underframe vs skeletal frame

underframe and skeletal frame both are nouns.

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

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

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

Other synonyms of underframe include frame, skeleton.

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