Compression/extension "machined" springs incorporate multiple features into a single component.
The ends of a compression spring can be flat and square to incorporate attachments. The slot is of constant width from end to end, thus there is no progressive touching of adjacent coils that would produce sound, debris or nonlinearity. Materials generally used to manufacture the springs include maraging materials such as 17-4PH CRES, 15-5PH CRES, Carpenter Custom 455, and Vascomax C300. Springs from 7075-T6 aluminum and Delrin are also common.
Helical Products Co. Inc., Box 1069, Santa Maria, CA 93456, (877) 353-9873, Heli-Cal.com