Machine tool orders during May suffered their largest monthly loss since September 1980, according to a report issued by the National Machine Tool Builder's Association (NMTBA) in McLean. Volume fell 25 percent in May, compared with the previous month.

Orders totaled $229.7 million in May, a 47 percent decline from May 1980. May machine tool shipments reached more than $412 million, 8 percent under April's figure but still ahead of the level of shipments of May 1980.

May orders were down in both the metal-forming and metal-cutting machine tool categories. Metal-forming orders declined to $48.9 million, a 35 percent fall from April's levels and the lowest since last October. The decline in orders, compared with May 1980, was about 19 percent. Metal-cutting machine tool orders stood at $180.8 million, the lowest level in nine months. May's total was 21 percent behind the previous month's and 52 percent below the orders received in May 1980.

"May orders are very disappointing," said James A. Gray, president of the NMTBA. "It is a distressing note for the country's economic recovery and industrial revitalization.

Blaming congressional inactivity on investment incentives proposed by the Reagan administration, Gray added, "Uncertainty has become the prevailing mood of the nation, and the low level of machine tool orders reflects that theme."

At the end of May, the machine tool industry's backlog was estimated to be $4.42 billion. At the rate of shipments during the first five months of 1981, this represents a backlog of approximately 10 months.