from end to end to check. Look down the bend line while closing the tooling
gap to zero. Start with no tonnage load, with the punch faces just touching, then
stroke the press slowly. Watch for the tooling to move slightly. If it does, you
need to recenter 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 make sure that all of the bolster (bed) set screws have been backed out or
removed so you are not trying to center your tooling using a distorted or curved
bolster. Also note that the New Standard style of press brake tools have no centering issues; these are fixed in location.
9. When bottoming, work your bend calculations based on the nose of
the punch, taking the bent/bend angle factor into account. This includes
the bend allowance and bend deduction. Remember that the radius expands
slightly when released from load during bottoming.
What angle did you need to achieve before releasing the part from pressure?
Divide that angle value by the angle value of the final bend—for example, 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
10. Modern press brake tooling will work fine for bottoming, assuming
you pick the right combination. Nonetheless, if you are bottoming correctly,
you will develop up to five times the tonnage load of air forming. So please be
aware that modern press brake tooling on average is about 70 Rockwell—it will
explode if overloaded.
Also be aware of the press brake’s centerline load limit. If you exceed that limit, you will upset the ram; the bed will remain permanently bent and no longer
deflect the way it was intended to. Also, beware of the sink tonnage and the
increased risk of exceeding the limit. Again, I would refer back to “The 4 pillars
of press brake tonnage limits,” available at www.thefabricator.com.
Remember that bottoming occurs about 20 percent above the material thickness, as measured from the bottom of the V die. Say you have 0.062-in.-thick material; 20 percent of that thickness is 0.012 in. Add that to your material thickness
of 0.062, and you get 0.074 in., which is approximately where bottoming occurs
in the V die in bend lengths of about 18 in. or less.
This means that, on the outside radius, you should not see any shininess or
other area that looks like it was hit too hard. The inside radius should have no
signs of the punch nose penetrating the material thickness.
If you use American, traditional-style tooling, all of the same rules apply with
a couple of additions and exceptions. First, these tools generally are surface-hardened only on the punch nose. If you overload one it will tend to make a
loud noise, and a large chunk of steel will hit the floor. With this style of tooling,
there’s only a chance of it exploding.
Also, these tools must be kept in sets, and all the tools need to face the same
direction. If you cut a punch length in sections, you can use those sections individually or mated together; but if not mated back together at the original cut
and with the tool facing the same direction, they may not successfully mate with
sections cut from the full length of tooling. The same applies to dies. How precise the tooling is made will have a big effect on the final mating results.
Bottoming: A Viable Option
If everything is done correctly, you should see an increase in production. Bottoming stabilizes the bend angle, and you’ll have fewer angle and dimensional
variations as well as fewer errors and corrections.
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 take into account the skill levels of the employees who will be performing the task. It takes only one mistake to damage your press brake permanently!
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. Accidents may happen. A part may be formed on the wrong tool at the
wrong time. An operator may pick up two pieces by mistake and form them at
the same time. Or he may just hit the part a little too hard while making adjustments. Whatever the error, the damage generally will be worse while bottoming.
Note that bottom bending is no longer the primary recommended method
of forming when you are discussing the topic with equipment manufacturers.
They too are moving the industry away from bottoming for all the reasons listed
previously. The simple fact is that very few people still understand how to perform bottom bending safely without coining the part, an older forming method that you do not want to perform. Nonetheless, bottom bending still can be
done, done correctly, and done safely.
Steve Benson is a member and former chair of the Precision Sheet Metal Technology Council of
the Fabricators & Manufacturers Association International®. He is the president of ASMA LLC, 2952
Doaks Ferry Road NW, Salem, OR 97301, email@example.com. Benson also conducts
FMA’s Precision Press Brake Certificate Program, which is held at locations across the country. For
more information, visit www.fmanet.org/training, or call 888-394-4362. For more information on
bending, check out Benson’s new book, The Art of Press Brake: the Digital Handbook for Precision
Sheet Metal Fabrication, © 2014, available at www.theartofpressbrake.com.
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Few people still understand how to perform bottom bending
safely without coining the part, an older forming method that
you do not want to perform. Nonetheless, bottom bending still
can be done, done correctly, and done safely.