Action of Three-High Rolls
In 1857 John Fritz was watching his men at their slow and fatiguing work at the two-high mill of the Cambria (Pa.) Iron Works. The thought struck him that by adding another or third roll at the top, the piece could also be given a pass on every trip back to the roller in front. The rolls would of course pull it through, the work would be less severe on the men, and, receiving passes in both directions, the piece would receive the full number in approximately one-half of the time which was then required, and, more important than all else, it would not have nearly so long a time to cool and could be finished at a more desirable temperature—a great advantage.
Strange to say the idea was immediately pronounced impracticable when he mentioned it and it was necessary for him to go through a long fight to obtain permission to make a trial.
The experiment was from the first successful but the mill burned one Saturday night, having supposedly been set afire by workmen who feared loss of their jobs. Rebuilt and manned by new workmen it ran with success.
The succeeding ten years saw the “three-high” type of mill come into extensive use both in America and Europe. Elevating or tilting tables have since been provided which mechanically raise the piece to the upper rolls, thereby relieving the workmen of this duty, which, with the great increase in size of ingots and pieces rolled soon became very arduous. To-day the “three-high” mill is just as important as ever.
The Reversing Mill
Having a fly wheel for the storing-up of power, the rolls must keep turning continuously in the established direction. In England, Nasmyth—the same man who invented the steam hammer—suggested that the fly wheel be dispensed with and the two-high rolls reversed after each pass. The piece would go back to the roller’s side receiving work in a regular pass on the way, just as in the three-high return. The idea was developed by Mr. Ramsbottom of the London and Northwestern Railway Company. By the use of powerful enough engines the desired end was accomplished and this type is now quite generally used for very heavy ingots which it is not economical to lift by tilting tables in the three-high mill process. There are certain other advantages also.
The above are the three general types of mills.