circ ular ▁cross - section ; ▁a ▁square ▁cross - section ▁is ▁possible . ▁ ▁Process ▁ ▁D ril led ▁holes ▁are ▁character ized ▁by ▁their ▁sharp ▁edge ▁on ▁the ▁entrance ▁side ▁and ▁the ▁presence ▁of ▁b urr s ▁on ▁the ▁exit ▁side ▁( unless ▁they ▁have ▁been ▁removed ). ▁Also , ▁the ▁inside ▁of ▁ ▁the ▁hole ▁usually ▁has ▁hel ical ▁feed ▁marks . ▁ ▁Dr illing ▁may ▁affect ▁the ▁mechanical ▁properties ▁of ▁the ▁work pie ce ▁by ▁creating ▁low ▁resid ual ▁str esses ▁around ▁the ▁hole ▁opening ▁and ▁a ▁very ▁thin ▁layer ▁of ▁highly ▁str essed ▁and ▁dist urbed ▁material ▁on ▁the ▁newly ▁formed ▁surface . ▁This ▁causes ▁the ▁work pie ce ▁to ▁become ▁more ▁sus cept ible ▁to ▁cor ros ion ▁and ▁crack ▁propag ation ▁at ▁the ▁str essed ▁surface . ▁A ▁finish ▁operation ▁may ▁be ▁done ▁to ▁avoid ▁these ▁det r iment al ▁conditions . ▁ ▁For ▁fl uted ▁dr ill ▁bits , ▁any ▁ch ips ▁are ▁removed ▁via ▁the ▁fl utes . ▁Ch ips ▁may ▁form ▁long ▁spir als ▁or ▁small ▁fla kes , ▁depending ▁on ▁the ▁material , ▁and ▁process ▁parameters . ▁The ▁type ▁of ▁ch ips ▁formed ▁can ▁be ▁an ▁indicator ▁of ▁the ▁mach in ability ▁of ▁the ▁material , ▁with ▁long ▁ch ips ▁suggesting ▁good ▁material ▁mach in ability . ▁ ▁When ▁possible ▁d ril led ▁holes ▁should ▁be ▁located ▁per pend icular ▁to ▁the ▁work pie ce ▁surface . ▁This ▁minim izes ▁the ▁dr ill ▁bit ' s ▁t endency ▁to ▁" walk ",