Tight-tolerance Acme and ball-screw and nuts feature minimal backlash. They are manufactured of bronze, plastic, and steel for high-wear resistance, and have manganese-phosphate-coated ball screws. Screw diameters are ½ to 9 in. Duff-Norton Co., Box 7010, Charlotte, NC 28241, (800) 477-5002, www.duffnorton.com