Design Specification for Machinability of Structural Sheet Metal Parts

Sheet metal is a comprehensive cold working process for thin metal sheets (usually below 6mm), including shearing, punching/cutting/composite, folding, riveting, splicing, forming (such as automobile body), etc…

B ultrasound, CT machine, inspection equipment, communications with the chassis cabinet and other equipment shell, small batch parts often use laser cutting off, bending, welding, painting, etc. The advantage of this process is that the manufacturing cycle is short compared to the molded parts market response speed.

Punching/laser cutting

Punching is divided into ordinary punching and precision punching; due to the different processing methods, the processing processability of the punching parts is also different. The punching processability is introduced below, which refers to the structural processability of ordinary punching. Laser cutting is the use of high-power density laser beam irradiation of the material to be cut so that the material is quickly heated to the vaporization temperature, evaporation of the formation of holes, with the movement of the beam on the material, the holes are continuously formed in a very narrow width (such as 0.1mm or so) slit, to complete the cutting of the material.

1.1 The shape and size of the punched parts are as simple and symmetrical as possible to minimize scrap when lining up the sample.

 

Figure 3.1.1 Layout of the punched part

1.2 The shape of the punched part and the inner hole should avoid sharp corners.

In a straight line or curve connection to a circular arc connection, arc radius R ≥ 0.5t. (t is the material wall thickness)

Fig. 3.2.1 Minimum values of the radius of the rounded corners of the punched part
1.3 Punched parts should avoid narrow and long cantilevers with narrow grooves
The depth and width of the projecting or recessed part of the punched part should generally be less than 1.5t (t is the material thickness). At the same time, narrow and long cantilevers with excessively narrow slots should be avoided to increase the strength of the edge of the corresponding die part. See Figure 3.3.1.

Figure 3.3.1 Avoid narrow and long cantilevers and recesses

1.4 Punching preference for round holes, punching has a minimum size requirement

Punching preference is given to round holes with minimum punching sizes related to the hole’s shape, the material’s mechanical properties and thickness.

 

 

Materials

Round hole diameter/b

Rectangular hole short side width

High carbon steel

1.3t

1.0t

Mild steel, brass

1.0t

0.7t

Aluminum

0.8t

0.5t

* The list of common material grades of the company corresponding to high-carbon steel and low-carbon steel is shown in Chapter 7, Appendix A. * t is the material’s thickness, and the punch’s minimum size is generally not less than 0.3mm.

Table 1 List of minimum sizes of punched holes

1.5 Hole spacing and hole edge distance for punching

The minimum distance between the punching edge of the part and the shape of the part and the hole has certain restrictions, see Figure 3.5.1. When the punching edge is not parallel to the edge of the shape of the part, this minimum distance should be no less than the material thickness t; when parallel, it should be no less than 1.5t.

Figure 3.5.1 Punching parts hole edge distance, hole spacing diagram

1.6 When punching bent parts and deep-drawn parts, the distance between the hole wall and the straight wall should be maintained

When punching bent or deep-drawn parts, the distance between the hole wall and the straight wall of the workpiece should be maintained (Figure 3.6.1)

Figure 3.6.1 Bending parts, stretching parts hole wall and the distance between the straight wall of the workpiece

1.7 Screws, bolts and countersunk seats through the hole

Screws, bolts and countersunk seats of the structure of the size of the following table are selected to take. For the countersunk head seat of the countersunk head screw, if the plate is too thin to ensure the hole d2 and countersunk hole D simultaneously, priority should be given to ensure the hole d2.

Table 2 Overholes for screws and bolts

 

Table 3 Countersunk head seats and over holes for countersunk head screws

Table 4 Countersunk head seats and over holes for countersunk head rivets

1.8 Limit values and design specifications for burrs on punched parts

1.8.1 Limit values for burrs on stamped parts

It is not allowed to exceed a certain height of the burr of the punching part. The limit value of the height of the burr of the punching part (mm) is shown in the following table.

Material wall thickness

 Material tensile strength  (N/mm2)

 

>100~250

>250~400

>400~630

>630

 

f

m

g

f

m

g

f

m

g

f

m

g

>0.7 ~1.0

0.12

0.17

0.23

0.09

0.13

0.17

0.05

0.07

0.1

0.03

0.04

0.05

>1.0 ~1.6

0.17

0.25

0.34

0.12

0.18

0.24

0.07

0.11

0.15

0.04

0.06

0.08

>1.6 ~2.5

0.25

0.37

0.5

0.18

0.26

0.35

0.11

0.16

0.22

0.06

0.09

0.12

>2.5 ~4.0

0.36

0.54

0.72

0.25

0.37

0.5

0.2

0.3

0.4

0.09

0.13

0.18

* Grade f (precision level) applies to parts with higher requirements; Grade m (medium level) applies to parts with medium requirements; Grade g (rough level) applies to parts with general requirements. Table 5 Limit values of burr height for stamped parts

1.8.2 Requirements for marking burrs in design drawings

* Burr direction: BURR SIDE.

* Part to be burr pressed: COIN or COIN CONTINUE. Generally, do not crimp the entire structural part fracture; this will increase the cost. Try to use in the following cases: exposed fractures; sharp edges often touched by human hands; holes or slots that need to pass cables; parts with relative sliding.

Figure 3.8.2.1 Example of marking burrs in sheet metal structure design drawings          

Bending

Minimum bending radius of bent parts

When the material is bent on its rounded area, the outer layer receives stretching, and the inner layer is compressed. When the material thickness is certain, the smaller the inner r, the more severe the stretching and compression of the material; when the outer corner of the tensile stress exceeds the material’s ultimate strength, it will produce cracks and fractures. Therefore, the structural design of bent parts should avoid too small a bending radius of the corner. The minimum bending radius of commonly used materials is shown in the following table.

Serial number

Materials

Minimum bending radius

08、08F、10、10F、DX2、SPCC、E1-T52、0Cr18Ni9、1Cr18Ni9、1Cr18Ni9Ti、1100-H24、T2

0.4t

15、20、Q235、Q235A、15F

0.5t

25、30、Q255

0.6t

1Cr13、H62(M、Y、Y2、冷轧)

0.8t

45、50

1.0t

55、60

1.5t

65Mn、60SiMn、1Cr17Ni7、1Cr17Ni7-Y、1Cr17Ni7-DY、SUS301、0Cr18Ni9、SUS302

2.0t

l bending radius is the inner radius of the bent part, and t is the wall thickness of the material.

It is the wall thickness of the material, M is the annealed state, Y is the hard state, and Y2 is the 1/2 hard state.

Table 6 commonly used metal materials minimum bending radius list

straight edge height of the bent part

The minimum straight edge height requirements under general conditions

Bending straight edge height should not be too small; the minimum height according to (Figure 4.2.1) requirements: h > 2t.

 

Figure 4.2.1.1 The minimum straight edge height of the bent part

Special requirements of the straight edge height

If the design requires the straight edge height of the bent part h ≤ 2t, then first increase the height of the bent edge, bending and then processing to the required size; or in the bending deformation zone after processing shallow groove, and then bending (as shown in the figure below).

Figure 4.2.2.1 Straight edge height requirements in special cases

Bending on the edge of the hole distance

Hole edge distance: first punching and then bending; the hole’s location should be outside the bending deformation zone to avoid the bending hole will produce deformation. The distance from the hole wall to the bending edge is shown in the table below.

Table 7 Hole edge distance on bent parts

Local bending process notch

The bending line of the bent parts should avoid the location of sudden changes in size

Local bending of a section of the edge, to prevent stress concentration at the sharp corner of the bending crack, the bending line can be moved a certain distance to leave the size of the abrupt change (Figure 4.4.1.1 a) or open the process slot (Figure 4.4.1.1 b), or punching process holes (Figure 4.4.1.1 c). Note the dimensional requirements in the figure: S ≥ R; slot width k ≥ t; slot depth L ≥ t + R + k / 2.

Figure 4.4.1.1 Design treatment of local bending

When the hole is located in the bending deformation zone, the form of the notch adopted

Figure 4.4.2.1 Example of cutout form

Bending edge with a bevelled edge should avoid deformation area

Figure 4.5.1 Bending edge with a bevelled edge should avoid deformation area

The design requirements of the dead hedge

The dead edge length of the dead edge is related to the thickness of the material. As shown in the figure below, the minimum length of the general dead edge L ≥ 3.5t + R.

Where t is the wall thickness of the material, and R is the minimum internal bending radius to kill the edge of the previous process (as shown in the figure below).

Figure 4.6.1 Minimum length of dead edge L

Process positioning holes added at design time

To ensure the accurate positioning of the blank in the mould, to prevent the bending of the blank offset and produce scrap, it should be added in advance in the design of the process positioning holes, as shown in the figure below. Especially for multiple bending and forming parts, all must be positioned with the process hole as the positioning reference to reduce the accumulated error and ensure product quality.

Figure 4.7.1 Process positioning holes added during multiple bending

Consider processability when marking bent part-related dimensions

 Figure 4.8.1 Example of bending parts labelling

As shown in the figure above, a) Punching first and then bending, L size accuracy is easy to ensure, easy processing. b) and c) If the size L accuracy requirements are high, it is necessary to bend first and then process the hole, causing processing trouble.

Bending parts of the rebound

Many factors affect the rebound, including the material’s mechanical properties, wall thickness, bending radius and the positive pressure when bending.

The larger the ratio of the bending radius to the plate thickness, the greater the rebound.

Examples of methods to suppress rebound from the design

The best part’s rebound, mainly by the manufacturer in the mould’s design, takes certain measures to circumvent. At the same time, the design of certain structural improvements to promote the rebound angle is less simple, as shown below: in the bending area, pressed reinforcement not only improves the stiffness of the workpiece but is also conducive to inhibiting rebound.

Figure 4.9.2.1 Example of a design to suppress spring back

Tensioning

The size of the radius of the corner between the bottom of the tensile part and the straight wall is required

As shown in the figure below, the corner’s radius between the bottom of the tensile part and the straight wall should be greater than the plate thickness, that is, r1 ≥ t. To make the stretching go more smoothly, generally take r1 = (3 ~ 5) t. The maximum corner radius should be less than or equal to 8 times the plate thickness, r1 ≤ 8t.

Figure 5.1.1 Radius size of the corner of the tension member

The radius of the corner between the flange of the tensile part and the wall

The radius of the corner between the flange of the tensioning member and the wall should be greater than 2 times the thickness of the plate, that is, r2 ≥ 2t. To make the stretching go more smoothly, generally take r2 = (5 ~ 10)t; the maximum flange radius should be less than or equal to 8 times the thickness of the plate, that is, r2 ≤ 8t. (see Figure 5.1.1)   

The diameter of the inner cavity of the circular tensioning parts

The diameter of the inner cavity of the round stretching parts should be taken as D ≥ d + 10t so that the pressure plate is not wrinkled when stretching. (See Figure 5.1.1)

Rectangular stretching parts adjacent to the radius of the corner between the two walls

The radius of the corner between the two adjacent walls of the rectangular stretching parts should be taken r3 ≥ 3t, and to reduce the number of stretching should be taken r3 ≥ H/5 as far as possible to pull out once.

Figure 5.4.1 The radius of the fillet between the two adjacent walls of a rectangular stretching part

The dimensional relationship between the height and the diameter of a round flangeless stretching part formed in one go

The height H to diameter d should be less than or equal to 0.4, i.e. H/d ≤ 0.4 when forming a round flangeless stretching part at one time, as shown in the figure below.

Figure 5.5.1 Dimensional relationship between height and diameter of a round flangeless tensioned part in one forming

Notes on dimensioning on the drawing for the design of tensioned parts

The thickness of the material after stretching changes due to the different magnitudes of stresses at various places. Generally speaking, the original thickness is maintained at the bottom centre, 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 stretching parts becomes thicker.

Standard method of product dimensions for drawn parts

In the design of stretching products, the dimensions on the product drawing should clearly indicate that external or internal dimensions must be guaranteed, not internal and external dimensions.

The labelling method of dimensional tolerances of tensioned parts

The inner radius of the concave and convex arc of the tensioned parts and the height dimension tolerance of the cylindrical tensioned parts formed at one time are double-sided symmetrical deviations, and the deviation value is half of the absolute value of the national standard (GB) 16-level accuracy tolerance and crowned with ± sign.

Forming

Reinforcement

The plate metal parts on the pressed tendons help to increase the rigidity of the structure, reinforcement structure and size selection see Table 6.

Table 8 Selection of reinforcement structure and size

Limiting dimensions of the pitch and edge distance

The pitch and edge distance limit sizes are selected according to the table below.

Table 9 limit the size of playing convex spacing and convex edge distance

Shutter

he louvre is usually used for ventilation and heat dissipation on various hoods or housings. It is formed by cutting the material with the edge of the convex die on one side and stretching the material with the rest of the die to form an undulating shape with an opening on one side.

A typical structure of a louvre is shown in Figure 6.3.1

 

Figure 6.3.1 The structure of the shutter

Shutter size requirements: a ≥ 4t; b ≥ 6t; h ≤ 5t; L ≥ 24t; r ≥ 0.5t.

Hole flap

The hole flanging type is more; this specification only concerns the inner hole flanging to be processed thread, as shown in Figure 6.4.1.

Fig. 6.4.1 Schematic diagram of the flanged structure of the inner hole with threaded holes

 

 

Table 10 Dimensional parameters of internal hole flanges with threaded holes