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Chapter 6 SCRAP – STRIP LAYOUT FOR BLANKING

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Chapter 6

SCRAP – STRIP

LAYOUT FOR

BLANKING

Chapter 6

SCRAP – STRIP

LAYOUT FOR

BLANKING

SCRAP – STRIP LAYOUT FOR BLANKING

Scrap Layout:In designing parts to be blanked from strip material, economical stock utilization is of high importance. The goal should be at least 75 per cent utilization. A very simple scrap-strip layout is shown in the next slide

Scrap Allowance:A scrap-strip layout having insufficient stock between the blank and the strip edge, and between blanks, will result in a weakened strip, subject to breakage and thereby causing misfeeds. Such troubles will cause unnecessary die maintenance owing to partial cuts, which defect the punches, resulting in nicked edges. The following formulas are used in calculating scrap-strip dimensions for all strips over 0.8 mm. thick

SCRAP – STRIP LAYOUT FOR BLANKING

This Fig shows: Single scrap-strip layout: t is the stock thickness; B is the space between part and edge of strip; C is the lead of the die, i. e., the distance from a point to the corresponding point on the next point; L is the length of the part; H is the part width; W is the width of the scrap strip.

t = specified thickness of the material B = 1.25 t when C is less than 64 mmB = 1.5 t when C is 64 mm or longer C = L + B, or lead of the die

Allowances for one-pass layouts

SCRAP – STRIP LAYOUT FOR BLANKING

Minimum Scrap-Strip Allowance: If the material to be blanked is 0.6 mm thick or less, the formulas above should not be used. Instead, dimension B is to be as follows:

Strip width W Dimension B0 - 75 mm 1.3 mm76 – 150 mm 2.4 mm150 – 300 mm 3.2 mmOver 300 mm 4.0 mm

Allowances for one-pass layouts

SCRAP – STRIP LAYOUT FOR BLANKING

View AFor work with curved outlines, B = 70 per cent of strip thickness t.

View BFor straightedge blanks: where C is less than 64 mm, B = t, where C is 64 mm to 200 mm, B = 1.25 t, where C is over 200 mm, B = 1.5 t.

Allowances for one-pass layouts

SCRAP – STRIP LAYOUT FOR BLANKING

View CFor work with parallel curves, use the same formulas as for view B.

View D For layouts with sharp corners of blanks adjacent, B = 1.25 t.

Allowances for one-pass layouts

SCRAP – STRIP LAYOUT FOR BLANKING

View ASingle-row layout intended for two passes through the die B = 1.5 t

View BDouble-row layout of blanks with curved outlines: B = 1.25 t

View CDouble-row layout of parts with straight and curved outlines B = 1.25 t

Allowances for one-pass layouts

SCRAP – STRIP LAYOUT FOR BLANKING

Allowances for one-pass layouts

Percentage of Stock Used

If the area of the part is divided by the area of the scrap strip used, the result will be the percentage of stock used.

If A = total area of strip used to produce a single blanked part, thenA = Pitch x Strip Width, and a = area of the part = L H.If P = 11.5 mm and W = 32 mm then A = 11.5 X 32 = 368 mm²If L X 9.5 mm and H = 27 mm then a = 29.5 X 27 = 256.6 mm²Percentage of stock used:

a  256.5  =  = 70% approx. A 368

EVOLUTION OF A BLANKING DIE

•Die Set Selection

•Die Block Design

•Punch Design

•Stripper Design

•Stock Stops

EVOLUTION OF A BLANKING DIE

Part to be blanked

EVOLUTION OF A BLANKING DIE

Die Set Selection

A commercially available standardized two-post die set with

150 mm overall dimensions side-to-side and front-to-back allows the

available 76 mm. wide stock to be fed through it. It is large enough

for mounting the blanking punch on the upper shoe (with the die

mounted on the lower shoe) for producing the blank shown in the

previous slide, since the guideposts can be supplied in lengths of from

100 to 225 mm.

Since the stock, in this case was available only in a width of 76

mm the length of the blanked portions extended across the stock left a

distance between the edges of the stock and the ends of the blank of 6

mm or twice the stock thickness; this allowance is satisfactory for the

3.2 mm stock.

EVOLUTION OF A BLANKING DIE

Die Block Design

By the usual ‘rule-of thumb’ method, die block thickness

should be a minimum of 20 mm for a blanking perimeter up to 75 mm

and 25 mm for a perimeter between 75 and 100 mm. For longer

perimeters, die block thickness should be 32 mm. Since the perimeter

of the blank is approximately 178 mm a die block thickness of 38 mm

was specified, including a 6 mm grinding allowance.

There should be a margin of 32 mm around the opening in the

die block; its specified size of 150 x 150 mm allows a margin of 45

mm in which four M10 cap screws and dia. 10 mm dowels are located

at the corners 20 mm from the edges of the block

EVOLUTION OF A BLANKING DIE

Punch Design

The shouldered punch (57 mm) long is held against a 6 mm

thick hardened steel backup plate by a punch plate 20 mm thick)

which is screwed and doweled to the upper shoe. The shut height of

the die can be accommodated by a 32-ton (JIC Standard) open-back

inclinable press, leaving a shut height of 240 mm. For the conditions

of this case study, shear strength S = 42 kg/mm², blanked perimeter

length L = 178 mm approx. and thickness T = 3.2 mm.

From the equation P = S L T

The pressure P = 42 kgs. X 178 mm X 3.2 = 23.92 tons.

This value is well below the 32-ton capacity of the selected press.

The shut height is 178 mm less the 1.6 mm travel of the punch into the

die cavity.

EVOLUTION OF A BLANKING DIE

Stripper Design The stripper that was designed is of the fixed type with a channel or slot

having a height equal to 1.5 times stock thickness and a width of 80 mm to allow for

variations in the stock width of 75 mm. The same screws that hold the die block to

the lower shoe fasten the stripper to the top of the die block.

If, instead of 3.2 mm stock, thin (0.8 mm) stock were to be blanked, a

spring-loaded stripper would firmly hold the stock down on top of the die block and

could, to some extent, flatten out wrinkles and waves in it.

A spring-loaded stripper should clamp the stock until the punch is withdrawn from

the stock. The pressure that strips the stock from the punch on the upstroke is

difficult to evaluate exactly. A formula frequently used is

Ps = 2.5 x L x t kgs.

Where Ps = stripping pressure, in kgs., L = perimeter of cut, in mm.

t = stock thickness, in mm.

EVOLUTION OF A BLANKING DIE

Stock StopsThe pin stop pressed in the die block is the simplest method for

stopping the hand-fed strip. The right-hand edge of the blanked

opening is pushed against the pin before descent of the ram and the

blanking of the next blank. The 4-8 mm depth of the stripper slot

allows the edge of the blanked opening to ride over the pin and to

engage the right-hand edge of every successive opening.

The design of various types of stops adapted for manual and

automatic feeding is covered in a preceding discussion.

EVOLUTION OF A BLANKING DIEDie Design

A Blanking Die

EVOLUTION OF A PROGRESSIVE BLANKING DIE

•Part Specification

•Scrap-Strip Development

•Press Tonnage

•Calculation of the Die

•Calculation of Punches

•springs

•Piloting

•Automatic Stops

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Part Specification

Linkage case cover

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Product study

Step 1, Part Specification

1. The production is of medium class; therefore a second-class die

will be used.

2. Tolerances required: Except for location of the slots, all

dimensions are in fractions. The slot locations, though specified in

decimals, are not very close. Thus a compound die is not

necessary; a two or three-station progressive die will be adequate.

3. Type of press to be used: Available for this production are presses

of 5-ton, 8-ton, or 10-ton capacity, with a shut height of 175 or

200 mm.

4. Thickness of material: Specified as 32 mm standard cold rolled

steel.

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Step 2, Scrap-Strip Development

B = 1.25t = 1.25 x 3.2 = 4 mmW = H + 2B = 60 + 8 = 68 mmC = L + B = 60 + 4 = 64 mm

Scrap-strip development for part

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Step 3, Press Tonnage It is now in order to determine the amount of pressure needed. Only the

actual blanking in the fourth stage need be calculated, since the work in the first

three stages will be done by stepped punches.

From Table, the shear strength S of cold rolled steel is 40 kgs/mm². The length L

of the blanked perimeter equals 60 x 4 = 240 mm. The depth of cut (stock

thickness t) equals 3.2 mm.

From the equation P = S L t

P = 40 kgs./mm² x 240 mm x 3.2 mm

= 30,720 kgs. Or 30.7 tons.

An available press only a 30-ton press with a 190 mm shut height and a

50 mm stroke. This press is selected. The bolster plate is found to be 300 mm

deep, 140 mm from centerline of ram to back edge of bolster, and 600 mm wide.

Shank diameter is 64 mm.

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Step 4, Calculation of the Die The die. The perimeter of the cut equals 240 mm and therefore the

thickness of the die must be 25 nm. The width of our scarp-strip opening is 60 mm

with 32 mm extra material on each side of the opening, it will be 60 mm + 64 mm =

124 mm or 130 mm width. The distance from the left side of the opening in stage 4

to the edge of the opening in stage 1 equals 3 C + 30 + 6 = 192 + 30 + 6 = 228 mm

and plus 62 mm = 290 mm or 296 mm long.

Therefore the die should be 25 x 130 x 296 mm long.

The die plate. As a means of filling in between the die and the die shoe, a

die plate of machinery steel is used. To secure the die plate to the die shoe M12

cap screws and dowels are used. A minimum of twice the size of the cap screw for

the distance from the edge of the die to the edge of the die plate is needed, which

will equal 25 mm. Twice this distance = 50 mm and 50 mm added to the size of the

die will result in a die plate of 25 x 180 x 346 mm. Figure in the next slide shows

the die and die plate fitted together and with the holes, which show the sharpening

portion and the relief portion.

EVOLUTION OF A PROGRESSIVE BLANKING DIE

The fitting of the die and die plate. Note the shear on stage 4,also the straight edge and the relief at die opening.

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Step 5, Calculation of Punches

Good practice requires 10 per cent of the metal thickness to be

removed from the basic dimension of the blanking punch. This same

value is used on the die opening, since holes are to be pierced in the

blank. The clearance rule will be applied to the die opening in Stages

1, 2, and 3, and to the punch in Stage 4 (see fig.).

For Stage 4: Blank to be 60 mm. square,

Stock thickness = 3.2 mm; 10% = 0.32 mm.

Punch = 60 – 0.32 = 59.68 mm.

Therefore the die opening will equal 60.01 to 60 mm and the

punch will equal 59.68 to 59.67 mm.

EVOLUTION OF A PROGRESSIVE BLANKING DIE

For Stage 2: Slot to be 8 mm wide by 34 mm long .

Die = 8 + 0.32 + 8.32 mm long = 34.32 to 34.33 mm.

Punch will equal 3.99 to 8.00 mm. wide, and 33.99 to 34.00 mm.

long.

The punch and the die opening will have straight sides for at

least 3 mm for sharpening, and then will have a taper relief of about

1½ deg. to the side. Figure 3-43 also shows a 3 mm shear for the die

at Stage 4 and a 3 mm shear for the punches of Stage 2, and also the

stepped arrangement of the punches for all stages.

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Illustrates calculation of clearance. Shear on punches, Die andStepped arrangement of punches to reduce cutting pressure.

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Step 6, springs

A solid stripper plate can be used for this job.

Step 7, Piloting

Figures 3in the previous slide illustrate the arrangement for

piloting. In this case it is direct piloting. However, if the part did not

have a center hole, and the slots and other holes were too small,

indirect piloting would have to be provided.

Step 8, Automatic Stops

Finger stops, illustrated in Fig., will act as stops when a new

scrap strip is being inserted but, after that, an automatic spring drop

stop must be used to halt the scrap strip. Figures in the next slide

illustrate details of the completed drawing of the die.

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Top and front sectional view of completed die.