EE was realigning elev chain on 623G by placing log between chain & sprkt while engaed to make chain skip tooth. The log rolled across outer housing pulling his hand across surface smashing/severing tip of right ring finger. He rtw w/in 2 days w/restrict due to pain meds. He chose not to take meds and was allowed to operate 623G as long as no meds were taken. EE retrained.