Gun drilling does not always require a dedicated deep hole machine. With through-spindle coolant and enough Z-axis travel, a standard VMC or HMC can produce accurate, straight, well-finished deep holes. This guide covers TSC pressure and flow, gundrill tooling, peck vs continuous cycles, the CNC drilling procedure, ejector retrofits, whip guides, and the L/D limits where a purpose-built machine wins.
A deep hole is commonly defined as a depth-to-diameter ratio of 5:1 or greater — within comfortable reach of a machining center fitted with long drills. Gun drilling is worth considering any time the tool list specifies an aircraft-length or other long-length drill.
| Machine requirement | Why it matters | Typical spec |
|---|---|---|
| Through-spindle coolant | Flood coolant cannot flush chips from deep inside the part | 150–1,000+ psi depending on diameter (see coolant section) |
| Rigidity | Lack of stiffness makes the drill bend, wander, and break | Box ways, thick castings, non-flexing column, low-vibration spindle |
| Z-axis travel | Long gundrills overhang the workpiece and trigger Z+ overtravel alarms | Clear the longest tool the ATC can hold — gundrills are usually hand-loaded |
| Spindle load / torque monitoring | The drill tip is invisible during the cut; monitoring stops breakage | CNC torque and force load monitoring |
| CAM support | A dedicated gundrill dialog stages pilot, entry, drilling, breakthrough, and withdrawal | E.g., a gun drill cycle with speed, feed, depth, and coolant pressure fields |
The relationship between pressure, flow, and drilled diameter is critical. Because flood coolant cannot help flush chips from deep within the part, the entire process depends on coolant delivered through the tool.
| Coolant parameter | Documented range | Guidance |
|---|---|---|
| Pressure range on machining centers | 150–1,000 psi (10–70 bar) | As drilled diameter decreases, required pressure increases |
| Internal coolant for gundrills on CNC | 300–1,800 psi (20–125 bar) | Forced through the tool to push chips back along the flute |
| Vertical machining minimum | ≈4 MPa (580 psi) | Toolmaker catalog guidance for vertical machines |
| Flow rate | 1–10 GPM | As diameter grows, flow rate — not pressure — becomes the bigger concern |
| Rule of thumb | Fill the hole volume per revolution | Example: a 3/8 in hole running 8 in deep at 300 RPM needs ~1.1 GPM |
Machining centers can accommodate two types of gundrill. The choice between them largely follows drilled diameter and tool life expectations.
| Gundrill type | Construction | Typical use |
|---|---|---|
| One-piece design | Solid carbide blank | Small diameters (under ~10 mm / Ü0.4 in); superior strength, stiffness, tool life in difficult materials |
| Three-piece design | Brazed carbide tip + hollow V-shaped tube + shank | Most popular; modular, so nearly any diameter from 0.031 in to 2 in is attainable |
Pecking breaks chips and delivers coolant to the tip, but it adds cycle time. Whether you need it on a machining center depends almost entirely on your coolant system.
| Cycle | What it does | Best for |
|---|---|---|
| G81 (continuous) | One uninterrupted feed to depth, no retract | Shallow holes (up to ~3–4×D) and solid-carbide drills that can shatter under pecking |
| G73 (high-speed peck) | Short retract (~0.010–0.020 in) each peck without fully leaving the hole | Breaking stringy chips at shallow-to-moderate depth (~3–5×D); fast, keeps the drill stiff |
| G83 (deep-hole peck) | Full retract to the R plane after each peck | Deep holes (5+ ×D) and long-chip materials (stainless, aluminum, titanium); complete chip evacuation |
The following documented procedure for gun drills on CNC machine tools (Guhring) is the closest thing to a standard for machining-center gundrilling:
Carbide drill with m7 tolerance, 1.5×D to 3×D deep. For holes over ~12:1, pilot ~0.001 in oversize and ~3:1 deep.
Hand-load the long drill to protect it from the ATC. Align the guide bushing with the pilot hole.
Approximately 200 RPM and 20 IPM, stopping just short of the pilot hole bottom. For depths exceeding 40×D, rotate the spindle counter-clockwise during entry.
Start high coolant pressure, then raise to the recommended RPM and feed to final depth without a peck cycle.
For through holes with oblique exits, reduce feed to ~40% about 1 mm before break-through.
Turn off coolant, drop speed to 200 RPM, and withdraw at maximum 200 IPM.
| Operation | Documented value | Context |
|---|---|---|
| Entry into pilot hole | ~200 RPM / ~20 IPM | Stop just short of pilot bottom (Guhring) |
| Entry for depth > 40×D | Counter-clockwise spindle rotation | Stabilizes the long drill during entry (Guhring) |
| Oblique breakthrough | ~40% feed, ~1 mm before exit | Reduces breakout burr and tool shock (Guhring) |
| Withdrawal | 200 RPM, max 200 IPM, coolant off | Prevents chips from being pulled back into the cut (Guhring) |
| Production gun-barrel drilling | Feed 1.5 → 2.5 IPM | Chlorine-free low-viscosity oil; output 1,500 → 2,800 barrels per shift (Houghton) |
| Inconel 718, Ø3.5 mm | 25 mm → 330+ mm per regrind | Improved gundrill tube and geometry (Mollart) |
If you own a lathe or turning center, the ejector — or double-tube system (DTS) — is the other low-cost route to deep holes without a dedicated machine. It can be retrofitted onto an existing lathe, boring mill, or machining center because it needs no pressure head and no workpiece face seal.
Existing lathe spindle bore must accommodate the double-tube diameter — typically 1.5–2× the drill diameter for the outer tube.
Deliver sufficient flow rate at moderate pressure. A common mistake is installing a high-pressure/low-flow pump suited to gundrilling.
Filtration to 50 μm is adequate — less stringent than gundrilling because of the larger coolant passages.
Handle the significant fluid volume returning from the bore. Below ~8 bar inlet pressure the Venturi suction collapses.
Whip guide supports every 40–60×D are still needed for deep holes.
For L/D > 12:1, pre-drill a pilot hole and start with conservative parameters.
The fundamental problem with deep holes on any machine is that the long, slender gundrill shaft lacks flexural rigidity at depth. Unsupported, it “whips” — deflecting in uncontrolled radial and axial directions until it vibrates, cuts oversize, or snaps.
| Support element | Guidance | Source basis |
|---|---|---|
| Unsupported drill length | Keep less than ~40 diameters unsupported if no steady rest is available | CNC Cookbook gun drill guide |
| Whip guide placement | Whip guides / anti-whip bushings are typically needed for drills over ~25×D | VMC deep hole guidance |
| Support spacing (ejector tubes) | Support the outer tube every 40–60×D | Ejector DTS retrofit notes |
| Guide bushing at entry | Contact the guide bush closely with the workpiece surface; locate as close to the part as possible | Gundrill troubleshooting guide |
| Pilot hole | On a CNC the pilot hole substitutes for the machine’s drill bushing as entry guidance | Guhring CNC procedure |
| Steady rest | A steady rest reduces whipping risk on lathes and on long unsupported overhangs | CNC Cookbook gun drill guide |
Purpose-built deep hole machines are engineered around tool support, coolant delivery, and stability. Their advantage grows with depth-to-diameter ratio — the point where they begin to win decisively is often quoted at ~40:1.
| Criteria | Machining center + gundrill | Dedicated deep hole machine |
|---|---|---|
| Max practical L/D | ~40:1; beyond that performance drops | 100:1 routinely, up to 400:1 |
| Whip guides | User-supplied; keep < 40×D unsupported | Built-in supports along the full stroke |
| Counter-rotation | Not available — one spindle only | Workpiece and tool contra-rotate; stabilizes the cut at extreme depths |
| Straightness | Good at moderate depth (~0.05 mm per 300 mm with proper support) | ~0.5 mm per 1,000 mm with self-guiding tooling and precision bushings |
| Coolant system | 150–1,000 psi typical; may need auxiliary pump and rotary union | 300–2,000 psi dedicated high-pressure system |
| Process monitoring | Machine spindle load / torque | Dedicated load, torque, coolant pressure and flow sensors with auto retract |
| Productivity | Limited by feed and machine rigidity | Higher feeds; BTA drills 5–6× faster than gundrilling |
| Capital cost | Uses an existing machine (plus retrofit) | $200k+ dedicated purchase |
The economics of machining-center deep hole drilling hinge on the cost of coolant delivery and the value of avoiding a dedicated machine purchase.
| Item | Documented figure | Context |
|---|---|---|
| Factory through-spindle coolant option | ≈2× the cost of an external/flood-coolant machine | OEM option pricing (REGO-FIX) |
| Factory TSC annual maintenance | 20–30% of the original purchase price | Seal inspection, regreasing, service |
| Through-tool coolant retrofit (reCool) | ≈half the factory TSC option; no annual maintenance | 100 bar (150 optional), ER 11–40, up to 12,000 RPM |
| Gundrill system on existing boring machine | ≈50,000 yuan vs ~1,000,000 yuan dedicated machine | ~1/20 the price; 3–5× drilling efficiency (Taiming) |
| Reconfigured machining center for deep bores | Add pumps, filtration, heat exchangers | Achieved up to 100:1 depth-to-diameter (GS Global Resources) |
| Cycle-time payoff example | Up to 70% cycle-time reduction; 3 parts in the time of 1 | In-house gundrilling on an HMC (Townsend Machine) |
| Symptom | Likely cause | Fix |
|---|---|---|
| Drill whips or breaks when the pilot is too short | Insufficient entry support for the long drill | Place a whip guide, contact the guide bush closely with the workpiece, and use a bush shape suited to the part |
| Rough bore finish | Excessive spindle runout | Minimize spindle runout before changing tooling |
| Bent hole / poor roundness and concentricity | Excessive concentricity error between the guide bush and the spindle | Decrease guide-bush-to-spindle concentricity; re-align the bushing |
| Rough finish plus bending | Improper clearance between guide bush and drill | Hold clearance at +0.003–0.008 mm; replace the bushing when out of tolerance |
| Hole drift or wander | Entry not square, or pilot too shallow | Spot-face / bell-mouth the entry; deepen the pilot (1.5–3×D) |
| Chips not evacuating | Insufficient coolant pressure or volume | Upgrade coolant supply — flood coolant cannot clear deep chips; ~30 psi shop pumps are inadequate |
| Oversize bore, inconsistent size | Holder taper wear, chips, or dirt raising runout | Clean and inspect the holder taper; use a hydraulic or shrink-fit holder at low runout |
| Poor cutting performance after regrind | Reground edge not concentric with drill diameter | Verify regrind quality; the gundrill cutting edge must be concentric with the drill diameter |
| Sudden coolant pressure drop (ejector retrofit) | Blocked inner tube or failing pump | Monitor pressure continuously; inspect the Venturi slots for wear or clogging |
| Sudden coolant pressure rise (ejector retrofit) | Blockage in the chip evacuation channel | Stop and clear the chip path before the Venturi suction is lost |