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How To Buy A Press Brake, and The Differences Between The Operations.
What Is A Press Brake?
A press brake is used for bending metal. It has a frame, bed, ram, and drive.
Press brakes are designed to deliver accurate vertical blows. The integral
parts of a press brake includes the housing, brake, bolster plate, bed,
ram, gibs, and drive.
1. The housing or end frames are heavily constructed to provide structural
support for the machine.
2. The brake is the part which carries sheet metal the full length of the
ram and bed without disrupting the forming process.
3. The bolster plate is secured to the press bed. It positions and supports
the die assembly.
4. The bed is a stationary mounting surface for the lower die or blade.
5. The ram carries the upper blade. It’s positioned on the front of
the housing and maneuvers vertically. It is designed for rigidity.
6. Gibs provide a sliding surface for the ram.
7. The drive gives vertical motion to the slide. It may be mechanical or
hydraulic.
The press brake is a very versatile type of fabricating equipment. It is
ideal for many metal forming operations with its ability to deliver force
in a confined longitudinal area. Any metal that can be punched or bent by
other processes can be formed on a press brake. The user must make choices
based on his anticipated production needs when choosing a press brake. What
must be determined is the tonnage required, length of bed, bolster width,
stroke length, drive system, and the control system. These choices depend
on the material being used, type of operation, and the rate of output. The
pressure needed to bend metal on a press brake depends on the hardness and
thickness of the metal and the width of the lower die. If you increase the
width of the lower die then less pressure is needed to fill it. If you decrease
the width of the die then your tonnage (striking force) requirements rise
dramatically. Any metal to be punched or bent can be formed on a press brake.
The materials generally used on a press brake are low carbon steel, alloy
steel, stainless steel, aluminum alloys, and copper alloys.
Press brakes are divided into two basic categories: mechanical drive systems
and hydraulic drive systems. The mechanically driven press brake has a fixed
tonnage and delivers more force at the bottom of its stroke than at the
halfway point. Mechanical drives will cycle its ram at more strokes per
minute than a hydraulically driven system of the same size. The electric
motor provides power to a flywheel which stores energy and provides speed
and consistency of motion to the drive shaft on a mechanical system. Mechanical
press brakes have been changed to increase output and for supplying varying
ram speed within a single stroke. The ram starts at high speed from the
top of the stroke and automatically changes into low speed for the operating
position of the stroke. At the bottom of its stroke, the ram again transfers
into high speed for its return. A control mechanism provides short, medium,
and long periods of time for the ram at slow speeds.
The air-friction clutch is another means of ram control on the mechanical
press brake’s capability. The air-friction clutch is reliable by means
of expansion of a tube to force friction shoes against a center. The air-friction
clutch provides a cushioning effect which extends die life. Today a press
brake operator can choose the high speed process for full cycle of the ram
or he can choose fast advance, low-speed bending, and high speed response
of the ram. This mechanical system increases productivity because once the
speed is selected, the machine will cycle automatically. Air-friction clutch
systems will have a higher degree of consistency. Ram leveling is an aid
to the user because it saves time in setting up the job. It is standard
on most mechanical press brakes. Ram leveling is accomplished by an independent
motor linked through a worm gear drive to a pair of adjusting screws inside
the rods joining the drive shaft to the press slide. Tilting adjustments
are completed by split couplings and clutches that release the drive on
one side of the ram. A calibrated device should be furnished at each end
of the ram to show the exact position of the side.
Hydraulic Press Brakes
Hydraulic press brakes are available with pressing capacities
up to 8,000 tons. A mechanically driven press brake of equal tonnage will
not deliver the same pressure at the bottom of their strokes, it is rated
at midstroke. The hydraulic press brake delivers its rated capacity over
the entire stroke. The hydraulically driven press brake’s tonnage
and ram speed are variable up to the machine’s rated limits. A hydraulic
drive allows a longer ram stroke than mechanically driven equipment. The
ram speed control on a hydraulic press allows the best adjustment of the
material being worked. The tonnage of a hydraulic press brake is a function
of the size of its cylinders, pump, and circuit capacity. The construction
of the press brake is
matched to these features.
The hydraulic press brake’s fixed tonnage can’t be surpassed
so the brake can be bottomed at full tonnage repeatedly without risk.
This is the hydraulic press brake’s advantage over the mechanical
press brake. The hydraulic press brake can’t be overloaded where the
die or press brake is damaged. The hydraulically driven ram will stop when
it reaches the selected tonnage. It can be withdrawn from any point on the
job. The hydraulic press brake has a preset tonnage control. This control
varies within the capacity of the machine. A light tonnage setting permits
the use of inexpensive dies for light bending. The press brake will operate
at a higher speed under a light setting. The hydraulic press brake allows
the operator simple stroke adjustment by mounting limit switches at the
desired top and bottom limits of ram travel. Every hydraulic press brake
is equipped with a micrometer adjustment for setting the precise position
of the ram at the bottom of the stroke. It is possible for the ram to be
positioned within a thousandth of an inch. A job requiring repetition can
be set up to produce identical parts in minutes. This capability is not
available with mechanical press brakes. The hydraulic press brake delivers
full rated power throughout its stroke and has a longer stroke than a mechanical
brake which is limited in stroke length by its crankshaft design.
There are different methods to keep the ram level on a hydraulic press
brake. Pressure can be exerted by two cylinders powering the ram to keep
it level. There are a wide variety of leveling and tilting controls: 1.
A servo electric system produces a ram tilt condition by a low voltage electric
signal which is increased and fed back to one of the two variable delivery
pumps. This system provides continuous correction to the ram level. 2. An
electronic system uses an electronic sensing device. 3. A proportioning
valve system checks the flow of fluid to the cylinders, prevents wavering,
and offers continuous correction with high accuracy. 4. A limit switch uses
a steel tape sensor that drives two highly sensitive limit switches through
spring-loaded cams. 5. A steel tape system enclosed in a glass tube transmits
data to a special level control unit. All of the above systems are used
for tilting the ram when required by the type of work.
Press brake manufacturers usually offer three alternatives for widening
a press brake:
1. The press brake can have a wide bolster plate and a removable angle bracket
for the ram.
2. Angle brackets on both the ram and bolster plate.
3. Removable angle brackets on the ram and bolster plate. Removable angle
brackets on the ram and bolster plate is usually the best choice because
it offers greater flexibility.
The widening of a press brake will depend on tonnage and the structure
of the frame. A large press brake should be widened no more than 36 inches.
If your work consists of punching and using progressive dies on wide stock
in heavier tonnages then a straight side press with four-point gibbing should
be considered instead of a a press brake. The straight side press will have
press brake adaptability with the structural strength of a press. Ram and
bed extensions add flexibility to the press brake. Extensions that are part
of the original equipment are better than add-on extensions because they
have the rigidity of factory installation. A press brake used for horning
operations like the closing of box ends should have extensions, preferably
on both sides. It’s counter productive to buy a smaller press brake
and add-on extensions later. It may cause deflection in areas of the bed
and ram outside the housings where die shimming will be a problem.
Mechanical Press Brakes
The basic power principle offers a rigid ram level. Mechanical
press brakes are easier to overload. Difficult to bring ram close to material
for scribed line work. Difficult to control bending speeds. Skilled operator
needed to slip clutch. Clutches require adjusting. Mechanical brake presses
don't enable you to adjust the stroke legnth. You must complete the revolution
and cycle the machine completly, you can't return the ram at any position
of the stroke.
Hydra-Mechanical Press Brakes
Rotary hydraulic cylinder turns the eccentric shaft by means of rigid mechanical
linkage to both ends of the ram, distributing power equally over its full
length. This basic power principle offers the operational control and safety
of the hydraulic principle plus the rigid ram alignment, accuracy and operating
speeds of the mechanical press brakes. With a Hydra-Mechanical press brake
design the operator is in full command of the job. He can meter out just
the right amount of fluid to move the ram the exact distance needed. With
a Hydra-mechanical press brake you can easily inch the ram down for scribed
line work, and easily locate the stroke bottom for setting up. This gives
more accuracy, less operating time, and less training is needed. The Hydra-mechanical
advantage allows you to instantly stop or start anywhere in the cycle, and
allow reversal of stroke direction at any position. Different from
a mechanical press brake that can only return the ram to the top after the
cycle is complete.
Small Press Brakes
A user should define what a small (25 ton or less) press brake
is going to be used for in production. The width of stock, gauge, and production
volume are requirements to help determine the tonnage, width between housings,
and required speed. If a small press brake is going to backup larger equipment,
the user should leave himself a flexible operating range. This usually means
purchasing a press brake with extra capacity to handle unknown possibilities.
A press brake barely exceeding the minimum requirements is not likely to
be efficient in the long run. When looking for a smaller press brake the
following should be considered:
1. The bed should be welded to the frame not bolted. The ram and bed should
be large enough to prevent deflection. Steel gearing is recommended.
2. It is advisable to have variable speed drives. The press brake should
have a reversing switch available to the operator. If there is an air friction
clutch, then clutch overload protection should be provided.
3. The ram adjustment, stroke, and speed of a small press brake will depend
on the work involved.
Other features to be considered are:
a) front operated back guage
b) two speed operation
c) positive lubrication
d) power ram adjustment is recommended during frequent die changes or tipping
of the ram for fade out work.
e) ram position indicators
Some small press brakes have design features and capabilities which are
unique. The dieless-type press brake has two wings instead of a conventional
bed. The material being formed is put over the wings and held in position
by the upper forming blade with a knife edge. The blade is mounted in a
hydraulically-actuated ram. The wings pivot on a hinge pin at the center
of the bend radius. Moving upward to the bend radius, the wings push the
metal against the stationary forming plate with a wiping action eliminating
whip-up. The dieless-type brake can be fitted with attachments and can make
multiple bends in one operation that would requrie several steps on conventional
equipment. Pneumatic press brakes are used for light work and have a cushioned
action which makes them useful on draw work. These press brakes are not
used for long runs except on very light stock.
Selection
A user should keep the following in mind when a selecting press
brake. If a machine is needed to exceed its rated tonnage periodically then
a mechanical press brake may be the best choice. If a user works with dies
having a high bottom section then a hydraulic press brake is recommended.
Speed is an important consideration and a hydraulic press brakes doesn’t
cycle its ram in strokes per minute as a mechanical press brake will of
the same tonnage.
In the past , only mechanical press brakes were used in high production
shops where product output per hour was important. Later hydraulic press
brakes overcame the slower output rate by adjusting ram speed within a single
stroke. A rapid speed, doubling the basic work speed, is used to do the
work and withdraw the ram. Some hydraulic machines offer a third speed which
is four times greater than the basic work speed. With this flexibility,
the hydraulic press brake can achieve an output rate on long production
runs close to comparable mechanically-driven press brakes.
Inspection Of Press Brake
The inspection standards here are separated into two groups: non-power inspection
and under power inspection.
Non-power inspection
The throat area in the side frames of large mechanical press
brakes should bechecked for frame failure. The ram of the press brake puts
great stress on the face of the throat when work to be bent is wider than
the housings. The user should look for any indication of cracks, breaks,
or welding to repair a break.
-Check for parallelism between the bed and ram with a surface gauge. Lower
the slide and zero the indicator at any point on the upper die holder and
then move the gauge along the lower die holder surface. The indicator will
show any deviation from parallelism. Requirements for bed-ram alignment
may vary but generally deviations shouldn’t exceed 0.001 inches/foot.
A slide which falls considerably below this probably has a sprung frame.
If the bed is bowed, dies will not match and the work won’t be uniform.
-Check the gibbing for visible signs of wear and excessive clearance. Excessive
clearance depends on the job. Gibs should be set for a minimum clearance
in blanking operations. In draw operations with dies fitted with heel blocks
it is sometimes better to set the gibs loosely. The gibs should be checked
closely because they’re important in maintaining slide alignment.
Gibs will also indicate the overall condition of a press brake.
-Examine bearings with a jack and an indicator. Use pressure on the bottom
of the ram, lift it, and with an indicator find out if there is excessive
wear in the machine’s bearings. Uncover the gear boxes and inspect
the gears for broken teeth or other signs of excessive wear.
Under-power inspection
Cycle the machine and check all the controls including the
inch controls. Verify what they do and what they’re supposed to do.
Listen to the meshing of gears and for any grinding or grating sounds. Check
for clutch slippage and any sounds in the clutch. In a hydraulic machine,
look for worn hose and leaks around cylinders. Also check to make sure the
work lights are working.
There is no simple formula for a complete analysis of press brakes. It
depends on the machinery operations involved like bending, punching, perforating,
trimming, blanking, notching or other operations involving different materials.
The advice and experience of your machinery dealer is invaluable in helping
you make an intelligent choice.
*This is one article in a series of How to
Buy Metalworking Equipment. Each article showcases and explains a particular
type of metalworking machine. They were originally published in the Metalworking
Machinery Mailer published by the Tade Publishing Group.
