Gun-Drilling

Over the years I have been asked the same questions repeatedly by new operators, engineers, and shop managers. Here are the answers, with links to detailed articles for each topic. 1. What qualifies as a deep hole? A hole is considered deep when the depth-to-diameter ratio (L/D) exceeds 10:1. Below that, standard twist drills can handle the job. Above 10:1, you need specialized tooling and coolant systems. Full article: Deep Hole Drilling vs Conventional Drilling ...

Carbide is the standard for gun drilling, but it is not always the best choice. In specific applications — high-silicon aluminum, hardened steels, and abrasive composites — PCD and CBN gun drills can outlast carbide by 10-50x. The catch is that these tools are expensive, brittle, and require a rigid setup. Here is what I have learned about when they work and when they do not. What PCD and CBN Are PCD (polycrystalline diamond) and CBN (cubic boron nitride) are superhard materials. PCD is the second-hardest known material after natural diamond. CBN is the second-hardest after PCD. ...

I get more questions about drilling small holes in Inconel 718 than almost any other topic. A recent Practical Machinist thread about drilling 3mm holes 160mm deep in Inconel 718 at HRC 40-45 got over 3,500 views. The user was struggling with drill breakage, centerline deviation, and surface finish. The root cause in that thread was the same one I see most often: coolant pressure at 8 bar (116 psi) when the job needs 100 bar or more. Here is what I have learned about getting this application right. ...

I have run gun drilling with both oil-based and water-based coolants across hundreds of jobs. The choice between them is not about which is better in general — it is about what matters more for your specific work: tool life and finish, or cost and cleanliness. Here is what I have found. The Core Difference: Lubrication vs Cooling Oil lubricates. Water cools. That simple distinction drives every other difference. A gun drill cutting edge needs lubrication to reduce friction between the tool and the workpiece. Oil does this naturally. Water-based coolants need chemical additives to approach the same level of lubricity. ...

Long workpieces cause problems that short ones do not. A 500mm shaft supported at both ends deflects under its own weight by a fraction of a millimeter. A 2000mm shaft deflects by several millimeters. That deflection pulls the hole off-center. Steady rests fix this. But only if they are set up correctly. A misaligned steady rest is worse than no steady rest — it forces the workpiece into a bend that the drill then follows. ...

High-speed rail axles are safety-critical components that need a through-hole running the full length. The hole allows an ultrasonic probe to pass through the axle for non-destructive inspection. Without the through-hole, internal defects cannot be detected. In my experience, the drilling quality directly determines whether the axle passes its final ultrasonic inspection. Axle Specifications and Material Grades A high-speed rail axle is typically 2-3 meters long with a 30-60mm bore. The material is high-strength axle steel like EA1N or EA4T. The surface finish requirement is Ra 1.6um as-drilled. The straightness spec is typically 0.5mm per meter. ...

Commercial vehicle brake calipers need precision bores for the brake pistons. The bore must be straight and smooth so the piston slides freely without leaking. A bore that is out of round or tapered by more than 0.02mm can cause the brake to drag or leak. I have machined calipers for Class 8 trucks, earthmovers, and military vehicles, and I have found that the bore quality is the single largest factor in caliper warranty returns. ...

Tool holder runout directly affects hole quality in deep hole drilling. I have measured the runout difference between collet holders and side-lock holders on dozens of gun drill setups, and the gap is significant enough to dictate which holder I use for each job. A difference of 0.01mm in runout at the holder can become 0.05mm or more at the drill tip. Problem Description Runout at the cutting edge causes the drill to cut unevenly. One side of the drill takes more material than the other, producing an oversized hole and accelerated tool wear. On gun drills, which rely on a single cutting edge and two guide pads, runout pushes one guide pad against the bore wall harder than the other, causing rapid pad wear and poor surface finish. ...

A manual lathe can be converted for gun drilling with the right equipment. The conversion costs a fraction of a dedicated machine but comes with real limitations. I’ve done this conversion on a 14-inch Colchester lathe and gotten usable results. The main requirements are a through-coolant toolholder, a high-pressure coolant system, and a guide bushing support. The toolholder mounts on the lathe carriage and holds the gun drill. The coolant system delivers fluid through the tool at the required pressure. The guide bushing supports the drill at the workpiece entry. ...

I spent the first three years of my career watching the coolant pressure gauge like a hawk. If it held steady at 1200 psi, I assumed everything was fine. Chips were packing? Must be a feed issue. Drill broke? Wrong parameters. Then a senior engineer walked over to my machine one afternoon, pulled out a bucket and a stopwatch, and showed me that my pump was delivering less than half the flow the drill actually needed. The pressure gauge never flickered. ...