Cutting tools tips: drilling deep

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by Manfred Lenz

Overcoming deep hole drilling challenges

Machinists often find deep hole drilling–at depths from 12 to 30 times drill diameter–daunting. Many are unsure about the process along with all that is required to maintain good hole straightness, tolerance/size and surface finish. Fortunately, there are some simple tips that will allow machinists to achieve near-perfect deep drilled holes while also increasing productivity as well as tool life.

1.Match pilot drill diameter
When deep hole drilling, a machinist must first produce a pilot hole, typically at a depth of two to three times the pilot drill diameter. Pilot holes should be the same size diameter as the deep hole drill to be used. This matching hole size creates a starting point and helps guide –almost like a bushing–the long drill, keeping it straight and preventing it from walking. Without a pilot hole at all, the long drill would vibrate back and forth at the start of the hole and eventually break.

2.Switch off the spindle when entering the hole
A common mistake machinists make is to feed an already rotating deep hole drill into a pilot hole. This causes the long drill to slap the sides of the hole, decreasing the tool’s life. Instead, leave the spindle off, fast feed the drill into the hole, then turn on the spindle when the drill tip is about 0.020 in. (0.5 mm) above where the pilot hole ends and begin to drill without pecking

Rapid feeding drills out of deep holes is a mistake as well. At the end of the drilling depth, the machinist should reduce the spindle speed to a few hundred rpms and retract the deep hole drill at a slow rapid to where drilling started. At that point, the machine spindle is switched off, and the drill exits the remainder of the hole.

Stopping a drill’s rotation before it enters a hole, retracting it slowly and at reduced rpms can increase drilling cycle times, but by barely a fraction of a second. The resulting gains in tool life far outweigh that little amount of added time.

3.Pay attention to drill geometry
Drill geometry is a key factor in successful deep hole drilling. Pilot drills, for instance, can have 140º point angles, while long drills may have 136º point angles. This ensures that the centre of the long drill will contact the material first while in the pilot hole and seat itself. Then, the corners make contact.

Some deep hole drills also have two land margins per flute. The drill tip does the cutting, while the land margins at the sides help hold the drill in place during operation. On long drills, land margins are located only at the very ends of the flutes for clearance that prevents drag. The more flute drag, the more heat generated and the higher the risk of drill breakage.

Solid-carbide drills are a must for producing hole depths greater than 12 times the drill diameter. Carbide tools are stiffer and less likely to wander as compared with HSS and cobalt tools in deep hole drilling. However, deep holes with large diameters–3 in. (76.2 mm or more–will require the use of insertable deep hole drills.

4.Ensure proper chip evacuation
The number one reason drills fail is due to inadequate chip evacuation. While most long drill geometries provide effective chip breaking, they must then evacuate the chips out of the hole. Those drills with both polished flutes and back tapers will work best.

Coatings minimize frictional heating and thus contribute to increased tool life. Coolant, however, is the most important factor for chip evacuation. Even one chip left in the hole can break the drill, so high pressure through-tool coolant is the only option. High pressure coolant forces them up the drill flutes and out of the hole. Through-coolant drills also eliminate the need for pecking cycles.

5.Use the right toolholder
Hydraulic and shrinkfit toolholder systems generate the least amount of runout, making them ideal for deep hole drilling applications. Both systems can cost a bit more, and precision collet chucks are one alternative, but they must be high quality and provide low runout.

A final and very important tip is to consult a tooling expert. A partnership between a shop and its tool supplier makes all the difference in choosing the right drill for deep holes, or any holes for that matter. SMT

Manfred Lenz is product manager for drilling, Seco Tools.


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