A stainless steel blank might be able to be formed on the same press brake as yesterday’s mild steel.
That isn’t to say that the posting of yesterday should be on today’s job.
The force requirement is different depending on the strength of the material. Springback is a change in the final angle following the retraction of the ram. When the stainless surface will be exposed on the finished product, tooling contact may cause cosmetic problems.
That’s why it isn’t simply a matter of if the press brake can make the bend, but if the entire process has been adapted for stainless.
APEX current Press Brake Range is the perfect combination of CNC Control with synchronized hydraulic systems and rigid machine structures to provide repeatable sheet metal bending.
There is still an inadequate amount of material information associated with “Stainless Steel”.
Before doing any calculations, determine the true grade and thickness.
APEX tonnage guidance suggests that using material strength data be used instead of relying solely on the generic terms “mild steel”, “stainless” or “aluminum”. It also suggests that actual thickness be measured as the thickness goes up in a squared term in the bending force calculation.
It is important when there is a switch from one stainless production to another.
A good set up begins with:
- material grade;
- actual thickness;
- bend length;
- target inside radius;
- minimum flange;
- required surface finish.
If one of those changes, do not assume that the “stainless” makes the jobs equivalent, but rather that the previous bend program needs verification.
The Tonnage Calculation is usually pushed up by Stainless.
One of the most common pitfalls is to figure a job with mild-steel and then applying the same figure to stainless.
APEX is very clear that this is a bad idea. Tonnage formula considers tensile strength, thickness, bending length, V-opening and punch radius, and states that stainless steel requires more tonnage due to the fact that many grades have higher tensile values, and are known for having higher springback.
This isn’t as simple as purchasing the largest tonnage machine that’s out there.
The required capacity should still be following the actual work piece.
A 1 m bend and a 3 m bend, in the same stainless sheet, do not provide the same load. Not either a broad or a narrow V-die.
Check bend length (not machine bed length) and machine/tooling capacity.
| Setup Item | What Must Be Rechecked for Stainless | Main Risk If Copied Directly |
| Material Data | Grade / tensile strength | Incorrect tonnage |
| Actual Thickness | Measure the sheet | Force and angle variation |
| V-Die Opening | Radius + load + flange | Excessive load or wrong radius |
| Angle Correction | Released bend angle | Springback error |
| Crowning | Long-bend result | Center-to-end variation |
| Tool Contact | Surface condition | Cosmetic marking |
| Stored Program | Material-specific settings | Wrong repeat-job setup |
Narrower V-Die Does Not Always Equal Better.
The logical approach to a small inside radius might be to think of making the V-opening smaller.
It has consequences to that decision.
A smaller opening results in a higher bending force, as well as a higher contact stress around the die shoulders. The larger opening will result in a lower force, but typically result in a larger natural radius and a longer flange for sufficient support.
APEX’s V-die selection guidance suggests approximately 8–10 × sheet thickness as a good starting range of V-die selection for stainless steel, as opposed to about 6–8× for conventional mild-steel air bending. These are guidelines only and not rules to follow for production.
The final radius and flange shape and the actual loading need to be confirmed with the actual material.
Springback is a Stainless Steel Job.
When the punch reaches bottom position then the bend is not completed.
It is completed once the ram is pulled back.
Stainless steel tends to have greater springback than mild steel, thus a bend that is acceptable when under load may open up after unloading. Springback is specifically identified by APEX as a material dependent variable and it is stated that the setups of other materials cannot be used on stainless.
Manual guessing isn’t the answer on a CNC machine.
Test bend, measure released part and input verified correction and store with material/tooling program.
That makes springback that can occur due to an operator memory lapse into a process variable recorded.
Protect the Surface so the First Cosmetic Part won’t be Scratched.
The finish on stainless steel frequently is important when it arrives at the press brake.
A good bend angle doesn’t always mean a useful part, as die-shoulder lines or drag marks, or protective film that has been damaged, can render it unusable.
This is directly impacted by the v-die geometry. When geometry permits, wider openings, larger shoulder radiuses, protective films, polished dies and roller-type dies can help minimize friction and marking, APEX notes.
Before production, inspect:
cleans or repairs surfaces of tools;
The condition of the protective film;
die shoulders;
movement during forming of sheets;
after the first bend take the finished surface.
Don’t wait until 20 identical marks are made before you determine that the tooling contact is unacceptable.
The Rewards of Air Bending Stainless Steel – When the Process is Under Control.
Air bending can be a valuable solution for many stainless parts as the bend angle is not determined by the sheet going all the way into matched tooling, but instead is controlled by how far the punch penetrates the sheet.
It also involves a lower force than bottoming and coining in general.
However, flexibility introduces a material behavior aspect.
APEX’s metal bending guide provides a larger die-opening starting point for stainless steel and correlates this with the forming space and material behavior.
If the extremely tight radius requirements dictate that the process be taken to a small V-opening or more severe forming condition, recalculate the load instead of simply operating it as an ordinary air-bending operation.
Long Stainless Bends Expose More Than Springback
A small number of samples is sufficient to demonstrate the angle is attainable.
A long panel indicates whether the machine will hold it on the working length or not.
The longer the bend the more visible the machine deflection, alignment of the tooling and crowning. Among the various types of applications for which crowning may be necessary to control variation between the center and ends are long bends, as identified by APEX’s V-die guidance.
Measure at least 3 places on long stainless panels:
- left;
- center;
- right.
If all three need the same correction look into the material or bend program.
In the event that the center is different from the ends, before making a change in the global angle correction, review crowning and deflection.
| Result on the Part | First Area to Check |
| Angle opens after release | Springback correction |
| Required tonnage is unexpectedly high | Material strength / V-opening |
| Inside radius too large | V-die opening |
| Short flange falls into die | V-opening / flange support |
| Surface has shoulder marks | Die contact / film / friction |
| Center angle differs from ends | Crowning / deflection |
| Repeat batch differs from previous job | Material + saved setup data |
Don’t Stainless Guess, Stainless Setup.
Repeat job should not start with “oh I think we used this die last time.”
Write the grade of the material, the actual thickness, punch, V-opening, CNC correction, bend sequence and 1st part measurement.
APEX’s press brake setup guide also includes the following steps as separate setups: drawing review, tooling selection, tonnage verification, tooling alignment, backgauge calibration, and crowning.
The more important this discipline is, the longer stainless jobs get, the more visible, the more tolerance-sensitive.
Test Stainless Job Before Specifying the Machine
Do not size a machine from similar mild-steel machines if stainless steel is to be a significant part of the production.
Provide stainless drawings, actual grades, thickness range, longest bend, radius and finish requirements to the supplier representative.
Use the press brake tonnage calculation and actual material data and verify the proposed tooling and V-opening vs. actual parts.
The best press brake stainless steel is not a press brake just that has enough force.
It is one that provides the shop with enough force, flexibility of tooling, compensation and process control to make the stainless part correctly after it is released from the load – and without damaging the surface along the way.
FAQ
Q1. Is it possible to use a normal CNC press brake to bend stainless steel?
Yes, if there are no restrictions in machine capacity and tooling and process is compatible with the stainless grade, thickness and bend length. Don’t assume that the same setup will work for stainless as it does for mild steel.
Q2. Is the tonnage of the press brake increased when working with stainless steel?
Yes, in comparison to mild steel in the same circumstances. However, actual force still needs to be determined based on tensile strength, thickness, bend length, V-opening and punch geometry.
Q3. What is the opening size for V-die to use when shaping stainless steel?
The APEX suggests 8–10× material thickness as a realistic range of starting diameters for conventional stainless air bending. Final selection is also to meet radius, minimum flange, tonnage and surface requirements.
Q4. Why is it easier to bend stainless steel without it being permanently distorted?
However, stainless will exhibit higher springback than mild steel and the angle released may vary more from the angle under load. One viable method for compensation is to make a test bend and then store the CNC correction.
Q5. What can be done to minimize tooling marks on stainless steel?
Maintain cleanliness of tooling and check die contact conditions prior to production. A larger shoulder radius, wider openings or roller dies may be beneficial in reducing marking, depending on geometry, protective film, polished tooling.