1、 Classification of common nut cold heading process
Hexagon nut is also a fastener with a wide range of surface. It has many production methods. Nuts with M24 specifications are generally produced by cold heading (pressing). The common cold heading process of nuts is as follows:
a. Cold heading of small diameter wire rod to produce nuts
This is a method of producing nuts by cold heading. The diameter of wire rod do = 0.60s ~ 0.70s, s - the size of nut. The working stations (procedures) of cutting material, shaping, upsetting, pressing six square and punching are used, as shown in Fig. 36-23.
It can be produced in three and four position automatic cold heading machine, and can also be produced in sequence on press. The production of the three position cold heading machine can save the shaping, but the nut larger than M12 specification can not be shaped, and the end quality and the uniformity of the bald angle are not well controlled.
b. Production of nuts by cold heading of larger diameter wire rod
This process is made by cutting, shaping, initial heading, pre forming, fine forming and punching. It is generally produced on the five position automatic cold heading machine. The clamp is equipped with turning mechanism.
c. Forming technology of hexagonal steel
This process is used less, generally used in the production of M20 or more large-size nuts, and is completed by the method of split sequence cold pressing on the press. The process is produced according to cutting material, initial pressure, fine pressure and punching.
2、 Analysis of cold heading (pressing) process of nuts
a. Cut off
Cutting off is the first and the key process in the multi station production of automatic cold heading machine or on press.
Because of the flatness of the cutting fracture and the size of horseshoe seal formed by the cutting plate pressing (see Fig. 36-25), they all have direct influence on the shaping and upsetting of the lower order.
The cutting length can be calculated from formula 36-22
Where lo - cutting length mm V-shape - Blank volume (mm 3) fo before nut punching - wire cross section area m M2
This is only a calculation value, and the cutting length should be adjusted by adjusting the material column in actual production. Sometimes, the weight method is used to measure whether the cutting material is accurate, that is, the blank weight is equal to the cut-off column weight. The diameter of the cutting die shall be 0.05-0.1mm larger than the maximum diameter of the material, and the gap between the cutter plate and the cutting die is about 0.1mm.
b. Plastic
As shown in figure 36-26, the shaping is to upsetting the end face of the material column and upsetting (pressing) the chamfer of 1-2 × 45 ° at the lower end, so as to trim the cutting defects to ensure the quality of the next ball pressing process.
The size of plastic shaping d = do+ (0.1-0.25) (mm)
Where do - wire diameter mm.
c. Upsetting ball
Upsetting ball is to upset (press) the shaped material column into a drum shaped ball. See Fig. 36-27. Its quality affects the clarity and quality of the end face, bald angle and edge of the nut. When determining the geometrical dimension of drum ball, according to experience, the dimension of DM and H should be as small as possible under the condition of chamfer 40 °.
In this way, when pressing the six square, the friction force of the corresponding parts is small. Under the action of the pressure type force, the metal has good fluidity and is easy to fill the six sides. If DM and H are larger, it is not easy to fill the hexagon when pressing the six sides. If the pressure force is increased to fill the six sides, the end face of the nut will generate a flying edge.
The drum ball size is as follows according to the experience data: dm= (0.7-0.8) d diameter Dmax ≤ smin where d diameter - nominal diameter of nut mmdmax - maximum diameter of drum ball mmsmin - minimum dimension of nut s square mm
According to the dimension of DM and D and the volume of nut blank, other dimensions of drum ball can be calculated as follows:
d. Press type
Pressing, that is, upsetting the six sides of the nut, so that it meets the requirements of the overall dimension of the hexagon nut.
Whether the deformation size is reasonable directly affects the quality of the product and the life of the die.
The main factors to be considered in the dimension of the pressing six-way are: the demoulding of the hexagonal billet in the hexagonal die and the expansion of the lower punching hole.
Therefore, it is required that there is an inclined angle γ (see Fig. 36-28) on the side of the nut, and its size is larger with the increase of specification. For example, for nuts with more than M10, γ is generally taken as 0 °.30 ′～ 1 °, if the angle is too large, the size difference between the upper and lower ports of the hexagon concave die is too large, which will make the six-way blanking (also known as the pressing lower die) not positioned stably in the sleeve die, which will easily cause the upsetting nut blank eccentricity and make the verticality of the nut（ β) The s dimension can not meet the standard requirements after punching and expanding. The actual value of 0.30 ′ - 1 ° of γ is determined by the actual production experience.
Besides this dimension, there are many dimensions directly related to the external dimension of the nut and the appearance of the product (see Fig. 36-29), which indicates the size of the nut pressing blank.
Among them, the geometric size of the concave is very important. D1 is a key size, which is small, and the punch is easy to produce burr; if it is too large, the punch is easy to appear bell mouth, which affects the integrity of internal thread.
The empirical data are as follows: 8:d1 = D small max+ (0.02-0.04) mmm8-m14:d1 = D small max+ (0.05-0.10) mmm14-m18:d1 = D small max+ (010-0.15) mmm18-m24:d1 = D small max+ (0.15-0.30) mm formula: D small max - maximum diameter of thread diameter in nut (mm) d= (1.05-1.1) d diameter formula, p= "" d diameter - nut nominal diameter (mm) < = ">
D size is too small, which is not conducive to upsetting and pressing of nuts, and is not conducive to metal flow, and unclear hexagon; D dimension is too large, and the bearing surface of nut is reduced, which affects appearance and fastening strength.
After the dimension of D1 and D is determined, the internal chamfer of the standard nut is about 120 °, generally 106 °, which is because the internal chamfer is smaller. According to the formula, the H dimension can be larger, which can save steel, and the deformation of nut during pressing is favorable, and the thickness of punching joint (i.e. the iron bean punched out of the punch) can be reduced, which is conducive to punching.
H= (d--d1) tg37 ° (formula 36-25)
The other important dimensions of the concave are H1 and α angle, which have an effect on the blanking of the hexagon from the hexagon die after the nut is upset and pressed.
H1 should not be too high, which will affect the nut hexagon billet to be washed out of the six-way lower die in time, and then the next blank enters the concave die, which causes heavy cap and failure.
The empirical data were as follows: H1 < 0.30mmm8-m10: h1= h1= "(0.6-1.0) mmm18-m24:h1= (1.2-1.6) mm < / P" ">
For the nut above M20, H1 of the press upper die is higher than the lower die (0.30-0.50) mm, which is more favorable for cold heading deformation.
α is generally taken as 10 ° - 15 °. After H1 and α are determined, D2 dimension can be calculated as follows
The top of the concave is a cone, and the angle of the cone is 150 °, then the angle of the cone is tg15 °, and the height of the whole concave is: h2 = H + H1 + tg15 ° (formula 36-27)
The size of the concave is not generally used as the inspection basis, and the size of the die is guaranteed. The above data are based on gb/t6170-2000 standard nuts. Not fully applicable for other types of nuts.
e. Punch
The size and quality of punch are all to meet the requirements of the next sequence tapping thread. The diameter of the inner hole of the nut is generally determined by the maximum size of the small diameter.
Considering that the hardness of steel has to affect the quality of punching, the hole diameter can be determined between the minimum and maximum size of the nut minor diameter, and the operator can flexibly master the hole size within the tolerance range. In fact, considering the factors of tapping, the tolerance of punching size is smaller than that of minor diameter.
3、 Problems that must be paid attention to in punching
1. the problem of s Square expansion after the nut punching
Punching is actually punching the blank. The punching surface of inner hole has punching plane and tearing surface (Fig. 36-30).
The punching force produced by the hole punching on the inner hole causes friction between the contact surface of the hole and the inner hole, which is opposite to the downward punching direction of the hole punch. The additional stress formed thus causes the radial tension, which makes the s Square expand radially, namely the expansion square.
Obviously, the expansion square is related to the punching rigidity and sharp edge of the hole, and also the material of the screw blank.
The expansion of low carbon steel is larger than that of medium carbon steel, and that of ordinary carbon steel is larger than that of high-quality steel with the same carbon content.
This can be explained by the increase of the cutting performance of steel with the increase of carbon content. Of course, because of the increase of carbon content and strength of steel, it also requires higher requirements for the strength and toughness of the pore punching.
In addition, the expansion square is related to the ratio of the opposite dimension (i.e. opposite side width) s of the nut to the height m of the nut. Table 36-4 lists the expansion square value after punching some specifications of the nut.
Even if these problems are noticed, the problem of "s" side is not solved because of the change of nut material (medium carbon steel or alloy steel). It is more prominent in M16 and above. In order to solve the problem of the s side's excess deviation due to the punch expansion, the following measures can be taken:
a. The size of punching hole is reduced and reaming is increased, and the reaming allowance is 0.5-1mm;
b. Two punching is used, and the second punch allowance is about 1 mm. There is no expansion in the second punch;
c. A hexagonal die is added in front of the punch die to prevent the nut s surface from expanding. The thickness of the hexagon die is slightly higher than the nut height M. the die mouth is rounded to facilitate the blank entering the die.
The die cavity should have a die taper of 0 ° 10 ′～ 0 ° 15 '. With this structure, even the hexagon thick nuts (gb/t56d=16, m=25; d=20, m=32; d=24, m=38) can also be produced by cold heading.
Table 36-4 expansion square value of part specification nuts after punching.
The six square die of upsetting nut should have taper, one is to make the blank easy to be ejected and demoulded, the other is to compensate the expanding square value of the punch, so that the dimension of the nut s Square is not too bad due to the expansion square. As shown in figure 36-31, the angle of γ above M10 is 0 ° 30 ′ - 1 °, and with the increase of nut specification, the γ angle also increases, and the maximum value should not exceed 1 °
d. The raised head size of the punch is improved, that is, H1 in the concave size of the blank at both ends of the blank after the nut is pressed.
If the part H 1 is increased properly, that is to say, the thickness of punching joint skin can be reduced, and the expansion of the punch can be improved. However, H1 should not be too high, which is not good for the blank to leave the boss, and it is easy to produce heavy materials (i.e. the first blank does not leave, and the second blank will come).
e. The problem of expanding square can be solved by using reverse punching.
2. roughness and roundness of holes
In order to achieve the minimum roughness and get a round inner hole, the gap between punch punch die of cold heading nut is smaller than that of the general punching die. It is hoped that more than 80% of the inner wall of the hole is bright (see Fig. 36-30), and the tear band shall not exceed 20% of the hole wall.
With small clearance punch, there is sometimes another quality problem: "slot hole", see figure 36-32. "Slot" is caused by secondary bright band produced during punching.
The quality of the punch hole is related to the geometry of punch punch and the clearance of punch die. There are three types of die for punching holes of cold heading nuts used in production
a. Punch die for convex table
As shown in figure 36-33.
The concave die has a boss at the edge of the die, which is suitable for punching the middle and small nuts under M12. The clearance between the concave and convex die is (0.03 ~ 0.15) mm.
Its advantage is that it is easy to locate when punching, and the punched hole fracture zone is less, and "bell mouth" is not serious.
The disadvantage is that when the punching speed is slow, there will be "slot hole". When replacing a new hole punch, the edge of the hole punch is sharp, it may also appear "slot hole". At this time, as long as the sand paper is used to round the edge of the hole, it can play the role of squeezing the punching surface during punching, and the appearance of "groove hole" can be avoided.
With this die, the punch boss H1 of six-way side should not be too high, too high. Iron chips are easily produced during punching, and it is stuck on the concave die surface to make the nut end face have indentation and affect the appearance.
b. Straight punch die
As shown in figure 36-34, the gap between these dies can be slightly larger than the above-mentioned die, and the life of such die is also long. The disadvantage is that burr is easy to be generated when the punching speed is slow, or one side is torn, which exceeds the ordinary fracture zone, sometimes extends to the chamfer in the nut (see Fig. 36-30), which causes incomplete buckle during tapping.
This phenomenon is easy to occur when the low strength nut is punched, which causes the quality instability.
c. Punch die with fillet
As shown in figure 36-35, the inner hole port of this kind of die has a round corner of r= (2-3) mm, and the gap between convex and concave die can be larger, which is generally used for above M14. The disadvantage is that the broken zone of the punched hole is large, that is, the "bell mouth" is large. Generally, the hole is reamed to make the hole round and smooth, and meet the dimensional requirements. When punching low strength nuts, it will also tear one side to the inner chamfer. The advantage is that the die has a long life.
