T-nuts, the screws and top jaws are subject
to a "centrifugal force" which decreases the gripping force in O.D. clam-
ping (and increases it in I.D. clamping).
For each speed there is a "Dynamic gripping force" (Fsd) which is deter-
mined as follows:
Fsd = Fso - Fct
where:
Fsd [kN] = Theoretical dynamic gripping force
Fso [kN] = Static gripping force
Fct [kN] = Theoretical centrifugal force
The "theoretical centrifugal force" is determined as follows:
Fct = M · R ·
where:
M [Kg] = Mass of master jaws + jaws,"T" nuts and screws
R [m]
= Radius of the gravity center of "M"
[rad/sec] = Chuck's angular velocity
To complete the calculations it is necessary to determine the "Mass
moment" M.R as follows:
M · R = (m
where:
m
[Kg] = Mass of 1 jaws with "T" nuts and screws
1
r
[m]
= Radius of the gravity center of "m
1
m
[Kg] = Mass of 1 gripping jaw
2
r
[m]
= Radius of the gravity center of "m
2
Z
= Number of the chuck's jaws.
The values for m1 · r1 are indicated in the following schedule:
Ø of
the
APLD-APLM
APRD
chuck
APC
APD-APM
APRC
170
0.027
0.027
0.035
215
0.059
0.059
0.072
260
0.105
0.10
0.13
315
0.19
0.18
0.24
400
0.47
0.44
0.54
The values for m
· r
are the mass and baricentric radius of the jaws only
2
2
and can be easily calculated by the user (for standard hard top jaws, the
weight and the baricenter are indicated in the catalogues).
18
INSTRUCTIONS AND SAFETY REGULATIONS
Fsmax
Fso
––––––-- = –––––
Ftmax
Ft
Fso= Ft . K
2,9 ; Fso= 30 . 2,9 = 87 KN
2
· r
+ m
· r
) · Z
1
1
2
2
"
1
"
2
TYPES
NTM
NTLM
NTC
NTLC
NTD
NTLD
-0.029
-
-0.029
-
-0.050
-
-0.050
-
-0.115
0.055
-0.12
-0.06
-0.17
-0.07
-0.18
-0.08
-0.40
0.21
-0.43
-0.24
Example: For a type 215 APD - 3 jaw chuck, with standard soft top jaws
in the most external position but not outside of the external diameter, at
4000 r.p.m., the calculation is as follows:
Fct = M · R ·
m
1
= –––– · n
30
therefore the theoretical centrifugal force is:
Fct = (0.059 + 0.043) · 3 · 419
Measuring the internal performance of the power chuck, the "effective
centrifugal force" (Fca), measured experimentally, is about 0,7 of the
theoretical one, therefore we have:
the "effective dynamic gripping force" is:
Example: continuing the previous example, we have the "real centrifugal
force":
and the "Effective dynamic gripping force"
this value is found in the diagram for a type 215 APD - 3 jaw chuck.
IMPORTANT: With standard jaws, NEVER exceed the maximum
allowed speed.
IMPORTANT: When using special jaws which are heavier than
standard jaws or in a more external position, it is necessary to calculate
the Fcs and Fsa and correspondingly reduce the speed.
IMPORTANT: We suggest using a dynamic gripping force measurement
system type DGM 270 to measure the "Effective dynamic gripping force"
in order to confirm the safe gripping conditions at each speed.
3.4 Draw coupling
To explain the concept of "Effective draw
coupling (force)", we must begin with the
"Effective dynamic gripping force" explained at
point 3.3.
The gripping force acts radially on the workpiece,
to create a coupling. This must be changed into
"effective draw force" (Fra), which acts tangential-
ly on the piece, multiplying it by the coefficient of
friction "f".
We have shown below the average values of the coefficient of friction "f"
for the different types of jaws and surfaces of the workpiece.
Schedule 4 - Coefficient of friction "f"
GRIPPING CONDITIONS
Turned soft top jaws
Hard top jaws (square teeths)
Hard top jaws (scharp teeths)
Jaws with carbide inserts
The draw coupling is determined by multiplying the draw force by the
arm "b" (clamping radius) (see drawing 4).
For machining on lathes, with a rotating piece, it is necessary to consider
the "effective dynamic draw coupling" (Tda) determined by multiplying
the "Effective draw force" (Fra) by the clamping radius (b).
NTRD
where :
Tda [N·m] = Effective dynamic draw coupling
NTRC
Fra [N]
-0.021
b
Example: clamping with a type 210 BHD-3 jaw chuck, speed 4.000
-0.038
r.p.m. in a finishing operation with soft top jaws on machined piece (f =
0,1) with a clamping on diameter of 160 mm (b = 0,08 m).
-0.09
Fra = Fsa · f = 75 · 0.1 = 7.5 kN = 7500 N
-0.12
Tda = Fra · b = 7500 · 0.08 = 600 Nm.
-0.33
Once the draw coupling has been calculated, it is necessary to determine
the "cutting coupling" (Tz), generated by the contact of the tools with the
workpiece. Verify that it is, at least, 2.5 times less than the Tda:
. r
2
= (m
+ m
· r
) · z ·
1
1
2
2
· r
= 0.059 Kg · m (see schedule above)
1
m
· r
= 0.72 · 0.060 = 0.043 Kg · m
2
2
3.14
= –––––– · 4000 = 419 rad/sec
30
= 53700 N ≅ 54 kN
2
Fca ≅ Fct · 0.7
Fsa = Fso - Fca
Fca ≅ Fct · 0.7 ≅ 54 · 0.7 = 38 kN
Fsa = Fso - Fca = 112-38 = 74 kN
Fra = Fsa . f
Rough piece
0.15
0.20
0.40
0.60
Tda = Fra · b
= Effective draw force
[m]
= Clamping radius
Tda ≥ 2.5 . Tz
2
drw.4
Worked piece
0.1
0.12
0.25
-
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