Cold heading forming process (6)

The following two aspects must be paid attention to when punching:

(1) The problem of square expansion after nut punching

Punching is actually punching out the blank. The punching surface of the inner hole has a punching surface and a tearing surface (Figure 36-30). The punching force produced by the hole punch on the inner hole causes friction on the contact surface between the hole punch and the inner hole, which is opposite to the downward punching direction of the hole punch. The additional stress thus formed causes radial tension, causing radial expansion in the s direction. , that is, the expansion formula. Obviously, the size of the bulge is related to the punching stiffness, the sharpness of the blade, and the material of the screw blank. Low carbon steel has a larger expansion area than medium carbon steel, and ordinary carbon steel has a larger expansion area than high-quality steel with the same carbon content. This can be explained by the fact that the cutting performance of steel increases with the increase of carbon content. Of course, as the carbon content of steel increases and its strength increases, its requirements for the strength and toughness of hole punching are also higher.

In addition, the bulge is related to the ratio of the nut's opposite dimension (i.e., the width across opposite sides) s to the nut height m. Table 36-4 lists the bulge values after punching some nuts.

Figure 36-30 Single sided tearing of punched holes

Even if these problems are noticed, often due to changes in the material of the nut (the material is medium carbon steel or alloy steel), the problem of S-square deviation due to square expansion cannot be solved, which is more prominent in M16 and above specifications. In order to solve the problem of S square being out of tolerance due to punching and expansion, the following measures can be taken:

a. Reduce the punching size, increase the reaming, and the reaming allowance is 0.5~1mm;

b. Use two punches, and the second punching allowance is about 1 mm. There is no expansion during the second punching;

c. Add a hexagonal die piece in front of the punching die to prevent the nut S surface from swelling. The thickness of the hexagonal die piece is slightly higher than the nut height m, and the die opening is rounded to facilitate the blank entering the die. The mold cavity must have a mold ejection taper of 0°10′~0°15′. With this structure, even hexagonal thick nuts (GB/T 56D=16, m=25; D=20, m=32; D=24, m=38) can be produced by cold heading.

Figure 36-31 Reference diagram for nut angles above M10

Table 36-4 Expansion values after punching of nuts of certain specifications.

Specifications (GB/T 6170) s/m

max Punching skin thickness (mm) Punching expansion value (mm) Nut material Hardness HB

M6 10/4.9 2.5 0.06～0.23 A3 103～107

M8 13/6.44 3.56 0.01～0.06 ML20 152～217

M10 16/8.04 5.1 0.01～0.06 ML20 200～246

M12 18/10.37 6.33 0.35～0.40 ML20

M14 21/12.1 7.85 0.23～0.33 ML20 180～204

M16 24/14.1 7.85 0.28～0.40 ML35 175～207

M20 30/16.9 10.1 0.70～0.80 ML20

The hexagonal die for upsetting the nut must have a taper. First, it makes it easier to eject the nut blank from the mold. Second, it compensates for the bulge value of the punching hole so that the s-square size of the nut does not exceed the tolerance due to the bulge. As shown in Figure 36-31, the γ angle above M10 is 0°30′~1°. As the nut size increases, the γ angle also increases, and the maximum should not exceed 1°.

d. Improve the boss size of the hexagonal punch, that is, h1 in the cavity size at both ends of the blank after the nut is pressed (see Figure 36-29). Appropriately raising the h1 part, that is, reducing the thickness of the punching and skinning, can improve the square expansion during punching. However, h1 should not be too high. If it is too high, it will be unfavorable for the blank to separate from the boss, and it is easy to produce heavy materials (that is, the first blank is not there). If it comes off, the second blank will come) and cause an accident.

e. Using back-punching holes can solve the problem of square expansion.

(2) Roughness and roundness of the hole

In order to minimize the roughness and obtain a rounder inner hole, the gap between the convex and concave dies for cold heading nut punching is required to be smaller than that of general punching dies. It is hoped that more than 80% of the inner wall of the hole will be a bright band (see Figure 36 -30), the tear zone does not exceed 20% of the hole wall. When punching with small gaps, another quality problem sometimes occurs: "slotted holes", see Figure 36-32. "Slotted holes" are caused by secondary bright bands produced during punching.

Figure 36-32 Schematic diagram of "slot hole" generated by nut punching

The quality of the punched inner hole is related to the geometric shape of the punching convex and concave dies and the gap between the convex and concave dies. There are generally three types of cold heading nut punching dies used in production:

a. Boss type punching die

As shown in Figure 36-33, the cutting edge of this type of die has a boss, which is suitable for punching nuts of medium and small sizes below M12. The gap between the male and female die is (0.03~0.15) mm. Its advantages are that it is easy to position when punching, the punched hole has fewer fracture zones, and the "bell mouth" is not serious. The disadvantage is that when the punching speed is slow, "slotted holes" will be produced. When a new hole punch is replaced and the cutting edge of the hole punch is sharper, "slotted holes" may also appear. In this case, just use sandpaper to punch the hole. The rounded corners of the mouth sand can play a role in squeezing the punched surface when punching, and can avoid the appearance of "slots". When using this kind of die, the hexagonal lower punch boss h1 should not be too high. If it is too high, iron filings will easily be produced during punching and stick to the die surface, causing indentations on the end face of the nut and affecting the appearance.

b. Straight punching die

As shown in Figure 36-34, the gap of this type of die can be slightly larger than the above die, and the lifespan is also longer. The disadvantage is: when the punching speed is slow, burrs are easily produced, or a piece is torn on one side, exceeding the ordinary fracture zone, sometimes extending to the inner chamfer of the nut (see Figure 36-30), resulting in failure to buckle during tapping. whole. This phenomenon is easy to occur when punching low-strength nuts, causing unstable quality.

c. Punching die with rounded corners

As shown in Figure 36-35, the inner hole port of this type of female mold has a rounded corner with r = (2~3) mm, and the gap between the male and female molds can be larger. It is generally used for M14 and above. The disadvantage is that the punched hole has a large fracture zone, that is, a large "trumpet". The hole is generally reamed to make the hole round and smooth to meet the size requirements. When punching a low-strength nut, it will also be torn to the inner chamfer on one side. The advantage is that the mold has a longer life.

Figure 36-34 Straight type

Figure 36-35 Punching die with rounded corners

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