Small-Diameter

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 seen this pattern more times than I can count. A shop is running 12mm gun drills with no coolant issues. Then they switch to a 6mm job and the pump starts making a rattling noise, the pressure gauge oscillates, and coolant flow becomes erratic. The diagnosis is almost always the same: the pump is cavitating because the small drill restricts flow more than the pump can tolerate. Why Small Drills Cause Cavitation A centrifugal pump needs a minimum flow rate to operate correctly. When the flow drops below this minimum, the pump recirculates fluid internally, heats up, and vapor forms at the impeller inlet. Those vapor bubbles collapse violently — that is the rattling noise you hear. ...

Swiss-type lathes are designed for machining small, slender parts. With live tooling and through-coolant capability, they can handle gun drilling for short deep holes. I have been running gun drilling operations on Swiss lathes for about four years, primarily for medical and precision instrument components. The approach is different from a dedicated gun drilling machine, but with the right setup it produces acceptable results for the right applications. Capabilities and Limitations A Swiss lathe is suitable for gun drilling diameters under 6 mm through lengths up to about 10:1 L/D ratio. Beyond that ratio, the coolant pressure and chip evacuation limitations become too severe. The main limitation is coolant pressure — most Swiss lathes have 200-500 psi coolant systems, while dedicated gun drilling machines run at 1000-3000 psi. ...

I’ve covered medical implant drilling elsewhere — this is about micro-hole drilling across all industries. The same principles apply whether you’re drilling fuel injector nozzles, medical components, or cooling channels in electronics. Below 3mm diameter, deep hole drilling changes character. The tools are fragile, coolant flow is restricted, and every parameter matters more than it does at larger diameters. Where Micro-Holes Show Up The industries that need micro-hole deep hole drilling: ...

The Job That Looked Simple on Paper I had a job drilling 3mm holes through 200mm of titanium 6Al-4V. The L/D ratio was about 66:1, which is well within the range of what a gun drilling machine can handle. I had drilled similar holes in steel without any trouble. The customer needed four holes in a part used for an actuator housing. The tolerance was ±0.05mm on diameter and the straightness requirement was 0.1mm over the full length. Nothing about this job looked unusual when I quoted it. ...