chapter 3 blanking die designing. 3.1 stamping machine and die

Chapter 3

Blanking Die Designing

3.1 Stamping machine and die

Its configuration and driving principle are shown schematically in Fig. 3.1.

1. Stamping machine The mechanical press is the most widely used stamping equipment.

The belt wheel 9 is located on axis loosely, which is rotated by the electromotor 4 through a belt deceleration system. When stepping on the pedal 12, the clutch 8 is connected. The slide block 11 driven by the crankshaft 7 and the connecting rod 5 moves downward from the highest limited position along the guide 2 of the press, and to carry out the stamping process.

1-table 2-guide 3-frame 4-electromotor 5-connecting rod 6-brake 7-crankshaft 8-clutch 9-belt wheel 10-belt 11-slide 12-pedal Fig. 3.1 Outline of a mechanical press and its driving principle

If the pedal is kept locking, continuous stamping can be carried out. The die set includes the punch and the die which are fixed under the lower end of the slide 11 and on the table 1 respectively.

If the pedal is released rapidly after stepping on, the clutch is disconnected. The slide would stop at the highest position after stamping under the action of the brake 6, thus a single stamping process is fulfilled.

The maximum force is occurred at a certain distance before the slide reaches the lowest limited position, or at a certain angle before crankshaft rotates to the lowest limited position. The deformation force demanded in the stamping process must be smaller than the nominal pressure.

The nominal stamping pressure, in unit of N, is the allowable maximum force acting on the slide.

2. Punching die

The structural design of the die affects the workpiece quality as well as the production cost directly. Therefore, the investigation on the structure and characteristics of the punching die is crucial to realize the blanking production and develop the blanking technique. The variety of the workpieces results in the variety of the structures of the punching dies. The blanking dies can be classified according to different features.

It can be classified as punching, blanking, trimming, cutting-off, parting, lancing, shaving and fine blanking dies.

(1) Classification according to the process property

It can be classified as single (simple), progressive (continuous) and compound dies.

(2) Classification according to the process combination

(4) Classification according to the stripping equipment

(3) Classification according to the guiding pattern between the punch and die

It can be classified as guideless die and dies with guide plate, guide pillar and guide tube.

It can be classified as dies with fixed stripper and elastic stripper.

It can be classified as dies with fixed stop pin, moved stop pin, pilot and pilot punch.

(5) Classification according to the stop gauge

(6) Classification according to the material of the punch and die

It can be classified as carbide, steel bonded carbide, steel strip, rubber pad and polyurethane pad dies.

3. Typical structure of blanking die

(1) Simple die

The die that only one process can be carried out in one press stroke is called simple die.

Its structure is simple ( see Fig. 3.2 ), so it can be easily manufactured. It is applicable to small batch production.

Fig. 3.2 Simple die

1-stop pin 2-guide bushing 3-guide pin 4-bolt 5-die shank 6-pin 7-fixed plate 8-upper bolster 9-punch 10-stripper 11- stock guide 12-die 13-lower bolster

(2) Progressive die

The die that several blanking processes are carried out at different positions of the die in one press stroke is called progressive die, as shown in Fig. 3.3.

In the operation, the locating pin 2 aims at the locating holes punched beforehand, and the punch moves downwards to punch by punch 4 and to blank by punch 1, thus the workpiece 8 is produced. When the punch returns, the stripper 6 scrapes the blank 7 from the punch, the blank 7 moves forward one step and then the second blanking begins. Above steps are repeated continually. The step distance of the blank is controlled by a stop pin.

1-blanking punch 2-locating pin 3-blanking die 4-punching punch 5-punching die 6-stripper 7-blank 8-workpiece 9-waste

Fig. 3.3 Progressive die for blanking and punching

Examples of multi-process combined types of progressive die

• 1 Punching and blanking• 2 Punching and cutting (a)• 3 Punching and cutting (b)• 4 Deep drawing and blanking• 5 Punching, bending and cutting• 6 Punching, flanging and blanking (a)• 7 Punching, cutting and bending• 8 Punching, coining and blanking• 9 Punching, flanging and blanking (b)• 10 Deep drawing, punching and blanking

Punching and blanking1-lower bolster 2-die 3-fixed stripper 4-punch plate 5-Bolster plate

6-upper bolster 7–blanking punch 8-punching punch 9-waste10-blank 11-locating pin 12-workpiece

Punching and cutting (a) 1-lower bolster 2-die 3-cutting punch 4-upper bolster

5–punching punch 6-bolster plate 7-punch plate 8-fixed stripper 9-blank 10-waste 11-workpiece

Punching and cutting (b) 1-lower bolster 2-stop block 3-fixed stripper 4-punch plate

5-bolster plate 6-upper bolster 7-cutting punch 8–punching punch 9–punching punch 10-blank 11-die 12-waste

Deep drawing and blanking 1-lower bolster 2-die 3-elastic stripper 4-elastic component

5-punch plate 6-bolster plate 7-upper bolster 8–punching punch 9-secondary drawing punch 10-first drawing punch 11-ejector block

12-blank 13-elastic component 14-workpiece

Punching, bending and cutting 1-lower bolster 2-die 3-punch-die 4-upper bolster 5-elastic component

6-stripping block 7-waste 8–punching punch 9-blank 10-blank holder 11-elastic component 12-workpiece

Punching, flanging and blanking 1-lower bolster 2-die 3-workpiece 4-elastic stripper

5-elastic component 6-punch plate 7-bolster plate 8-upper bolster9–blanking punch 10-flange punch 11-punching punch 12-waste

13-ejecting plate 14-elastic component

Punching, cutting and bending 1-lower bolster 2-bending punch 3-punch-die 4-upper bolster

5-punching punch 6-punching punch 7-bolster plate 8-punch plate 9-fixed stripper 10–blank 11-die 12-waste 13-workpiece

Punching, coining and blanking 1-lower bolster 2-die 3-workpiece 4-elastic stripper 5-elastic component

6-punch plate 7-bolster plate 8-upper bolster 9-blanking punch 10-coining punch 11-punching punch 12–blank 13-waste

Punching, flanging and blanking 1-lower bolster 2-elastic component 3-flanging punch 4-workpiece 5-punch-die 6-upper bolster 7-punching punch 8-elastic component

9-elastic stripper 10–blank 11-waste 12-die

Deep drawing, punching and blanking 1-lower bolster 2-elastic component 3-die 4-ejector block 5-elastic stripper 6-elastic component 7-punch plate 8-bolster plate 9-upper

bolster 10-first drawing punch 11-secondary drawing punch 12-punching punch 13-blanking punch 14 punching die 15-waste

16-locating pin 17-bolster plate 18-workpiece

(3) Compound die

The die that several processes are carried out at the same die position in one press stroke is called compound die, as shown in Fig. 3.4.

The main characteristic of the compound die is that the part 1 is both the punch and the die. The outside circle of the punch-die 1 is the cutting edge of the blanking punch, while the inside hole is a deep drawing die.

When the slide moves downwards along with the punch-die 1, the blanking process is done first by the punch-die 1 and the blanking die 4, the blanked workpiecet is pushed by deep drawing punch 2, and then the deep drawing die moves downwards to carry out deep drawing operation. The ejector 5 and the stripper 3 push the deep drawn workpiece 9 out of the die when the slide returns. The compound die is suitable for mass production and high accuracy blanking.

1-punch-die 2-deep drawing punch 3-press plate (stripper) 4- blanking die 5-ejector 6-blank 7-stop pin 8-blank 9-deep drawn workpiece 10- finished part 11-waste

Fig. 3.4 Compound die for blanking and deep drawing

Examples of multi-process combined types of compound die

1 Punching and blanking

2 Punching, bending and cutting

3 Cutting and bending

4 Blanking, deep drawing and punching

5 Cutting, bending and punching

6 Blanking, deep drawing, punching and flanging

7 Blanking and deep drawing

8 Punching and flanging

Punching and blanking1-lower bolster 2-elastic component 3-blank holder 4-blanking die

5-punching punch 6-stripping plate 7-workpiece 8-blank 9-punching waste 10-punch-die

Punching, bending and cutting1-lower bolster 2-cutting die 3-bending punch 4-punching punch

5- knockout rod 6-workpiece 7-punching-bending die 8-waste

Cutting and bending1-lower bolster 2-punch 3-workpiece 4-upper bolster

5-punch-die 6-konckout plate 7-fixed stripper 8-blank 9-cutting die

Blanking, deep drawing and punching1-lower bolster 2-blanking die 3-fixed stripper 4-punch-die

5-upper bolster 6-punching punch 7-konckout plate 8-workpiece 9-waste 10-punch-die 11-blank holder 12-ejector rod

Cutting, bending and punching1-lower bolster 2-punch-die 3-punch-die 4-upper bolster

5-punching punch 6-konckout plate 7-fixed stripper 8-blank

9-workpiece 10-cutting die

Blanking, deep drawing, punching and flanging1-lower bolster 2-blanking die 3-fixed stripper 4-punch-die

5-upper bolster 6-punching punch 7-knockout rod 8-knockout plate 9-punch-die 10-workpiece 11-waste 12-blank holder 13-ejector rod

Blanking and deep drawing1-lower bolster 2-blanking die 3-fixed stripper 4-punch-die

5-upper bolster 6-knockout plate 7-workpiece 8-waste 9-drawing punch 10-blank holder 11-ejector rod

Punching and flanging1-lower bolster 2-elastic component 3-blank holder 4-flanging die 5-knockout plate 6-punching punch

7-punch-die 8-workpiece 9-waste

According to the preceding analysis on the types of the die structure, the die parts can be classified into two categories by its function.

These parts take part in the performance of the technological process and contact with the blank directly, which include the working parts (performing the stamping forming directly); locating parts (guaranteeing an excellent guide for the blank feeding, controlling the feed distance of the blank, obtaining a correct position of the blank); stripping, holding and ejecting parts (pressing the blank, knocking out the workpiece or waste from the die).

1. Technological structure parts

These parts neither take part in the technological process nor contact with the blank directly, which include the guiding parts (guaranteeing the correct relative position between the punch and the die to improve the quality of the blanking workpiece); the supporting and holding parts (installing the die to the press for transferring the working pressure); and the fastening parts (connecting and fastening separate parts of the die), etc.

2. Assistant structure parts

The classification of the die is shown in Fig. 3.5.

Fig. 3.5 Classification of the die

1 Punch

Working parts 2 Die

3 Punch-die

4 Stop pin and pilot

5 Stock guide (Guide rule)

Locating parts 6 Locating pin (Locating plate)

7 Side guide

8 Kicker

Stripping, holding and knocking out par

9 Stripper

10 blank holderts

11 Ejector

12 Knock out

Technological structure parts

Fig. 3.5 Classification of the die

13 Pillar

14 Guide bushingGuiding parts

15 Guide plate

16 Guide tube

17 Upper and lower bolster

18 Fastening plate of the punch and die

Supporting and clamping parts 19 Shank

20 Bolster plate

21 Limite

r

22 Bolt

Fastening and other parts 23 Pin

24 Others

Assistant structure parts

3.2 Design of the main blanking die parts and their

standards

3.2 Design of the main blanking die parts and their

standards

3.2.1 Significance of standardization for blanking die

The National Standard of the Cold Stamping Die of the People’s Republic of China (abbr. as National Standard bellow hereinafter) are GB2851~2875-90.

These standards include: die carrier and their technological conditions, die parts and their technological conditions, and typical combination of the die and their technological conditions.

The adoption of these standards, may simplify the die design, improve die quality, save plenty of human labour and material resources, and decrease die cost.

After adopting the standard die carriers or the typical combination, only the working parts and the locating position are needed to be specifically designed.

During die designing, the standardized parts should be selected correctly and the non-standard parts should be designed with reference to the standard parts. The standard of the die in this book refers to the National Standard.

The standard die carriers are composed of the upper bolster, lower bolster, pillar, guide and bushing, etc.

There are totally twelve types of standard die carriers. The sliding guide die carriers includes diagonal pillar die carrier, back pillar die carrier, narrow form back pillar die carrier, center pillar die carrier, circular form center pillar die carrier and four pillars die carrier. The ball-bearing die carriers includes diagonal pillar die carrier, center pillar die carrier, four pillars die carrier; die carrier with spring guide plate diagonally arranged, and die carrier with spring guide plate in the middle.

The die carrier with two pillars is usually adopted.

The back pillar die carrier can be used to blank the wide strip plate for convenience of plate feeding as well as operation.

The eccentric moment occurred during blanking may deflect the die and reduce die life and workpiece quality. Therefore, it is suitable for the medium and small workpieces with medium complexity and ordinary tolerance demand.

So the die life can be lengthened and the workpiece quality improved. But the width of the strip plate is limited by the distance between pillars, therefore such die carriers are adopted in the compound die or for the workpiece with a high tolerance demand.

For the center and diagonal pillar die carriers, the die deflexion doesn’t occur during stamping.

The diagonal pillar die carrier is usually adopted in the progressive die.

In order to prevent the upper bolster to be turned 180° in a mistake, the diameters of the two pillars and bushings of the center or diagonal die carrier are not the same. Their difference is generally 2~5mm.

For the workpiece with a high tolerance demand, the four pillars die carrier is usually adopted.

The typical combination of the cold blanking die consists of the die carrier, punch plate, die plate, stripping device, guiding device, bolt and pin, etc.

According to the situation of the die carrier, the guiding, stripping and stock guiding methods, and the process combination, etc., there are fourteen typical combinations of the cold blanking die in the National Standard.

3.2.2 Design of the working parts and their standards

There are three kinds of standard punches with circular form in the National Standard, as shown in Fig. 3.6.

Which kind of punches should be selected is determined by the dimension d in the working portion.

1. Punch

(a) A type circular punch (b) B type circular punch (c) quick-change circular punch

Fig. 3.6 Standard punches

A type circular punch is adopted for d=1.1~30.2mm, B type for d=3.0~30.2mm, quick-change circular punch for d=5~29mm.

To fix the circular punch on the fastening plate, the hole-basis transition fit m6 is adopted for A and B types of circular punches, and the hole-basis clearance fit h6 for the quick-change circular punch.

The length L of punch should be determined by the die structure. When using the fixed stripper and stock guide (see Fig. 3.7), the length L of punch is:

(3-1)

where H1 is the thickness of the fastening plate in

mm; H2 is the thickness of the stripper in mm; H3 is

the thickness of the stock guide in mm; H is the additional length mainly determined by the depth of punch entering into the die (0.5~1 mm), the total wearing repairing amount (10~15 mm) and the safe distance between the stripper and punch plate when the die is in the shut state (15~20 mm).

1 2 3L = H + H + H + H

Fig.3.7 Determination of the punch length

When the distance between punches in the same die is very small, a riveting structure can be used for the circular punch (Fig. 3.8a); a jacket structure is usually used for the small hole punching (Fig. 3.8b); the quick-change type is used for the small punch which is vulnerable to damage during blanking (Fig. 3.8c);

The non-standard punches and their fastening patterns are shown in Fig. 3.8.

Fig. 3.8 Non-standard punches and their fastening patterns

(a) (b) (c) (d)

(e) (f) (g)

the non-circular punch, with is a bit large size, can be fastened to the die bolster directly by bolts, pins or bolts and location groove instead of fastening plate (Fig. 3.8d); a circular step structure can be used with a stop gauge in the fastening portion, when the working portion of the punch is non-circular (Fig.3.8e).

The small punch can also be fastened to the punch plate by low-melting alloy, inorganic or epoxy resin adhesive, as shown in Fig 3.9.

(a) by epoxy resin adhesive (b)by low-melting alloy (c) by inorganic adhesive

Fig. 3.9 Other methods to fix the punch

(a) (b) (c)

In general, the strength of punch is enough, so it’s unnecessary to be checked. But in the case of quite slender punch, or punch with small cross-section for thick blank, both the compressive capacity and the longitudinal bending strength of

the punch should be checked out.

In blanking, the compressive stress σc of the

punch should be less than the allowable compressive stress [σc] of the punch:

(1) Check the compressive capacity

c c＝P

F (3-

2) For circular punch, substituting P=πdtτ into Equation (3-2), we obtain:

4

c

d

t

(3-3)

where, P is the blanking force in N; F is the minimu

m area of the cross-section of punch in mm2; d is th

e punch diameter in mm; t is the blank thickness in

mm; τ is the shearing strength of material in MPa;

[σc] is the allowable compressive stress of punch in

MPa, which is determined by the heat treatment of

material and the structure of die.

In blanking, the stability of punch subjected to axial compression is usually checked by the Euler formula.

(2) Check the instability bending stress

The punch with a guiding device is equivalent to a bar with one end fixed and the other end hinged supported. Considering the safety coefficient n, the allowable blanking force without instability bending is:

2

2

2

EJP

nl

(3-4)

The maximum length of the punch without instability bending is:

where P is the blanking force in N; E is the elastic modulus of punch material, E = 2.2×105 MPa for normal die steel; n is the safety coefficient, for quenched steel, n=2~3; J is the inertia moment to the axis of the minimum cross-section of punch, for the circular punch with diameter d, J= πd4/64 ≈ 0.05 d4 mm4; l is the length of punch in mm.

2

max

2

EJl

nP

(3-5)

Substituting the values of E, n and J of the circular punch to Equation 3.5, we obtain the maximum length on condition that the instability doesn’t occur for the circular punch with a guiding device is:

Similarly, the maximum length for the noncircular punch with a guiding device on condition that the instability doesn’t occur is:

2

270mac

dl

P (3-6)

max 1200J

lP

(3-7)

The punch without a guiding device is equivalent to a bar with one end fixed and the other end freed. Considering the safety coefficient n, the allowance maximum blanking force without instability bending is:

Similarly, the maximum length for the punch on condition that the instability doesn’t occur is:

2

24

EJP

nl

(3-8)

In the case of circular punch: 2

max 95d

lP

In the case of noncircular punch: max 425J

lP

(3-9)

(3-10)

2. Die

The patterns of the die cutting edge are

illustrated in Fig. 3.10.

Fig.3.10 (a) and (b) are the dies with the straight wall cutting edge.

The strength of the cutting edge is high, the dimension of its working portion keeps unchanged after repairing and its manufacture is convenient. It is suitable for stamping the workpieces with complex shape or high tolerance demand.

Fig.3.10 Shapes of the die cutting edge

(a) (b)

(c) (d)

But the waste or the workpiece in such circumstances is prone to be accumulated inside the hole of the cutting edge, so increasing the expanding force, the ejecting force and the wearing of hole wall. The worn cutting edge forms the shape of inverse cone, which may induce the workpiece jumping from the opening-mouth of the hole to the surface of the die, and cause difficulty in operation.

Fig.3.10a type cutting edge of die is suitable for the non-circular workpiece. Fig.3.10b type is suitable for the circular workpiece, the die with the workpiece or waste to be ejected and the compound blanking die.

The workpiece or waste is easy to fall down from the die hole. The workpiece or waste wouldn’t accumulate easily inside the hole of the cutting edge. The friction and expanding force exerting on the hole wall are small, therefore the wearing of the die as well as the mending amount of the die per mending operation is small. But the strength of cutting edge is a bit lower.

Fig.3.10 (c) and (d) show the die with conical shape cutting edge.

The height h and pitch a, β of the straight wall of die are determined by the workpiece thickness and the manufacturing method, as shown in Table 3.1.

The dies with conical shape cutting edges are suitable for stamping thin workpieces with simple shape and low tolerance demand.

The dimension of cutting edge increases after repairing, but in general its influence on the workpiece dimension and the die life is weak.

Table 3. 1 Main parameters of die cutting edge

Workpiece thickness t (mm)

h(mm)

α β Notes

<0.5 ≥4

15′~30′

2°

The values of α and β are suitable for manual bench work. When manufacturing by the spark machining, α=4′~20′(the smaller one is used for the compound die), β=30′~50′. When manufacturing by the linear cutting with an inclined device, β=1°~1.5°.

0.5~1 ≥5

1.0~2.5 ≥6

2.5~6.0 ≥830′~1

°3°

The outer dimension of the die should guarantee the strength as well as rigidity of the die.

According to Fig.3.11, the die thickness H

d, the wall thickness c from the cutting edge to the die edge can be determined at first, and then to select the standard die dimension. There are four types of standard die, i.e., the die plates with rectangular or circular form, the single circular die and the stepped circular die fixed in the fastening plate.

Fig. 3.11 Determination of the outer dimension of die

When the die is fastened and located by bolts and cylinder

pins, the value c should guarantee that the distance between

the bolts, the bolt and pin, or the hole and cutting edges is

not too small, otherwise the die life would be decreased. The

minimum hole distance refers to Table 3. 2.

Bolt dia-

meterM4 M6 M8 M10 M12 M16 M20 M22

N 7 11 14 17 19 24 28 32

A 8 10 12 14 16 20 25 27

C 5

Table. 3.2 Minimum hole distance between bolts, bolt and pin, and between hole and cutting edges

The minimum value can be selected according to Table 3.3.

For the punch-die of compound die or the die with multi cutting edges, due to the distance between cutting edges is determined by the dimension of the blanking workpiece, from the view of strength, the wall thickness is limited by its minimum value.

Table 3.3 Minimum distances between cutting edges of the punch and die (mm)

3. Inserted structure of the punch and die

The solid punch or die of large or medium size with complex shape and locally thinning is difficult in forging, machining and heat-treating, or would be discarded due to local wearing in working. Therefore in such case the inserted structure is adopted (see Fig. 3.12).

Fig. 3.12 Inserted structure

The general designing rules of the inserted structure are:

Change the manufacture of the complex internal shape into the manufacture of the outer shape if possible. Sectionalize at the sharp angle when sharp angle exists.

(1) Convenience in manufacturing, decrease the human labour of the worker and the distortion during heat treatment.

When sectionalizing an arc shape, the sectionalized line should be located in the straight line, 3~5 mm from the intersection of the curve and the straight line. The sectionalization line should be perpendicular to the cutting edge, as shown in Fig. 3.12 (b).

According to the shape of the workpiece, the sectionalization can be done along a symmetrical line. Therefore the sections with the same shape and size can be machined or ground simultaneously, as shown in Fig. 3.12 (a).

(2) Convenience in repairing, replacing and adjusting.

The local protrusive or concave part, which is weak or easy to be worn, should be made as a whole, or adopted an inserted structure, as shown in Fig. 3.12 (c).

The possibility to adjust the relative position between holes should be guaranteed in the inserted structure, as shown in Fig. 3.12 (d).

The sectionalized line of the punch and that of die should be staggered about 3~5 mm to avoid

burring in blanking.

3. Satisfy the technological demand of blanking.

There exist various methods to fasten the inserted structure, such as heat fit, conical fit, frame fit, bolt fastening, fixing by bolts and pins, and by casting the low-melting alloy and epoxy resin, etc.

To guarantee the eligibility of stamping workpiece with sound outline, the blank should be located correctly in the die. The locating by controlling the feeding distance is called stop locating, and the locating perpendicular to the feeding direction is called stock locating.

3.2.3 Design of the locating parts and their standards

1. Stop parts

There are three kinds of stop parts in the National Standard, i.e., stop gauge, kicker and pilot.

(1) Stop gauge

According to the structure pattern, the stop gauge can be divided into fixed stop pin, spring ejecting stop gauge, back trip stop gauge and initial stop gauge.

The fixed stop pin can be divided into types A and B according to the National Standard.

The dimension of the fixed stop pin can be determined according to the workpiece thickness. Generally the height h of the head of stop pin is greater than the workpiece thickness t.

The numbers of stop pin is determined according to the shape and dimension of workpiece. For the narrow workpiece, two stop pins are used to maintain the stability of the stop.

The spring ejecting stop gauge is divided into spring, wring spring and rubber ejector according to the National Standard.

The spring ejecting stop pin is usually installed in the stripper and can extend and shrink. Fig. 3.13 shows the back trip stop gauge.

There exists a 45° slant plane in the stop pin of the back trip type stop gauge. When feeding, the scrap hits the slant plane and uplift the stop pin. After the scrap crosses over the stop pin, the slice spring makes the stop pin press into the hole, then pulling the strip backwards till the stop pin withstands the scrap and locate.

1-back trip type stop pin 2-slice spring 3-bolt

Fig.3.13 Back trip stop gauge

• During each feeding, two actions should be done in the opposite directions, i.e., feeding the blank firstly, and then pulling it backwards. The back-trip stop pin is usually installed in a fixed stripper.

It is mainly used in the progressive die, (see Fig.3.3 piece 2), and installed on the working end surface of the blanking punch.

Before blanking, the pilot is inserted into the pre-punched hole, to locate the relative position between the hole and outer edge, and correct the error of the feed and stock locating, thus to achieve a relative accurate workpiece.

(2) Pilot

The pilot can be classified into four types: A, B, C and D, where types A and D shown in Fig. 3.14 consisting of two portions for guiding and locating.

The portion for locating is a cylinder. The standard is determined according to the dimension d of the locating hole. In order to insert the pilot into the hole smoothly, the clearance fit h6 is usually adopted for the cylinder diameter. The cylinder height is adopted as （ 0.5~0.8 ） t ( t is the blank thickness).

(a ) A type pilot (b) D type pilot

Fig. 3.14 Pilot

For the progressive die using stop pin and pilot, the distance e between the stop pin and pilot is as follows:

where, c is the feed distance in mm; D is the diameter of blanking punch in mm; d is the pole diameter of stop pin in mm; 0.1mm is added to enables the strip to be pulled back or pushed forward during locating.

In Fig. 3.15(a):

In Fig. 3.15(b):

0.12 2

D de c (3-11)

0.12 2

D de c (3-12)

Fig.3.15 Position of the stop pin and pilot in the progressive die

(a)

Fig.3.15 Position of the stop pin and pilot in the progressive die

(b)

It is better to use a kicker instead of pilot, if the

thickness of the blanking material is too thin (less

than 0.5mm), to insert the pilot into the hole may

bend the hole edge; or if the diameter of the punching hole or the size

of blanking punch is too small, the installation of a

pilot would reduce the strength of punch, so it is

better to use a kicker instead of pilot.

The cross-section of IA and A is rectangle, which Ⅱis easy to be manufactured, when the cutting edge is worn out, the burrs would occur on the side of the strip blanked away (Fig. 3.17a), and then influence the normal feeding of the blank.

(3) Kicker

There are six types of standard kicker, IA, IB, IC, and A, B, C, as shown in Ⅱ Ⅱ Ⅱ Fig. 3.16.

When adopting types IC and IIC, the burr occurs on the concave part of the strip side (Fig.3.17b), so the preceding disadvantages may overcome, but it is more difficult in the manufacture of the kicker.

Fig. 3.16 Types of kicker

（ a ） （ b ）

Fig. 3.17 The working condition of the kicker

Kicker is equivalent to a blanking punch. The nominal size of its length is equal to the feed distance plus (0.05~0.1) mm. The negative tolerance is 0.02 mm. The dimension of the hole of the kicker is matched manufactured according to the actual size of the kicker based on the clearance fit prescribed through blanking.

2. Stock parts

• The standard type of the stock part is the stock guide. The die with stock guide is shown in Fig. 3.2 (piece 11). If the dimensional tolerance of blank width is large, a spring side pressing device can be used to guarantee the strip closely touching or contacting one side of stock guide to feed the blank correctly.

3.2.4 Design of the stripping and ejecting parts and their standards

The stripping parts include the fixed stripper, spring stripper and waste cutter etc.

1. Stripping parts

The fixed stripper is installed in the die and stock guide, as shown in Fig.3.2 (piece 10). The spring stripper is installed in the upper die by the rubber (or spring) and bolts, as shown in Fig. 3.4 (piece 5); it can also be installed in the lower die, as shown in Fig. 3.4 (piece 3);

Some of them can also be controlled by the elastic buffer or gas cushion installed in the lower die seat or beneath the working table of press, as shown in Fig.3.18.

The outline of the stripper is consistent with that of the die, the hole shape of the stripper is consistent with the outline of the punch, and certain amount of clearance should be kept.

(a) elastic buffer (b) gas cushion

Fig. 3.18 Elastic buffer and gas cushion

During cutting, a circular or rectangle cutter can be installed to strip the plate from the punch instead of the stripper. The waste cutter is installed closely beside the punch.

The height of the cutter edge is about t+2 mm lower than that of the punch edge, as shown in Fig. 3.19 (t is the blank thickness).

Fig. 3.19 Waste cutter

In the compound die, in order to eject the workpiece out of the punch-die, an ejector (also called stripping device) is usually used, as shown in Fig. 3.4.

2. Ejecting part

The ejector consists of the ejecting block and the standard ejecting pin, ejecting plate, screw ejecting lever or stepped ejecting lever.

3. Selection and calculation of rubber

The rubber is usually used as the elastic component in blanking die because its allowable load is larger than that of spring, and the installation and adjustment of the rubber is easy.

The plane dimensions and the height of rubber should be determined during selection. The plane dimensions of rubber is calculated according to the pressure P that the rubber can support:

P = Fq (3-13)

where, F is the area of the cross-section of rubber in

mm2; q is the pressure per unit area related to the co

mpressive strain in MPa, and is determined by the c

ompressive characteristic curve of the rubber, as sh

own schematically in Fig. 3.20.

(a), (c) rectangle (b) circular tube (c) cylinder Fig. 3.20 The compressive characteristic curve

of the rubber

The selected height of rubber should meet the demand of technology and the structure space of die. The working life of rubber should also be taken into account. Generally, the maximum reduction of rubber should not exceed 45% of its free height H.

The rubber installed in the die, should usually be pre-compressed for 10~15% H to reach the demanded stripping force. The compressive working stroke is (25~30)% H.

The free height H of the rubber is:

0.25 ~ 0.30 workH

H (3-14)

The height calculated according to Equation 3.14 should be checked by Equation 3.15:

5.15.0 D

H (3-15)

where, D is the diameter of the rubber in mm.

If H/D >1.5, then the rubber should be divided into several sections. Steel pad should be filled up between every two sections.

The marks and the mechanical properties of the rubber generally used in the die are shown in Table 3.4.

The oil-resistance of the rubber is bad, so it should not contact with oil to avoid corrosion and damage.

Table 3.4 Mechanical properties of rubber usually used 

Types of

rubberMark Slit force

(N/ Cm2)

Percentage elongation

(%)

Permanent deformation

(%)

Shore durometer number

(A)

Ordinary rubber

1120 ≥300 ≥250 ≥35 60~75

1130 ≥600 ≥300 ≥35 60~75

1140 ≥800 ≥350 ≥35 55~70

1250 ≥1300 ≥400 ≥30 50~65

Oil-resistant rubber

3001 ≥700 ≥250 ≥25 60~75

3002 ≥900 ≥250 ≥25 60~75

The standard of the spring for stripping or ejecting during blanking is included in the National Standard and the standard JB425-62 issued by Mechanical Ministry for cold blanking die.

4. Selection of spring and its calculation

The spring can be selected according to the above Standard. It should satisfy the structure demand of die, and its force and stroke should meet the demands in designing.

The characteristic curve of spring is shown in

Fig. 3.21.

The usually used spiral spring is taken as an

example to illustrate the procedure of selection.

Fig. 3.21 Characteristic curve of the spring

(1) According to the stripping force Ps, the

numbers of spring are pre-determined based on the structure of die, and then the force applying on the each spring Ps/n is calculated.

(2) Calculate the total reduction F of spring, which is the sum of the pre-reduction F0, the

working stroke F′ and the total repairing amount F″ of punch, that is:

0F = F + F' + F'' (3-16)

where ， F′ is the working stroke of stripper,

usually taking its value as t+1 mm; F″ is the total

repairing amount of punch, usually 4~10 mm; F0 is

the pre-reduction of spring, under this pre-reduction

the pre-compressive pressure of spring P0 is equal

to Ps/n according to the characteristic curve.

(3) According to the pre-compressive pressure Ps/n and the total reduction F, the spring is selected, to meet the following conditions:

1) The maximum allowable reduction of the spring F1 should be greater than the total reduction of the spring F during working, that is:

The maximum allowable reduction F1 is determined by the difference between the free length of spring H to the length withstanding the maximum working load H1.

FF 1 (3-17)

2) The maximum working load of the spring P1

should be greater than the total pressure P

corresponding to total reduction F:

(3-18) 1

0 1

0

P F

P PF

The guide plate, the standard pillar and bushing are the guiding parts for the upper and lower die.

3.2.5 Design of the guiding parts and their standards

There are eight kinds of standard pillars: types A, B, C; types A, B of small patterns; types A, B of demountable patterns and clamping ring fixed pattern.

There are five kinds of standard bushings: types A, B, C; small pattern and clamping ring fixed pattern.

The pillar and its fitted bushing are pressed into the installation holes of the lower and upper die seats respectively.

For B type pillar and bushing, the hole-base shrink fit H7/r6 is adopted;

for small pillars and bushings, the hole-base transition fit H7/m6 and shrink fit H7/r6 are adopted;

for the clamping ring fixed pillars and bushings, the hole-base transition fit H7/js6 is adopted.

For the non-standard pillars and bushings, the methods of fastening by the low-melting alloy and epoxy resin are also adopted.

Due to the sliding friction between the pillar andbushing, the hardness and abrasion-resistant abilities are required for the fitting surface.

The surface is usually carburized with steel 20. The carburizd depth is 0.8~1.2 mm. The clearance fit H7/h6 and H6/h5 are usually adopted for the fit of the pillar and bushing.

The bushing length L can be determined according to Fig. 3.22.

The length of the bushing should guarantee that the pillar and bushing always keep in contact during working.

.

Fig. 3.22 The determination of the pillar length

3.2.6 Design of the supporting and clamping parts and their standards

1. Upper and lower die seats

The upper die seat is used to fasten the parts of the upper die, and then the upper die is fastened to the slide block of press by the shank.

The upper die seat plays the roles of transferring and undergoing the blanking force simultaneously.

The lower die seat is used to fasten the parts of the lower die.

The standard die seat is made of HT20-40, ZG45, QT45-17 by casting; the upper die seat with shank is made of A3 through forging and machining.

The lower bolster is fastened to the working table by bolts and press plate and plays the roles in undergoing blanking force and dispersing it to the working table.

2. Fastening plate of the punch and die

It is used to fasten the punch and die. There are fastening plates of circular, rectangular and single punch plate in the Standard.

3. Bolster plate

It is used to undergo and disperse the pressure transferred by the punch, decrease the pressure per unit area applying on the die bolster, and prevent indentation pressed on the surface of the die bolster contacting with the punch.

Generally, the bolster plate is adopted in all the typical combination of the standard cold blanking die.

For the non-standard structure, whether using the bolster plate or not, is determined by the amount of the pressure that the die seat bears.

The pressure per unit area P transferred by the punch can be calculated by the following equation:

1.3 c

p LTp

F F

(3-19)

where, p is the blanking force in N; F is the contact ar

ea between the punch and the die seat or bolster plate i

n mm2; [σc] is the allowable compressive stress of the

upper die seat in MPa; for cast iron HT20-40, [σc]=98

MPa; For Q195,Q215, Q235 steel, [σc]=117~157 MPa.

4. Shank The medium and small blanking die is fastened to the slide block of the press by shank, its diameter d matches with the slide hole of the press.

There are seven types of shank in the National Standard, i.e., pressing-in shank, screwing-in shank, flanged shank, self-centering shank, pushing-in and movable shank, grooved shank and universal shank.

The concentricity and perpendicularity of the pressed shank are high.

The flanged shank is suitable for large die.

The movable shank is supported by a spherical plate to guarantee the movable connection of the upper die and the slide block of press.

It is usually used in the high tolerance blanking die with a guiding device. Its disadvantage is that the center of the die is difficult to aim at the center of the slide block during installing, so it is also difficult to overcome the error due to eccentricity.

3.2.7 Fastening parts

There are bolts and cylinder pins.

The bolt is used to connect the parts and is subjected to the tensile stress.

The cylinder pin plays the role in locating and is subjected to the deflection force (shearing stress).

3.2.8 Material of the blanking die parts and their heat treatment rules

The material of the blanking die parts and their heat treatment rules are shown in Table 3.6.

The selected material of die and punch is relevant directly to the die life as well as die cost. Recently the different kinds of die steel commonly used for cold blanking are as follows:

Part Selected material Heat treatment rulesPunch with simple

shapeT8A, T10A, Cr6WV,

9Mn2V, Cr12MoV, Cr12

Quench HRC50~60, trail tempered HRC40~50, Quench HRC58~62, trail

tempered HRC40~50

Die bolster and circular dieT10A, 9Mn2V, Cr12, Cr6WV, Cr12MoV Quench HRC58~62

Punch, die and inserted block with complex hape Cr12, Cr12MoV Quench HRC58~62

Punch and die of the plate die T7A Quench HRC45~48Upper and lower die seats HT20-40, Q235, QT40-17, ZG45  

Shank Q235, 45  

Punch plate, die block Q235, 45  

Stripper Q235, 45 45: The hardness can be determined as needed

Stock guide Q235, 45 45: Hardened and tempered HRC28~32

Kicker T10A, 9Mn2V, Cr6WV, Cr12Quench HRC56~60Quench HRC58~62

Cutter T10A Quench HRC56~60

PillarBushing

2020, GCr15

carburizd depth 0.8~1.2 mm, Quench HRC58~62, Quench HR

C62~66

Stop pin 45 Quench HRC43~48

Bolster plate 45, T7AThe hardness can be

determined as neededEjecting plate 、 ejecting pin 、

ejecting lever45 Quench HRC43~48

Table 3.6 Materials for manufacturing the die parts and their heat treatment rules

The carbon tool steel T8A, T10A are the cheapest and the most widely used.

Its hardness after annealing is lower than that of the alloy steel. It has good machining property, and the process of forging, annealing and quenching is easy to be mastered. It is suitable to manufacture the working parts of the blanking die with small size and simple shape.

But its hardenability and abrasion-resistantce are bad, its quench distortion is large and its working life is short.

Comparing with T10A, 9Mn2V has a higher hardness and abrasion-resistance, and also has a good machining property. The substitution of T10A by 9Mn2V results in lower cost and higher quality.

The low alloy tool steel, such as CrWMn, 9CrSi, 9Mn2V etc, can be quenched by oil, therefore it has a good hardenability and a small quench distortion.

If the thickness of the plate is smaller than 4 mm, the cutting edge of the die can undertake 20,000~22,000 times of blanking before repairing, and the die life is twice times higher than that of the carbon tool steel.

CrWMn has a high hardenability, hardness and ab

rasion-resistantce and small quench distortion. It is

suitable for the die with complex shape and the die

parts manufactured through linear cutting.

High-carbon high-chrome die steel, such as Cr12, Cr12MO, Cr12MOV etc, has a high strength, good hardenability and abrasion-resistance and small quench distortion.

The carbon content of Cr12 (chromium 12) is a bit higher. The distribution of carbide is non-uniform severely, which results in a decrease in the strength as well as hardness.

The Cr12 series die steel has rather high combination properties.

The carbon content of Cr12MOV is low and its strength and toughness is better as compared with Cr12. It is usually used to manufacture the die withstanding large blanking force and with long life expectancy. It can also be used to manufacture the die with complex shape and small quench distortion demand.

High-carbon medium-chrome die steel includes

Cr6WV, Cr4W2MOV etc. The Cr6WV is advantageous in small chrome content, better strength and impact toughness as compared with Cr12.

Due to the small content of carbon and chrome, its abrasion-resistance and hardenability is not as good as Cr12, but its quench distortion is small. Its life expectancy is almost the same as Cr12.

Cr4W2MoV (120Cr4W2MoV) is a new brand of

die steel for cold forming to substitute Cr12. As compared with Cr12, it is characterized in finer size and more uniform distribution of eutectic carbide. Therefore its hardenability, hardening-quenching capacity, mechanical property and abrasion-resistance are a bit higher.

The alloy elements, such as W, Mo and V, etc. improve the stability of the steel and make the die undergo possible chemical heat treatment possible.

3.3 Structural design of the blanking die

1. Principle and general procedure of die designing

The steps of die designing include collecting necessary data, determining the structure scheme based on sufficient analysis and investigation, drawing the working principle diagram, the final assembly diagram, and then drawing the part diagram of the die according to the final assembly diagram, , and then drawing the part diagram of the die according to the final assembly diagram.

• The necessary data to be collected include: the parts drawing of the product, the productive batch of the product, the process chart, the process specification and the die design program. The manufacture datum of each operation, the type of the die structure (simple die, progressive die or compound die), the capacity of the press, the quality of the blanking workpiece and other necessary items should be record in the process chart and the process specification.

• The detailed structure and dimension of the die are determined according to the production batch and workpiece demand, etc. For small batch production, the structure of die should be simple, the manufacture period should be short and the cost should be as low as possible. For large batch, the perfect structure, high productivity and long working life should be pursued.

• Furthermore, the structure measures to adjust, utilize, maintain, transport and deposit the dies should be taken into account.

2. Determination of the pressure center of die

The acting point of the resultant force in blanking is called the pressure center of die. In order to guarantee the die to work smoothly, the pressure center of die should align with the axis of shank and the center line of the slide block of press to prevent the deflection of die during working which would result in non-uniform clearance and wearing. Usually, the pressure center of die is laid on its symmetrical center point. The determination of the pressure center of die is carried on mainly for the complex, multi-punch punching and progressive dies.

• For the workpiece with complex shape, it can be divided into straight-line sections (the pressure center of die is situated at the middle of the straight) and arc sections and then to calculate its pressure center.

• The method to find the resultant force of the parallel force system is usually adopted to obtain the pressure center of die. The procedure to obtain the pressure center of the die for multi-punch blanking by analytic method is shown schematically in Fig. 3.23.

Fig. 3.23 Method to find the pressure center of die

(1) Draw the outline of the working portion of punch proportionally; (2) Select the co-ordinate X-Y. The selection of the co-ordinate should simplify the calculation. (3) Calculate the perimeter of each graphic outline (or linear section) L1 、 L2 、 L3 、 L4……Ln ( representing the blanking force), and the distance of gravity center in each graphic outline to the coordinate axes χ1 、 χ2 、 χ3 、 χ4…… and y1 、 y2 、 y3 、 y4……yn. (4) According to the mechanic principle that “the moment of the resultant force to a certain axis is equal to the sum of the moment of each component to the same axis”, the calculation equation of the distance from the die pressure center to the axes of X and Y can be obtained:

• The determination of the pressure center of die can also be obtained graphically by the linear polygon method.

1 1 2 2 3 30

1 2 3

...

...n n

n

L x L x L x L xx

L L L L

1 1 2 2 3 30

1 2 3

...

...n n

n

L y L y L y L yy

L L L L

3. Determination of the shut height of die

• The outline dimension of die should fit with the selected press. For example, the lower die of the blanking die should be able to install on to the working table of the press, the dimension of the shank should be consistent with that of the installing hole in the slide block, the ejecting pin of the ejecting device in the lower die should be able to penetrate through the bolster hole of press. The shut height of die should be consistent with that of the press.

• The shut height of die refers to the distance H between the top surface of the upper bolster and bottom surface of the lower bolster when the die is in the lowest working position. The shut height of press refers to the distance between the lower surface of the slide block and the upper surface of the bolster installed on the working table when the slide block is in the lower dead point.

• It is called the maximum shut height Hmax when the length of the connecting rod is adjusted to the shortest value, It is called the minimum shut height Hmin when the length of the connecting rod is adjusted to the longest value, as shown schematically in Fig. 3.24.

• The value of the shut height of die is between the value of the maximum and minimum shut height of press, usually:

min max10mm 5mm H + H H

Fig. 3.24 Selection of the shut height

• If the shut height of die is larger than the maximum height of press, the die cannot be used in this press; oppositely, if the shut height of die is less than the minimum height, the die can be used by adding a bolster.

凸模的孔形根据凸模，以 H7/m6 配作。按表面粗糙度 0.8 加工

凹模

卸料板

凹模凸模

凸凹模

推料板

顶出器

凸模固定板

顶板

上底板

下底板