introduction to press working

[born_for projects]

Introduction to Press working
Press working may be defined as chip less manufacturing process by which various components are made from sheet metal.
This process is also called cold stamping
These process are genrally carried out at room temprature
Classification of press operatons




The operations done on sheet metal can be classified into two types
Cutting operations - In cutting operations the work piece is stressed beyond its ultimate strength. the stress caused in metal by applied forces will be shearing stresses e.g-blankng,piercing ,trimming, shaving
Forming operations- the stresses are below the ultimate strenth of metal. In these operation there is no cutting forces but only contour of work piece is changed to get the desired product



Various press working operations
BLANKING
PIERCING
SWAGING
PLANISHING
COINNG
EMBOSSING
BENDING
DRAWING



Press working eqipments
Sources of power
Method of actuation of ram/rams
Number of slides
Type of frame
Type of work for which press has been designed



Types of press
Fly press
Open back inclineable press
Straight side single crank press
Ecentric press
Double actiom crank press
One point press
Two point press
Double and triple action press
Friction screw press
Hydraulic press




Basic elements of press
Capacity “The capacity of press is generally governed by aspects like tonnage, stroke, space for mounting, number of stroke per minute and length of stroke
Press action “ refers to whether machine has single action of slide or double or triple
Mechanism “ refers to that the machine is crank operated, eccentric operated, hydraulic operated or pneumatic operated
Frame design “ in most of cases the open back inclinable frame which facilitates ejecting the part by gravity is preferred



Accessories of common press
Base
C- frame
Ram
Pitman
Crank shaft
Crown
Clutch
Brake
Tie rods



PRESS TOOL COMPONENTS
Working components “ dies ,punches and their sections
Structural components “ upper shoe punch holder), lower shoe ( die holder) and shanks
Guiding components “ guide posts and bushings
Feeding components - feed the stock strip or blank to the stamping station
Locating and locking components “ locating pins and rings
Stripping components “ strippers, push off pins ,knockouts
Fastening components “ all fasteners



Press working terminology
Bed
Bolister plate
Die set
Die
Shut height
Pitman
Stroke
Die block
Lower shoe
Punch
Upper shoe
Punch plate
Back up plate



COMPUTATIONAL CAPACTIES AND TONNAGE REQUIREMENTS
1. Cutting force
2. Smallest hole that can be punched
3. Clearence in die and punch
4. Die block design
5. Punch design


CUTTING FORCE CALCULATIONS
P = 3.14*D*T*Fs for round hole
P= L*t*Fs for other profiles
where P=cutting force reiqured
D=diameter of hole
T=thickness of material
Fs=shearing force or ultimate shear stress
L= perimeter of material




Smallest hole that can be punched
Fc=3.14/4*d*d*fc
Fc=p=3.14/4*d*d*fc=3.14*d*t*Fs
D=4t*Fs/Fc
Here d=dia of smallest hole
t=thickness of material
Fs=ultimate shear stress
Fc=allowable compressive strength of punch material
P=cutting force




Clearence in die and punch
Clearence in die is given in case of piercing operation
Clearence in punch is given in case of blanking operation
Value of clearence is 3% to8% of t where t is thickness of material
Other clearences given in die are angular clearences



Cutting force calculations
C=LTS
C=cutting force in tons
L=length of cutting edge, inches
T=thickness of stock material, inches
S=shear strength of stock material, tons



Method to reduce cutting forces
Providing stepped or staggerd punches
Providing single shear
Providing double shear


Centre of pressure
When the shape of blank to be cut is irregular, the summation of shear force about the centre line of pressure may not be symmetrical due to this the bending movements will be introduced in press ram producing undesireable deflection and misalignment
To avoid this centre of pressure must be calculated while laying punch position on punch holder




Types of die sets
Simple die set
Compound die set
Progressive die set
Multiple die set



Types of punches
Plain punches or headed punch
Peen headed punch
Quilled punch



Critical stages in shearing action of metal
Plastic deformation
Penetration
Fracture



Strip layout
Economic stock utilizations of high importance a design engineer should aim for a goal at least there must be 75% stock utilization.
In preparing layout it should be observed that the distance between nearest two points of blank and between blanks and also edge of sheet to first blank in no case be less than sheet thickness




Sheet metal shaping operations
Various sheet metal shaping operations are
Bending
Forming
Drawing




About bending operations
Bending is the metal working process by which straight length is transformed into curved length
During this operation the outer surface of metal is under tension and inner surface is in compression




Types of bending
Edge bending
V-bending
U-bending



Forming dies
Solid form dies
Pressure pad forming dies
Curling dies
Embossing dies
Coining dies
Bulging dies
Assembly dies


Drawing operation
Drawing is operation of producing cylindrical or box shaped components with straight or tapered edges
In drawing the area of developed blank before drawing should be same as the surface area of metal in drawing
Thickness of material remains unaltered