Irregular surface drilling

Drilling irregular surfaces can lead to excessive, uneven forces on drill cutting edges, which leads to premature wear. It is important to follow the guidelines and reduce feed when necessary.

Learn more about the challenges and strategies for each drill type.

• To keep the cutting force balanced between the central and peripheral insert on an acceptable level, the pre-drilled hole should not be larger than DC/4

Challenges:

• Chip evacuation is affected, might become more problematic
• Deburring in the crossing is hard.  Burr formation must be as small as possible
• Causes more tool wear than conventional drilling

Guidelines:

• For holes with different diameter: drill the larger hole first to reduce burr formation
• Start with lowest recommended feed during the crossing of holes
• Not recommended in long-chipping materials due to chip evacuation while crossing holes

• Use shortest drill possible to minimize vibration tendencies and decrease the effects of deflection
• Start with lowest recommended cutting speed and 1/3 of lowest recommended feed (or lower) until fully engaged and then return to normal feed
• 4–5×DC the inclined entry can be angled up to 15º
• 6–7×DC the inclined entry can be angled up to 10º

• Use shortest drill possible to minimize vibration tendencies and decrease the effects of deflection
  • Recommended start values for cutting speed and lowest recommended feed (or lower)  
• 4–5×DC the inclined exit can be angled up to 15º
• 6–7×DC the inclined exit can be angled up to 5º 

• Use shortest drill possible to minimize the drill bending outwards from the center, similar to an inclined surface
• Reduce feed to 1/3 of the initial penetration rate for a concave surfaces
• Radius of curved surface should be bigger than drill radius
• Not recommended for 6–7×DC drills

• Reduce feed rate to ¼ of normal rate to avoid chipping
• If unstable conditions, make a pilot hole (preferably with short CoroDrill^® 870) or spot-mill to create a flat surface

• The hole must be perpendicular to the surface
• Min. recommended radius of surface
  • Convex: 4×DC
  • Concave: 1×DC
• Ensure stable conditions and use shortest possible drill
• Reduce feed during entrance
  • Convex: ½ of normal rate
  • Concave: ¼ of normal rate
• Hole quality and tool life could be affected compared with favourable conditions
• Make a pilot hole or spot-mill to create a flat surface if radius is smaller than recommended

• Not recommended due to risk of chipping on cutting edge

Challenges:

• Chip evacuation is affected, might become more problematic
• Deburring in the crossing is hard.  Burr formation must be as small as possible
• Causes more tool wear than conventional drilling

Guidelines:

• Always drill the larger hole first in order to minimize internal burr formation
• Min. recommended diameter on the hole to be crossed is 2×DC (to ensure that the centre of the tip enters the workpiece first)
• Ensure stable conditions and use shortest possible drill
• Reduce feed to ¼ of during entrance and exit of holes
• Reduce retract feed
• Hole quality and tool life could be affected compared to favourable conditions

• Maximum recommended angle from centre to corner is 6 degrees to ensure the tip enters the workpiece first
• Ensure stable conditions and use shortest possible drill
• Reduce feed to 1/3 of normal rate during entrance
• Make a pilot hole (preferably with short CoroDrill^® 870) or spot-mill to create a flat surface if larger angle or longer lengths

• Maximum recommended angle is 30 degrees
• Ensure stable conditions and use shortest possible drill
• Reduce feed to 1/3 of normal rate during hole exit
• Reduce retract feed
• Hole quality and tool life could be affected compared with favourable conditions

• Maximum recommended angle from centre to corner is 6 degrees
• Stable conditions
• Use shortest possible drill
• Reduce feed to of normal rate during entrance
• Hole quality and tool life could be diminished compared to drilling in favourable conditions
• Make a pilot hole or spot-mill to create a flat surface if there is a larger angle or longer length

• Can damage drill when penetrating the surface and/or exiting the hole
• Pay special attention for small diameter drills, deflection may occur causing miss-alignment, hole wander and even tool breakage
• Reduce feed to 25% of recommended rate until full diameter is in cut to reduce risk of chipping

• Possible to drill if radius is > 4 times drill diameter and the hole is perpendicular to the radius
• Feed: 50–100% of recommended rate until full diameter is in cut
• Alternatively, mill a small flat on the surface before drilling

• Possible to drill if radius is >15 times drill diameter and the hole is perpendicular to the radius
• Reduce feed to 30% of recommended rate until full diameter is in cut
• Alternatively, mill a small flat on the surface before drilling

• Not recommended due to risk of chipping on cutting edge

• For drilling depths below 12×D, reduce feed to 25% of recommended rate when crossing the existing hole
• For holes with different diameters: drill the larger hole first to reduce burr formation

• Inclinations up to 10 degrees
  • Reduce feed to 30% of recommended rate until full diameter is in cut
• Inclinations of more than 10 degrees
  • Not recommended for drilling
• Alternative for large inclinations – mill a small flat surface, then drill the hole

• Cannot be used - only parts of the cutting edge will cut and the tip may not contact the surface first
• High risk of chipping