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THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. Title of the invention. - AN IMPROVED PUNCH AND DIE ASSEMBLY
2. Applicant(s)
(a) NAME : LARSEN & TOUBRO LIMITED
(b) NATIONALITY : An Indian Company,
(c) ADDRESS: L & T House, Ballard Estate, Mumbai 400 001,
State of Maharashtra, India
3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF THE NVENTION
The present invention relates to an improved punch and die assembly for piercing and stripping for holes. More particularly, the invention is concerned about piercing and stripping for holes which are adjacent to each other and/or the diameter of holes is less than the material thickness.
BACKGROUND OF THE INVENTION
Press working is one of the major processing techniques for converting metal sheets into different forms, shape and size. It is essentially carried out with the help of Press which has got the stationary bottom bolster and moving Ram at the upper side. Normally the Punch sub assembly is clamped to Ram and Die sub assembly is clamped to bottom bolster. The metal strip / coil are fed through feeder gap of the press tool. The force (tonnage) is applied from Ram to punch while descending through metal strips / coils into the Die which is firmly clamped . at the bottom bolster.
The press tools are used for piercing (making holes) and blanking (producing components with contours) of the metal components, formed by passing strip / coils through them. While piercing the punch (male parts) enters the metal strip / coil making desired holes with the support from die (corresponding female part). On the return of punch after piercing, it is required to separate punch from the metal strip / coil, which is termed as stripping in press working.
Press tool, mainly consist of two halves, namely a die half and a punch half. The die half of the press toot is secured with the stationary platen of the press and the punch half is attached to the moving platen. The word punch refers to the entire upper half of the press tool but, more specifically, it refers to any so called male section which punches or forms metal within a female section, the female being known as die section. The sheet metal strip is passed between the two halves thru provisions given on the die section. Then the punch section presses or punches the metal into female die section for piercing a hole or for forming the shape on the metal sheet. The required force for this operation is supplied by the

press on which the tool is loaded by it's up and down movements. The stripper acts to separate the punch from strip after the piercing / punching is over. There are two types of strippers i.e. fixed stripper and Floating (spring loaded) stripper.
There are many components made in sheet metal which requires holes of different diameters with different sheet thickness combinations which are used in Automobiles, Electrical sectors, Home appliances etc. These holes for different usages can be achieved on sheet metals by drilling, any other machining operations, press tool operation etc.
JP 2003275828 discloses a punching method and device for workpiece with opposing flange attached, in which the machine punching through the outer edge of the flange and also cutting off the punched-out part has bisected two punch members that are abutted to each other so as to surround the workpiece, with the first and second punches formed respectively on the upper and lower ends of each punch member. These punches each combined with an upper and lower dies simultaneousiy punch out each outer edge of the first and second flanges of the workpiece. In addition, the cutters which are provided for each punch member close to the punches bisect each annular slug produced in the upper and lower sides.
US 5,555,760 discloses an actuator-punch assembly with forming die installation which is providing an arrangement for punching formed work-pieces which is readily programmable and which allows punching to be carried out in the same station.
When piercing holes are to be made for diameters more than two times material thickness, the stresses developed during punching and stripping are normal. The stress concentration is adverse while punching holes of diameters less than material thickness.
The strength of punches which pierce or forms the sheet depends on the design and requirement of the component. While piercing holes in metal for less than material thickness, it induces very heavy compressive stress in the punch which

leads them to breakage due to buckling of the punches. This buckling may be combined effect of stress and deflection induced on punch while piercing. Whenever multiple punches are loaded closer due to component requirement, it becomes more difficult to guide the punch with individual stripper bushes because of space constrain. One prior method used for piercing closely located holes was doing the cutting at different stages for easy loading of stripper bush for guidance and location of the punches. But in this case the too! length or cycle or number of tools required will increase which adds to the cost of the component.
Thus, there is a need to improve punch and die assembly, to avoid breakage of punch and providing guidance of punch closer to the height where it enters the material, for piercing and stripping for holes which are adjacent to each other and/or the diameter of holes is less than the material thickness. Guidance of punch closer to the cutting edge can be achieved by floating stripper bush concept. In the present invention, the stripper bush which holds and strips the metal sheet from punch is kept floating to provide maximum guidance for punch.
OBJECTS OF THE INVWENTION
An object of the present invention is to overcome the problems/disadvantages of the prior art.
Another object of the present invention is to provide an improved punch and die assembly for piercing holes of diameters less than material thickness with very close center distance at the same station.
Another object of the present invention is to provide guidance means for providing guidance to plurality of punch means.
Further object of the present invention is to provide stripping media means for stripping.

Yet, another object of the present invention is to provide an improved punch and die assembly with cost effectiveness.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided an improved
punch and die assembly for piercing and stripping for holes in metal sheets, said
holes being adjacent to each other having diameters less than material thickness
comprising:
plate means ;
guidance means;
pin means;
plurality of stripping medium means being located between plurality of said plate
means with plurality of said pin means;
plurality of punch means in proximity to said plurality of stripping medium means;
wherein said punch means having body diameter so as to strengthen said punch
means against a buckling load and a cutting diameter so as to give protection
against deflection while piercing;
wherein said guidance means adapted to provide guidance to plurality of said
punch means during punching operation;
whereby in said punching operation during downward stroke said plurality of
punch means being guided by said guidance means to pierce through the said
metal sheet wherein said metal sheet being firmly held by said stripping medium
means being compressed by the downward stroke and exerting force on the said
sheet metal means till said plurality of punch means complete piercing holes at
the same station and said stripping media means stripping said sheet metal
means during upward stroke.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig 1 illustrates an improved punch and die assembly showing ail the parts.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
According to the invention there is provided an improved punch and die assembly for piercing holes of less than material thickness which are very near to each other. The present invention is used to load single or multiple punches (1) for piercing holes of diameters less than material thickness with very close center distance at the same station. Due to multiple setting for optimizing loading pattern and component tolerance requirement, it is necessary to do punching of both the holes at the same station. The different setting calls for shift in centre of pressure, which needs to be near Press shank position. The multiple setting for the component in same piercing tool brought in the requirement of modular unit construction for adjusting the various punching with respect to centre of the pressure. Though modular units provided optimized piercing location but it leads to sub-press construction to de-link variation in centre of pressure from shank position of Press. In sub-press construction the punch assembly is not connected to Ram of the Press. Sub-press construction means no physical connection between top half of press tool and press. The advantage in this is out of parallesim between bolster and ram of press does not reflect adveserly on tool. The Ram of the Press while in operation, descends to hit the upper half of the press tool which are floating (mainly by coil springs) covering all the cutting punches.
According to preferred embodiment of the present invention, the guidance of the punch closer to the cutting edge is achieved through floating coiled spring-loaded stripper bush (4). The guidance is provided to body diameter (6-8 mm) as well as cutting diameter (2.4 - 2.6 mm) on 3.15 MS sheet components. There is a gradual section change by giving taper from cutting diameter to body diameter. The body diameter is made bigger to strengthen the punch (1) against the buckling load. The guidance at cutting diameter gives protection against the deflection caused while piercing. The stripper bush is made to rest on component material thickness providing guidance very close to cutting action. Jig grinding (expensive but most accurate tooling process) is done in stripper bush for body diameter as well cutting diameter to ensure perfect concentricity. The floating

action for stripper bush is provided by coil spring around the same. Piercing punches also ensure to have perfect concentricity while doing cylindrical grinding. As the two punches doing the piercing the holes are very near to each other, polyurethane is used as stripping media which gives very high load characteristics compared to limited deflection within the constrained space. Two horizontal plates are provided to accommodate the polyurethane pad (2) outside of the piercing punches with retainer pins (3) as shown in the figure 1. The stripping action is by polyurethane pad (2) construction and retained with pins outside the cutting action column, providing guidance at cutting as well as holding / body diameters to strengthen against buckling / high compressive load failures. The cutting clearance between the punch (1) and die (6) is kept double than norma! to reduce the load the load while piercing as higher cutting load results into buckling of the punches.
During downward stroke of the ram, first the stripper bush (4) touches the metal sheet (5). At this point the punch (1) does not touch the sheet. On further downward movement of punch half, the polyurethane pads (2) get compressed and they exert force on the sheet (5) and holds it till the punch (1) touches and pierce into the sheet (5) for making cutting operation. During cutting operation, the punches are closely guided by the stripper bush (4) loaded by polyurethane pads (2). After cutting operation completes and the ram goes up, the stripper bush (4) strips the sheet (5) out of the punches. Then the sheet (5) is fed further forward for another cycle and the process continues. Two retainer plates houses both the polyurethane pads (2) with help of stepped pin locked with quarter pins. The quarter pin facilitates quick change while changing location with this modular design concept. The collar of piercing punch (1) is kept above the upper retainer plate ensuring stripping action while returning of the Ram i.e. upward action, Mean while coil spring around stripper bush (4) lifts the modular unit to clear the feeder gap facilitating the component movement. The coil spring lifts the weight of punching sub assembly with stripper bush (4), retainer plates, poly-urethane pads (2), and retainer pin (3) with quarter pin to lock. The piercing diameter and guiding diameter are provided with the taper at the step for maximum strength. Due to taper there is a gradual increase in cross section and gives added strength to punch (1) while cutting especially item with hardness of HRc 58-60.

The junction of taper with piercing diameter is given radius to avoid stress concentration while manufacturing, heat treatment and press working.
ADVANTAGES OF THE INVENTION
1. This modular concept is easy for assembly and change in setting.
2. Punch breakage reduce drastically.
The invention has been described in a preferred form only and many variations may be made in the invention which will still be comprised within its spirit. The invention is not limited to the details cited above. The top plate, punch, polyurethane pad, retainer pins, retainer plates, stripper bush, sheet metal and die block as stated do not limit the scope of the present invention. The structure thus conceived is susceptible of numerous modifications and variations, all the details may furthermore be replaced with elements having technical equivalence. In practice the materials and dimensions may be any according to the requirements, which will still be comprised within its true spirit.
punch -1
Polyurethane pad-2 retaincr pin-3 stripper bush-4 sheet metal -5 die block-6

We Claim:
1. An improved punch and die assembly for piercing and stripping for holes
in metal sheets, said holes being adjacent to each other having diameters
less than material thickness comprising:
plate means ;
guidance means;
pin means;
plurality of stripping medium means being located between plurality of
said plate means with plurality of said pin means:
plurality of punch means in proximity to said plurality of stripping medium
means;
wherein said punch means having body diameter so as to strengthen said
punch means against a buckling load and a cutting diameter so as to give
protection against deflection while piercing;
wherein said guidance means adapted to provide guidance to plurality of
said punch means during punching operation;
whereby in said punching operation during downward stroke said plurality
of punch means being guided by said guidance means to pierce through
the said metal sheet wherein said metal sheet being firmly held by said
stripping medium means being compressed by the downward stroke and
exerting force on the said sheet metal means till said plurality of punch
means complete piercing holes at the same station and said stripping
media means stripping said sheet metal means during upward stroke.
2. Assembly as claimed in claim 1, wherein said punch means having body diameter ranging from about 6 mm to about 8 mm.
3. Assembly as claimed in claim 1, wherein said punch means having cutting diameter ranging from about 2.4 mm to about 2.6 mm.
4. Assembly as claimed in claims 1 to 3, wherein said punch means is substantially taper shaped profile.

5. Assembly as claimed in claims 1 to 4, wherein said punch means comprising collar means located above said upper plate means.
6. Assembly as claimed in claim 1, wherein said guidance means is a floating coiled spring-loaded stripper bush.
7. Assembly as claimed claim 6, wherein said stripper bush means having substantially appropriate concentricity in said body diameter and said cutting diameter.
8. Assembly as claimed in claim 1, wherein said stripping medium means are polyurethane pads.
9. Assembly as claimed in any of the preceding claims having cutting clearance between said punch means and a die block double than known cutting clearance.
10. An improved punch and die assembly for piercing and stripping for holes which are adjacent to each other and/ or for diameters less than material thickness as herein substantially described and illustrated with the accompanying drawings.