As I'm specialized in technical translations, I have worked with these dictionaries that have proved to be very useful. The terms sometimes are "official" as in SKF or OSHA.
Eng-Fr-Spa glossary of machine tools:
www.acta.es/glosarios/máquinas_herramientas-i.htm
Occupational Safety & Health Administration (OSHA):
http://www.osha.gov/dcsp/compliance_...erms_ensp.html
Bearings, SKF. Monolingual (Spanish). Terms and definition.
http://www.skf.com/skf/support/html/...Page=a&lang=es
Technical dictionary (general) English-Spanish
http://www.sapiensman.com/ESDictionary/index.htm