Cold heading process and accuracy method for fasteners

Cold heading is a forging method that uses a mold to upset metal bars at room temperature, usually used to manufacture the heads of screws, bolts, rivets, etc. It can reduce or replace cutting processing. Forging materials can be copper, aluminum, carbon steel, alloy steel, stainless steel, and titanium alloy. Cold heading is often carried out on specialized cold heading machines, facilitating continuous, multi-station, and automated production. On a cold heading machine, it is possible to sequentially complete processes such as cutting, heading, accumulation, forming, chamfering, wire rolling, diameter reduction, and edge cutting.

Let’s briefly introduce how to use cold heading knowledge.

1、 Brief Analysis of the Characteristics of Cold Heading Technology

(1) According to the theory of metal plastic deformation, a certain pressure is applied to the metal billet at room temperature to produce plastic deformation in the mold cavity, forming according to the specified shape and size.
(2) It is necessary to choose high-quality metal materials with good plasticity, which have strict standards for chemical composition and mechanical properties.
(3) There are multiple models and series of cold heading bolt and nut forming machines available, with reliable equipment performance, high efficiency, and stable quality.
(4) The forming and forging force of the workpiece is large, and the configuration of power equipment is expensive.
(5) The workpiece has good surface quality and high dimensional accuracy, but due to the presence of cold work hardening during the forging process, the deformation should not be too large.
(6) The cold heading process is applicable to large batches of workpieces of various specifications.

2、 Reasons for affecting upsetting accuracy

(1) Chemical composition of raw materials: Pure metals have better plasticity than alloys, and impurity elements usually cause brittleness to decrease plasticity. Various alloys have different effects on plasticity.
(2) Raw material metallographic structure: Different properties, shapes, sizes, quantities, and distribution states of multiphase structures have varying degrees of impact on plasticity. Defects such as grain and segregation, inclusions, bubbles, and porosity can reduce the plasticity of the metal.
(3) Process deformation temperature: As the temperature increases, the plasticity increases, but this increase is not a simple linear increase.
(4) Metal strain rate: The increase in strain rate has both a decrease in metal plasticity and an increase in metal plasticity. The combined effect of these two factors ultimately determines the changes in metal plasticity.
(5) Deformation mechanics conditions: Compressive strain is beneficial for the development of plasticity, while tensile strain is unfavorable for plasticity. The plastic processing method with a three-dimensional compression principal stress diagram and a two-dimensional compression principal strain diagram is extremely conducive to the development of metal plasticity.
(6) Other problem factors: In the case of discontinuous deformation, the plasticity of metals can also be improved, especially when low plasticity metals undergo hot deformation.

3、 Selection of Cold Heading Machine Equipment

(1) The connection between the crankshaft and the body, as well as the impact connecting rod, is made of high wear-resistant alloy copper tiles, which have high bearing capacity, long service life, and low maintenance costs.
(2) The body is cast from ductile iron with added alloy, which has high tensile strength and good wear resistance.
(3) Adopting a two-stage gear transmission system, with high transmission efficiency and large transmission torque.
(4) Equipped with pneumatic clutch brake to reduce motor power and energy consumption.
(5) The cutting system adopts a guide plate to drive the cutting rod, which moves back and forth, and the cutting force is transmitted in a straight line. The force is large, stable, and has good dynamic balance.
(6) The multi-station cold heading machine adopts a tension closed clamp to transport the workpiece, and the clamp system can flip or translate, which is conducive to the arrangement of the forming process.
(7) Equipped with a variable frequency speed regulation device, it can achieve stepless speed regulation within a certain range.
(8) Equipped with fault detectors and safety protection devices, the equipment automatically shuts down in case of failure, providing great protection for equipment and molds.
(9) Install a thrust device on the feeding box to improve the feeding accuracy.
(10) The lubrication system oil circuit design is simple and efficient, which can effectively protect the punching rod and workpiece while ensuring circulation filtration.

4、 Selection of raw materials for workpieces

(1) The raw materials must undergo spheroidization annealing treatment, and their metallographic structure is spherical pearlite.
(2) In order to minimize the tendency of material cracking and improve the service life of the mold, it is also necessary to ensure that the cold drawn material has the lowest possible hardness to improve plasticity.
(3) The full size accuracy of raw materials should generally be determined based on the specific requirements and process conditions of the product, while multi-station cold heading machines generally have lower requirements for the accuracy of diameter reduction and strong shrinkage dimensions.
(4) The surface quality of raw materials requires that the lubricating film be dull and dark, and the surface must not have defects such as scratches, folds, cracks, burrs, rust, oxide skin, and pits.
(5) The total thickness of the decarburization layer in the radial direction of the raw material is required to not exceed 1% of the diameter of the raw material.
(6) In order to ensure the cutting quality during cold forming, it is required that the raw materials have a hard surface and a soft core.
(7) Raw materials should undergo cold heading tests, as cold work hardening increases deformation resistance. The lower the sensitivity of the material to cold work hardening, the better.

5、 The selection of cold heading forming oil

(1) has excellent extreme pressure lubrication performance, high-temperature resistance, extreme pressure wear resistance, rust resistance, and high-temperature oxidation stability. Improving the accuracy and smoothness of processed parts can prevent mold sintering, meet the multi station forming processing of standard and non-standard parts, and be fully capable of processing cold heading stainless steel workpieces.

(2) It can reduce oil fumes and “drifting mist” to a very low limit, meeting the strict requirements of multi mold punch machining nuts with strong shrinkage molds.
(3) It has good processing performance for products with large deformation, such as hollow, high-strength bolts, sleeves, stainless steel hollow, semi hollow rivets, etc.
(4) It can effectively protect the punching rod (needle) and mold, extend their service life, and reduce overall costs.
(5) Good heat dissipation effect, avoiding excessive temperature in the workpiece and mold during the cold forming process.
(6) High temperature resistance and good performance, not easy to produce oil sludge. Low odor, low smoke, protecting the operating environment.
(7) It has a certain degree of rust resistance and can meet the rust prevention requirements between processing processes.
(8) It can meet the rigorous cold heading stainless steel forming process and the large deformation of carbon steel forming process.

Asuda Hardware products co., ltd is a professional stainless steel screw manufacturer and stainless steel screw supplier