For a person who is just starting their adventure with the metallurgical industry, steel bar is one of the most simple materials used in manufacturing processes. We define 'bar' as a metallurgical product that can be described as having a diameter smaller than its length. Not always circular – can be rectangular or hexagonal.
It is worth mentioning that the name 'rod' came from the old measuring unit – in different ancient countries and regions 'rod' meant different lengths. For example, – the ancient Roman 'rod' had between 2.77 and 3.08 meters (from 9 ft 1,1 to 10 ft 1.3 in). Such measuring unit was also used in Poland.
On that note – rods are contemporary and used mainly in the machining processes as a source of material for parts manufacturing. They are used most often in lathes, especially equipped with automatic feeders, often – as a whole – numerically controlled.
What are the materials used to produce bars?
Bars (also known as 'blank', 'slug' or 'billet') currently are made out of a vast variety of materials, and such selection is strongly connected to the desired application. Ferrous metals, non-ferrous metals, other alloys, and polymers – these are example materials. Steel rod and their parameters are regulated by international and country norms. In Poland, rods are described with three key features: dimension, type, and class. In assigning particular material to a class the most often used criteria such as 'smooth rods' (classes A-0 or A-I) and 'ribbed rods' (A-II, A-III or A-IIIN).
On the other hand – aluminum rods are made out of many types of alloy and are categorized also based on the material rigidity and mechanical properties. Such features are the most important in selecting aluminum rods to manufacture desired parts.
On top of that – by selecting the right alloy for the job it is important to take into consideration further bettering processes of machined parts – eg. anodization or other galvanic processes. Other types of bars often used in machining are made out of copper, brass, titanium alloys, or polymers (like POM-C or POM-H). In addition – the rod that has a dimension ratio below 0,001 is called 'a wire'.
What is the technology of manufacturing rods?
W In metallurgy exists a categorization of rods – we divide these parts into drawn and rolled bars. It is strongly related to the rod manufacturing process. It is based on the susceptibility of selected material to deform it beyond the limits of its elasticity. Forces that affect material can cause the material and lead to – so so-called – crack phenomenon.
Layers of atoms in the crystal structure of a metal move, but remain overall cohesion. The most popular methods of rod manufacturing are:
- rolling,
- pulling,
- less often – forging.
What is the technology of rolling bars?
To manufacture such bar machining shops usually use rolling mills – devices with very hard rolls that shape metal by sheer force. The material used in such manufacturing is colloquially called 'slab' – it is a long element of selected material, most often in the shape of a prism.
To change its form – the slab is heated up to the crystallization temperature and often – beyond. Heated-up material is formed in a rolling mill. These devices often have other elements that can shape bars even further – like specialized grooves and flanges. During that process, the slab is shaped into the form of a bar, and high temperature removes stress from the material. On top of changing shape, the nanostructure of a bar is also different. Slab and rod material has different mechanical properties such as rigidity and elasticity.
The biggest advantage of the rolling process is its repeatability and possibility to maintain high-speed production. On top of the most popular 'hot rolling' in particular situations, many shops also use a 'cold rolling' process.
What is the technology of manufacturing drawn bars?
Drawing technology is used in applications that require high precision of manufacturing bar while maintaining the mechanical properties of the material. The downside of that method is a decrease in the elasticity of the produced bar, but an important advantage is an increase in part rigidity. The base material for the drawn bars process is usually a rolled bar.
The whole process is based on – literally – drawing material through hardened dies that have smaller dimensions than processed material. The bar is pulled by a bench broach. Its specialized jaws hold one of the bar sides and pull through the die. To increase precision and desired parameters of drawn bars manufacturing process is divided into a few steps with smaller and smaller dimensions of the dies to provide the best precision.
Pulling does not change the internal structure of the material – in other words: focuses on increasing the length of the material by decreasing its dimension. The final material usually does not require additional processing; pulled properly achieves the desired dimension and its surface is smooth. The process itself is often used on top of rolling to achieve the required client parameters of the final product.
Drawn and rolled bars – the key differences
To summarize – each method has its advantages and disadvantages. The most important difference is the cost of the process. Due to the complexity of the pulling process, usage of hardened dies, and other specialized equipment – it is more expensive. That is why the drawn bars are often more expensive than rolled.
On the other hand – precision of manufacturing, high quality of bar surface, and ability to keep initial material properties drawn bars are often used as a material for further manufacturing (eg. machining). Such material is often used in production parts for industries like automotive, airplane, or medical. Rolled bars are significantly cheaper and much more flexible. The selection of the right manufacturing process to produce bars is strongly related to the client's requirement, application, and desired further machining/processing.
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