In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform. The concept is similar to the rolling of dough.
Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is known as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is known as cold rolling.
In terms of usage, hot rolling processes more tonnage than any other manufacturing process, and cold rolling processes the most tonnage out of all cold working processes.
Roll stands, holding pairs of rolls, are grouped into rolling mills that can quickly process metal, typically steel, into products such as structural steel (I-beams, angle stock, channel stock, and so on), bar stock, and rails. Most steel mills have rolling mill divisions that convert the semi-finished casting products into finished products.
There are many types of rolling processes, including ring rolling, roll bending, roll forming, profile rolling, and controlled rolling.
Interestingly, the invention of the rolling mill in Europe may be attributed to Leonardo da Vinci in his drawings. The earliest rolling mills were in crude form, but the same basic principles were found in the Middle East and South Asia as early as 600 BCE.
The earliest rolling mills were slitting mills, which were introduced from what is now Belgium to England in 1590. These passed flat bars between rolls to form a plate of iron, which was then passed between grooved rolls (slitters) to produce rods of iron.
What is a Rolling Mill?
A rolling mill is an extensively used process and the most expensive one in terms of equipment costs. It accounts for almost 90% of the metal produced by forming.
In the rolling process, the job is drawn through a set of rolls due to interface friction, and the compressive forces reduce the thickness of the workpiece or change its cross-sectional area.
The types of rolls used in rolling mills depend upon the shape, size, and gap between the rolls and their contour.
Because of workability and limitations in equipment, rolling is done in the following steps: that is, some passes through the rolls may be required to get the required configuration.
During rolling, initial passes result in a reduction of cross-sectional area, while intermediate passes try to bring the shape closer to the final shape, and the final or finishing passes bring the material to the required shape and size. Plates, bars, rounds, structural shapes, sheets, and strips can be obtained by rolling.
Types of Rolling Mills
The following are the 6 different types of rolling mills used in many industries:
- Two high-rolling mills
- Three high-rolling mills
- Four high-rolling mills
- Cluster rolling mills
- Planetary rolling mills
- Tandem or Continuous mills
#1. Two High Rolling Mills.
As the name suggests, two-high rolling mills use two mill rolls of sheet metal stacked on the rolling stand. One roll processes through the machine clockwise, and the other roll moves counterclockwise.
These cold-rolling mills excel at working soft metals like copper and lead. And because they’re smaller than three- and four-high rolling mills, the two-high models are often less expensive than larger options. However, because of their size, they can’t always press the metal as thin as a three-high rolling mill can.
#2. Three High Rolling Mills.
Three-high rolling mills generally use two large working rolls, plus a smaller backup roll. The first and third rolls rotate in the same direction, and the middle roll rotates in the opposite direction. Often used to create cladded material, these mills are fantastic for metals like steel, aluminum, and stainless steel.
Although three-high rolling mills have many benefits, if you need an extremely small strip thickness, you might need to use a larger machine, such as a four-high rolling mill.
#3. Four High Rolling Mills.
With a four-high rolling mill, two working rolls of metal are placed close to the workpiece, and two larger rolls are placed at the very top and bottom of the roll stand.
Although they are expensive, these massive machines can press all sorts of sheet metal into extremely thin strips. Industries like aerospace and the military often require the precision metalwork that a four-high roll mill provides.
Regardless of the working roll and backup roll quantity, the mill’s actual configuration is also an important element in how custom metal products are manufactured. The three main roll mill configurations are the cluster, planetary, and continuous rolling mills.
#4. Cluster Rolling Mill.
A cluster rolling mill is a special type of four-high rolling mill in which each of the two working rolls is backed up by two or more of the larger backup rolls, which are able to roll hard materials.
It may be necessary to employ rolls of a very small diameter but of considerable length to achieve the desired outcome. Hard materials, including stainless steels, titanium alloys, and nickel alloys, may be rolled in these mills.
#5. Planetary Rolling Mill.
In the planetary mill type of rolling machine, a large backup roller is surrounded by some planetary working rollers, as shown in the figure.
Each planetary roll gives a continuous reduction. It is employed to reduce large thicknesses in a single pass of a steel strip. Its rolling capacity is higher than a cluster machine but less than a tandem rolling machine.
#6. Tandem or Continuous Mill.
With a continuous rolling mill, the same amount of material must pass through each stand in a given period of time using a series of rolling mills. If the cross-section is reduced, the speed must be increased proportionately.
The speed of each set of rolls is synchronized so that the input speed of each stand is equal to the output speed of the preceding stand. The uni-coiler and wind-up reel not only feed the stock into the rolls, but also provide back tension from front tension to the strip.
Types of Rolling Process
The rolling of metal can be done in a variety of ways, and these methods are utilised differently depending on the application.
The quantity of product, the condition of manufacturing, and other factors all play a role in the decision-making process when choosing the type of rolling process to use. Rolling Process can be broken down into the following categories according to these requirements:
Thread and Gear Rolling:
Thread and gear rolling: By rolling over the metal input with Dies, a thread is formed. This method cuts threads and gears.
Shape Rolling:
This rolling process forms metals (Workpieces). Rolls I-Section, H-Section, etc. Metal is shaped with rollers. The ultimate shape determines roller combinations and shapes. It’s precise.
Ring-rolling:
This roller has a Drive Roller, an Idler roller, and an Axial roller. Drive and Idler rollers revolve in the same direction. Idler rollers rotate inside metal, and as the hole diameter increases, they move closer together.
Tube Piercing:
Two Rollers plus a stationary mandrel make up this rolling procedure. Metal slides towards the mandrel as rollers turn it. The mandrel’s compressive tension causes the metal to move toward it, creating a hole. It makes thick-walled, seamless hollow tubes.
Skew Rolling:
Ball bearings are rolled this way. Metal is run through a specific roller to make bearing balls. It’s an easy way to mass-produce ball bearings.
Transverse Rolling:
This rolling creates a tapered surface. Metal is transferred between two rotating rollers. Rollers have a tapered section that causes surface tapering. It is utilised during the production of tapered shafts, leaf springs, etc.
Roll-bending:
This rolling method bends metal. When metal passes through the rollers, it curves along their direction. Roll bending boosts a vehicle’s aerodynamic efficiency by bending chassis tubing.
Flat-rolled:
It’s the basic rolling method used to make bloom, slab, etc. from ingots. Input flat metal is output with reduced thickness. This technique of rolling reduces metal thickness only at the output.
Controlled Rolling:
As the name suggests, it is one type of rolling process that is done in a controlled way. Industries use it frequently. In steel industries, grain size is predetermined, and rolling is controlled to produce the same grain size.