Best Practices in Deep hole Drilling

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Tips and Tricks to achieve best possible results
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Tips and Tricks to achieve best possible results
Select a dedicated, specially designed deep hole
drilling machine
The Machine Tool

• Machine Base is designed to withstand extremely
  high loads impart better dampening
  characteristics to produce precise holes
• High speed drilling spindle capable of
  withstanding heavy drilling thrust
• Use of high torque AC spindle motors
• Heavy thrust feed drive motor
• High Pressure Coolant system and efficient swarf
  removal provisions
• Use of Tool monitoring system for critical
  applications
• Tube guiding and vibration dampening using tool
  steadies

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Tips and Tricks to achieve best possible results
Appropriate selection of deep hole drilling setup
Component counter rotation creates a straighter
axis and keeps the drill in the center when the
drill feeds, achieving excellent hole straightness

• There are three possible modes of operation
• Tool rotating and feeding - component stationary
• Tool stationary and feeding - component rotating
• Tool rotating and feeding - component
  counter-rotating
• Counter rotate the part where straightness is
  critical
• For a 5.56 mm bore size at 530 RPM of component
  we achieve hole straightness of 0.076mm - 0.127mm
  over a length of 710mm.
• Without counter rotation the hole straightness
  would be 0.508mm - 0.762mm
• Net improvement in straightness with counter
  rotation in this case is about 6 times

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Tips and Tricks to achieve best possible results
An accurately matched guide bush ensures that the
drilling starts straight, stays accurate and
continues that way till the end What starts good,
ends great!
Guide bush Design Accuracy

• The Guide Bush is an important accessory that
  guides the tool into the component while starting
  the drilling process.
• Net improvement in straightness with counter
  rotation in this case is about 6 times
• Hardness of a alloy steel / Carbide guide bush
  has to be 58-62 RC
• Replace the guide bush if the clearance between
  the guide bush and drill exceeds acceptance
  thresholds
• Make sure the guiding length of the bush is
  enough to support the gun drill lug
• Manufacturing tolerance on OD-ID concentricity
  has to be between 0.005mm TIR
• Make sure the guide bush and gun drill are
  matched prior to use. A minor deviation of
  (0.127mm 0.203mm) between the gun drill and
  guide bush can cause a major deviation in hole
  straightness

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Tips and Tricks to achieve best possible results
Tool or Insert Selection
Selection of appropriate tooling results in
better bore straightness, tolerance as well as
surface finish

• Tool geometry
• Carbide Grade and Coating on Tool
• Support pad configuration

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Tips and Tricks to achieve best possible results
Tool Insert Condition
Check the cutting edge of gun drill or the insert
at regular intervals

• A worn out or damaged tool can cause greater
  damage and spoil output quality
• In very long parts or materials that are
  difficult to machine, intermittent checks might
  be required
• A Gun drill regrinding machine can be used to
  re-sharpen the Tool.

Gundrill regrinding machine
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Tips and Tricks to achieve best possible results
Coolant Pressure
Pumping the coolant oil at optimum pressure is
essential for successful deep hole drilling

• It ensures that the chips are evacuated
  efficiently
• It removes process heat
• It provides lubrication to guide pads
• Straightness of the hole and bore tolerance are
  affected by coolant pressure

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Tips and Tricks to achieve best possible results
Coolant Type Additives
Type of coolant oil used and additives have a
profound impact on hole surface finish, hole
tolerance and most importantly tool life

• Use of straight cutting oil yields best results
• Viscosity recommended is 10-12 CST at 100ºF
  (40ºC)
• Use of EP ( Extreme pressure )additives are
  mandatory
• 2 - 2.5 Sulfur to the oil provides anti-weld
  properties
• 4 - 4.5 Chlorine helps lubrication of burnishing
  pads allowing better chip evacuation
• Oil should have anti-foaming properties
• Use only specially formulated Gun Drill oil
• Water soluble coolants can also be used for Gun
  Drilling (Cast Iron)

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Tips and Tricks to achieve best possible results
Coolant Temperature
Use of oil chiller to remove process heat and
keep the oil at 100 F (38C) or below is highly
recommended

• Removing process heat helps to improve tool life
  and produces good hole tolerance and surface
  finish.

Image Source Advance Cooling
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Tips and Tricks to achieve best possible results
To obtain good hole straightness, it is necessary
that all elements like LM guideways or box type
ways and sub-assemblies like spindles, bush box,
work head and tool head slides and steadies are
aligned accurately
Machine Accuracy Alignment of moving elements
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Tips and Tricks to achieve best possible results
Cutting Parameter Selection

• To obtain good output results, it is necessary
  that recommendations from tooling manufacturers
  on cutting speed, feed and coolant pressure be
  followed
• These recommendations always need to be fine
  tuned to achieve desired chip formation and
  optimal tool life for an individual application
  in production mode

Image Source Botek
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Tips and Tricks to achieve best possible results
Tube Support and Guiding mechanism

• The tubes used in gun drilling are very long and
  slender
• They need to be guided to reduce whipping
• Whipping leads to reduction in tool life and the
  drill losing its axis and wavering off-course
• Guiding is recommended at internals of every
  30-40 L/D ratios
• It is better that the tube steadies move with the
  spindle so that at any given point un-supported
  tube length remains equal

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Tips and Tricks to achieve best possible results
Workpiece Material Homogeneity Uniform Grain
Structure
A major factor contributing to the success of
deep hole drilling is the quality of input
material

• Straightness, hole tolerance as well as surface
  finish are dependent on the uniformity in grain
  structure and homogeneity of the material
• Hard spots or uneven heat treatment affects the
  chip formation resulting in chip clogging and
  excessive wear of the cutting edge
• Our recommendation is to buy material from
  reliable sources to ensure consistent material
  properties

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Tips and Tricks to achieve best possible results
Swarf needs to be filtered appropriately by using
a high pressure coolant filtering and
recirculating system specially designed for deep
hole drilling
Coolant Filtration

• Hole Tolerance and Surface Finish are affected if
  coolant oil is not filtered appropriately.
• Tool life is also affectedly by impurities being
  present in oil
• Machine elements like High Pressure Pumps and
  Coolant Unions need quicker replacements if oil
  is incorrectly filtered.