Mfg Tooling -09 Progressive dies.pdf

8/10/2019 Mfg Tooling -09 Progressive dies.pdf

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Nageswara Rao Posinasetti

February 6, 2012 1P N Rao

Power Presses

Operations deal with cutting and

forming of sheet metal. Press

Bed or Bolster plate Frame Ram or slide


Press types

Open back inclinable (OBI) press Straight side press Press brake Double action press Triple action press Knuckle press Hydraulic press

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Open back inclinable(OBI) press

Gap frame press

Fig. From Dr. John G. Nee ( revised by), Fundamentalsof Tool Design, Fourth Edition, 1998, SME

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Rated capacities of ANSI standard single-point,

gap-type mechanical power presses (metric)

Press capacity,kN (tons)

Rating distance abovebottom of slide stroke, mm

(in)

Non geared Geared20 (2.25) 0.5 (0.02) --

50 100 (5.6 11.2) 1.0 (0.04) 1.6 (0.06)

140 3 00 (15.7 33.7) 1.0 (0.04) 3.0 (0.12)

400 1000 (45 112) 1.6 (0.06) 6.0 (0.24)

1400 2240 (157 252) -- 6.0 (0.24)



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Single-action, straightside, eccentric shaft

mechanical press

Fig. From Dr. John G. Nee (r evised by), Fundamentalsof Tool Design, Fourth Edition, 1998, SME

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Press brakeUsed forbending

Fig. From Dr. John G. Nee ( revised by), Fundamentalsof Tool Design, Fourth Edition, 1998, SME

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Knuckle press

Coining operations

Fig. From Dr. John G. Nee (revised by), Fundamentalsof Tool Design, Fourth Edition, 1998, SME

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Hydraulic press

Slow operating cycleBetter adjustability


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Press Tool Operations

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Spring back

One of the principal concerns in a sheetmetal operation, is the spring back of themetal.

When the metal is deformed, it is firstelastically deformed and then plastically.

When the applied load is removed, theplastic component of the deformationremains permanently, but the elastic partsprings back to its original shape.

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Shearing action

The metal is brought to the plastic stageby pressing the sheet between twoshearing blades so that fracture is

initiated at the cutting points. The fractures on either side of the sheet

further progressing downwards with themovement of the upper shear, finallyresult in the separation of the slug fromthe parent strip.



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Fig. From Dr. John G. Nee (revised by), Fundamentalsof Tool Design, Fourth Edition, 1998, SMEP N Rao

Clearance When correct clearances are used, a clean

break would appear as a result of theextension of the upper and lower fracturestowards each other.

With an insufficient clearance additionalcut bands would appear before the finalseparation.

Ductile materials require smallerclearances and longer penetration of thepunch compared to harder materials.



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Fig. From Dr. John G. Nee (revised by), Fundamentalsof Tool Design, Fourth Edition, 1998, SMEP N Rao


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Clearance

This clearance depends essentially on the

material and thickness of the sheet metal. This clearance can be approximated per

side C, as

where, t = sheet thickness, mm and = material shear stress, MPa


Clearance

BlankingIt is a process in which the punchremoves a portion of material from thestock which is a strip of sheet metal of the necessary thickness and width. Theremoved portion is called a blank and isusually further processed to be of someuse, e.g., blanking of a pad lock key.


Clearance

Piercing Also sometimes called punching, thepiercing is making holes in a sheet.

It is identical to blanking except of thefact that the punched out portion comingout through the die in piercing is scrap.

Normally a blanking operation willgenerally follow a piercing operation.



Clearances as percentage of stock thickness

Material Round Other contours

Soft aluminium < 1 mm

> 1 mm

Hard aluminium

Soft copper alloys

Hard copper alloys

Low carbon steel

Hard steel

Silicon steel

Stainless steel

2

3

4 to 6

2

4

2

3

3

4 to 6

3

5

5 to 8

3

5 to 6

3

5

4 to 5

5 to 8



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Punching/Shearing Force The force required to be exerted by the

punch in order to shear out the blank fromthe stock can be estimated from theactual shear area and the shear strengthof the material.

P = L t where P = punching force, N = shear strength, MPa L = Length of cut



http://www.youtube.com/watch?v=Z26pEs8bocs&feature=rel

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Stripper Due to the release of the stored elastic

energy in the stock left on the die, thestock tends to grip the punch as thepunch moves upward.

This necessitates the use of a stripper toseparate the punch from the stock.

The force required for stripping dependsbesides the material, on other factorssuch as the position and size of thepunched hole.


Stripping force Thicker materials or small hole in the

middle of a strip require more strippingforce than thin material or a hole towardsone of the edges.

A punch which has smooth side wallswould strip very easily.

Similarly more effort is required to strippunches that are close together.


Stripping force A general estimate of the stripping force

may vary from 2.5 to 20% of the punchforce but 5 to 10 percent is good for most

of the applications. Stripping force, Ps = 1.5 L t L = perimeter of cut, in t = stock thickness, in Ps =Stripping force, tons



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Stripping force A general estimate of the stripping force

may vary from 2.5 to 20% of the punchforce but 5 to 10 percent is good for mostof the applications.

Stripping force in tons, Ps = 20,600 L t L = perimeter of cut, mm t = stock thickness, mm Ps =Stripping force, kN


Shear To reduce the required shearing force on

the punch, for example to accommodate acomponent on a smaller capacity punchpress, shear is ground on the face of thedie or punch.

The effect of providing shear is todistribute the cutting action over a periodof time depending on the amount of shearprovided.


Shear Thus the shear is relieved of the punch or

die face so that it contacts the stock overa period of time rather thaninstantaneously.

It may be noted that providing the shearonly reduces the maximum force to beapplied but not the total work done inshearing the component.


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Angular clearance In the shearing operation, first the

material is elastically deformed and thenplastically, and finally removed from thestock strip.

After the final breaking, the slug willspring back due to the release of storedelastic energy.

This will make the blank to cling to the dieface unless the die opening is enlarged.


Angular clearance The draft provided depends on the

material, thickness and shape of the stockused. For thicker and softer materials,generally higher angular clearances areprovided.

The normal value is from 0.25 to 0.75 degper side but occasionally a value as highas 2 deg may be used.


Angular clearance The die opening increases after every

sharpening of the die because of theprovision of the angular clearance.

So, to maintain the die size as per thedesign, the angular clearance is providedin the die opening along with a straightportion called as die land or cutting land.


Angular clearance The length of the cutting land is about 3

mm for sheets which are less than 3 mmthick.

For greater thicknesses, die land same asthe material thickness has been found tobe a good practice.



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Sheet Metal Die Design

Progressive dies,

Compound dies, and Combination dies.

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http://www.youtube.com/watch?v=q5Wlr6Co5SI

http://www.youtube.com/watch?v=rx5p-2uZ6jc&feature=related

http://www.youtube.com/watch?v=sJnt_djvclo&feature=related


Screws and Dowels

Dowels alignment Screws Joining Two dowels are required for proper

alignment


Die Construction

Screws and dowels


Die Block

The die block size essentially depends onthe workpiece size and stock thickness.Though sometimes, the type of blank

contour and the type of die may alsoinfluence the choice of die block size.

A number of thumb rules are availablewhich are essentially based on practicalexperience.



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Die Block

The die block should

be able to withstandthe impact of thepunch striking thematerial.

The value of A shouldbe 1.25 times the diethickness for smallerdies and 1.5 to 2.0times for larger dies.


Sectional die construction


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Fig. From P. N. Rao, Manufacturing Technology, TataMcGraw Hill, 1998.


Fig. From P. N. Rao, ManufacturingTechnology, Tata McGraw Hill, 1998.

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Punches

Plain punches Pedestal punches Punches mounted in punch plates

Perforator punches Quill punches Backup plate Slug ejection



Plain Punch

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Pedestal Punch



Pedestal Punch

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Punches mounted in punch plates



Punches mounted in

punch plates

< 1 in

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Perforator Punch

< 1 in



Perforator Punch

< 1 in

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Method of preventing

rotation of punch in a

punch plate



Quill Punch

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Slug ejection

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Pilots

The pilots in progressive dies are used inorder to bring the stock into the correctposition for the succeeding blanking orpiercing operations.


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Pilots

The fit between the pilot size and the

pierced hole determines the accuracy ofthe component produced. Too tight a fit between the pilot and

pierced hole results in friction which wouldspoil the component as also, there wouldbe excessive pilot wear.


Pilots

0.050 to 0.100 mm average work,

0.025 to 0.050 mm close work, and 0.013 to 0.018 mm high precision

work.


Pilots

Stock thickness,

(mm)

0.130 0.400 0.750 1.500 3.000

Pilot diameter, mm

3.0

4.5

6.0

7.5

9.0

12.5

20.0

0.05

0.08

0.10

0.10

0.10

0.13

0.13

0.08

0.13

0.20

0.20

0.20

0.25

0.25

0.13

0.20

0.25

0.25

0.30

0.40

0.40

--

0.25

0.40

0.40

0.50

0.75

0.75

--

--

--

--

0.75

0.75

1.00


Pilots

The indirect pilot is not attached to thepunch and is on its own. It enters apreviously pierced hole.

The indirect pilot should be positioned atsome distance away from the blankingpunch to provide for the necessary stocksupport during the entry of the pilot.


Pilots

Solid pilots are sometimes harmful as incase of misfed stocks whose thickness isabove 1.5 mm.

The pilot in such a case would break. Though, with thinner sheets, it may

simply pierce the sheet with no damage tothe die.



Hence for thicker stocks, it is desirable to

use spring loaded pilots

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Stripper and Stock Guide

The stripper removes the stock from the

punch after a piercing or blankingoperation. The function of a stock guide is as the

name implies, guide the stock through thevarious stations.

The strippers are classified into two types:channel or box stripper and spring oper-ated stripper or pressure pad.



Channel

Stripper

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Die Stops

Stops are essentially used for stopping thestock while feeding, so as to register thecorrect position under the punch.

While using in conjunction with a pilot, thestrip is stopped at a position from which,the pilot brings it to the registry position.



Solid Stop

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Documents

Mfg Tooling -09 Progressive dies.pdf