What is Gun Hole Drilling?
Gun drilling is a process of drilling deep holes which in simplest terms are long depth to diameter holes. Gun drilling machines are a type of deep hole drilling machine which are used when the desired hole length is not possible on conventional CNC Machines. Gun drilling is a deep hole drilling process that uses a machine with a name derived from its application of drilling gun barrels. This method uses a long drill paired with high pressure cutting fluid to achieve deep straight and concentric holes. Specialized deep holes drill machines can achieve holes up to a staggering 400:1 D:d (Depth to Diameter).
BTA (Boring and Trepanning Association) drilling machines are another type of deep hole drilling machine. BTA drilling machines are capable of drilling larger diameter holes at rates considerably faster than gun drilling machines.
This article will discuss when you should consider using them. But first, what really is a deep hole?
Deep Hole Classification:
There is much debate as to what a deep hole is, but we can consult our depth-to-diameter ratio D:d. Deep holes were once defined as a three times depth to diameter ratio back in the 1920s. Now, it’s common to refer to a deep hole as ten times depth-to-diameter. Increased mainly due to the capabilities of modern CNC machines. Most CNC mill or lathe machines will have little difficulty drilling holes up to ten times depth-to-diameter. There are a few techniques and processes that will increase our odds of successfully creating deep holes. Let's briefly talk about the options we have on conventional CNC lathes and mills.
Drilling with conventional CNC lathes and mills.
Under five times depth-to-diameter most CNC turning and milling machines equipped with standard flute drills will not have problems drilling holes. At five to seven times Ø peck drilling cycles can be introduced.
Pecking is a method that temporarily raises the drill. It’s best to visualize a woodpecker or oil drilling machine. Introducing a pecking drilling technique helps to evacuate material from the bore. The pecking process is repeated several times per hole depending on depth and material. The drill will raise .5-1 times diameters. Chips left in a bore can cause heat, loose tolerances and additional tool wear, thus removing chips is essential to keeping tolerances and long tool life. Chip removal becomes our limiting factor for deeper depths on traditional CNC lathes and mills.
Above seven times diameter it’s recommended to switch to a parabolic drill also known as helix drills. Parabolic drills have a different head and spiral geometry that aid in chip evacuation. Parabolic drills increase the drilling depth near five times deeper than standard drills before requiring pecking. 1.5-2 times diameter before pecking is a significant time savings over a full production run. Many machinists simply prefer parabolic drills as their standard tool because they offer increased feed rates which further reduce machining time. The benefits don’t come free as they are more expensive to their standard counterparts. When drilling with a parabolic drill it’s advised to use a machine equipped with through tool/spindle coolant. We don't often use parabolic drills as we have gun drilling machines.
At and above ten times Ø it’s still possible to continue to use a parabolic drill combined with a custom drill cycle. While the brave could continue to use parabolic drills at twenty times Ø The rest of use should consider a different approach.
Drilling deep holes with gun drilling machines:
Above twenty times diameter and with 1-50mm Ø holes a gun drilling machine is your weapon of choice. Gun drills themselves can be used on specially configured CNC and lathes however it’s preferred to use a gun drilling machine. A gun drill machine uses a long drill with a single-lip and runway configuration. These tools are called gun drills or deep hole drills. Gun drills can have either brazed tips or tool inserts, allowing replacement if the cutting edge becomes damaged. However if the shaft becomes bent or damaged then the entire tool has to be replaced.
The negative space on the tool called a runway extracts waste material from the hole. High pressure cutting fluid supplied internally through the tool forces the cut material to follow the runway. The cut material and cutting fluid eventually finds its way into a chip holding tank. The chip holding tank is a box shaped chamber located in the center of the machine. The collected material is then moved to a metal drum via conveyor belts for recycling.
There are three configurations of deep hole drilling machines. Ordered by drift reduction and improved concentricity.
rotating cutting tool and stationary workpiece.
rotating workpiece and stationary cutting tool.
rotating cutting tool and counter rotating workpiece.
Tools that counter rotate the workpieces produce better concentric holes and increased drilling speeds.
At Anoesis we use deep hole drilling as a replacement for parabolic drill applications. Workpieces are first machined on our CNC mills, bar stock lathes or swiss lathes then move to the deep hole drilling machine. They are placed across supports near the secondary spindles. The secondary spindle engages with the workpiece and counter rotates it. Counter rotating the workpiece eliminates drift and ensures hole concentricity above forty depth-to-diameter.
As the cutting tool is quite long compared to its diameter, sliding braces called whip guides are needed. Spacing between the whip guides will decrease as the main spindle housing moves forward. On the whip guide you will see an orange silicone seal that blocks the chip box housing holes.
Our machines have a max hole dimension of Ø25 at a 200mm depth.
Common part examples are thru-hole axles, bike cranks, air tool internals and suspension components. Deep hole drilling machines greatly reduce the possibility of defective parts as they are designed for one type of operation and are mechanically simple having only one axis.
Going deep and wide with BTA drilling machines:
From 20-200mm BTA drilling machines will be your tool of choice. BTA (Boring and Trepanning Association) machines come in two forms, ones with brazed tools or tools with specialized inserts. On BTA machines fluid is introduced from the outside of the tool forcing material through hollow cavities within the tool itself. BTA machines are three to four times faster than gun drilling machines. The machines can also utilize contour boring techniques to further increase the diameter of a hole. BTA machines utilize pull boring which is essentially pulling the tool back through the hole while it’s kept under tension. This method can be applied to existing holes truing or piloting them.