How is a fixed or precision cut defined and how is it performed? What are the technical possibilities and what degree of precision can be expected. The greater the precision required, the longer and more expensive the operation becomes. What do I have to pay attention to when commissioning a precision cut?
At Montanstahl 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.
In fact, at the trade fairs and events we attend, we often positively surprise many professionals who were looking for our special profiles but had given up hope thinking they were unattainable.
Commercially available delivery lengths and length tolerances
Steel and stainless steel bar products, such as angle sections or flat bars, are usually manufactured in a production process (usually hot rolling) that requires a certain length tolerance of the bars.
There are various so-called commercial lengths that are common on the market. For example, standardised commercial profiles such as beams, U-profiles, angles, etc. made of structural steel are produced in commercial lengths of six, twelve or fourteen metres.
The same products made of stainless steel, on the other hand, are only available from stock in six-metre commercial lengths. However, special lengths can also be realised as custom-made products.
For different product and material types, there are sometimes different commercial delivery lengths. Another example is the drawn flat steel h11, which is offered on the European market in the usual commercial lengths of 3 – 4 m, but in Switzerland is mainly sold in the commercial length of 3 – 3.2 m. These two commercial lengths are already available from stock.
These two commercial lengths already show a considerable difference in length tolerance.As a rule, rough length tolerances are defined in the plus range so that a guaranteed minimum length can be calculated and planned. Commercially available length tolerances are e.g. minus zero +50mm, +100mm or +200mm.
This is usually due to the production technique, but also to the corresponding input material.
The billing method also plays a role:
- For bar steel that is billed by the metre (e.g. rectangular and square tubes), the length tolerance is kept very tight, as the excess length is not billed.
- For bar steel, which is invoiced by kilogram, the length tolerance is more generous, as the actual weighed weight is calculated.
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. These have to be sawn off. This is usually done by a so-called collar cut, in which a collar consisting of several bars is sawn off in one operation. The degree of accuracy is correspondingly low, so that the collar cut is called a rough cut.
Steel cutting methods
As a rule, length cuts that are below the ten-millimetre mark are already referred to as precision cuts or fixed cuts. The tolerance range can be narrowed down to a length accuracy of one hundredth of a millimetre. In the area of bar steel, however, this is extremely rare.
For bar steel, fixed cut tolerances of -0/+5mm to -0/+1mm are usually required.
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 precision cuts that are made deviating from the right angle to the bar length.
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 due to the system. This depends on the useful 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. These affect 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 surface roughness of the cut and the amount of swarf produced. 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 stock lengths, if they are commercial stock lengths, not only the cost of solid cuts must be taken into account, 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.
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