Although the development of modern technology in mechanical engineering has made it possible to replace many metal parts with more practical solid-state plastics and composites, the need for steel elements remains. Metal processing technologies remain relevant, but new methods and tools are also emerging in this area. So, the knurling of the thread, replacing the traditional cutting, made it possible to optimize the production process of manufacturing parts and improve the quality of the screw connection in principle.
Features of the rolling process
The technology refers to varieties of transverse knurling, but in this case the emphasis is on the use of rollers in relation to cylindrical workpieces. The method also focuses on the principles of extruding a screw profile, which allows you to gently form a thread, adhering to the technical specifications to the smallest dimensional indicators. The following can be attributed to the features of the thread rolling process:
- The absence of destruction of the internal structure of the metal billet. This also applies to corrosion-resistant, heat-resistant and special types of steels. It is the soft deformation effect that excludes undesirable processes of excessive pressure on the metal.
- Hardening of the outer layers of the workpiece occurs, and the load capacity of the element also increases.
To these advantages it is worth adding the characteristics of the screw profile itself. Due to the sliding of the knurled surface, the embossed surface acquires optimal hardness and roughness with a microstructure that is favorable for contact with the texture of adjacent surfaces.
Knurling with two rollers
In the implementation of this method, thread rolling machines-semiautomatic machines are used, which allow metric, trapezoidal and other screw profiles to be performed with high accuracy. Complex corrugations are also performed on running gears and small-modular helical gears. The process of forming a thread is carried out by rolling a profile, which is applied previously. This is a kind of knurling of notches on the thread, formed due to the forced rotation of the rollers. In the process of movement, the machine performs radial movement of functional elements by applying force from a hydraulic drive. In turn, the cylindrical workpiece is located between the rollers on the supporting part or in the cartridge of the gripping device. It rotates under the influence of friction, which is formed when the rollers come in contact with the surface of the part and grows as the deformation profile is introduced.
Characteristics of roller segments
Rollers themselves for rolling are only an integral part of a universal machine, however, according to the principle of their action, they can also act as independent carvers. In any case, it is important to consider two main parameters when choosing this segment - tensile strength and profile diameter. As for strength indicators, the rolling of the thread with rollers is able to withstand up to 1400 MPa, while maintaining accuracy of up to 0.1 mm. The disadvantage of this method is precisely the limitation on the thickness of the cylinder. For example, the diameter range of workpieces of a standard format varies from 1.5 to 15 mm on average. The thread pitch in this case will be up to 2 mm, and the length will be about 80 mm. At the same time, the technology is quite costly, given the complexity of manufacturing rollers and machines that serve the working infrastructure.
Knurling with holders and cylindrical heads
This equipment is used as part of a cylindrical non-power tool. As operating equipment, universal metal-cutting units can be used. For example, as a machine for rolling threads with holders and cylindrical heads, turning, turning-revolving and spindle automatic machines may well be used. The main technological feature of the use of the equipment itself is the completeness and high accuracy of the process. The same heads provide final processing with support for high requirements regarding runout parameters, alignment and thread stability. That is, after applying this operation, there is no longer a need for special refinement. But along with the advantages of using holders and knurling heads, there are also disadvantages, which include low productivity, which excludes the possibility of using the method in a large-scale manufacturing format.
Knurling dice
This technology, on the contrary, has been successfully used in hardware production in the serial production of fasteners with conventional accuracy. The use of flat dies is characterized by high performance, while requiring the connection of equipment that is simple in design. This ensures both the reliability of the workflow and versatility in the manufacture of parts of various sizes. For example, the diameter range for thread rolling in this case will be 1.7-33 mm. The maximum length of the thread will be 100 mm, and the step offset is within 0.3-3 mm. Of the negative aspects of the use of dies can be called low rates of hardness of parts, since the tool only works with materials whose tensile strength does not exceed 900 MPa. On the other hand, dies of special modifications make it possible to roll on self-tapping screws and screws in one threaded pass.
Manual knurling
Power-driven mechanized machines do not always give the expected exact result. They perform well in continuous processing and in performing complex tasks associated with the deformation of solid-state metal. But, for example, knurling is best done on a manual machine without a drive. Manual force will be enough to extrude small turns on the cylindrical surface of the metal, while maintaining high accuracy. The work uses compact machines, the device of which is formed in two parts - a bed and working equipment with three rollers. The process of rolling the thread on the knitting needles is carried out through the handle connected to the head through the shaft. The spoke integrates into a collet mechanism with an adjustable socket. It is important to foresee extreme values for the diameter of the workpiece. On average, cylindrical parts 1.5-3 mm thick are suitable for such machines.
The technology of knurling "on the passage"
Special technique for forming long threads over 250 mm. The features of this method include the axial feed of the workpiece, as well as the formation of the angle of elevation of the rollers along the screw line relative to the knurling contour. If we talk about the machines used, then the optimal unit will be with an inclined spindle, the design of which will allow the use of roller segments with ring cutting. The screw configuration will also be diverse - left and right, single and multi-start profiles with strict exposure to a certain step are possible. The maximum rolling diameter of this type of thread reaches 200 mm with a pitch of 16 mm. In practice, threaded rods with a trapezoidal or metric profile are often made in this way. To achieve a high processing speed, the machines are provided with a special transmission, the outboard bearings of which are forcibly lubricated by the built-in mechanism. This allows you to achieve speeds of about 600 rpm.
Conclusion
The knurling technology gives many advantages to the manufacturer, which is expressed in the operational qualities of the part itself and the optimization of the work process. But, choosing this method of forming screw profiles, one should take into account its weaknesses. The main disadvantage of thread rolling is the rapid wear of the processing tool. For different tools, profile coils can be erased, end facets wear and work area chipping. To eliminate or minimize such effects allows regular maintenance of fixtures, expressed in timely dressing, sharpening and processing of protective chemicals for metal.