What is Rolling?
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.
This is a major manufacturing process for TMT Bars, Metal Sheets, cross sections of large length like beam, railroads etc. It is one of a metal forming process in which the metal work piece is compressed between a set of rolls where it reduces its cross section area and increases its length.
Reinforcement steel is extracted from a mould that determines a particular shape (dia) e.g. 8 mm, 10 mm etc.
What is Rolling Margin?
When mould is brand new, the sectional weight of particular dia steel extruded from mould would be exact as per IS Code. But after certain time mould become little bit bigger and after that the same dia steel weight will be more in same length.
If we produce 8 mm Steel in new Mould then it weight will be 8*8/162=0.395 KG per meter.
But, if we produce same dia Steel (8 mm) in old Mould then weight would be more than 0.395, it may be 0.4 kg per meter.
Allowable deviation in rolling margin in reinforcement steel as per Indian standards 1786-1985 for a Batch;
8mm to 10mm +/- 7%
12mm to 16mm +/- 5%.
20mm and above +/-3%.
According to the tolerance chart, there are three condition of steel occurred during manufacturing process Under weight, Standard weight & over weight. We should not use/procure over weight steel because it will affect our construction cost. This is invisible wastage that we can not measure also.
So, now we check how it's affecting our cost;
According to Calculation (BBS) you are required 8 mm dia steel 3000 Rmt.
So, total weight will be 3000*0.395=1185 Kg, Where 0.395 is nominal weight according to calculation 8*8/162=0.395 kg per meter.
Now you will give order to supplier to supply 1185 Kg Steel for your 3000 Rmt requirement.
But, if steel will be manufactured in old mould and we consider it up to +7% then weight per meter will be 0.395+(0.395*7%)=0.423 kg per meter. and you have to procure 1269 Kg steel instead of 1185 Kg for same running meter because steel always comes in weight not in running meter. The difference will be 1269-1185= 84 Kg. Means you have to pay much amount for same requirement.
This is the very less quantity for example. Imagine how this deviation will affect huge construction project like Metro, Monorail, Railway tracks etc. Even for your own house when you going to construct.
How to Calculate weight of Steel;
Weight as per IS Standard = Dia x Dia / 162.
In another way, rolling margin is the difference between theoretical weight & actual weight of Steel, which happens due to change shape of mould.
I hope this article will help to you;
Thanks a lot!!!