There are, however, other advantages of this system, which may be noted as follows. When a single emery-wheel is used there is evidently twice the amount of wear to take a given amount of metal off (per traverse) that there is when two wheels are used, and furthermore the reduction of every wheel diameter per traverse is evidently twice as great with one wheel as it is with two. From some experiments made by Messrs. Morton Poole, it was found that using a pair of 10-inch emery-wheels it would take 40,000 wheel traverses along an average sized calender roll, to reduce its diameter an inch, hence the amount of error due to the reduction of the emery-wheel diameters, per traverse, may be stated as 1⁄40000 of an inch per traverse, for the two wheels.
Fig. 688.
Now referring to [Fig. 688], let r represent a roll and w w the two emery-wheels.
Suppose the wheels being at the end of a traverse, the roll is 1⁄40000 inch larger at that end on account of the wear of the emery-wheels, then each wheel will have worn 1⁄40000 inch diameter or 1⁄80000 inch radius, hence the increase of roll diameter is equal to the wear of wheel diameter.
Fig. 689.
Now, suppose that one wheel be used as in [Fig. 689], and its reduction of diameter will be equal to that of the two wheels added together, or 1⁄20000 inch, this would be 1⁄40000 in the radius of the wheel, producing a difference of 1⁄20000 difference in the diameter of the wheel.