ROLL-FORMED SECTION The process Roll forming is process in which strip or coiled sheet metal is fed continuously through a series of contoured rolls arranged in tandem. Only bending takes place , the metal gauge essentially remains constant. the number of pairs of rolls required to form a particular part depends on :-
Material • Shape to be formed • The precision required Other operations like notching ,piercing, embossing, welding can be incorporated into the operation.
The most common operation is a “ flying” cutoff operation after operation is completed. • Punch –press operation are normally located ahead of rolling
APPLICATIONS Roll forming is a continuous bending operation in which a long strip of metal (typically coiled steel) is passed through consecutive sets of rolls. Roll forming is ideal for producing parts with long lengths or in large quantities. A variety of cross-section profiles can be produced, but each profile requires a carefully crafted set of roll tools. Roll forming can be carried out with plated , galvanised ,lithograped ,vinyl-coated etc…
Dual layered components can also be made at one time.These may include bimetallic parts such as those made when a thin stainless-steel facing sheet and a carbon steel support sheet are formed together. • Long pieces like railroad-car and truck-trailer trim members can be made with this process.Maximum length of part is dictated only by application requirements and handling conditions.
Applications • Roll forming is best applicable for constant complex cross-sectional shapes. • Parts often are long length (short part made by cutting strips). • Possible thickness rolled is 0.13- 25 mm. commercial limit is 0.25- 4 mm. • width range is 25 mm- 2.5 m. commercial maximum width is 1 m. but 400 mm widest stock processible on commonly available machines.
applications • Deep forms are difficult (100 mm is practical depth limit). • Roll formed components are applicable for mass production. • Building industry – roof and sliding panels, joists, windows frames, downspouts, architectural trim and copper electrical conductors. • Hot rolling- sheet metals , rail tracks. • Ring rolling – turbine , pipes, pressure vessels.
applications • Appliance parts made by roll forming includes:- panels for stoves, refrigerators, lighting fixture parts. • Curtain rods and tracks for sliding doors, drawer handles and metal picture frame members made by rolling. • Curling (longitudinal bending) can be rolled eg. Bicycle fenders and wheels, barrel hoops, drum rings etc.
The rolls are straight and rigid cylinders. • The width of the strip is much larger than its thickness and no significant widening takes place. • The co-efficient of friction µ is low and constant over the entire roll-job interface. • The yield stress of the material remains constant for the entire operation , its value being the average of the values at the start and at the end of rolling.
On this assumption,we calculate the following parameter- • Torque and power required to drive the rolls • Roll separating force • Power loss in bearing
Economic Production Quantities • Roll Forming-Mass Production process • Production rates are rapid • Setup and tooling costs high • For process to be economic at least 100,000ft/yr must be produced • In some cases 500,000ft/yr is required to make the process justifiable-depending on configuration and efficiency of alternative methods
Set up times are lengthy due to the large number of tooling elements that are interrelated • Once operative it is an extremely fast process • Speeds upto 300 ft/min, though average speeds of 50 to 100 ft/min • Low labour cost
As compared with extrusion or machined parts there is a savings in materials cost • Overall this contributes to very low cost of production which more than makes up for the high initial setup and tooling costs
Material Properties • All normally formable materials may be used. • Best results with ductile alloys but extreme ductility is not required. • Deformation parallel to grain lines of the material prevents fracture.
Suitability for Cold Rolling • Determined by tensile properties • Tensile properties depend on crystal structure • Cold rolling changes crystal structure and thereby the tensile properties • Material selection should consider change in properties due to cold working.
σ σ ε ε Elastic Properties • Suitability for rolling depends on two properties: • Magnitude of yield point stress • Extent of plastic region
σ ε Springback • Strain in the plastic region is partly elastic and partly plastic • On removal of deforming force, elastic strain is recovered • Plastic strain is retained This recovered elastic strain is called springback. Applied strain should exceed required strain by this value.
Materials Used • Steels – low carbon, stainless, high alloy steels • Aluminium, Brass, Bronze, Copper • Clad, plated, anodized, galvanized, organically finished materials • Pre painted stock if coated with durable, non brittle, baked finish.
Design Recommendations • Parts designer must understand the process thoroughly • Must be in contact with the engineer who understands the roll forming operation, equipment and tooling i.e. sequence of operations • High tooling costs, tool wear and forming problems can be reduced by reviewing the sequence of operations.
Bending Radii • Bending radii at both inside and outside corners must be generous- atleast equal to stock thickness or preferably twice stock thickness • Radii less than one stock thickness possible- sharp bends- • requires sharp corners on the forming rolls • Reduces roll life significantly
Approaches for sharp bends • For stock upto 0.8mm in thickness external bead can be used • For stock greater than 0.8mm a formed groove one third to one half of stock thickness facilitates bending.
Part Length • Form the stock in long lengths and cut to length after forming-one continuous automatic operation • Sometimes there are minimum length limitations for roll formed pieces • There are entrance and exit flare distortions at each end • Parts shorter than 3 times the centre line spacing of rolls of the machine employed will not form satisfactorily.
Depth of Form • Deep profiles require correspondingly larger diameter forming rolls -more costly • Differential amount of pull exerted on the stock due to the different peripheral speeds • Slippage and roll wear occur • Maximum form depth generally 100mm
Some forming machines equipped with differential gearing • So that large diameter rolls can be run at lower rpm-to match peripheral speed to stock speed • Such machines can form deeper sections
Symmetrical/Balanced Forms • Perfectly symmetric types are good for roll forming • Even if perfect symmetry is absent, it can still be used by having equal bending on either side of centerline • Non symmetrical forms require straightening as a part of forming
Vertical Sidewalls • Exactly vertical sidewalls should be avoided to reduce troubles for roll former • Excessive roll wear and scoring of the work piece is thereby reduced • This is done by including a draft angle of ½ degree
Blind Corners • Some feasible blind corners and radii are shown here • When precise bends are needed, these should be avoided • Controlling both sides of stock by rolls, facilitates accurate forming
Minimum Length • Minimum possible practical length for formed legs is 3 times stock thickness This Not this
Dimensional factors & Tolerances • Factors causing dimensional variations – spring back, variations in hardness, thickness, and yield point of material, tooling deviations and tooling wear, setup and adjustment of tooling and machine deflection • Close tolerances are held with thinner stock and smaller parts • Tighter ones are held at extra cost