As the strike headed into a third month, "The company rejected a proposal from the union that was unacceptable," said company spokesman Rusty Dunn in a statement.
Dunn said a federal mediator met with union negotiators "without any success."
The Chicago Tribune reported Saturday that machinists aligned with Local 851 of the International Association of Machinists and Aerospace Workers are holding out for cost-of-living raises and better healthcare benefits.
The company has also proposed eliminating seniority and pension agreements, freezing wages for six years and doubling healthcare contributions from employees.
About 750 union workers are on strike, although about 50 have crossed the picket lines, the Tribune reported.