TPT September 2014

Article

Caparo

2.2 Over storage in the floop (accumulator) – Over storage in the accumulator is also a cause of slit edge waviness or camber. In the accumulator the slit is bent in the “U” shape so if the storage in the accumulator is more than desired there will be instances of bending or edge waviness. This camber present in the skelp again leads to the mismatch defect.

2.5 Position of guide rolls – Arrangement of the guided rolls is required to be correct. If the position of the guided rolls is incorrect then un-centred slit can enter in the mill, another potential reason for the mismatch defect.

Position of guide rolls

‘U’ bending in the floop

2.6 Weld rolls configuration – The configuration of the weld/ squeeze rolls must be appropriate. The pressure applied by the squeeze out roll must be the same. If the applied load is not equal then it can be a reason for the mismatch.

2.3 Work done by the break down passes – The breakdown passes are the most important rolls in the tube mill. They are responsible for imparting the initial profile to the skelp. We know that for the same diameter (OD) of ERW tubes, more width of the skelp is required in thin gauge tubes than in thick gauge tubes. Hence, if the same breakdown passes are used for both kinds of tubes, there will be instances of untouched and un-worked portions in the skelp, which might further behave erratically. The problem will be compounded if other defects such as skelp waviness or unaligned movements are present.

Various types of welding rolls

A self-centring chuck, also known as a scroll chuck uses dogs (usually called jaws), interconnected via a scroll gear (scroll plate), to hold onto a tool or work piece. Because they most often have three jaws, the term three-jaw chuck without other qualification is understood by machinists to mean a self- centring three-jaw chuck. The term universal chuck also refers to this type. These chucks are best suited to grip circular or hexagonal cross-sections when very fast, reasonably accurate (±0.005" [0.125mm] TIR) centring is desired.

Breakdown pass (thin gauge)

Breakdown pass (thick gauge)

Self centring mechanism used in tube mill

Breakdown pass (thin gauge)

Breakdown pass (thick gauge)

2.4 Pressure applied by the rolls – The pressure applied by the rolls sets on the edges of the slit must be equal. If the pressure applied is not equal then there is going to be a length and gauge variation in the slit edge – a possible reason for the mismatch defect.

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September 2014