Views: 93 Author: Site Editor Publish Time: 2021-08-17 Origin: Site
Sheet metal processing is a comprehensive cold processing process for metal sheets (usually below 6mm), including shearing, punching, bending, welding, riveting, mold forming, and surface treatment. Its notable feature is the same-part thickness-uniformity.
Non-mold processing: The process method of sheet metal processing through counting punches, laser cutting, plate shears, bending machines, rivet machines and other equipment. It is generally used for sample production or small batch production, and the cost is relatively high. The processing cycle is short and the response is quick.
Mold processing: The sheet metal is processed through a fixed mold. Generally, there are blanking molds and forming molds, which are mainly used for large batches of the shovel, and the cost is low. The pre-mold cost is high, and the quality of the parts is guaranteed. The pre-processing cycle is long and the mold cost is high.
Blanking: digital punching, laser cutting, shearing machine.
Forming: bending, stretching, punching.
Other processing: pressure riveting, tapping, etc.
Welding: the connection method of sheet metal.
Surface treatment: powder spraying, electroplating, wire drawing, silkscreen, etc.
Sheet metal blanking methods mainly include digital punching, laser cutting, plate shearing, die blanking, etc. Numerical control is the current commonly used method. Laser cutting is mostly used in the proofing stage, and the processing cost is high. Mold blanking is mostly used for mass processing.
Below we mainly introduce the blanking of sheet metal with the number of punches.
Numerical punching is also called turret CNC punching machine, which can be used for blanking, punching, drawing holes, rolling bars, punching shutters, etc., and the machining accuracy can reach +/-0.1mm.
The thickness of the plate that can be processed by CNC is:
| Cold rolled sheet | ≤4.0mm |
| Aluminum plate | ≤5.0mm |
| Stainless steel plate | ≤2.0mm |
1. There is a minimum size requirement for punching. The minimum size of the punching hole is related to the shape of the hole, the mechanical properties of the material, and the thickness of the material.
2. The number of punched holes and hole margins. When the minimum distance between the punching edge of the part and the shape is not parallel with the shape of the part, the minimum distance should not be less than the material thickness t; when it is parallel, it should not be less than 1.5t.
3. When drawing a hole, the minimum distance between the drawing hole and the edge is 3T, the minimum distance between two drawing holes is 6T, and the minimum safety distance between the drawing hole and the bending edge (inside) is 3T+R (T is the sheet Gold thickness, R is the bend fillet).
4. When drawing the bending parts and drawing parts for punching, a certain distance should be kept between the hole wall and the straight wall.
The forming of sheet metal is mainly the bending and stretching of sheet metal.
1. Sheet metal bending
● Sheet metal bending mainly uses bending machine tools.
● The basic principle of the bending process sequence: bending from the inside to the outside, bending from small to large, bending the special shape first, and after the previous process is formed, it will not affect or interfere with the subsequent process.
● Common tool shapes
Common V groove shape
● The minimum bending radius of the bending part:
When the material is bent, the outer layer is stretched while the inner layer is compressed on rounded corners. When the material thickness-timing, the smaller the inner r, the more severe the stretching and compression of the material; when the tensile stress of the outer fillet exceeds the ultimate strength of the material, cracks and breakage will occur, therefore, the structure of the bent part The design should avoid too small bend radius. The minimum folding radius of the company's commonly used materials is shown in the table below.
● Height of the bending part:
Under normal circumstances, the minimum straight edge height should not be too small, the minimum height requirement: h>2t
If the straight edge height h≤2t of the bending part is required, the height of the flange must be increased first, and then processed to the required size after bending; or after the shallow groove is processed in the bending deformation zone, the bending is performed.
● 1.6 The minimum bend straight edge height with beveled sides:
When the bent side has a beveled part, the minimum height of the side is: h= (2~4) t> 3mm
● The hole margin on the bending part:
Hole margin: punch holes first and then bend. The holes should be located outside the bending deformation zone to avoid deformation of the holes during bending. The distance from the hole wall to the flange is shown in the table below.
● Partially curved process cut:
The bending line of the bending piece should avoid the position of sudden size change. When a certain segment of the edge is partially bent, in order to prevent the sharp corners from stress concentration and bend cracks, the bending line can be moved a certain distance to leave the sudden size change, or process grooves or holes can be punched. Note the size requirements in the figure: S≥R; slot width k≥t; slot depth L≥t+R+k/2.
● Bending edges with beveled edges should avoid the deformation zone:
● Design requirements for sheet metal hemming (dead edge):
The length of one side of the sheet metal fold is related to the thickness of the material. As shown in the figure below, generally the minimum length of the dead edge is L≥3.5t+R. Among them, t is the wall thickness of the material, and R is the minimum inner bending radius in the previous process of the dead edge.
● Added process positioning holes:
In order to ensure the accurate positioning of the blank in the mold and prevent the blank from shifting during bending and producing waste products, process positioning holes should be added in the design in advance, as shown in the figure below. Especially for parts that are bent and formed multiple times, the process hole must be used as the positioning reference to reduce the cumulative error and ensure the quality of the product.
● When marking the relevant dimensions of the bent part, the manufacturability should be considered:
As shown in the figure above, a) Punch holes first and then bend, so that the dimensional accuracy is easy to guarantee and the processing is convenient. b) and c) If the dimension L requires high precision, it is necessary to first bend and then process the hole, which is troublesome.
● There are many factors that affect the springback of the bending parts, including the mechanical properties of the material, the wall thickness, the bending radius, and the positive pressure during bending. The larger the ratio of the inner corner radius of the bending piece to the plate thickness, the greater the springback. Examples of methods to suppress springback in design.
The springback of bent parts is currently mainly circumvented by manufacturers taking certain measures during mold design. At the same time, improving certain structures from the design to reduce the rebound angle is as shown in the following figure: pressing the stiffener in the bending zone can not only improve the rigidity of the workpiece but also help suppress the rebound.
The drawing of sheet metal is mainly done by numerical control or general punching, and various drawing punches or dies are required.
The shape of the stretched parts should be as simple and symmetrical as possible and stretched as much as possible at one time.
For parts that need to be stretched multiple times, the traces that may be produced on the surface during the stretching process should be allowed.
Under the premise of ensuring the assembly requirements, a certain inclination of the stretched sidewall should be allowed.
● Requirements for the radius of the fillet between the bottom of the drawing part and the straight wall:
As shown in the figure, the fillet radius between the bottom of the tensile piece and the straight wall should be greater than the plate thickness, that is, r1≥t. In order to make the stretching process more smoothly, r1= (3~5) t is generally taken, and the maximum fillet radius should be less than or equal to 8 times the plate thickness, that is, r1≤8t.
● The radius of the fillet between the flange and the wall of the stretched part
The fillet radius between the flange and the wall of the stretched part should be greater than 2 times the thickness of the plate, that is, rz≥2t. In order to make the stretching process more smoothly, generally take r2=(5~10)t, the largest flange The radius should be less than or equal to 8 times the plate thickness, that is, r2≤8t.
●The diameter of the inner cavity of the circular drawing part
The diameter of the inner cavity of the circular stretching piece should be D≥d+ 10t, so that the pressing plate will not be wrinkled when it is stretched.
●The radius of the rounded corners between two adjacent walls of the rectangular stretched part
The fillet radius between the two adjacent walls of the rectangular stretched part should be r3≥3t. In order to reduce the number of stretching, r3≥H/5 should be selected as much as possible, so that it can be pulled out at one time.
●When a round non-flange stretched part is formed at one time, the size relationship between its height and diameter is required
When a circular flangeless tensile part is formed at one time, the ratio of the height H to the diameter d should be less than or equal to 0.4, that is, H/d s0.4.
● The thickness change of the stretched part material:
Due to the different stress levels on the stretched parts, the thickness of the stretched material changes. Generally speaking, the center of the bottom maintains the original thickness, the material at the bottom rounded corners becomes thinner, the material at the top near the flange becomes thicker, and the material at the rounded corners around the rectangular stretched part becomes thicker.
● Marking method of product size of stretched parts
When designing a stretched product, the dimensions on the product drawing should indicate that the external or internal dimensions must be guaranteed, and the internal and external dimensions cannot be marked at the same time.
● Marking method of dimensional tolerance of tensile parts
The inner radius of the concave-convex arc of the drawing part and the height dimensional tolerance of the once-formed cylindrical drawing part are the double-sided symmetrical deviation, and the deviation value is half of the absolute value of the 16-level accuracy tolerance of the national standard (GB) and is prefixed with a sign.
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