8 Quick Tips Regarding Blanking of Sheet Metal

Estimated reading time: 38 minutes

Blanking of Sheet Metal is a cold stamping process that uses a die to separate one part of the sheet or strip material, which is previously placed between the die’s convex and concave edges, from the other part in the form of a tear, so as to obtain a flat blank or a manufactured part of the desired shape and size. This article focuses on 8 quick tips regarding the blanking of sheet metal.

The working principle of blanking of sheet metal

When the die edge is sharp and the gap between the convex and concave die is normal, the separation process of the sheet plate material is roughly through three stages: elastic deformation, plastic deformation, and fracture separation. The figure below gives the whole process of sheet plate material punching deformation.

Process requirements for blanking processing

The use of the Banking of Sheet Metal process can complete the processing of more complex shape parts, punching parts material thickness t is generally unlimited, but the current level of technology can be achieved: thin, ultra-thin material punching, t < 0.5 ~ 0. 05mm, t_min < 0.01mm; thick material, ultra-thick material punching, t> 4. 75 ~ 16 mm, t_max ≤ 25 mm, punching t_max ≤ 35 mm; more commonly used In order to improve quality of punching and simplify the manufacture of dies, there are specific requirements for the processed punching parts in the following aspects.

1. Precision

Generally speaking, the economic accuracy of the inner shape of blanking of sheet metal parts is IT12~IT14 grade, and it is generally required that the accuracy of drop parts should preferably be lower than IT10, and that of punching parts should preferably be lower than IT9 grade.

2. Commonly used plates

The common plates generally suitable for general blanking of sheet metal are mainly: carbon structural steel plate, high-quality carbon structural steel plate, low-alloy structural steel plate, electric silicon steel plate, stainless steel plate and other ferrous metals, as well as pure copper plate, brass plate, aluminum plate, titanium alloy plate, nickel-copper alloy plate, and other non-ferrous metals such as insulating glue wood plate, cardboard, fiber plate, plastic plate.

3. Profile or bore of punched parts

The shape of the inner hole of the blanking of sheet metal part should be designed as simple and symmetrical as possible. Avoid sharp corners. Generally, there should be R>0.5t (t for material thickness) or more rounded corners. The projection overhang and groove of the punched part should not be too long, and its width b should be greater than twice the material thickness t, i.e. b > 2t. The punch size should not be too small, otherwise, the strength of the convex die is not enough. Generally, for mild steel punching, the minimum punch size allowed is approximately equal to the material thickness, and the specific values for other materials are shown in the following table.

Minimum size of the punched hole with free cam die

Material	Minimum diameter of the punch	Minimum side length
Material	Round holes	Rectangular holes
Hard Steel	1.3t	t
Soft steel and brass	t	0.7t
Aluminum	0.8t	0.6t
Cloth and paper laminated wood	0.4t	0.35t

Minimum size for punching holes with sheathed dies

Material	Minimum diameter of the punch	Minimum side length
Material	Round holes	Rectangular holes
Hard Steel	0.5t	0.4t
Soft steel and brass	0.35t	0.3t
Aluminum and zinc	0.3t	0.28t

4. The distance between holes and between holes and edges of punched parts

The distance between the hole and the hole and the edge of the punched part should not be too small, otherwise, strength of the concave die is not enough and it is easy to break, and the edge of the workpiece is easy to produce expansion or distortion deformation. The minimum distance value should be taken as a≥t (for round holes), or a≥1.5t (for rectangular holes).

The structure form of the punching die and its selection

According to the different combinations of the punching process, the punching die can be divided into simple punching die, compound punching die, and progressive punching die. According to the different materials of the blanking parts, the blanking dies can be divided into two categories: metal blanking dies and non-metal blanking dies. Different forms of die structure are suitable for different production batches and different manufacturing accuracy of sheet material processing.

1. Simple punching die

A simple punching die is also called a single process die, it is a blanking of sheet metal process that can only complete one kind of punching or drop material in one stroke of punch ram. According to the different ways of guiding, it can be divided into an unguided die, guide plate die and guidepost die.

• Unguided die

The diagram below shows an open type unguided die, in which the convex die and concave die are fixed on the upper and lower die base with screws and pins through the fixed plate, and the fixed stopper pin is used to position the die. The advantages of this die are simple structure and low manufacturing cost, but the disadvantages are that the die has no guide device, the movement of the convex die can only rely on the punch slider guide, it is not easy to ensure a reasonable uniform clearance during the work, the accuracy of the parts is not high, the installation of the die is difficult, the working part is easy to wear, the productivity is low, the safety is poor, so this die is only suitable for the production of small quantities, the accuracy requirement is not high, the shape is relatively simple parts (blanks) Therefore, this type of dying is only suitable for punching and cutting of simple parts (billets) with low volume and accuracy requirements. Generally speaking, unguided single process blanking dies are usually used in the following situations.

The dimensional accuracy of blanked parts is not high, usually lower than the IT12 grade. The thickness of the blanked material is large, usually t≥1mm. The shape of the blanked parts is round, square, rectangle, rectangle or multi-angle and similar or close, regular and simple geometry, and the blanked parts are round, straight, without sharp angle and tooth, small tab, and small branch bud, overhanging wall and other blanking shapes. The output of blanking of sheet metal parts is not large.
No requirement for punching surface quality, burr, and flatness of blanking of sheet metal parts.The large size of punching parts, the recommended minimum size of punching parts: length×width×material thickness≥ 25mm× 10mm× 1mm; smaller size and thinner material thickness of the blanking of sheet metal workpiece, for safety reasons, not recommended to use open die punching.

• Guide plate die

The guide plate die is different from the unguided die as shown in the figure below, because it is equipped with a guide plate in the upper part of the concave die. During punching work, the die is always moving in the hole of the guide plate, and the guide plate is also used for unloading the material. The strip is fed by a hook-shaped stopper pin fixed on the die and a guide.

The advantages of this kind of die are that the clearance between the convex and concave dies can be guaranteed during work, which improves the accuracy of the manufactured parts, long service life, easier installation, and better safety. The disadvantage is that it is more troublesome to make the die, the hole of the guide plate needs to be matched with the convex die, and the stroke of stamping equipment needs to be small in order to ensure that the convex die always stays away from the guide plate when working. It is generally used for punching work of single-process punching or multi-process progressive die with a simple shape and small size of sheet thickness t> 0. 5mm. For parts with complex shapes and large sizes, this structure is not suitable, and it is better to use the die structure with a guide pillar and a guide sleeve type guide.

• Guide pillar die

The guide pillar die is shown in the figure below. The lower end of the guide pillar is pressed into the hole of the lower die holder and the guide sleeve is pressed into the hole of the upper die holder, and the gap fit between the guide pillar and the guide sleeve is often H6/h5 or H7/h6. The guide pillar and the guide sleeve are used to guide the die when it works. The convex die of the mold is fixed to the upper die base through the convex die fixing plate with screws and pins, and the concave die is fixed directly to the lower die base by screw and pin screws and pins. After the strip is fed in, it is positioned in front and left, and right with fixed stop pins to ensure the correct position of the strip on the die. The upper die is equipped with an unloading plate to unload the material. The advantages of this kind of die are that it has a good guiding effect, ensures uniform clearance between the convex and concave dies, improves the accuracy of the parts, reduces the wear and tear of the working parts, and is easy to install. The disadvantage is that the mold is complicated and costly to manufacture, and it is suitable for the work of blanking of sheet metal parts with large production volume and high precision requirements. Generally speaking, guided single process blanking dies are usually used in the following situations.

The dimensional accuracy of punching parts is higher, generally higher than ITI2 level, and can reach IT10 level or even some higher.

The material thickness t of punching parts is generally unlimited, but the current process level can be achieved as follows: thin and ultra-thin material punching, t<0. 5~0.05mm, t_min≤ 0.01mm; thick and super-thick material punching, t>4. 75~16mm, t_max≤25mm, punching t_max≤35mm; more commonly used punching material thickness t≤ 3mm, more material thickness range for t > 0.5~2mm.

The production nature of the applicable stamping parts is batch and mass production.

The quality, burr, and flatness of the punching surface of the stamped parts are required.

Restrictions on the size of punching parts are: using the standard die frame, the recommended maximum concave die size for punching parts is L×W≤ 630mm×500mm; the diameter of the smallest round hole for punching is d_min≥ (0.5~0.6) t, and the recommended d_min≥ t; the maximum thickness of punching material is t_max≤ 12~16mm, and the recommended t_max≤10mm, and t>10mm for hot punching.

2. Composite punching die

A compound punching die is a die that performs more than two processes simultaneously at the same station of the die during one punching stroke of the press. The most important feature of this type of die is that it has a convex die that can drop the material and a concave die that can punch the hole, which can realize the punching of the inner hole and the shape at the same time. The most common compounds for punching are: punching and dropping die, notching and dropping die, etc.

The following figure (a) shows the processed punching and drop part, and the following figure (b) shows the flip-type compound die (drop concave die 11 is mounted on the upper die), and the whole set of dying is guided by guide pillar 12 and guide sleeve 2. When punching, the unloading plate 14 first presses the strip material to play a leveling role, and as the press slider continues to go down, the falling concave die 11 presses the unloading plate 14 down with the convex die 9 and the convex and concave die 13 to work together to punch out the shape of the parts, and when the press slider goes up, the unloading plate 14 unloads the strip material from the convex and concave die under the action of the polyurethane block 15, and the beater bar 7 is pushed by the crossbar of the press, and the parts are removed from the convex and concave die through the beater plate 8, the push rod 6 The punching scrap is then funneled directly from the convex and concave die holes to the press table.

The following figure (c) shows a front-loading compound die, the working process of which is similar to that of the inverted type. The punched out parts are ejected by the lower top cylinder of the press or by the top bar 14 through the discharge block 12 via the elastic buffer, while the strips and punched scrap are pushed out by the crossbar of the press through the discharge plate 9 and the beater bar 8 of the upper die.

Inverted laminate film is easier to lead out due to punching waste can leak out from the press table hole and the workpiece is pushed down from the upper die, which is easy to operate, safe, and can ensure high productivity. Therefore, it should be used in preference. However, when punching, because the rigid pushing device does not have a flattening effect on the workpiece, the flatness and dimensional accuracy of the workpiece is lower than when using the flexible pushing device, so it is mainly used for punching thick materials.

However, the top plate and the unloading plate of the front-loading compound die are elastic, and the workpiece is flattened at the same time by the pushing device of the lower die, and the strips are taken out by the unloading device of the upper die, so the three are mixed.

The compound die can complete several processes in one die and one stamping stroke to increase production efficiency exponentially. Generally, when the dimensional accuracy or the position accuracy such as coaxial and symmetry of the stamping workpiece is required to be high and the production batch is large, the composite die can be considered to be used for discharging, and for the stamping workpiece with more complicated shape and the repositioning may produce larger processing error, the composite die can also be used. The following figure (d) gives some of the shapes of parts that are suitable for processing with compound dies.

3. Progressive punching die

The progressive die refers to the die that completes more than two stamping processes at the same time in different stations of the same die during one stamping stroke of the press, also called jump-step die and continuous die.

In a progressive die, in addition to the general structure of a normal die, structural parts such as the starting stopper, side pressure device, guide pin, and side edge are also required. The figure below shows a punching and drop feed progressive die with a guide pin to set the distance and feed the material by hand. The workpiece is shown in the upper right corner of the figure. The upper and lower dies are guided by guide plates. Die shank 1 is connected to the upper die holder with threads. The set screws 2 with riding seam are used for tightening to prevent loosening of the threads during punching. The distance between the punching die 3 and the drop die 4 is the feeding step A.

The feed is initially positioned by the fixed stopper pin 6 and finely positioned by the two guide pins 5 mounted on the drop die. The structure of the guide pin and the drop die is H7/r6, which should be connected in such a way that it can be easily dismantled when the die is resharpened, so the hole where the pin is mounted is a through-hole. The shape of the head of the guide pin should be such that it can be inserted into the punched hole when guiding, and it should have a slight clearance with the hole. To ensure the correct spacing of the first part, in a progressive die with a guide pin, an initial stopper is often used. It is installed in the middle of the guide plate under the guide plate. When punching the first part of the strip, the first two holes are punched by pushing the starting stopper pin 7 by hand, so that it sticks out from the guide plate against the front end of the strip. In the next punching process, the fixed stopper pin 6 is used to control the feeding step for initial positioning.

Compared with single-process die and compound die, progressive die constitutes a kind of stamping die with complex structure, a high number of parts, high precision, and heat treatment requirements, complex die assembly and manufacturing and requires precise control of step, which is suitable for the production stamping parts with larger batch size or smaller shape size and thinner material thickness.

4. Non-metal punching and cutting dies

According to the different organization and mechanical properties of non-metal materials, there are two types of punching methods for non-metal materials: cutting with a sharp-edged convex die and cutting with a common punching die.

• Cutting with a sharp-edged convex die.

The sharp-edged convex die is mainly used for cutting fibrous and elastic materials such as leather, felt, cardboard, fiber cloth, asbestos cloth, rubber, and various thermoplastic films.
The structure of the sharp-edged convex die is shown in the figure below. In the following figure, (a) is the outer beveled edge for drop material, (b) is the inner beveled edge for punching, and (c) is the beveled edge on both sides of the convex die used for cutting vulcanized hard rubber sheets in the heated state to ensure that the cut edge is vertical; and (d) is the felt seal compound die structure. The bevel angle α of the sharp-edged convex die is shown in the following table.

The value of the bevel angle α of the sharp-edged convex die

Material name	α/(°)
Baked hot hard rubber	8~12
Leather, felt, cotton textiles	10~15
Paper, cardboard, horse manure paper	15~20
Asbestos	20~25
Fiberboard	25~30
Red cardboard, paper glue board, cloth glue board	30~40

It is designed so that the direction of the bevel of its pointed edge should be against the scrap. When punching, a piece of hardwood, ply, polyurethane rubber sheets, non-ferrous metal sheets, etc. is placed under the sheet to prevent damage or chipping of the edge and it is not necessary to use a concave die. It can be installed in small tonnage presses or processed directly by hand.

• Ordinary blanking die of punching

For some hard non-metallic materials such as mica, phenolic paper glue board, phenolic cloth glue board, epoxy phenolic glass cloth glue board, etc, the punching die of a common structure can be used for processing. Since these materials have a certain degree of hardness and brittleness. In order to reduce the surface crack, delamination, and other defects should be appropriate to increase the crimping force and counter-top force, reduce the die gap, the lap is also larger than the general metal materials. For material thickness greater than 1.5 mm and the shape of a variety of complex paper and cloth glue board parts, the blank needs to be preheated to a certain temperature before punching and cutting.

The determination of the main process parameters of punching and cutting

In order to ensure the quality of the blanked parts, the following process parameters should be determined when developing the blanking process and designing the relevant punching dies.

1. Calculation of punching force

The punching force is the main basis for the selection of suitable presses and is also data necessary for the design of dies and the calibration of die strength. For punching with a normal flat edge, the punching force is calculated as:

The total punching force required for punching is the sum of punching force, unloading force, pushing force, and top part force. Whether all these forces should be taken into consideration when selecting a press should be treated separately according to different die structures.

When the tonnage of punching equipment does not meet the need of punching force, it can be achieved by adopting measures such as step punching (designing different die punch structures with different heights), beveled edge punching (repairing the convex or concave die into a beveled edge shape) or hot punching (heating the punched material above the blue brittle temperature zone).

2. Determination of die clearance

The blanking gap Z is the difference in the size of the working part between the blanking die and the concave die. The blanking gap has a great influence on the blanking process. Its size directly impacts the quality of the punched part and also has a significant impact on the die life. The blanking gap is the most important process parameter to ensure a reasonable blanking process. In actual production, the value of the reasonable clearance is determined by experimental methods. Since there is no absolute reasonable clearance value and the specific requirements of each industry are not the same, each industry and even each company has its own blanking clearance table, which is often chosen by referring to the relevant blanking clearance table when determining specific clearance value. Generally speaking, a smaller reasonable clearance is good for improving the quality of the punched part, while a larger reasonable clearance is good for improving the lives of the die. Therefore, a larger reasonable clearance should be used to ensure the quality of the punched part.

In addition, the double-sided clearance Z for punching can be calculated according to the following formula:

Z=mt

m————Coefficient, see table below;
t————Sheet thickness, mm.

Machinery manufacturing and automobile, tractor industry m value

Material name	m value
08 steel, 10 steel, brass, pure copper	0.08~0.10
Q235, Q255, 25 steel	0.1~0.12
45 steel	0.12~0.14

The m-value of the electrical instrumentation industry

Material Type	Material name	m value
Metal Material	Aluminum, pure copper, pure iron	0.04
Metal Material	Hard aluminum, brass, 08 steel, 10 steel	0.05
Metal Materials	Tin-phosphor bronze, beryllium alloy, and chrome steel	0.06
Metal Materials	Silicon steel sheet, spring steel, high carbon steel	0.07
Non-metallic materials	Paper cloth, leather, asbestos, rubber, plastic cardboard, adhesive cardboard, adhesive sheet, mica sheet	0.02 0.03

3. Determination of working part size of convex and concave dies

In blanking operation, the size and accuracy of the working part of the die is the primary factor affecting the tolerance level of the size of the blanked part, and the reasonable clearance of the die also depends on the size of the working part of the die and its tolerance to ensure. Therefore, when determining the size of the working part of the convex and concave dies and their manufacturing tolerances, it is necessary to take into account the law of punching deformation, tolerance grade of the punched part, die wear, and manufacturing characteristics.

• Basic principles of punching convex and concave die size calculation

When punching, the diameter of the hole determines the size of the convex die, and the clearance is obtained by increasing the size of the concave die. In the case of drop, the profile size determines the size of the concave die, and the clearance is obtained by reducing the size of the convex die. As the concave die wears out, it increases the size of the drop part, and the convex die wears out, it decreases the size of the punched part. In order to improve the die life, the size of the concave die should be made to the minimum limit size of the drop part and the size of the convex die to the maximum limit size of the punched part when making a new die.

• Method to ensure the clearance of punching die

Specify the dimensions and tolerances of the convex die and concave die separately and manufacture them separately. The clearance requirement is ensured by the size and manufacturing tolerance of the convex and concave dies. This processing method provides interchangeability of convex and concave dies, short manufacturing cycle time, and is easy to manufacture in batches.

The clearance is ensured by the method of single-matching the convex and concave dies to each other. After machining, the convex and concave dies must be matched to each other and are not interchangeable. Usually, the concave die is chosen as the reference die for drop parts and the convex die is chosen as the reference die for punching parts. The dimensions and tolerances are marked on the part diagram of the benchmark die, and the part diagram of the matching non-benchmark die is marked with the same basic dimensions as the benchmark die, but no tolerance is marked, and the punching clearance is matched according to the actual dimensions of the benchmark die, and the clearance value is guaranteed to be within Z_min~Z_max. This method is mostly used for dies with complex shapes and small clearances.

• Convex and concave die single-match processing method

The principle of determining the basic size of the convex and concave dies is to ensure that the working parts of the die have the maximum amount of wear within the qualified size.

Punching equipment

The equipment used for sheet metal blanking is mainly crank presses. Crank presses are divided into open presses and closed presses according to the structural characteristics of the machine body. The open type press worktable is open in the front, left, and right side, which is easy to install and adjust the die and operation, but the rigidity is poor, the tonnage is 25kN~4MN, the figure below shows several types of open type press; the closed type press is frame type processing, open in front and back, the rigidity is better, the tonnage is more than 1. 6MN.

Although there are more types of crank presses, the working principle is basically the same. Simply put, it is to increase the force and change the form of motion through the crank structure (crank linkage mechanism, crank elbow mechanism, etc.) and use the flywheel to store and release energy to make the crank press produce large working pressure to complete the stamping operation. The following is an example of JB23-63 crank press to explain its structure and movement principle. JB23-63 crank press belongs to open type tiltable press, see the figure below.

When the press is in motion, motor 1 transmits the motion to the large pulley 3 through the V-belt, and then to the crankshaft 7 through the pinion 4 and large gear 5. The upper end of the connecting rod 9 is mounted on the crankshaft, and the lower end is connected to the slider 10 to change the rotational motion of the crankshaft into the reciprocating linear motion of slider, and the highest position of the slider 10 is called the upper stop (dead) position, while the lowest position is called the lower stop (dead) position. Due to the need for production process, slider sometimes moves and sometimes stops, so it is equipped with clutch 6 and brake 8. Since the press has short time for process operation during the whole working time cycle, most of the time is no-load empty range. In order to make the motor load evenly and use equipment energy effectively, thus the flywheel is equipped and the big belt pulley takes off the pulley action at the same time.

When the press is working, the upper die 11 of the used die is mounted on the slider, and the lower die 12 is mounted directly on table 14 or with pad 13 on the table surface, it can get the suitable closing height. At this time, the material is placed between the upper and lower dies, that is, it can be punched or another deformation process to make the workpiece.

The key points of punching die design and application

The punching process is completed by punching die, which is the key to ensure the shape, size, and accuracy of the blanked parts. Therefore, the punching process of the sheet material depends largely on the design of an economic, reasonable, and practical punching die.

1. Punching processability

Need to carefully analyze the punching process of the parts, so that the developed processing technology and die structure can meet the needs of processing. For example, for the punching process of the plate with sharp corners, the process can generally be arranged by using the principle that two straight lines intersect to form a sharp corner. In figure (b) below, a straight edge of the convex die intersects with one side of the strip to obtain the workpiece shown in figure (a) below; in figure (d) below, the strip is fed from the right to the left, one side of the profile is punched out first, and then the workpiece is punched out to obtain the workpiece shown in figure (c) below. If the workpiece is to be punched in a single pass, the working parts such as cam and concave dies are often inserted to facilitate subsequent maintenance and replacement.

Another example is that for the dense holes on the plate material, the punching process is poor parts punching, if the one-time punching is not strong enough concave die, and the punched parts are prone to hole edge material convex deformation, at this time, the available interval position, only half of the concave die, the first time with the stopper pin B blocking material, punching a few holes I, the second time with the stopper pin A blocking material, punching a few holes II. After punching once, the strip can be turned over and the remaining holes can be punched with stopper pin B, as shown in the figure below.

2. Processing accuracy

It is necessary to carefully analyze the processing accuracy of the punched parts, so as to determine the appropriate processing method and design the corresponding die structure. Such as processing 2.5 mm thick 20 steel made of punching parts processing, if the roughness Ra requirements are not less than 0.8 ~ 1. 6μm, and processing hole accuracy up to IT9 level, then the use of ordinary punching die simply can not meet the requirements, at this time, the need to use precision punching or the use of extrusion light processing technology.

3. Punching sequence of a single process punching die

When a single process punching die is used to process plate material, the punching sequence should be reasonably arranged, mainly with the following principles.

• First drop the material before punching or notching, and the positioning reference of the subsequent processes should be consistent to avoid positioning errors and size chain conversion.
• When punching holes of different sizes and close to each other, in order to reduce the deformation of the holes, the larger holes should be punched first and then the smaller ones.

4. Progressive die punching sequence

When using a progressive die to process sheet material, the arrangement of punching sequence is mainly based on the following principles.

• Punch or notch first, and drop or cut off last. The first punched hole can be used as the positioning hole for the subsequent process, and when the positioning is also required high, the process hole (generally two) can be punched specifically for positioning.
• When using fixed distance side edge, the cutting process of fixed distance side edge is arranged to be carried out at the same time as the first punching, so as to control the feeding distance. When two fixed pitchside edges are used, they can be arranged as one in front of the other or side by side.

5. The relationship between production volume and processing accuracy

It is necessary to carefully analyze and take into account the relationship between the production volume and processing accuracy of the punched processed parts, so as to determine the appropriate die type and design the corresponding die structure. For example, a simple punching die as shown in the figure below can be used for punching parts with small production volume and low processing accuracy. The convex die 2 and concave die 3 are positioned on the upper and lower templates by the mounting plate 4 where the hole types correspond to each other, and the rubber sleeve 1 is used to press and dematerialize the material. However, if the processing accuracy is high, even if the production batch is not large, a guide plate die or die holder-guided punching die should be used.

6. Force of mold structure

The working parts of the designed die and the force of the die structure need to be carefully analyzed so that measures can be taken during the design process or the structure of the die can be designed to be improved. For example, for the punching of small holes such as plates or pipes, the working conditions of the convex die are poor, and it is easy to break after the force is applied. In other words, the thickened part of the small die 3, the unloading plate 5 and the guide bush 4 are sliding together to play a guiding role, and the clearance between the working part of the die and the guide bush should be slightly larger to improve the die life.

For example, when punching the various notches shown below in the shear or fallen semi-finished products, since the punching notch is not a closed structure, the horizontal pressure on the side of the die and concave die edge will not be balanced to cancel each other, in order to eliminate the lateral force may cause the notched die to shift and lead to uneven punching clearance and other effects, or make the notched die skewed or even broken and other fatal defects, therefore, in the mold design, the following processing methods are usually used.

• Reinforce the strength and rigidity of the notched convex mold

For example, increase the part size and choose high strength to die material to make it resist the frequent action of lateral force.

• Set the positioning part corresponding to the punching notch

Block 5 as shown in Figure (a) below.

• Anti-bias load structure

To reduce the lateral force on the slot in the blanking plate 3, guide pin 1 and guide pin 2 are designed on the upper die. In the case of offset blanking, guide pin 2 is pressed against the plate under the action of the respective spring, and guide pin 1 is inserted into the guide hole under the action of the spring to The guide pin 1 is inserted into the guide hole by the spring to balance the bias load.

The main working principle of the b and c solutions is: before the notch is punched and the lateral force is not generated, the notch punching die is pressed against the concave die or block to complete the protection of the notch punching die, the assembly requirements, the notch punching die and concave die or block for no clearance or small clearance (generally about 1/3 of the standard single-sided punching clearance) with a clean surface and reliable guide.

7. Small-size blanking parts

For the small-sized blanking parts with simple shapes, multiple varieties, and small batch production, the general-purpose blanking die is generally used to complete the processing of the parts. The general-purpose die structure is simple, and the operation of the corresponding upper and lower dies can be changed simply in the same die to realize the punching of parts of different shapes and sizes. Therefore, it is useful for production organization and management, shortening the production cycle of parts and reducing manufacturing costs.

The following figure (a) shows the universal blanking die structure where the upper and lower die holders are connected into a single die holder. Figures (b) and (c) below show the separate structure of the upper and lower die holders, mostly open-type dies.

The C-shape punching dies shown in (a) below have a high coaxiality because the concave die hole and the guide bushing assembly hole on the C-shape holder are machined in one clamping.

The convex die 5 in the die is both the convex die for the punching process and is guided by the guide sleeve 4 mounted in the hole in the die holder 1, and the head also acts as a die handle to connect with the press slide. To ensure the accuracy of the punching die, the convex die 5 and the inner hole of the guide sleeve 4 should be machined to a clearance fit of H6/h5 with a coaxiality of no more than 0.003mm; the concave die 2 is mounted directly in the lower hole of the die holder. The unloading plate 3 is fixed in the middle of the die holder with screws.

The entire die has a compact structure and good processing performance. Different shapes of the shaft and hole parts can be punched by changing the die shank convex die 5, concave die 2, and unloading plate 3 (working size and shape change).

Figure (b) below shows another general-purpose drop and punch die structure that can punch square, rectangular, and other shapes. The whole set of dies is very versatile. When punching holes of different shapes and diameters, it is enough to change the convex die 3 and concave die 10.

When it is necessary to drop material, part of the positioning plate 5 can be removed and the convex die 3 and concave die 10 can be replaced with drop material convex die and concave die for drop material punching. When punching is required, the convex die 3 and concave die 10 can be replaced and the three positioning plates 5 can be positioned according to the shape of the drop part to punch holes.

The following figure (c) shows the structure of a general-purpose punching die. The lower end of die shank 1 is designed with fine threads, and the external rotation of the upper fastening cone sleeve 2, whose cone angle is 60°; the upper part of the convex die 3 is also designed as a cone, and the cone surface is stuck in the fastening cone sleeve and relies on the automatic centering of the cone surface; the upper fastening cone sleeve 2 has wrench grooves on the outer edge, and the hook wrench plate can be used to fasten the convex die 3; the hard rubber sleeve is used to unload the material on the convex die 3; the concave die 4 is also designed with a conical outer edge, and with the concave die seat The concave die 4 is also designed to have a conical outer edge and is fastened to the concave die seat through the lower fastening taper sleeve 6 using fine threads.

Installation method of punching die

Correct installation of punching die is a prerequisite to ensure the quality of sheet blanking processing and the safety of the die, the safety of the equipment and the personal safety of the operator. The general installation principle of the punching die on the press is: first fix the upper die on the press slide, and then adjust the fixed lower die according to the position of the upper die. In the process of die installation, the press must be adjusted accordingly.

The installation of the punching die is divided into two types: unguided punching die and guided punching die. The installation methods are as follows.

1. Installation of unguided die

The installation of unguided punching dies is more complicated, and the methods are as follows.

• Before die installation, check the press and die.
• Check the installation condition of the punching die. The closing height of the punching die must match the loading height of the press. The closing height of the die must be measured before installation. If the closing height of the die is too small to meet the above requirements, add a ground flat pad on the press table to make it meet the above requirements before mounting the die.
• Place the punching die at the center of the press, see the figure below. The upper and lower dies are padded with pad 3.
• Loosen the nut on press slide 4 and turn the press flywheel by hand or pry bar to lower the press slide to contact with the upper die plate 6 and make the die shank of the punch enter the die shank hole of the slide.
• After the height of the slider is adjusted, fasten the die shank to the press slider.
• Adjust the clearance of the convex and concave dies, i.e., pad the edge of the concave die with a cardboard1 or copper sheet equal to the thickness of the one-sided clearance value of the convex and concave dies, and adjust the clearance of the convex and concave dies with the transillumination method and make it even.
• After the gap is adjusted, insert bolt 10 into the groove of the press table and fasten the lower die to the press by means of pressure block 8, pad 9, and nut 7. Note that the tightening of the bolts should be done symmetrically and in a staggered manner.
• Start the press for test punching. In the process of test punching, if the die clearance needs to be adjusted, loosen the nut 7 slightly and use a hand hammer to gently hammer the lower die in the direction of adjustment according to the distribution of the die clearance until the die clearance is suitable.

2. Installation method of guided punching die

The installation and adjustment of guided punching die are more convenient and easier than that of unguided die due to the guide pillar and guide bush guidance.

• Make the technical preparation, cleaning of the die and press table, and inspection of the press before installation according to the installation requirements of unguided punching dies.
• Put the closed die on the press table.
• Separate the upper die from the lower die and pad the upper die with a wooden block or pad iron.
• Lower the press ram to the lower pole and adjust it to make contact with the upper plane of the upper die plate.
• Fasten the upper die and lower die to the press slider and pressure table respectively, with the screws fastened symmetrically and in a staggered manner. The slider should be adjusted so that when it is on the upper pole, the convex die does not escape from the guide plate or the guide sleeve does not fall farther than 1/3 of the length of the guide column.
• After fastening firmly, carry out test punching, and transfer to formal production after passing the test punching.

The quality and precision of sheet blanking

The quality of the sheet blanking process refers to the quality of the cut-off surface, size and shape accuracy, etc. The surface roughness of the punched parts is generally below Ra12.5μm, and the specific values can be found in the table below.

Approximate surface roughness of the shear surface of the punched part

Material thickness t/mm	≤1	＞1~2	＞2~3	＞3~4	＞4~5
Surface roughness Ra/μm	3.2	6.3	12.5	25	50

The dimensional accuracy of sheet metal blanking parts has a direct influence on the manufacturing accuracy of the punching die. The higher the accuracy of the punching dies, the higher the accuracy of the punched parts. The dimensional accuracy of the blanked parts provided in the table below refers to the data of punching and processing of common materials such as aluminum, copper and soft steel with reasonable clearance. In the table, the general punching accuracy and higher punching accuracy refer to the punched parts obtained by processing with punching dies of IT8~IT7 grade and IT7~IT6 grade manufacturing accuracy, respectively.

Tolerance of distance between holes mm

Material thickness	Distance from the center of processed parts for general blanking accuracy	Distance from the center of processed parts for general blanking accuracy	Distance from the center of processed parts for general blanking accuracy	Distance from the center of the machined part for higher blanking accuracy	Distance from the center of the machined part for higher blanking accuracy	Distance from the center of the machined part for higher blanking accuracy
Material thickness	Below 50	50~150	150~300	Below 50	50~150	150~300
Below 1	±0.1	±0.15	±0.2	±0.03	±0.05	±0.08
1~2	±0.12	±0.2	±0.3	±0.04	±0.06	±0.10
2~4	±0.15	±0.25	±0.35	±0.06	±0.08	±0.12
4~6	±0.2	±0.3	±0.4	±0.08	±0.10	±0.15