UNC Resources Inc., announced yesterday that it has agreed to acquire National Tool Co., a Richmond, Ind., manufacturer of custom and standard automated heavy-machine tools.

UNC, A Fall Church-based company engaged primarily in uranium mining and milling, said it intends to continue Natco's trade name and its manfacturing activities and that Natco's current management has agreed to stay with the company. Natco's sales were approximately $35 million in 1980.

"We believe Natco will complement our other manufacturing activities while providing a substantial opportunity for growth in the commercial market," Keith A. Cunningham, UNC president and chief executive officer, said yesterday. UNC in continuing to seek diversification by internal growth, through major acquistions in the resource area and other business opportunities, he added.

UNC would not disclose the purchase price for the company, which was owned by about 25 individuals, according to a UNC spokesman.

Last week, UNC notified the government about the proposed acquisition under the terms of the Hart-Scott-Rodino Act, which gives the antitrust agencies time to review proposed combinations that are over a certain size.

Assuming no antitrust problems, Cunningham said Natro would be merged into UNC's Manufacturing and Services Group, which is engaged predominantly in production of nuclear reactor cores and related components for the U.S. Navy.

Natco's manufacturing activities and its 675 employes are all in Richmond, Ind., where the company's facilities are situated in 143 acres.

A UNC spokesman said the company is "still interested" in acquiring Western Airlines. However, it hasn't made any major moves in that direction and hasn't purchased enough stock in Western to require reporting to the Securities and Exchange Commission. The threshold is 5 percent.