In practical drilling, "deep" starts at 3–4× diameter. A 1/2" drill in a 2" hole is 4× diameter. A 3/8" drill in a 2" hole is 5.3× diameter. Once you cross this threshold, chip evacuation becomes the dominant failure mode — not tool wear, not insufficient speed. The chips cannot get out efficiently, they repack against the cutting edge, and the drill either breaks or produces an oversize, rough hole.
Every decision in deep hole setup — peck cycle depth, coolant pressure, drill geometry — is aimed at solving this one problem.
Peck drilling retracts the drill periodically during the cut to clear chips. The critical setting is the peck increment — how deep the drill goes before each retraction. Set it too large and chips pack before the drill retracts. Set it too small and you waste cycle time and create unnecessary friction at the retract point.
A common starting rule: peck increment = 1× to 1.5× drill diameter. A 1/2" drill pecks every 0.5"–0.75". For soft materials that produce long stringy chips (aluminum, low-carbon steel, 304 stainless), use the shorter increment. For short-chip materials (cast iron, brass, free-machining steel), you can go longer.
The retract type matters too. Full retract — pulling the drill completely clear of the hole — gives the best chip clearance but adds the most cycle time. Chip break retract — backing off 0.05"–0.1" without fully clearing — breaks the chip but does not evacuate it. Use full retract for depth beyond 5× diameter. Chip break retract can work from 3–5× in materials that do not produce long stringers.
Flood coolant poured over the top of the workpiece does very little useful work at the bottom of a deep hole. The coolant does not reach the cutting zone; it only wets the shank and maybe the top of the hole. For holes deeper than 3× diameter, coolant delivery method matters as much as the coolant itself.
Through-spindle coolant (coolant fed through the center of the drill body and out the tip) is the correct solution for deep production holes. It delivers coolant directly to the cutting edge and uses the pressure to flush chips upward out of the flute. If your spindle has through-coolant capability and your drill is center-drilled for coolant passage, use it for any hole deeper than 4× diameter.
If through-spindle coolant is not available, the next best option is high-pressure flood directed into the hole entrance during retract. On a peck cycle, the drill retracts, coolant flushes the hole entrance, chips wash out with the flood, and the drill re-enters a cleaner hole. This requires good fixturing to keep the part stable when the flood is applied.
Mist coolant is largely ineffective for deep holes. It conditions the flute but does not move chips. Do not rely on mist alone for holes beyond 3× depth in steel or aluminum.
For holes 3–5× diameter: cobalt HSS in standard helix geometry works for most materials. Good quality point geometry (split point preferred to prevent walking on entry) and 135° included angle for harder materials.
For holes 5–8× diameter: parabolic flute drills are worth the additional cost. The wider flute channel moves chips more efficiently without relying as heavily on peck cycles. In some setups, a parabolic drill at 6× diameter in one pass outperforms a standard drill with aggressive peck cycles in both cycle time and hole quality.
For holes beyond 8× diameter: gundrilling is the correct process. Gundrill geometry — single-flute, brazed carbide tip, through-coolant — is purpose-built for L/D ratios from 10× to 50×. If your production requires consistent 10× depth holes, the investment in gundrilling capability or outsourcing is smaller than the cost of broken HSS drills and scrapped parts.
Standard speeds and feeds assume reasonable chip evacuation. In deep holes, reduce both as depth increases. A practical rule: reduce surface footage by 10–20% for holes beyond 4× diameter. Reduce feed by a similar amount. The slower cutting generates less heat, produces more manageable chips, and reduces the load when chips partially pack.
Do not try to compensate for a bad deep-hole setup with higher feeds. Higher feeds make more chips faster, which makes the packing problem worse, not better. When a deep hole drill starts squealing or showing signs of chatter, the first adjustment is usually a feed reduction, not a speed change.
Deep holes require a good entry. If the surface is not perpendicular to the drill axis, the drill flexes on entry, enters at an angle, and the deviation compounds as depth increases. Use a spot drill or center drill to create a conical seat before the deep drill engages. The seat registers the drill tip and keeps entry straight.
If drilling through-holes that exit at a non-perpendicular surface (angled exit), the drill is unsupported on the breakthrough side. Back the material with a sacrificial plate to support the drill tip on exit and prevent the exit-side burr from deflecting the drill on breakthrough.
Deep blind holes require depth control. A drill press with a depth stop calibrated to the finished depth, accounting for the drill tip geometry, prevents over-drilling blind holes. Set the stop, confirm with a test hole in scrap, then run production.
A dull drill in a deep hole setup breaks. It does not just cut slow — the increased thrust from a dull edge packs chips faster and the drill fails. Resharpened drills going into deep work need to be geometrically correct, not just "sharp enough."
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