The Flower & Strip Width
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Tube roll design begins with the selection of the most appropriate breakdown roll forming type for the application. Then a sequence of shapes representing the desired amount of bending at each mill station is developed. When we stack these shapes on top of each other they look like a flower. Hence the term, "Forming Flower". This visual representation allows the roll designer to visually and mathematically analyze and correctly distribute the work done at each type of station.
The most technical aspect of flower development involves the computation and control of the developing girth of the flower as bending occurs. Metal bends about a fixed length neutral axis. The neutral axis is initially positioned halfway between the upper and lower surfaces of the flat strip. As bending takes place the metal outside the neutral axis effectively stretches and thins while the metal inside the neutral axis is in compression and effectively thickens. The simultaneous thinning and thickening within the metal causes the location of the neutral axis to shift as the metal is bent. Concurrently, the inner surface of the metal decreases in girth to become the inside of the tube and the outer surface increases in girth as it becomes the outer surface of the tube. If strip width is correctly calculated the strip will grow in girth to fully load the first fin pass, have a small allocation of metal to load the other fin rolls, as well as an allocation for upset at the weld when required.
The flower images on this page represent the actual developing flower of a one inch diameter tube. It shows light and heavy wall strips in grey and blue respectively. The girth of the weld diameter is shown in red and the starting strip widths are shown in the flat for comparison. Follow the girth growth of both strips. As bending and growth progress the two girths will tend to equalize before the first fin pass with the heavy wall reduction allowance being slightly greater to enhance strip edge conditioning. After that the fin rolls will confine and reduce the still growing girth and leave a specified amount for the desired weld roll loading and bead formation. Weld upset allowance for high frequency welding is shown in orange. Fin pass girth reduction allowances are shown in pale blue and will progressively decrease at each fin pass. Although shown at the end of the strip the reduction amount is actually dispersed throughout the total girth at each fin pass.
Mathematical duplication of this phenomenon by the designers' software is critical to knowing the changing outer girths of the flower. This in turn determines roll surface contours and is critical in accurately calculating the correct amount of metal to start the process with. I.E. the "strip width". Most roll design software uses static girth modeling parameters for flower development and separate unrelated formulas for strip width calculating. At JMC Rolls we literally grow our flowers. Our software was developed specifically for this purpose. Girth growth and/or reduction is based on neutral axis positioning at each station. It is calculated at each pass from the weld back thru the fins and back to the flat strip. The only variables being the desired fin pass loading and weld upset allowance. This means our strip width recommendations are derived from dynamic data as opposed to the static pinch of this and that formulas that are commonly used throughout the industry.