METAL-FOLDING MACHINES

Metal-folding machines, less tool- and labor-intensive than press brakes, meet the needs of short-run, just-in-time, or cell-type bending applications for complex or large sheet-metal parts in short- - run operations and, in particular, for panel bending.

Advantages include easier material handling because the machine supports the work rather than the operator; the bending process eliminates material slippage; sheet-size intolerance folds into the first bend to hold critical dimensions for fitup; cycle time may improve, compared to press-brake bending, due to simplified material handling; operator health and safety hazards are reduced from less material handling and workpiece contact during bending; and less tool changing is usually required than on a press brake.

Folding-machine components include upper and lower clamping jaws, folding beam, sheet support and backgages, tooling, and CNC controls.

BASIC COMPONENTS OF A FOLDING MACHINE

To select a folder, consider size and capacity, not tonnage, feeds and speeds, and range of motion. Motion-control devices determine accuracy and repeatability, collision avoidance, and control sophistication. Mechanical and servo-hydraulic sources provide power, with the latter operating at increased capacity and higher speeds.

Folding-beam motion drive and control systems determine bending accuracy and repeatability. Least accurate are chain drives, with rotating cams and proximity switches, at plus or minus 1 degree of bend angle. Gear drives and electronic potentiometer achieve plus or minus 0.5 degree or better. Servo-hydraulic rotational drives with electronic feedback reach plus or minus 0.1 degree. Dual-drives on beams minimize bend-angle irregularities on thicker materials. Servo- hydraulic machines provide the fastest-folding beam speed and cycle times.

The upper and lower clamping jaw and folding beam are tooled. A variety of tooling has been developed to avoid collision with the part. Segmented tooling leaves tabs unbent, and enables bending of internal flanges or of four-sided boxes. Select tooling for its geometry-height, front and rear clearance, and toe angle; its mass to resist deflection; hardness; and tool-change method. Tool mounts include multiple bolts, quick-action clamps, and automated clamping.

Generally, CNC controls offer a minimum of three-axis motion control for the upper clamping-beam stroke-up, backgage stop position, and folding-beam travel for angle. Some controls offer programming for one-- line service diagnostics, tool setup calculation, and computer-aided design/computer-aided manufacturing (CAD/CAM) integration.

ROLL-- BENDING MACHINES

Roll-bending machines, using two to four rolls to rotate and bend the metal as it passes between them, roll metal into cylinders or cylindrical segments, flattened and elliptical cylinders, and truncated cones. Metal thickness can range from 16-gage (0.061 in.) to 10in. thick plate. Maximum thickness depends on machine power, minimum thickness by ability of the metal to be curved without damage.

Fabricators can select from initial-- pinch, four-roll double- pinch, and combination pinch-pyramid bending rolls.

Initial-pinch machines require inserting the workpiece into the machine twice in order to prebend both ends of a cylinder and ensure closure of the seam. To prebend the first end, the operator inserts the plate into the machine, which clamps it and pinches it between the top and bottom rolls. A side roll, moving diagonally toward the top roll, sets bend radius. The operator then removes the plate from the bender, rotates the plate 180 degrees to insert the second end into the rolls, then rolls the cylinder to completion.

Four-roll double-pinch machines roll plate in one pass. Driven top and bottom rolls clamp and pinch the plate, and two side rolls move diagonally toward the top roll. By using first one side roll and then the other, the plate is prebent and rolled completely. Four-roll machines are also used to roll cones.

Pinch-pyramid machines, also called three-roll double-pinch machines, can also prebend both ends of a plate with a single insertion into the bending machine, for reduction in material- handling time and cost. These machines can act as single-pinch machines or as pyramid machines, allowing the fabricator to select the best procedure for a particular job. The wide opening between the rolls allows short, heavy plate to be rolled and, with accessory tooling, rolling of structural sections and bar stock on edge. Pinch- pyramid machines drive all three rolls-the top roll is fixed and the two lower rolls move in a straight path or an arc toward the top roll. Cone rolling is done by tilting the lower rolls.

ANGLE-BENDING MACHINES

Angle-bending machines form strucrural-steel shapes of all kinds- channel, beams, angles, and tubes of square, rectangular, and round cross section, and aluminum extrusions. Small-capacity machines form angle to 3-by-3-in. legs; large-capacity machines bend angle to 6-by- 6-in. legs.

The relative position of the roll shafts differentiates four styles o\f machines: pyramid, three-roll single-pinch, three-- roll double-pinch, and four-roll double-pinch. Pyramid and three-roll double-pinch machines constitute over 95 percent of all machines in the field.

The pyramid machine, the simplest design, positions two fixed rolls on one side of the work and a moveable roll on the opposite side. Pyramid machines cannot prebend to form the ends of the section being formed as it enters and leaves the machine; ends of the section remain straight. This is a disadvantage, as prebending allows the fabricator to use the entire length of the part, important on costly material and on large sections where loss of material from scrapping straight ends can be significant.

The three-roll double-pinch type uses one fixed roll and two moveable rolls. Rolls adjust to bend symmetrically or asymmetrically and prebend both ends of the workpiece in a single pass. Bending distance-the distance between the tangent contact points of the rolls- increases with diameter rolled, broadening the capacity of the machine.

The three-roll single-pinch bender uses a fixed roll, a moveable roll that clamps the workpiece against the fixed roll, and an adjustable side roll that controls bending diameter. This type of machine can prebend the leading end of the material; to prebend the trailing end, the operator must insert the tail in a separate pass, which adds handling and cycle time. Section height is limited to 2- by-2-in. angle leg.

Four-roll double-pinch machines use two opposing rolls to clamp the section and two side rolls, allowing prebending of both ends of the stock in one pass. Four-roll machines adapt to CNC controls- clamping material between opposing rolls sets a fixed reference point, necessary for programming. Position of the side rolls can be CNC-programmed to control bending geometry, allowing for repetitive rolling of complex curves. The fourth roll makes this machine the most costly. Angle size is limited to 1 1/2-in. leg.

Horizontal-roll-shaft machines, which pass work in the vertical plane, serve well for small sections where operators can handle parts and tooling manually or by overhead crane. These machines fit in areas with limited floor and overhead space.

HOW ROLLS STACK UP

ON ANGLE-BENDING MACHINES

Vertical-shaft machines, which pass work standing on edge, require enough floor space to accommodate the workpiece as it enters and leaves the rolls. Operators can position conveyors or supports to hold large sections. Easier maneuvering of parts close to the floor make vertical-shaft machines safer and easier to operate than horizontal-shaft machines.

SHAPE BENDING

Techniques and machines exist to shape-bend almost any ductile material, including small-diameter tubing or massive structural-iron beams. Accuracy of automatic bending equipment permits locating the plane of bend to plus or minus 0.10 in. and positioning the bend to plus or minus 0.005 in. at high production rates.

To choose a machine, convert the largest cross sections of the parts into section-moduli ratings for mild steel. Then compare these ratings with the section-modulus capacity of the bender.

Ram benders, for tubular and solid sections, move a ram that forces the workpiece between two fixed or pivoting dies. Final ram position controls the bend angle with fixed-radius tooling. Fixtures on the machine allow bending relative to stock ends or previous bends. Large sweeping curves can be bent in small increments, indexing the pipe for each bend.

Power press bending, the most common process, offers a wide variety of techniques and tooling. Two or four-post die sets maintain a high degree of accuracy-- tolerances range from plus or minus 0.02 to 0.06 in., depending on tools, material, and equipment, with allowance for springback and pulling or stretching.

Roll benders consist of three or four rolls set in a pyramid arrangement in either a horizontal or vertical plane. These adapt to the production of rings and coils, with a short, straight section at each end of the stock left unbent. Tools consist of rolls grooved to match the stock, adjusted by spacers, especially for the bending of structural shapes and bars.

Rotary-bending machines wrap the workpiece around a tool called the bend die or form block for either rotary-compression bending or rotary-draw bending. Stretch forming, a form of rotary-- draw bending, bends special-shaped workpieces, such as angles, through their deepest leg.

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