Gun Drill,Deep Hole Drilling
Deep hole drilling is a machining process used to create holes with a depth-to-diameter ratio of at least 10:1. The process typically involves the use of specialized drilling machines that deliver high-pressure coolant through the drill bit. This coolant helps in flushing out chips from the hole, cooling the cutting area, and reducing tool wear.
Deep hole drilling is a critical process in many manufacturing industries, used to create deep and precise holes in metal parts.Some common deep hole drilling processes include Gun Drilling, BTA Drilling (Boring and Trepanning Association), and the use of traditional carbide twist drills. Each method presents distinct advantages for deep bore drilling, contingent on the requisite hole diameter and precision.
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What is gun drill？
Gun drilling is a type of deep hole drilling process used to create deep and straight holes in various materials with extremely high precision and surface finish.
Generally, deep holes have a depth-to-diameter ratio (L/d) of ≥100. Examples include cylinder bores, axial oil holes in shafts, hollow spindle holes, and hydraulic valve holes. Among these holes, some require high machining precision and surface quality, and some materials have poor machinability, necessitating a very high depth-to-diameter ratio. Gun drills, with effective diameters ranging from 1 to 50 millimeters (0.04 to 2.00 inches), are capable of drilling deep holes that exceed the depth achievable by traditional machinery and tools (such as twist drills). This is because gun drilling involves forced chip removal, where high-pressure cutting oil enters the gun drill through the drill shank, reaches the cutting zone, and cools and lubricates the cutting edge. The iron chips and cutting oil are discharged along the “V” groove of the drill rod. The deeper the hole, the more the machining efficiency is realized.
Gun drilling was initially used for gun barrel manufacturing due to its excellent hole machining performance. It is now widely applied in industries such as shipbuilding, automotive, engines, locomotives, military, chemical machinery, oil nozzles and pumps, mining machinery, and hydraulic components.
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Classification of Gun Drills
Brazed Gun Drills
Solid Carbide Gun Drills
Indexable Gun Drill Tools
Advantages Of using Gun Drill
Due to the unique tool structure of gun drills, during deep hole machining, there is no need for chip evacuation, allowing for one-pass machining to the desired depth. The deeper the hole, the more the machining efficiency is realized. Depending on the material and hole diameter, the machining speed can reach 30 to 100 mm/min or higher. Gun drills are not only suitable for specialized deep hole drilling machines but can also be used on other traditional machine tools, such as machining centers and CNC lathes.
(Note: Unlike specialized deep hole drilling machines, machining centers rarely use guide bushings for alignment. Therefore, to use a gun drill on a machining center, a pilot hole must first be pre-drilled in the workpiece. To achieve optimal machining results, the pilot hole must meet specific requirements for diameter tolerance, depth, and other aspects.)
Two different setups for deep hole drilling
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Deep Hole Drilling on Conventional Machine Tools
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Step 1: Pilot Hole:
The workpiece is drilled with a pilot hole to guide the deep hole drill. The pilot hole has a length of 1.5 times the diameter (L=1.5xD) and a slight oversized diameter for clearance.
Step 2: Deep Hole Drilling:
The deep hole drill is then inserted into the pilot hole to continue drilling to the desired depth.The coolant cycle is shown, where high-pressure coolant is pumped into the drill to cool the cutting edge and flush out chips through the drill's internal coolant channels.
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Deep Hole Drilling Machines
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Setup:
The workpiece is clamped securely in place.
A drilling bush holder and drilling bush guide the drill bit to ensure straightness.
The spindle unit drives the rotation and feed of the drill bit.
Coolant Cycle:
The high-pressure coolant system is more advanced, with a dedicated high-pressure pump and filters.
Coolant is supplied directly to the cutting edge through the drill's internal channels.
The coolant carries away chips from the drilling area, ensuring smooth operation and preventing overheating.
Gun drill vs twist drill
Twist drills are commonly used drilling tools for general drilling needs.Twist drills are commonly used drilling tools for general drilling needs.Common twist drills can be used for holes with a depth to diameter ratio of 5:1 and High performance twist drills with through-tool coolant for holes with a depth to diameter ratio of 10:1.For holes with the depth to diameter ratios mentioned above, twist drills offer the following advantages:
Stability and Efficiency:
Twist drills provide good stability and can efficiently complete drilling tasks for relatively shallow deep holes.
Wide Applicability:
This ratio of hole depth is common in many routine machining processes, and twist drills are capable of handling such tasks.
Cost-Effective:
Twist drills are relatively inexpensive and have a long service life, making them suitable for mass production.
Ease of Operation:
Twist drills are simple in structure and easy to operate, making them suitable for use on various machine tools.
For drilling needs with a ratio of 5:1, Twist Drills are ideal due to their stability, cost-effectiveness and ease of use. For deeper holes, such as those with Depth to Diameter Ratio of 10:1 or higher, more advanced tools and equipment are required, such as high-performance twist drills, specialized deep hole drilling tools or gun drills.
Frequently encountered problems in gun drilling and the reasons for them
Common Issues in Gun Drilling and Their Solutions
Most frequent questions and answers
- Internal cutting edges are too high.
- Insufficient back clearance.
- Cutting edges are not centered or the size is too small.
- Excessive wear or chipping of cutting edges.
- Low cutting speed or excessive feed rate.
- Excessive feed rate.
- Loose central frame or guide bush, unable to support the tool shank.
- Guide bush wear.
- Severe wear or chipping of cutting edges.
- Insufficient coolant pressure, causing chip jamming.
- Coolant viscosity is too high, leading to poor chip removal.
- Burrs at the oil inlet or excessive oil inlet depth.
- Chips are not effectively broken or removed.
- Excessive wear on the central cutting edge, improper selection of side cutting edge clearance.
- Tilting or misalignment of the guide bush or workpiece.
- Misalignment of the tool, machine, and spindle centerline.
- Workpiece material deformation.
- Excessive wear on the drill head outer edge.
- Guide bush wear.
- Workpiece material hardness and machinability are inconsistent, causing tool deflection.
- Machine rigidity issues, affecting accuracy.
