Deep Hole Drilling

Deep hole drilling creates long, straight, high-aspect-ratio bores using gun drilling or BTA tooling with controlled chips, coolant, and tight runout.

Overview

Deep hole drilling produces bores with high length-to-diameter ratios using dedicated gun drilling (single-lip) or BTA/STS systems. High-pressure coolant and chip evacuation enable straight holes, good surface finish, and consistent diameter in materials that would wander or pack chips in conventional drilling.

Choose it for holes typically deeper than ~10× diameter, especially when straightness, concentricity, and repeatability matter (hydraulic manifolds, barrels, long shafts, molds). It’s commonly run on specialized deep-hole machines or capable CNCs with the right tooling, fixturing, and coolant systems.

Tradeoffs: higher setup and tooling cost than standard drilling, and it demands stable fixturing and clear requirements for datum, runout, and bore spec. Very small diameters, cross-holes near the entry, interrupted cuts, and hard inclusions increase risk and cost; secondary operations like honing/reaming may be needed for tighter size or finish.

Common Materials

  • 4140 Steel
  • 17-4 PH Stainless
  • Aluminum 6061
  • A2 Tool Steel
  • Inconel 718

Tolerances

±0.001" to ±0.003" (diameter); straightness typically 0.001"–0.005"/ft

Applications

  • Hydraulic cylinder rods
  • Rifle and cannon barrels
  • Mold cooling channels
  • Oilfield drill collars
  • Aircraft landing gear struts
  • Long pump shafts

When to Choose Deep Hole Drilling

Choose deep hole drilling for long bores where L/D is typically >10:1 and you need controlled straightness and consistent diameter over the full depth. It fits low to medium volumes where setup is justified and the hole is a primary functional feature. It’s most effective when the part can be rigidly supported and you can define datums, depth, and bore requirements clearly.

vs CNC Drilling

Choose deep hole drilling when the hole is long enough that conventional twist drills tend to wander, bellmouth, or pack chips. Dedicated gun/BTA tooling with high-pressure coolant holds straighter bores and more stable diameter over depth, with fewer peck cycles and less risk of drill breakage at high L/D.

vs Boring (CNC boring bar)

Choose deep hole drilling to create the initial long bore efficiently and with better chip evacuation at high L/D. Boring bars become deflection-limited as depth increases; deep hole drilling maintains stability and can hit size/finish that would be impractical to bore from solid.

vs Reaming

Choose deep hole drilling when you need to generate the full-depth hole from solid; reaming requires a predrilled hole and is sensitive to runout and stock variation. Deep hole drilling provides a predictable pre-bore that can then be reamed or honed if tighter size or finish is required.

vs Honing

Choose deep hole drilling for material removal and creating the bore geometry; honing is a finishing step, not a hole-making method. Deep hole drilling is the cost-effective way to establish the long, straight bore before honing to final size, roundness, and surface finish.

Design Considerations

  • Call out hole diameter, total depth, and whether depth is measured from a specific datum face or from the entry surface
  • Specify straightness/concentricity requirements and reference datums; avoid vague notes like “drill straight hole”
  • Keep entry features simple and provide a stable start surface; counterbores or spotfaces can reduce walk and bellmouth
  • Avoid intersecting cross-holes or keyways near the entry region; place interruptions deeper or add process allowances
  • Allow room for tool runout and breakout at the far end; specify acceptable exit condition or leave extra stock for facing
  • If tight size/finish is required, plan a finishing allowance and call out the final process (ream/hone) instead of over-tightening the drill tolerance