This set of Manufacturing Processes Questions and Answers for Freshers focuses on “Roll Forming – 4”.
1. The minimum bend radius for bending material in roll forming is one stock thickness.
Explanation: The minimum radius used is one stock thickness; however 2 times stock thickness is preferable. Bending radius smaller than one stock thickness is possible to use but the life of rolls reduces significantly.
2. The parts shorter than_____ the centreline spacing of rolls of the machine employed will not feed or form satisfactorily.
a) 2 times
b) 3 times
c) 4 times
d) 5 times
Explanation: It is preferred to produce the stock in longer lengths and cut the desired length after forming. This operation needs to carry out in a continuous automatic manner. A thumb rule on this is that, the parts shorter than 3 times the centreline spacing of rolls of the machine employed will not feed or form satisfactorily.
3. Depth of roll formed section should be kept as small as possible.
Explanation: Smaller depth helps to simplify tooling, reduce tooling wear and preserve the stock finish. The maximum form depth that has been established for average-size forming machines is 100 mm.
4. The waviness can be seen in the formed product if unformed areas are less than _____ wide.
a) 100 mm
b) 120 mm
c) 125 mm
d) 150 mm
Explanation: Wide parts show some kind of waviness unless unformed areas are less than 125 mm wide. Waviness or other type of irregularity in wide areas can be avoided with the help of longitudinal stiffening ribs.
5. Waviness at the edges can be avoided by_____
a) longitudinal stiffening ribs
b) incorporating a flange
c) using roller support at the edges
d) waviness can never be avoided
Explanation: Waviness or unevenness at the edges can be avoided by incorporating a flange, hem, or rib near or at the edge. Longitudinal stiffening ribs are used to minimize irregularity in the areas of the product.
6. In roll forming, exact vertical side walls are avoided.
Explanation: In roll forming, exact vertical side walls are avoided to reduce excessive roll wear and scoring. A draft angle equal to 0.5° or more is preferred.
7. Why blind corners and radii should be avoided while roll forming?
a) It needs an additional setup
b) It is time consuming
c) Skilled labour is a need for that
d) Less accuracy
Explanation: Blind corners and radii are feasible but should be avoided. Because they are less accurate than the corners formed with rollers in contact with both sides of the stock.
8. Forming of any metal, however, must be achieved with stresses above the yield stress.
Explanation: If the material is stressed below yield point, it would spring back to its original shape. Therefore, material should be stressed beyond the yield point. But bear in mind that stress should not exceed the maximum tensile stress; otherwise the product will crack or tear during forming.
9. It is difficult to form materials with high yield and ultimate tensile strength. Why?
a) High ductility
b) High fatigue strength
c) Lower creep resistance
d) Lower elongation
Explanation: The larger the difference between yield and ultimate tensile strength, larger is the elongation, thus, better the chance to form the metal. It is very difficult to form metals with extremely high yield and ultimate strength and having a near zero elongation. These materials will crack at the sharp bends because the elongation in the outside fibres would be larger than maximum elongation.
10. What will be the theoretical elongation of outside fibre in ratio to the neutral axis, if inside bend radius is equal to the material thickness and the bending factor is equal to 0.33?
Given: 1) r = t
2) k = 0.33
Solution: Let, bend angle = α
Length of the outside fibre= lo =(r + t)*α
Length of the neutral axis = ln = (r + 0.33t)* α
Therefore, theoretical elongation = ( lo – ln )/ ln
= [(r + t)*α – (r + 0.33t)* α]/ (r + 0.33t)* α
As, r=t and k=0.33,
Elongation= 0.503 = 50.3%.
Sanfoundry Global Education & Learning Series – Manufacturing Processes.
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