Hidden sandstone in coal seams can wreck the $12-million machines that chew through tons of coal. Mining through unexpected hard rock can even cause fatal cave-ins. Stolar Inc., of Raton, N. Mex., has developed a system that uses radio signals to detect sandstone deposits before the machines get to them. The system constructs an image of the rock in the coal seam, in a way that is similar to a medical CAT scan.

The American dairy industry turns out 48 billion pounds of cheese whey a year. about half of this is dried and sold; the rest is waste. WaCon Corp., in Denver, is carrying out a pilot project to turn this waste into cheap animal feed.Eventually, dairies might license the fermentation process from WaCon. Then the dairies would offer the feed to the farmers that sell them milk.

Fast Breaks, in Bloomington, Ind., is a campus food-delivery service without a kitchen of its own. Company employees take orders from phones in McDonald's and Taco Bell, and sell the restaurants' food at a markup. The restaurants get more business at no extra cost, while Fast Breaks saves enough on facilities and ads to make a profit on such low-cost items as burger and tacos.

When Checker Motors Corp. quit making taxicabs in 1982, American cab companies could no longer buy cars specifically designed to be taxis. But now The London Coach Co., of Mt. Clemens, Mich., has stepped into the breach by offering another classic -- the roomy London cab. The company imports the car body and chassis from England and adds a Ford engine and other components, thus fulfilling taxi regulations as well as the "buy American" rules in many cities.

Many people buy home computers only to discover that they have limited applications. Two laid-off software writers and a former ad agent started Diversions Inc., of Belmont, Calif., to give people an inexpensive new use for computers. The company sells special printer ribbons and colored pens that enable people to make ironon transfers for T-shirts. Customers create designs on their computers and print them out on paper with Diversions' ribbons. After using the colored pens, they iron the finished design on a shirt.