Here is one more re-post from theArtofPressBrake’s forum page, the question was about Bottom bending aluminum on a press brake. Re-posted here because of its relevance to the trade.

QUESTION:

Our team still has some problems with over-bending… at least in some materials or parts.  A part came up the other day and they want to just bottom bend them to make sure they get their 90 degrees.  As a result they are thinking about Bottom Bending all Aluminum.  There is good discussion going back and forth on both sides of this thinking.  Can I get your two cents on the subject?  Good Idea? Bad Idea?, Advantages?, Disadvantages? In regards to tooling, machinery, safety, productivity, etc.?

I appreciate your time.  Hope you are doing well!!

ANSWER:

Brian:

Bottom bending can be done with softer grades of aluminum; H-series for example. Assuming your guys have a full grasp of the process and are capable of preforming the process safely the following list of best practices for bottom bending should help.

1. Aluminum, the harder the grade, the greater the amount of springback that you will need to deal with; Very soft aluminum may exhibit no springback. Septembers Bending Basics column will teach you how to predict springback so keep an eye out for next month’s issue.
2. If everything is done correctly you should see an increase in production; bottoming stabilizes the bend angle. Fewer angle and dimensional variations, fewer corrections and errors.
3. As a general rule aluminum loses its integrity if the material is creased. I would recommend that you review this article “How are bending turns sharp”. This article goes into the tonnage vs. land area. I would also recommend “The four pillars of tonnage” as your guys will be working with higher tonnages and the increased likelihood of damage from the bottom bending tonnages. If you make aircraft parts with that crease, that alone will make the parts unacceptable. I highly recommend that you review the material’s specifications for minimum acceptable inside bend radius. For the most part a one-to-one relationship is recommended and in some cases a sharp radius may invalidate the materials warrantee.
4. On that note – Do not put a crease in the inside radius, use a nose radius as close to 1-to-1 as possible. Note bottoming a sharp bend is often performed with mild steel but caution must be used to avoid coining; where the punch and die are less than material thickness.
5. Bottoming, especially if the bend is sharp makes the aluminum more susceptible to the grain in the material spreading or cracking. The closer to that one-to-one relationship, the better and the less likely that this cracking will occur on the outside of the bend. But, you still may need to pay attention to the grain direction. Also, if the part has a “grained finish”, grain it opposite of the material grain direction.
6. Because you should not crease the bend at center as you would with a coin or “sharp” bend radius, you will not be able to use that crease as a means of compensating for springback as you could if you were working with mild steel. The only way left bottom bend aluminum is to use the 88-degree punch; an 85-degrees will take the bend too far. That means that you cannot achieve enough negative springback (springforward) to force the bend back to angle. This means two things; first, that without being very skilled and knowledgeable you will be unable to bottom bend a piece of material were the springback exceeds 2-degrees. There are ways to exceed that 2-degree limit with only an 88-degree punch, but not by much. It also means that bottoming is best used for light gauge materials and bends of 90-degrees.
7. Assuming the use of a 90-degree die and an 88-degree punch you will have 2-degrees of angular clearance. With the idea being, that in bottoming we try to match the punch angle to the materials springback. So to deal with bends having a springback greater than 90-degrees but no more than 2-degrees; 1-degree for example, you will need to cheat a little. To deal with bends that that have other bend angles, purchase an roll of 1/8-wide, very cheap, vinyl pin stripping tape, the cheaper the better. Place a piece high on the punch face. This has the effect of changing the punch angle. It is rare that you will be able to use a piece of this tape on both sides of the punch. This usually does not work. This is a long standing practice to use tape, most will use masking tape which works for a while but needs to be replaced on a regular basis. This does not happen with the pin stripping tape; that tape is made from vinyl. Also never cover the entire face of the punch with tape that only changes the center of the tool and not the angle.
8. Select your die width as a “Perfect” bend. A bend where width is determined from a one-to-one relationship between the inside bend radius and the material thickness. Even if the punch radius is less than one-to-one use this formula. The formula for a perfect die width: (2 x material thickness) x 3.429435 This formula is a highbred version of the formula taught in my class. The reason for the change will be explained in the September Bending Basics Column in the Fabricator Magazine. Radius Bends with an inside radii greater than 120% of the material thickness generally are not suited for bottoming. The exception would be forming larger radius bends into urethane which can have a similar affects to those of true bottom bending.
9. Take your time setting up the press brake! Make sure everything is clean before installing the tooling. Most importantly – CENTER the tooling! Even if it takes walking back and forth from end to end while checking. Looking down the bend-line while closing the tooling gap to zero. No tonnage load just touching, then stroke the press slowly. Watch for the tooling to move slightly, if it does, you need to re-center that end. If you move one end of the tool you will change the other, so you will need to return to the opposite side of the machine and check the alignment there. You need to do this until you see no movement in the tooling on either end. Also check and make sure that all of the bolster (bed) set screws have been back out or removed so you are not trying to center your tooling using a distorted or curved bolster. If you are using the “New Standard” style of press brake tooling, there are no centering issues; these are fixed in location.
10. Develop your Bend Deductions (BD), Bend Allowance’s (BA), etc. based on the nose of the punch; less the Bent/Bend angle factor. Remember that the radius expands slightly when released from load, same with the springback. What angle did you need to achieve the part to before releasing from pressure? Divide that angle value by the angle value of the final bend, e.g. 92/90. That division problem yields a value of 1.0222. Multiply that value by the radius on the nose of the punch and you have your radius for your bend deduction calculations. Again I would refer you to the September edition of Bend Basics; it should be out in another week.
11. Modern press brake tooling will work fine for bottoming assuming you pick the right combination of press brake tooling. Nonetheless, if you are bottoming correctly you will develop up to 5-times the tonnage load of air forming. So please be aware of the following: 
    1. Modern press brake tooling on average is about 70-Rockwell it will explode if overloaded.
    2. Be aware of the press brakes Centerline Load limit. If you exceed that limit you will upset the ram and bed will remain permanently bent and no longer deflect the way it was intended to.
    3. Beware of the sink tonnage and the increased risk of exceeding the limit. Again I would refer to back to The four pillars of tonnage column.
    4. Remember that bottoming occurs about 20% above the material thickness and on the back side of the bend you should not see in the area of the bend, shiny or areas looking like it was hit too hard. On the inside of the bend there should show no signs of punch nose penetrating to less that material thickness.
12. If you are use American traditional style tooling all of the same rules apply with a couple of additions and exceptions.
    1. These tools are generally surface hardened on the nose of the punch only. That means that if you overload one it will tend to make a loud noise and a large chunk of steel will it the floor. Only a small chance of exploding this style of tooling.
    2. These tools must be kept in sets. Cut one must be re-mated with cut one on the next tool and all of the tool need to face the same direction.

Bottoming is a viable option for your forming department but the decision really comes down to a few internal calculations on your part. Bottom Bending will improve production rates and overall quality but, your calculations also need to include the issues related to the skill levels of the employees that will be preforming the task. It only takes one mistake to permanently damage your press brake!

While ram upset is always a possibility regardless of the method you are using, it is at least five times more likely if you are bottoming because of the increased tonnage. When an accident does happen, part on the wrong tool at the wrong time, picking up two pieces and forming at the same time or just hitting the part a little too hard while making adjustments. Whatever the error, generally speaking the damage will be worse while bottoming.

Note that Bottom Bending is no longer the primary recommend method of forming when you are discussing the topic with Original Equipment Manufacturers. They too are moving the industry away from bottoming for all the reasons listed above and the fact that very few people still understand how to safely preform bottom bending without coining the part, an older method that you do not want to perform.

Nonetheless, Bottom Bending can still be done, done correctly and done in a safe manner.