Troubleshooting Gun drilling Operations

Hole making in all its many forms can be challenging. Difficulties with chip evacuation, coolant starvation, drill walk and poor surface finish—these are just a few of the headaches that can turn one of the most common of all metalworking operations into an awful day. Unfortunately, the list of potential problems tends to grow as hole depths increase.

Gun drilling is a deep hole drilling process that uses a long, single-flute tool to produce holes in metal at high depth-to-diameter ratios. It is usually performed on dedicated gun drilling machines but is also possible on specially equipped mills and lathes.

Wherever it’s performed, gun drills can experience various issues that affect the quality of the holes they produce, including drill walk, poor surface finish, incorrect hole size, and inadequate tool life. Here are some of the most common issues and how to identify them, a step that is critical to avoiding broken tools, scrapped workpieces, and lost productivity:

Hole Straightness
This issue is usually caused by misalignment of the tool and workpiece, a dull or damaged drill, or an unstable setup. To correct it, use a dial indicator to ensure the tool and workpiece are properly aligned in all axes. If the workpiece is less than secure, switch to a more stable clamping method—a well-maintained, hydraulic three-jaw chuck is generally sufficient for holding most workpieces, while a collet chuck or hydraulic toolholder should be used to grip the drill. Also, a steady rest might be required to support very long workpieces. Finally, check the drill for wear and avoid using too small of an outside angle (more on this shortly).

Surface Quality
Rough surfaces can be due to inadequate coolant, excessive feed rate, or improper tool geometry. Begin by checking and adjusting the coolant flow rate and pressure. For smaller diameter drills (below 0.5 inch or 12.7 mm), a pressure of around 500 to 1000 psi (35-70 bar) is commonly used, with a flow rate of 1 to 2 gallons per minute (gpm). For larger drills (above 0.5 inch), 300-500 psi (20-35 bar) is more suitable, with a flow rate of 2 to 5 gpm or even more. Also, check and adjust the cutting parameters—in this instance, a lighter feed rate or higher spindle speed might be needed. Increasing the gun drill’s outer angle may help as well but be sure to check with your cutting tool provider for recommendations.

Incorrect Hole Diameter
For oversize holes, the spindle RPM might be too high, the feed rate too low, or a combination of the two is possible. On the flip side, undersize holes are often caused by insufficient spindle speeds or overfeeding the tool. As with all troubleshooting steps, adjust one variable at a time, document what was changed, and monitor the results before making additional changes. If these steps don’t correct the problem, the culprit could be the coolant pressure—too much tends to make drills cut larger, just as too little leads to undersize holes. Lastly, reach out to your gun drill manufacturer for advice. You may be using an incorrect tool geometry for the material or machining conditions.

Higher-than-expected tool wear is another common problem, one that can be caused by misalignment, improper feeds and speeds, insufficient coolant flow and pressure, or the wrong cutting tool for the workpiece material. Of course, the drill itself might be the culprit, which is why it’s crucial to partner with a knowledgeable supplier of high-quality cutting tools and lean on them for advice when troubles arise. Sometimes all it takes is a minor tweak to the drill geometry or a different coating to conquer these and many other problems. Happy drilling.