An 18-year-old man already serving two life terms plus 100 years in prison for rape was convicted of armed robbery in Montgomery County yesterday and sentenced to 70 more years after shouting obscenities through a day-long trial.

"I'll kill you," Daniel Dennis Collins told the Circuit Court jurors at the end of a bizarre proceeding in which he served as his own lawyer. "That's gonna come back up on you . . . . All you . . . gonna be scared if I get back on the streets."

Judge L. Leonard Ruben gave Collins consecutive sentences totaling 70 years on robbery, assault and handgun charges and specified that he serve them after serving sentences he already faces in Prince George's, Howard and Charles counties.

"I think there's a substantial likelihood that Mr. Collins will never walk the streets again," said Assistant State's Attorney John J. McCarthy.

Collins was sentenced earlier this year in Howard County to life plus 50 years in prison for charges connected to a crime spree in January 1985 after he escaped from the Clifton T. Perkins Hospital Center in Jessup, where he was undergoing psychiatric evaluation before trial on other rape and related charges.