Methods of Reducing Cutting Forces
• It sometimes become necessary to reduce force to prevent praise
overloading.
• One method of reducing cutting force is to step punch lengths.
• Punches or group of punches progressively become shorter by about one
stock-material thickness. A second method is to grind the face of punch or
die at a small shear angle with the horizontal.
• This has the effect of reducing the area in shear at any one time.
• shear also reduces stock to the press and smooth out the cutting operation.
• This shear angle chosen should provide a change in punch length from 1 to
1.5 times the stock thickness.
Stepping Punch
• Shear that is equal to or greater than the stock thickness is called full shear.
• cutting forces are reduced by approximately 30% when full share is applied.
• Double share is preferred over single shear because it does not set up lateral
force components. A double shear angle on punches should be concave to
prevent stretching the material before it is cut. The shear angle may be applied
either on the punch face or die face, depending upon whether the operation is
blanking or punching, because shear will distort the work material. In other
words, this shear angle for blanking operation will be the die member, while in
the piercing operation the shear angle will be on punch member.
Double Shear on Die
Single Shear on punch
Single Shear on Die
Springback is defined as the elastic recovery of the material after
unloading of the tools. Springback results in a dimensional change in the
bent part.
• Spring back occurs when a metal is bent and then tries to return to its
original shape.
• There are two basic views as to why Spring Back occurs, one states that it
is due to the displacement of molecules and the other considers Spring
Back in terms of a stress-strain diagram.
• One of the reasons for Spring Back is that as the material is bent the inner
region of the bend is compressed while the outer region is stretched.
• This means that the molecular density is greater on the inside of the bend.
generally the compressive strength of material is greater than it tensile
strength.
• This means that pressure will permanently deform the outer regions of
the piece before it deforms the inner regions.
Drawing
• Deep drawing die design is the process of designing
a die used for the deep drawing process, which is a
metal forming process in which a sheet metal blank
is drawn into a forming die by a punch to form a
desired shape. The design of the die is critical to
achieving the desired shape and minimizing defects
such as wrinkling, tearing, and excessive thinning of
the material.
• The following are some of the key considerations in deep drawing die
design:
1. Material selection: The material used for the die must be able to
withstand the forces and temperatures involved in the deep drawing
process. Typically, tool steels or carbide materials are used.
2. Die shape: The shape of the die must be designed to allow for the flow of
material and minimize stresses and strains. The die must also be
designed to allow for easy removal of the finished part.
3. Punch shape: The shape of the punch must be designed to match the
desired final shape of the part. The punch must also be designed to allow
for easy removal of the finished part.
1. Die clearance: The clearance between the punch and die is critical to
achieving the desired part shape and minimizing defects. Too much
clearance can cause wrinkling, while too little clearance can cause tearing
or excessive thinning.
2. Lubrication: Lubrication is essential to minimizing friction and preventing
defects such as galling or scoring. The type and amount of lubricant used
must be carefully selected based on the material being formed and the
specific die design.
3. Cooling: Heat buildup can cause deformation or cracking of the die.
Therefore, cooling channels must be designed into the die to dissipate
heat and maintain the desired temperature.
4. Die maintenance: Regular maintenance of the die is essential to ensure
consistent quality and prolong the life of the die. This includes cleaning,
lubrication, and repair or replacement of worn or damaged components.
