Chapter 6 - Designing for Press Brake Forming
The focus
of this section is on forming, and particularly on bending, the most common type of
forming, and the process most closely identified with the press brake.
Equipment Characteristics
Press brakes are usually in the capacity range of 20 to 200
tons, with bed lengths ranging from 4 to 14 feet (1.2 m to 4.3 m), although much larger
and smaller tonnages and bed sizes are in use. They may be powered by mechanical,
hydraulic or mechanical-hydraulic means. They may be "up-acting" or
"down-acting", depending on the direction of the ram's power stroke. Figure 1
shows a down-acting CNC hydraulic press brake.
Figure 1. Elements of a typical CNC hydraulic press brake.
Press brakes may be equipped with one of several types of
back gauges and depth stops including manually placed and adjusted gauges, pins which
engage holes in the workpiece and computer numerically controlled programmable units which
adjust settings after each stroke.
Operation
Most press brakes are manually fed. The operator holds the
workpiece between the punch and die against the appropriate gauge, providing the pre-set
dimension for the bend (Figure 2).
Figure 2. In this section drawing
of a press brake, the workpiece is in position, showing relationship of backstop gauge,
ram, bed and tooling.
When the blank is properly positioned the machine is
activated causing the ram to move toward the bed, and the workpiece is formed between the
die and punch. Then the ram returns, allowing for removal of the workpiece.
One type of press brake operation is air bending of sheet
metal into a straight line angle. As shown in Figure 3, the punch pushes the workpiece
into the die cavity. Throughout the entire operation, the workpiece touches only the tip
of the punch and the two edges of the lower die. When the force of the upper die is
released, the workpiece "springs back" to form a final angle. The amount of
spring back is directly related to material type, thickness, grain and temper.
Figure 3. Example of air bending. The punch
pushes the workpiece into a die cavity. The workpiece touches only the tip of the upper
die and the two edges of the lower die.
To minimize set-up time, most tools for air bending are
made with the same angle in both the punch and die. Commonly an 80° or 85° die angle is
used to allow for sufficient spring-back to obtain a 90° final angle.
Figure 4. In "coining" or "bottoming"
a punch and die is manufactured to the desired final bend angle. The workpiece is formed
completely into the die.
In situations requiring dimensional accuracy and angular
precision, another forming process is required (Figure 4). This process is called
"Coining" or "Bottoming." Coining requires having a punch and die
manufactured to the desired final bend angle and forcing the workpiece completely into the
die. Coining reduces spring-back, however this process is limited by the tonnage capacity
of the press brake.
Advantages and Limitations
The fundamental advantage of the press brake as a forming
tool lies in its flexibility. The use of standard vee-dies allows economical set-ups and
run times on small lots and prototypes. Almost any part size and formed shape can be
accommodated with the standard die sets, eliminating the cost and lead time associated
with press form tooling. Figure 5 shows the complexity of parts that can be manufactured
on a press brake.
Figure 5. Examples of press brake forming.
Modern press brakes with programmable back gauges using
multiple die set-ups have made this forming process much more competitive for longer runs.
In cases where product designs require specially shaped
tooling, press brake die costs and lead times are relatively modest.
The enormous range of workpiece sizes which can be
accommodated in the press brake is another significant advantage. Parts may be as long as
the ram (within tonnage limits) and part width is constrained only by the ability to
remove the workpiece from the machine after forming.
Since die changes are accomplished quickly, a variety of standard shapes can be
created at modest cost, providing considerable flexibility in configuration of the final
product. It should be remembered, however, since each bend is gauged separately, every
bend or operation introduces the potential for an additional dimensional variation (Figure
6).
Figure 6. Since each bend is gauged
separately on a press brake, every bending operation introduces an additional dimensional
tolerance.
 |
| tolerance schedule: |
note: |
operation sequence |
- A ±machine tolerance
- B ±form tol.
- C ±mach. tol. & ±form
tol.
- D ±form tol.
- E ±mach. tol. & ±form
tol.
- F ±mach. tol. & ±2 (form
tol.)
- G ±mach. tol. & ±2 (form
tol.)
- H ±mach. tol.
- J mach. tol.
- K ±mach. tol. & ±3 (form
tol.)
- L ±mach. tol. & ±3 (form
tol.)
- M ±mach. tol. & ±3 (form
tol.)
- N ±form. tol.
- O ±mach. tol. & ±4 (form
tol.)
- P ±2 (form tol.)
- Q ±mach. tol. & ±4 (form tol.)
& ±2 (angularity variance)
- R ±mach. tol. & ±4 (form tol.)
& ±2 (angularity variance)
|
- form tolerance includes:
- 1. material thickness variation
- 2. angularity variance
|
- shear oversize
- n.c. pierce, notch and return
- back gauge 3x
- front gauge 1x
-
- 1. form B from backgauge
- 2. form D from backgauge
- 3. form Kfrontgauge (D against gauge)
- 4. from Nagainst backgauge
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Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association
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