U.S. spending on machine tools in September rose 48% from the previous month and 6% from the year-ago September, according to a study by two industry groups to be released today. Companies spent an estimated $713 million for machine tools in September, the Assn. for Manufacturing Technology and the American Machine Tool Distributors' Assn. said in a joint statement. That nearly doubled the revised estimate of $482 million spent in August and surpasses the estimated $673 million spent in September 1997, the groups said. For the first nine months of 1998, an estimated $5.9 billion worth of machine tools were bought, down 7% from the year-ago period. Analysts consider demand for machine tools a reliable indicator of the strength of the manufacturing sector and the overall economy. The groups cited spending at the annual International Manufacturing Technology Show, the nation's largest, as a key reason for the increase. The industry was hurt by the 54-day General Motors strikes that ended in late July, along with the usual summer spending slowdown. Figures were submitted by companies participating in the U.S. Machine Tool Consumption report.