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Irregular surface drilling

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.

Irregular surface drilling



​Can damage drill or inserts when penetrating the surface and/or exiting the hole.

Indexable insert drills
  • Reduce feed rate in order to avoid insert damage
Solid carbide drills
  • Reduce feed rate to ¼ of normal rate to reduce risk of chipping on the cutting edges
Exchangeable-tip drills
  • Reduce feed rate to ¼ of normal rate to avoid chipping
  • If unstable conditions, make a pilot hole or spot-mill to create a flat surface​
 

Convex surface drilling



Indexable insert drills
  • Centre of the drill in contact with workpiece first, giving normal torque
  • No adjustment is needed
Solid carbide drills
  • Minimum recommended radius of the convex surface is 4 times the drill diameter
  • The hole should be perpendicular to the surface
  • Feed 50-100% of normal rate
Exchangeable-tip drills
  • Minimum recommended radius of the convex surface is 4 times the drill diameter
  • The hole should be perpendicular to the surface
  • Stable conditions
  • Use shortest possible drill
  • Reduce feed to ½ 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
 

Concave surface drilling



Indexable insert drills
  • Should enter with the centre insert first
  • Drill engagement varies depending on the concave surface radius and hole diameter in relation to the height of the drill point
  • The periphery of the drill will be engaged first if the concave surface radius is small in relation to the hole diameter
  • Reduce feed to ⅓ of normal rate to reduce drill deflection
Solid carbide drills
  • Possible to drill if radius is >15 times drill diameter and the hole is perpendicular to the radius
  • Reduce feed to ⅓ of normal rate​
Exchangeable-tip drills
  • Minimum recommended concave surface radius is 1 x the drill diameter (to ensure the centre of the drill tip enters the workpiece first)
  • Stable conditions
  • Use shortest possible drill
  • Reduce feed to ¼ of normal rate during entrance
  • Hole quality and tool life could be diminished compared to drilling in favourable conditions
 

Pre-drilled holes



Indexable insert drills
  • Pre-drilled hole should not be larger than DC/4 to keep the cutting force balance between the central and peripheral insert on an acceptable level
  • Possible to use three flute drill to open out pre-hole
Solid carbide drills
  • Not recommended due to risk of chipping on cutting edge
Exchangeable-tip drills
  • Not recommended due to risk of chipping on cutting edge
 

Cross hole drilling

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
General guidelines:
  • Feed reduction is generally necessary as the drill crosses the existing hole
  • For holes with different diameters: drill the largest hole first to reduce burr formation ​
Indexable insert drills
  • When crossing a hole with diameter larger than ¼ of the drill diameter, reduce feed to ¼ of normal rate
Solid carbide drills
  • Reduce feed to ¼ of normal rate when crossing the existing hole
 
Exchangeable-tip drills
  • Minimum recommended diameter on the hole to be crossed is 2xDC (to ensure that the centre of the tip enters the workpiece first)
  • Stable conditions
  • Use shortest possible drill
  • Reduce feed to ¼ of normal rate during entrance and exit of cross hole
  • Reduce retract feed
  • Hole quality and tool life could be affected compared to favourable conditions​
 

Drilling inclined entrances/exits

Generates uneven and excessive forces acting on the cutting edges
  • Intermittent cutting as the drill enters/exits the workpiece
  • Increases chance of vibration
  • Can distort the drilling profile
  • Causes more tool wear than conventional drilling
General recommendations:
  • Stability is crucial. A small length to diameter ratio will help to keep the tolerances
  • Milling a small flat surface is recommended when entering workpieces with a large inclination​
Indexable insert drills
  • Use a drill as short as possible to minimize vibration tendencies and decrease the effects of drill deflection
  • If the surface is angled more than 2 degrees, reduce feed to ⅓ of normal rate
  • The feed reduction will also help to reduce burring on exiting​
Solid carbide drills
  • For inclinations up to 10 degrees, reduce the feed to ⅓ of normal feed rate until the full diameter is in cut
  • Inclinations of more than 10 degrees are not recommended for drilling
  • An alternative for large inclinations is to mill a small flat surface, then drill the hole​
 

Drilling inclined entrances​

Exchangeable-tip drills
  • Maximum recommended angle is 6 degrees
  • Stable conditions
  • Use shortest possible drill
  • Reduce feed to ⅓ of normal rate during entrance
  • Hole quality and tool life could be 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 entry angle or longer length
 

Drilling inclined exits​

Exchangeable-tip drills
  • Maximum recommended angle is 30 degrees
  • Stable conditions
  • Use shortest possible drill
  • Reduce feed to ⅓ of normal rate during exit
  • Reduce retract feed
  • Hole quality and tool life could be diminished compared to drilling in favourable conditions
  • It is recommended that tip margins are kept in contact with the hole surface on both sides
 

Drilling asymmetrically curved surfaces

Causes the drill to bend away from the centre line of the hole

  • Initially only the periphery of the cutting edge is in contact
  • Similar but not identical to inclined surfaces
Indexable insert drills
  • Since the centre insert will not be initially in the cut to balance the cutting forces, reduce the entrance feed to ⅓ of normal rate
Solid carbide drills
  • Cannot be used - only parts of the cutting edge will cut and the tip may not contact the surface first
  • High risk of chipping
Exchangeable-tip drills
  • Maximum recommended angle from centre to corner is 6 degrees
  • Stable conditions
  • Use shortest possible drill
  • Reduce feed to ⅓ 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
 
 

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