Deep Hole Drilling

Deep hole drilling creates long, straight, precise bores at high length-to-diameter ratios using specialized tools with internal coolant and chip evacuation.

Overview

Deep hole drilling (including gun drilling and BTA drilling) produces very deep, accurate bores with excellent straightness and surface finish, far beyond what standard twist drills can handle. It uses specialized tooling with internal coolant delivery and chip evacuation to maintain tool stability and control heat over extreme length-to-diameter ratios.

You use deep hole drilling when you need L/D ratios of roughly 10:1 to 100:1 or more, consistent diameter, and minimal drift in steels, stainless, high-temp alloys, or aluminum. The process excels for through-holes and deep blind holes but needs good fixturing, rigid setups, and clear access from at least one side. Tradeoffs include higher setup cost, more specialized suppliers, and limits on very small or very large diameters, so it makes the most sense when hole performance is critical or when you have multiple identical deep holes per part or per batch.

Common Materials

  • Aluminum 6061
  • 4140 steel
  • 17-4 PH stainless steel
  • Stainless 304
  • Inconel 718
  • Titanium Grade 5

Tolerances

±0.001" to ±0.003" on diameter; straightness ~0.001"–0.005"/inch depending on depth, material, and tooling.

Applications

  • Rifle and shotgun barrels
  • Hydraulic manifold deep ports and galleries
  • Crankshaft and camshaft oil passages
  • Mold and die cooling channels
  • Landing gear and actuator rods
  • Medical instrument shafts and cannulas

When to Choose Deep Hole Drilling

Choose deep hole drilling when hole depth exceeds about 10× diameter and you need reliable straightness, good surface finish, and controlled diameter through the full depth. It fits best for production or recurring work with identical or similar deep bores where setup cost can be amortized. It is especially useful in tough materials where standard drills wander, overheat, or break at extreme depth.

vs CNC Drilling

Choose deep hole drilling over standard CNC drilling when the hole depth is more than about 10× diameter or when straightness, runout, and finish over long distances are critical. Deep hole tooling manages heat and chip evacuation much better, so you can reach depths that would be unreliable or impossible with conventional twist drills.

vs CNC Turning

Select deep hole drilling instead of relying only on turning operations when you need an internal bore much deeper than you can reasonably drill on a lathe with standard tools. It provides better control of straightness and diameter on long bores in shafts, rods, and barrels than a typical turret-drill setup.

vs EDM Drilling

Use deep hole drilling instead of EDM drilling when the hole diameter is moderate, the material is machinable, and speed and cost matter more than extreme material hardness capability. Deep hole drilling removes material faster and more economically for long bores where you don’t need the tiny diameters or exotic-alloy flexibility of EDM.

vs Standard Boring / Reaming

Choose deep hole drilling over boring/reaming when you must generate the entire depth with a guided tool rather than step-drill then finish. Deep hole drilling can produce a near-finished hole in one controlled pass, with less risk of cumulative alignment error over very long distances.

vs 3D Printing

Pick deep hole drilling instead of 3D printing when you only need a precise, long, straight bore in an otherwise conventional machined or forged part. It delivers far better dimensional control and surface finish inside deep holes than most printed processes, and fits easily into traditional machining workflows.

Design Considerations

  • Aim for hole depth-to-diameter ratios above ~10:1 to justify deep hole drilling; below that, simpler drilling may be more economical
  • Provide a clear, uninterrupted entry surface and avoid starting the hole on angled or rough faces to improve tool guidance and reduce wander
  • Keep cross-holes, keyways, and large interruptions away from the first several diameters of the bore to avoid tool deflection and chatter
  • Specify required straightness, concentricity, and surface finish on the drawing, not just diameter, so the shop can select the right tooling and process
  • Design with standard gun drill diameters where possible and note any critical diameter bands or transitions along the depth
  • Allow adequate runout space and fixturing length at one end of the part so the shop can properly support and guide the deep hole drill