11.9.1 Centre Drill
Only use on turning centres to drill for tailstock centre location or for the specific chamfer angle.
11.9.2 Spot Drill
Before drilling with a twist drill, first produce a centering hole. This supports the drill body before the point starts to cut and act as a lead for a tap; an unsupported drill may not start concentric resulting in a hole out of position or a broken drill. The point angle of the centring drill is traditionally between 90o – 100o; a 90o angle presents the simplest solution producing a 45o chamfer with ease of programming.
11.9.3 Drilling (Twist Drill)
To maintain the accuracy of a drilled hole, after centering, use light feeds to produce a straighter hole than that when forcing the drill using higher rates. When drilling a deep hole (>3×D), for materials that produce continuous chips, the method used is a peck drilling or chip removal operation. Withdraw a standard drill after a maximum depth of 5×D; gradually reduce this depth to 1×D for deep holes. For extra deep holes reduce the speed and feed progressively to half speed and minimum of 0.025 mm/rev (0.001”/rev), the drill supplier normally provides formulae for these and other calculations. The following graph is a Guide to the expected diameter oversize in normal drilling operation.  |