Precision Cutting of Steel Bars
Release time：2023-12-12 Click：25
What is fixed or precision cut and how is it performed? What are the bar length possibilities and what degree of precision can be achieved? What do I have to pay attention to when commissioning a precision cut?
At Jinnuo we use the best technology to be able to offer our customers the product they want, and thanks to precision cutting technology we are able to meet even the most particular demands of bar lengths.
In fact, we often positively surprise many purchasing manager and enginners, who look for steel profiles but but struggle to find them in the required specific bar length or personalized shape.
Commercially available bar lengths and length tolerances
Steel and stainless steel bars, such as angle sections or flat bars, are usually manufactured using a production process (usually hot rolling) that requires a certain length tolerance of the bars.
There are various so-called commercial lengths; these are the lengths in which steel bars and flats can be easily found on the market. For different product and material types, there are different commercial bar lengths.
For example, standardised commercial structural profiles (such as beams, U-profiles, angles, etc.) in carbon steel are delivered typically in lengths of 6, 12 and 14 metres. The same products made of stainless steel, on the other hand, are only commonly available in 6-metre lengths.Another example is drawn flat steel h11, which is offered on the European market in the usual commercial lengths of 3 – 4 m, but in Switzerland it’s mainly sold in the length of 3 – 3.2 m.
Montanstahl keeps on stock most commercial lengths of stainless steel structural profiles. That said, across the metallurgical sector, customers value customization more and more. To accomodate this trend in customer preference, we regularly manufacture ad-hoc tailor-made profiles and delivery bars in special lengths, based on the customer’s wishes.
Length tolerances also play a key role when it comes to supplying steel bars. As a rule, rough length tolerances are defined in the plus range so that a minimum length is always guaranteed and can be used for calculations and planning. Commercially available length tolerances are -0/+50mm, -0/+100mm or -0/+200mm. The length tolerance varies based on the production technique and the starting material.
The billing method of the steel profile also plays a role:
When steel bars are billed by the metre (e.g. rectangular and square tubes), the length tolerance is kept very tight, as the excess length is not billed.
When steel bar steel are billed by kilogram, the length tolerance is more generous, as the real weight of the bar shipped is used for billing purposed.
Depending on the production technology, a head cut of the bar may be necessary. In hot rolling, for example, the bars are ticked off with a guillotine, which leads to deformations at the bar ends. To remove the imperfections at the extemities of the bar, companies usually carry out a so-called collar cut, in which a collar consisting of several bars is sawn off in one go. Naturally, the degree of accuracy is fairly low, which is why the collar cut is often also called a rough cut.
Steel cutting methods
As a rule, length cuts that are below the 10mm mark are considered precision cuts or fixed cuts. The tolerance range can be narrowed down to a length accuracy of 0.01 mm, although such length precision is extremely rare.
The typical length tolerances required for steel bars cut-to-length, that is with a precision cut, is of -0/+5mm to -0/+1mm. Depending on how the tolerance is required, the length can be in the plus range (-0/+) or both in the minus and plus range (±). A length tolerance that defines a maximum length is rather rare, but in principle also possible (+0/-).
Mitre cuts are specific type of precision cuts where the bar is not sawn perpendicularly at a straight angle. Instead, the bar is cut at a agle different from 90°, most commonly at a 45° angle but other degrees are also possible.
There are general rules for this, which state that the cutting angle must be specified from the perpendicular in the direction of the nominal length. For the different types of profiles (beams, angles, U-profiles, T-profiles and rectangular or square tubes) there are various schemes with drawings indicating the respective cutting positions.
The list can be found here.
Generally, mitre cuts can only be made up to a certain maximum angle depending on the width of the crosscut saw. The angle tolerance for bar steel is usually ±1°, the other tolerances are defined as for 90° fine cutting.
There are different cutting methods for making fixed cuts, which detemine the accuracy and depend, among other things, on the material and the cross-section of the bar.
Band saw: with or without coolant
Circular saw: with or without coolant
CNC-controlled circular saw or band saw
Laser cutting (tube cutting laser)
CNC milling
Costs and latest considerations for precision steel cutting
Different processes lead to different cutting accuracies with a corresponding cost structure.
For example, computer-controlled saws are ideal for cutting different partial lengths from a longer bar, as they eliminate the two-eye principle (two-man rule) when checking dimensions before cutting.
Other parameters that influence cutting accuracy are:
Cutting speed has a significant impact on productivity and associated costs, but can be at the expense of cutting accuracy.
The shape of the blade affects the roughness of the cutting surface and the amount of swarf produced, which can be a significant cost factor in the case of expensive stainless steels or nickel alloys.
Finally, it must be borne in mind that when cutting commercial stock lengths, not only the cost of precision cuts must be taken into account from a financial perspective, but also the cost of scrap from the cut material.
Another factor that is often underestimated is the time required. Cutting steel or stainless steel takes a long time. Depending on the thickness of the material and the mass to be cut, it can take up to an hour per cut.