Practice and steel properties.
Most press brake workers select V-die opening out of their practice while implementing particular task. Though their selection is based upon practice, it may also bring about fabricated metal sheet improper performance, which in its turn will lead to producing a defective eventual work piece.
Steel is an admixture containing combined mineral components such as iron and carbon that get mingled at intense temperature. Proportion of minerals differ and due to this technical properties of steels may vary. Thus, steels can never exist with unchanging properties and have the same performance. During fabrication and bend processes steel materials mostly possess established combination, consequently have similar performance (not exactly the same).
Prior to coming to practical rules about determining V-die opening to accomplish a particular task, first we need to analyze the factors of these rules and further affect of V choosing on different parameters during bend procedure.
Thickness, stuff and tonnage.
Certain steel types are more resistant compared to others types. During bend procedure of these materials this has to be considered as they have more strength. Now we are not going in details on which element determines how resistant the steel will be and the way of resistance calculation. However, it must be mentioned that this persistence is often referred to as stretch resistance. It can be called UTS (Ultimate Tensile Strength). This is a property all of us ought to consider while buying steels.
What to do with a high UTS ?
Mild steels normally have got 42 kg/mm² UTS, stainless steels possess 70 kg/mm². It should be noted that the pressure demanded during bend process depends on UTS level. Tonnage for bend procedure through a press brake is estimated as t/mtr or else t/ft. Note as well materials of more thickness require bigger V-opening, as plate metals cannot match with a small V.
This is one of the important specifications while making V-die opening. Sometimes workers hold view that piece semi diameter depends on punches, not V opening. To have a more clear view on this, let us take a bridge, that hangs upon cliffs. The more the distance between two ends of a cliff, the farther a bridge hangs, forming a bigger semi diameter. Thus, a big V-opening creates a bigger radius upon pieces. However, UTS also affects it. The same goes with comparison of a wooden bridge and a bridge made out from ropes. In conclusion, a strong piece requires bigger radius. Practice has shown that resulting semi diameter R usually can be 1/8 for V-opening for materials possessing ½ thickness while bending mild steels. This formula is used by the majority of workers: R=V/8
This is possible to adjust in accordance with different materials:
R×0.8 in case of aluminum (low UTS).
R×1.4 in case of stainless steels (high UTS).
The least leg (flange).
This is one of the most important aspects to consider when choosing a proper V-opening for a piece. It is compulsory to get metal sheets and shoulders of dies directly contacted together. Otherwise, if flange size does not meet the requirement, it will lead to the interruption of bend procedure. To conclude, the big V-opening requires the least flange to be big as well. The least flange can be determined by a particular formula. Take b as the inside least flange run, V as the die-opening. In case of 90 degrees bend the least inner flange will be:
The least inner flange= V×0.67
It is possible to adjust it to various bending angles:
b×1.6 in case of 30 degrees.
b×1.1 in case of 60 degrees.
b×0.9 in case of 120 degrees.
Choosing a proper V-opening for bend procedure.
Choosing of dies has a great influence on bend procedure. It influences radius, pressure, the least flange as well as piece final appearance. What is the key to determining a proper V-opening? The main thing is that the proper V-opening is not the same for all materials and thicknesses while accomplishing bend procedure. In case it is apparent from practice that R, which is the radius on a piece, is 1/8 for V-opening, a perfect V-opening in case of particular thicknesses will be V×8, as in case of a V-opening like this, R equals the material thickness being bent.
What is the reason of R equality to thickness?
It must be noted that in the process of bending steels, not any portion disappears or accumulates. Thus, when having smaller result radius compared to the thickness being bent, an excess portion will appear. This in its turn brings about unacceptable deformation. The-less-than-thickness radius causes over-boarding on each side of bending. This leads to imperfect piece look and makes pieces less strong. To avoid radius deformations as well as to keep it in proper size consider this formula:
So, in case of thicknesses no more than ½ a proper V-opening will be 8×T. It is necessary to mind whether the least flange and pressure are no more than press brake specification. If T is greater than 1/2”, result radius gets bigger. Consequently, V=T×10 in case T<1/2”.
In case of compatibility of V=T×8 and a, b points (the least flange and pressure) choose that die, then go on to work. But in case V-opening has an affect on the least flange, then enlarge the flange on the piece or apply a less V-opening. In case result V-opening demands pressure, which exceeds press-brake machine capacity, using a bigger V-opening is the most preferable option to reduce the demanded pressure. Decision should be made prior to bend procedure to provide ideal pieces meeting to any customer`s requests.