In most foundries, grinding wheel costs can vary immensely per ton of castings poured.
This figure may vary from one foundry to another depending on the metal or alloy, size of casting, number and size of gates and risers, and on the type of equipment used—swing frame, floor stand or portable. Part of the cost involves wheels that may be used to notch or cut off gates and risers, but this article will concentrate on the wheels used for the snagging operation.
The grinding wheel can affect wheel costs as well as production rates. One major contributing factor to high grinding costs may be the use of the lowest priced grinding wheel obtainable. While this buying policy does offer the lowest wheel cost, it can ultimately be more expensive in both yearly purchases and labour costs. Furthermore, the cheapest wheel may also reflect inconsistent quality, which could result in a variation of grinding wheel performance from wheel to wheel and from order to order.
In order to extend longevity of the wheel with the main objective of a lower wheel cost, a wheel that is actually too hard for the application at hand is often used. The loss in the material removal rate and operator efficiency therefore offsets the costs of the wheel savings, as many tests have proved conclusively.
Using a too soft wheel, on the other hand, while contributing to a happier operator, will most certainly increase the total wheel consumption. Management may now ask how all these factors should be evaluated.
Some foundries measure grinding wheel efficiency in terms of the amount of metal removed versus the amount of grinding wheel used. Others may measure the grinding wheel cost per kg of metal removed. Both these methods may result in a two-dimensional view where a three-dimensional view with grinding time added will in most instances supply a better result.
In some instances, foundries do not realize how important wheel selection can be, relying only on operator opinion for the best wheel to use, without measuring wheel consumption or metal removed. Although this may seem like a simple solution, it is not likely to result in determining the efficient grinding wheel, or the lowest grinding cost. Operators most likely will tend to prefer a soft fast cutting wheel.
The above-mentioned approaches will most likely provide a very high ratio of metal removed to wheel wear resulting in a low production rate, or high productivity but with a relatively high wheel consumption. The ultimate aim is to strike a happy medium between rate of metal removed and wheel consumption.
The missing third dimension in these approaches to wheel efficiency and grinding costs is the time element. Labour and overhead costs in most cases contribute more than the wheel cost in determining the total grinding cost. The only accurate way to determine a fettling operation cost is to determine the weight of metal removed, weight of wheel used, and the total contact grinding time of the test. Contact time rather than total elapsed time is important as it is not subject to operator effort. Accurate measurement of the contact grinding time per casting or per test run is of utmost importance.
With contact grinding time readily available, accurate means of determining the weight of metal removed while grinding must also be obtained. The castings selected for test, after chipping and sorting of scrap, should be weighed before and after grinding. Larger castings should be weighed individually and small castings can be weighed by skid load. To ensure accurate grinding wheel cost information, the wheel should be weighed at full and then at stub diameter. The difference represents the weight of usable abrasive consumed during the test.
The castings selected for the test should all be of the same metal or alloy, and approximately the same size and shape. In reality, these last two conditions are not always possible. It is then that the value of determining contact time becomes even more relevant. It helps to balance out the effect of just such variables in the work pieces. Furthermore, when a wheel is used by more than one operator, using contact time in calculating the wheel efficiency and costs helps to eliminate variables in operator technique and operator opinion of a wheel.