How punch press works
The design principle of the punch press is to convert circular motion into linear motion.
The flywheel is driven by the power of the main motor, and the gears, crankshaft (or eccentric gear) and connecting rod are driven by the clutch to achieve the linear motion of the slider.
The motion from the main motor to the connecting rod is circular.
There must be a transfer point between the connecting rod and the slider for both circular and linear motion.
There are roughly two types of mechanisms in its design, one is ball type and the other is pin type (cylindrical type).
Through this mechanism, the circular motion is converted into a linear motion of the slider.
Punching presses pressure on the material, causing it to deform plastically.
Therefore, a set of molds (upper and lower molds) must be used to obtain the required shape and precision.
The material is placed between them and deformed by the pressure exerted by the punch press machine.
The reaction force caused by the force exerted on the material during processing is absorbed by the mechanical body of the punch press.
Classification of punch press
1. Classified by slider driving force
The driven force of slider can be divided into the mechanical driving force and hydraulic driving, so according to the different driving force used the punch press can be divided into:
(1) Mechanical punch press (Mechanical Power Press)
Most of the general sheet metal stamping processes use mechanical presses.
Depending on the liquid used, there are oil-based hydraulic presses and water-based hydraulic presses.
At present, the majority of oil-based hydraulic presses are used, while water-based hydraulic presses are mostly used for large machines or special machines.
2. Classified by slider movement
According to the slider movement mode, there are single-action punch press, double-action punch press and triple-action punch press.
At present, the most used are single-action punching machines with one slider, while double-action and triple-action punching machines are mainly used in the drawing process of automobile bodies and large machined parts, the number of which is very small.
3. Classified by slider drive mechanism
(1) Crank Press
A press that uses a crank mechanism is called a crank press.
The most common reasons for using the crank mechanism are ease of manufacture, correct determination of the position of the bottom end of the stroke, and the fact that the slide movement curve is generally suitable for various processes.
Therefore, this type of press is suitable for punching, bending, drawing, hot forging, warm forging, cold forging and almost all other press operations.
(2) Crankless Press
A crankless press is also known as an eccentric geared press.
The eccentric gear press structure is superior to the crankshaft structure in terms of shaft rigidity, lubrication, appearance, and maintenance, while the disadvantage is that it is more expensive.
Eccentric geared presses are more advantageous for longer stroke lengths, while eccentric geared presses are more advantageous for longer stroke lengths.
(3) Knuckle Press
The use of an elbow joint mechanism on the slide drive is called knuckle press.
This type of punch press has a unique slider motion curve where the slider speed becomes very slow near the lower dead center (compared to a crankshaft punch). .
It also correctly determines the position of the dead center under the stroke.
Therefore, this press is suitable for compression processing such as stamping and finishing, and is most often used in cold forging.
(4) Friction Press
A press that uses a friction drive and screw mechanism on an orbital drive are called a friction press.
This press is ideal for forging, crushing, bending, forming, drawing, etc., and is versatile in its functions.
It was widely used before the war because of its low price.
The disadvantages including the position of the bottom end of the stroke cannot be determined, poor machining accuracy, slow production speed, and overload in the event of a control error, the need for skilled operators, so it’s now being phased out.
(5) Screw Press
The use of a screw mechanism on the slide drive mechanism is called a screw press.
(6) Rack Press
The use of rack and pinion mechanism on the slide drive mechanism is called rack-and-pinion type press.
Screw presses have almost the same characteristics as rack and pinion presses, and their characteristics are roughly the same as those of hydraulic presses.
Previously, it was used for extrusion of pressed liners, scraps and other articles, oil press, bale, and shell extrusion (hot press thinning). The rack presses, however, have now been replaced by hydraulic presses and are no longer used except in very special circumstances.
(7) Link Press
Punch presses that use a variety of linkage mechanisms on the slide drive mechanism are called linkage presses.
The purpose of using the linkage mechanism is to shorten the cycle time while keeping the drawing speed within the limit during the drawing process.
Speeding up the approaching stroke from the upper dead point to the starting point of machining by reducing the speed change of extension machining, and reverting from the lower dead point to the upper dead point.
The speed of the stroke allows for shorter cycle times than crankshaft presses for increased productivity.
This type of press, which has a wider bed table surface, has been used since ancient times for the deep drawing of cylindrical containers with narrow bed surfaces, and more recently for the deep drawing of automobile body panels.
(8) Cam Press
Punching machines that use a cam mechanism on the slide drive mechanism are called cam presses.
This type of press is characterized by making the appropriate cam shape so that the desired slide motion profile can be easily obtained.
However, due to the nature of the cam mechanism, it is difficult to transmit large forces, so this type of press has very little capacity.
4. Classified by body shape
Classified by body shape, the punch press can be divided into:
(1) C-frame punch press
(2) H-frame punch press
At present, most of the punching machines used in the general stamping industry are of C-frame press, especially the small punching machines (under 150 tons).
Large machines are mostly of the straight side press (H-frame).
Those who use a C-frame press body are called C-frame presses and those who use a straight frame body are called straight side presses, or H-frame press.
(1) C-frame Press
As the body of C-type punch press is not symmetrical, the reaction force during punching will cause the deformation of the front and back openings of the body, which will deteriorate the parallelism of the tooling.
This is the biggest drawback of C-frame press.
It is therefore generally used at about 50% of the nominal pressure.
However, it is still popular because of its good operability, good mold accessibility, easy mold change and other favorable factors.
At the same time, the price of the C-frame press machine is also relatively inexpensive, so it is currently the mainstream of the press machine.
However, the punch press capacity is about 300 tons or less.
(2) Straight Side Press
Because of its symmetrical body, the straight side press has the advantage of being able to withstand the eccentric load during operation, but the accessibility of the die during operation is poor.
Generally, this type is used in large machines above 300 tons, with an integrated body and three-stage body.
Selection of punch press
When choosing a punch press, it is important to clearly understand the purpose for which it will be used.
For this reason, it is important to fully understand the processing methods, work methods, press functions, technology trends, etc., as well as to correctly grasp market trends, situations and trends, etc.
However, because of the complexity of the processing method and the function of the press, it is not easy to understand, and in many cases, the selection is not based on a true understanding of the purpose of use.
Particular care should be taken when selecting a punch press especially for rationalization, as there is no room for error.
The wrong choice of the press will result in a reduction in the efficiency of the press and may result in wasted investment in equipment.
Punching capacity has three main elements.
- Pressure capacity
It means the position of the slider at the lower dead center, which is capable of generating tons of pressure, also known as “nominal pressure” or simply “capacity”.
Units are expressed in tonnes.
- Torque capacity
It is the position where pressure capacity is generated, also known as capacity generation point, expressed in mm.
- Working capacity
The maximum effective capacity, expressed in kg-m, that can be produced in one stroke.
How to choose the right punch press
Correctly determining processing methods and work methods
(1) Correct determination of processing methods and engineering
There are various methods of stamping, and sometimes stamping is used in combination with machining.
When selecting a press, first examine whether the desired processing method is appropriate for the target product and whether it is the right processing method and whether it is the right number of processing works.
When deciding on the processing method, the type of press to be selected is also largely determined.
(2) Level of production
If a lot exceeds 3,000 to 5,000 pieces, it is advantageous to use automatic feeding.
When the production volume is large due to many processes, continuous processing and transfer processing should be considered, and high-speed automatic punching and transfer processing should also be considered.
In addition, it is necessary to consider automatic machines such as high-speed automatic presses and transfer presses.
The choice of the universal or automatic press is based on the amount of production, but basically the necessary amount should be considered when necessary.
Always maintain appropriate inventory levels, and consider not only the current but also future production levels, market conditions, and technological trends.
(3) Relationship between shape, quality and size of materials
The shape and quality of the material are determined by the processing method, crop yield, and material utilization rate.
The shape of the material depends on whether it is a roll of material, a fixed-size material or a semi-processed product, and the size of the material, the method of operation is also very different.
(4) How to supply materials, take out products, and dispose of wastes
The above operations are collectively known as material handling.
In production plants, material handling accounts for a large proportion of all-round operations.
Therefore, material handling is not just a position of partial digestion of production, but must be considered from the perspective of rationalization of the whole plant.
The functions required in a press machine are very different depending on the material handling consideration.
(5) How to use the die cushion
Additional die buffers should be considered for single action presses for drawing operations.
Since the die cushion provides high performance, it is possible to perform difficult drawing operations even without the use of a double-action punch press.
A locking device is required as an accessory device to improve the performance of the die cushion.
Select the capacity of the punching machine suitable for processing.
(1) Calculate the machining pressure and the machining pressure stroke curve.
Calculate the maximum pressure required for machining and the change in pressure during the machining stroke.
Multi-processing should find out the pressure stroke curve of each process, overlap and find out the composite pressure stroke curve.
Determine the maximum pressure to be processed and the pressure stroke curve to determine the pressure capacity to be selected.
The decision on work capacity should be based on the frequency of processing (number of cycles per minute).
Punch presses equipped with an automatic feeding device should preferably have a motor with one horsepower increase.
It is preferable not to choose a press capacity that is just above capacity for saturation use, but rather a common capacity of 75-80% of nominal capacity.
(2) Eccentric load, degree of concentrated load
In the case where two or more dies are used on one punching machine or when a continuous punching die is used, eccentric load is of course present. However, most other stamping processes also have eccentric loads.
Since the design of punching capacity is usually based on the center load, it is important to note that the pressure capacity may be reduced if there is an eccentric load.
Therefore, for eccentric load work, select a punching capacity with a sufficient margin.
Most cold forging processes are extreme concentrated loads.
For concentrated load work, choose a press with a small die space as much as possible.
(3) Calculation of the reduction in the effective capacity of the die cushion.
When a die cushion is installed, the extension capacity of the press is equal to the capacity minus the cushion.
Normally, the capacity of the buffer is 1/6th of the nominal capacity of the press.
This may seem small at first glance, but it is definitely not a low value when compared to the effective drawing capacity available for drawing processing near the center of the punch press, but appropriate value.
Although a high cushioning capacity of one-third of the press capacity is required for processing, the effective extension capacity near the midpoint of the stroke on a standard press is significantly reduced (in extreme cases, the ability to push the cushion is lost), so extra care should be taken.
Therefore, for such a high cushioning capacity, the torque capacity of the press should also be selected as high.
If the buffering capacity is too high, the structure will be uneconomical due to the mismatch of torque capacity, so if it is necessary, a double-acting press should be considered.
Clarify the dimensional accuracy of processed products
The necessary accuracy of the processed product is determined by the use of the product and its connection with the following process.
There are many reasons for poor machining accuracy, such as deviations in plate thickness of the material, volume of the stock block (extrusion process), material (related to deformation resistance), degree of lubrication, and die wear, etc., depending on the production process.
If high processing accuracy is required, choose a press with high rigidity or a press with large capacity (sufficiently generous pressure capacity for its processing).
However, C-frame presses are not very effective even with large capacity presses and care should be taken to select a press with high rigidity.
Fully understand the function of the punch press
(1) Adequate investigation of press specifications
Press specifications indicate the main capacity and size of the press and are the basis for selecting a press.
The indication of press capacity usually indicates only pressure capacity, and should be checked by combining torque capacity and working capacity.
The press used for blasting should have a shorter stroke and a higher SPM.
For example, the height of the die, the width of the working surface, the T-groove for the die assembly on the working surface, the pin hole for the buffer, etc. must be determined in relation to the die to be used.
It should be noted that excessive demands on the specifications of the press due to the limitations of the plant may adversely affect the function of the press.
Therefore, if the specifications are fine, you should choose a punch press with standard specifications and one that already functions reliably and has a proven track record.
(2) Selection of press attachments
The adequate inspection of attachments should also be carried out as proper use of attachments increases productivity.
In order to increase production, the best method is to install an automatic feeder for continuous processing.
If it is difficult to process continuously due to the complexity of material supply and product extraction, a timer can be used for continuous operation to improve productivity.
The use of conveyors or UNLOADER units can also be beneficial for increased productivity.
For example, if the die is changed frequently, the quick change (Q.D.C.) device for the die and the die holder are also beneficial.
Consider slow running devices, material changeovers, product stacking devices, etc.
Punching presses with roller feed shall be equipped with a variable-speed device.
In addition, it is necessary to prepare for the future need to install buffers and feeders or rotary cams.
However, if you are equipped with too many complicated attachments, it will increase the failure rate and increase the maintenance trouble, so it is necessary to select the appropriate attachments for the press.
(3) The elasticity of the function should be checked
In order to maintain a good crop rate, the function of the press must be in line with the tendency of the market to replace machines with new ones.
That is, it should examine functions that can be adapted to both high and low volume production, as well as those that can be changed over with future models.
For example, the driving power of the automatic feeder is usually taken from the crankshaft of the punch in order to easily perform synchronization.
If a separate drive feeder is used, it can be easily moved due to its independent power, can be freely combined with any press, and can be adapted to changes in work.
(4) Choose a punch press that is easy to maintain and has a high degree of reliability.
Stamping products are produced in large quantities and usually require several processes, so if a press machine breaks down, it will have a significant impact on the overall production.
For this reason, it is important to select a press that is easy to maintain, easy to inspect, and has a high degree of reliability, especially with regard to the stability and durability of the clutch, brake, and electrical operation.
Punching work is highly hazardous, so safety measures should be taken and safety equipment should be included in the selection of a punch press.
Even in the event of a malfunction, there will be no accident and safe operation is possible.
Therefore, the operation of presses requires a variety of interlocking devices, shear plate and hydraulic overload safety devices, and safety devices for two-hand operation, linear and mechanical operations.
(6) Noise and vibration
Noise and vibration in stamping plants are restricted by law due to public health concerns.
The operating environment is an important issue for the future.
Therefore, it is necessary to check noise and vibration countermeasures for press equipment in the future.
(7) Related to automation
In recent years, the production pattern has changed and the production lot tends to decrease, so the frequency of exchanging molds and materials has increased.
Automation is also necessary to save labor and time.
In other words, the time has come to systematize the automatic stamping and processing line for the selection and configuration of dies and materials, adjustment of punching machines, production operation, and management of production volume.
With today’s technology, it is not a fantastic dream that an unmanned press line can produce the desired product and maintain the required accuracy by remote instructions.
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