In design of a press tool for shearing operation, all the various
forces involved in its construction will be considered. These two major forces
must be considered and become part of the design,among which two major forces,
namely cutting force and stripping force.
A. Cutting
Force (Fc)
Cutting
force is that force which is applied to shear the material in the course of the
cutting operation. Blanking, punching, and shaving are some of the operations,
it plays a very critical role since it acts as an indicator of the efficiency,
quality, and safety of the processes involved in manufacturing.
The
following are some factors which influence the magnitude of this force:
1. Material:
This will depend upon the shear strength of material to be cut, which again is
dependent on the nature itself - mild steel, aluminum, or composite material.
Usually, a higher tensile strength will require more cutting force.
2. Material
Thickness: It is a requirement of more force to actually shear this material.
In comparison to the applied force and thickness, normally it is a straight
line but varies with the material properties.
The
cutting force plays an important role in the design of the press tool in
shearing operations (i.e. Blanking, Piercing, Punching, Trimming etc.). Either
way, it is a problem either in lifting the production process or hindered by
it. Proper calculations of the force involved in cutting to ensure appropriate
sizing of the tooling and suitable free operations without lots of risks of
stress ensure that it will never be prone to early failure and wear and tear.
Cutting
Force is Calculated as;
Where,
Fc=
Cutting Force, N
L= Cutting
Length, mm
t=
Thickness of sheet, mm
τmax
= Shear strength of Sheet Material, N/mm2
B.
Stripping Force (Fs)
It is the
force that would have to be applied for the withdrawal of the cut workpiece
from punch or die after the process of shearing has taken place, and it is
denoted as Fs. Among the vital forces that carry out a clean separation without
damaging the material or the tooling, this is one of them.
The
factors involved are:
1. Material
Adhesion: Owing to friction and elastic properties of material, the material
gets stuck in die on shear cut. The stripping force must overcome adhesion
forces, so that it is easily ejected from the die.
2. Die
Geometry: The stripping action depends a lot on the die geometry. This would
include clearance between workpiece and the die as well as the design of
stripping plates or mechanisms.
3. Surface
Finish: Finish on the die as well as on the workpiece is friction-related. That
is, the smoother the surface, the lesser would be the friction. With it comes
decreased stripping force.
4. Material
thickness: It affects the amount of stripping force just as it does with the
cutting force. The bigger the thickness, the bigger the pulling out force is,
especially when the material becomes liable to deformation at the point of
cutting.
Where,
Fs=
Stripping Force, N
Fc=
Cutting Force, N
Total Force: Total force is helpful to
determine the amount of right tonnage that needs to use for press tool operation.
It is calculated as,
Total Force = Cutting Force + Stripping Force
Where,
P = Required
Press Tonnage, N
Fs =
Stripping Force, N
Fc =
Cutting Force, N
Examples:
1. Calculate the required press tonnage for following data:
Operation : Blanking
t =Sheet Thickness, 2mm
τ = Shear strength, 400 N/mm2
Solution:
A. Cutting Force:
t =Sheet Thickness, 2mm
τ = Shear strength, 400 N/mm2
Cutting Length(L) = 197.08mm
Fc = 197.08 X 2 X 400
Fc = 157664 N
Fc = 15.8 Ton
B. Stripping Force
Fs= 10% of 157664
Fs= 15766.4 N
Fs= 1.6 Ton
C. Total Force
P= 157664+15766.4
P= 173430.4 N
P= 17.3 Ton
Required Press Tonnage for above operation is 17.3 Ton.
2. Calculate the required press tonnage for following data:
Operation : Blanking
t =Sheet Thickness, 2mm
τ = Shear strength, 280 N/mm2
Solution:
A. Cutting Force:
t =Sheet Thickness, 2mm
τ = Shear strength, 280 N/mm2
Cutting Length(L) = 205.66 mm
Fc = 115169.6 N
Fc = 11.5 Ton
B. Stripping Force
Fs= 10% of 115169
Fs= 11516.9 N
Fs= 1.2 Ton
C. Total Force
P= 115169.6+11516.9
P= 126686.5 N
P= 12.7 Ton
Required Press Tonnage for above operation is 12.7 Ton.
3. Calculate the required press tonnage for following data:
Operation : Blanking
t =Sheet Thickness, 1.5 mm
τ = Shear strength, 400 N/mm2
Solution:
A. Cutting Force:
t =Sheet Thickness, 1.5mm
τ = Shear strength, 400 N/mm2
Cutting Length(L) = 267.04 mm
Fc = 267.04 X 1.5 X 400
Fc = 160224 N
Fc = 16.02 Ton
B. Stripping Force
Fs= 10% of 160224
Fs= 16022.4 N
Fs= 1.6 Ton
C. Total Force
P= 160224 + 16022.4
P= 176246.4 N
P= 17.6 Ton
Required Press Tonnage for above operation is 17.6 Ton.
4. Calculate the required press tonnage for following data
Given;
Operation : Piercing
t =Sheet Thickness, 1.5 mm
τ = Shear strength, 530 N/mm2
Solution:
A. Cutting Force:
t =Sheet Thickness, 1.5 mm
τ = Shear strength, 400 N/mm2
Cutting Length(L) = 78.54 mm
Fc = 78.54 X 1.5 X 400
Fc = 47124 N
Fc = 4.7 Ton
B. Stripping Force
Fs= 10% of 47124
Fs= 4712.4 N
Fs= 0.4 Ton
C. Total Force
P= 47124 + 4712.4
P= 51836.4 N
P= 5.2 Ton
Required Press Tonnage for above operation is 5.2 Ton.
5. Calculate the required press tonnage for following data
Given;
Operation : Piercing
t =Sheet Thickness, 2 mm
τ = Shear strength, 530 N/mm2
A. Cutting Force:
t =Sheet Thickness, 2 mm
τ = Shear strength, 530 N/mm2
Cutting Length(L) = 2(πd) = 2(78.54) = 157.08 mm
Fc = 157.08 X 2 X 530
Fc = 166505 N
Fc = 16.7 Ton
B. Stripping Force
Fs= 10% of 166505
Fs= 16650.5 N
Fs= 1.7 Ton
C. Total Force
P= 166505 + 16650.5
P= 183155.5 N
P= 18.32 Ton
Required Press Tonnage for above operation is 18.32 Ton.
6. Calculate the required press tonnage for following data
Given;
Operation : Blanking and Piercing
t =Sheet Thickness, 2 mm
τ = Shear strength, 400 N/mm2
Solution:
A. Cutting Force:
t =Sheet Thickness, 2 mm
τ = Shear strength, 400 N/mm2
1. Blanking Operation
Cutting Length(L) = 371.76 mm
Fc = 371.76 X 2 X 400
Fc = 297408 N
Fc = 29.74 Ton
2. Piercing Operation
Cutting Length(L1) = 69.28 mm
Cutting Length(L1) = 38.11 mm
Fc = (69.28+38.11) X 2 X 400
Fc = 85912 N
Fc = 8.6 Ton
Total Force (Fc)= 29.74+ 8.6 =38.34 Ton
B. Stripping Force
Fs= 10% of 38.34
Fs= 3.8 Ton
C. Total Force
P= 38.34+3.8
P= 42.14 Ton
Required Press Tonnage for above operation is 42.14 Ton.
