TECHNICAL TIP #58; DRILLING; USE OF SPOTTING DRILLS:
Spot drills are commonly found in 90º and 120º point angles, while most drills come with 118º and 135º points. When is each used and why? For jobber and longer length drills, better positioning and size control can be achieved by first spot drilling. Spotting drills typically have short flutes, short overall lengths and no body clearance or margins. Eliminating margins allows chucking close to the point so that they will produce a true start or center. Spot drills with 90º point angles are used when you want to pre-chamfer the hole, and should only be used with HSS or cobalt drills.
120º spot drills to precede either 118º or 135º HSS drill points generally work well. Carbide following drills will be prone to chipping if the spotting angle is less than the drill point angle. Spotting for carbide drills should always have a flatter angle than the drill point angle, so that the chisel edge area of the drill makes contact first. A 120º spot angle is ideal for a carbide drills with a 118º drill point. A 140º spot angle for carbide drills with 135º points is also ideal.