Springback is not a material property

Every so often a drawing arrives with a bend angle called to a quarter of a degree and a note underneath saying the material is S275, as if that settled it. It does not settle it. Two plates from two casts, both certified S275, both inside spec, will come off the same die at angles that differ by more than the quarter degree the drawing is asking for.

What actually varies

S275 is a minimum-yield specification, not a target. The mill has to clear 275 N/mm². Nothing stops it clearing that by a wide margin, and mills routinely do, because the chemistry that gets them safely over the line on one order is not the chemistry they happen to be running on the next. Higher actual yield means more elastic strain stored in the bend, and more of the bend comes back when the ram lifts.

Thickness moves too. A plate ordered at 6 mm and delivered at 5.85 mm is inside mill tolerance and will not form the same way.

What the machine can and cannot know

The brake knows where the ram is. It does not know what is under it. That is the whole problem in one sentence: the control can hold ram position to a hundredth of a millimetre and still hand you a part at 91.4° instead of 90°, because the position it was told to stop at was worked out for a plate that behaved slightly differently.

Air bending makes this worse and better at the same time. Worse, because the angle depends on how far into the vee the punch goes, so every variable in the material shows up in the angle. Better, because the correction is one number on the control and you can make it after the first part instead of grinding a new tool.

What we do about it

The brake is set from the previous job’s numbers, the first part is bent, and then somebody walks it to the surface table and measures it. If it is out, the overbend gets adjusted and the next part is measured too. Only when two consecutive parts land does the run start.

That is the whole method. It is not clever and it does not need to be. It costs two pieces of material and about fifteen minutes, and it is the difference between a bend angle held to half a degree and a bend angle that was assumed.

The part of this that ends up on your drawing

If the angle matters, call it and we will hold it. If it does not, leave it at the shop standard, because holding half a degree on a bend nobody is going to measure is time somebody paid for.

The forming animation on the front page is this argument drawn: the ram takes the bend 3.8° past the angle on the drawing and the plate comes back to it. Hot form the same part and there is nothing to come back — which is the other half of the reason we ask what the part is for.