skeletal frame vs underframe

skeletal frame and underframe both are nouns.

Word	Noun	Adjective	Verb	Adverb
skeletal frame	Yes	No	No	No
underframe	Yes	No	No	No

As nouns, skeletal frame and underframe are synonyms defined as:

skeletal frame and underframe: the internal supporting structure that gives an artifact its shape

Other synonyms of skeletal frame include frame, skeleton.

skeletal frame (noun)	underframe (noun)
the internal supporting structure that gives an artifact its shape	the internal supporting structure that gives an artifact its shape