Gun Drilling Coolant Pressure & Flow Rate Calculator
Gun drills need higher pressure the smaller the diameter. For a hole 50×D deep in neat oil it takes about 131 bar (1,901 psi) at Ø2 mm, 52 bar at Ø10 mm and 35 bar at Ø20 mm, while the flow rises with size (about 2.3, 20 and 51 L/min). Beyond 20×D each extra diameter of depth adds 0.4%.
Gun Drilling Coolant Pressure, Flow Rate & Pump Power Calculator
Typical coolant pressure and flow for single-flute gun drills and BTA heads by diameter and depth, the flushing velocity in the chip flute, pump power P = Q·p / (600·η) and filtration.
Why Does a Small Gun Drill Need More Pressure?
In a single-flute gun drill the coolant goes down a hole through the drill and has to push every chip back up the V-flute against the full length of the hole. The coolant passage shrinks with the square of the diameter while the friction along the hole does not, so small drills need much higher pressure to deliver enough flow. A flow that is too low lets chips pack in the flute, the drill seizes and breaks. Simple rules that ignore the diameter — some online tools use "20 bar + 5 bar per L/D" — overestimate deep large holes and underestimate small ones.
| Drill Ø | Coolant Pressure | Coolant Flow | Flute Velocity | Pump Power at 80% Efficiency | Filtration |
|---|---|---|---|---|---|
| Ø2 mm | 126 bar (1,833 psi) | 2.3 L/min (0.60 gpm) | 48 m/s | 0.60 kW | 5 µm |
| Ø3 mm | 100 bar (1,455 psi) | 3.9 L/min (1.04 gpm) | 37 m/s | 0.82 kW | 5 µm |
| Ø4 mm | 85 bar (1,235 psi) | 5.8 L/min (1.53 gpm) | 31 m/s | 1.03 kW | 5 µm |
| Ø5 mm | 75 bar (1,087 psi) | 7.8 L/min (2.07 gpm) | 27 m/s | 1.22 kW | 10 µm |
| Ø6 mm | 68 bar (980 psi) | 10.0 L/min (2.65 gpm) | 24 m/s | 1.41 kW | 10 µm |
| Ø8 mm | 57 bar (832 psi) | 14.8 L/min (3.91 gpm) | 20 m/s | 1.77 kW | 10 µm |
| Ø10 mm | 50 bar (732 psi) | 20.0 L/min (5.28 gpm) | 17 m/s | 2.10 kW | 10 µm |
| Ø12 mm | 46 bar (660 psi) | 25.6 L/min (6.76 gpm) | 15 m/s | 2.42 kW | 10 µm |
| Ø16 mm | 39 bar (560 psi) | 37.7 L/min (9.96 gpm) | 12 m/s | 3.03 kW | 15 µm |
| Ø20 mm | 34 bar (493 psi) | 51.0 L/min (13.46 gpm) | 11 m/s | 3.61 kW | 15 µm |
| Ø25 mm | 30 bar (434 psi) | 68.9 L/min (18.20 gpm) | 9 m/s | 4.30 kW | 15 µm |
| Ø32 mm | 26 bar (377 psi) | 96.1 L/min (25.39 gpm) | 8 m/s | 5.21 kW | 15 µm |
Pressures are given in bar and psi and flows in L/min and US gpm, using the exact conversion factors in NIST Special Publication 811 (unit conversion factors) (1 bar = 14.504 psi, 1 US gallon = 3.785 L).
How Much Does Depth Add?
The calculator uses P ≈ 173.7·D−0.57 × (1 + 0.004·max(0, L/D − 20)) bar, fitted to tool-maker guidance. Below about 20×D the pressure is set by the drill itself; beyond that the hole length adds losses. The calculator adds 0.4% per diameter of depth beyond 20×D: the Ø10 mm drill needs 52 bar at 50×D and 71 bar at 150×D. If the machine cannot reach the pressure, reduce the feed and peck less aggressively rather than drilling with starved coolant.
Gun Drill or BTA?
A gun drill sends coolant through the tool and returns the chips along the outside flute. A BTA (STS) head does the opposite: coolant flows around the outside of the boring tube and the chips return through the tube, so it needs more flow at lower pressure. The calculator applies about 70% of the pressure and 1.6× the flow for BTA, typical for heads from roughly Ø16 mm upward. Check the tool maker's chart for the exact head.
Oil or Emulsion?
Neat gundrilling oil is the standard: it lubricates the guide pads that burnish the hole and gives the best tool life and finish. Water-soluble emulsions cool better and need slightly less pressure for the same flow, but the pads run with less lubrication, so tool life on steel usually drops. Keep the concentration in range with the refractometer Brix calculator.
| Coolant / Size | Viscosity | Pressure / Filtration | Note |
|---|---|---|---|
| Neat gundrilling oil | 10–20 cSt at 40 °C | Baseline pressure | Best lubricity at the guide pads; standard choice for gun drills |
| Water-soluble EP emulsion (8–12%) | ~1 cSt (water-like) | About 10% less pressure for the same flow | Cooler, but less pad lubrication: shorter tool life on steels |
| Filtration Ø < 5 mm | — | 5 µm | Fine chips block the small coolant holes |
| Filtration Ø 5–15 mm | — | 10 µm | Standard gundrill filtration |
| Filtration Ø > 15 mm | — | 15 µm | Larger passages tolerate coarser filtration |
Pump Power and Filtration
Filtration targets are easier to specify as a cleanliness code: ISO 4406 (fluid cleanliness code) counts particles above 4, 6 and 14 µm, so a filter rating and a target code together tell the supplier what the coolant system has to achieve. Hydraulic power is P[kW] = Q[L/min] × p[bar] / 600; divide by the pump efficiency (80% here) for the motor size. The Ø10 mm example needs about 2.2 kW. Filter to 5 µm for drills under Ø5 mm, 10 µm from Ø5 to Ø15 mm and 15 µm above: fine chips plug small coolant holes before anything else goes wrong. The deep-hole section of Machinery's Handbook (Industrial Press) covers feeds and speeds for the same tools. For the drilling cycle itself see the G83 peck drilling calculator, and for straightness of long bores the boring bar L/D calculator.
Cite this page
ToolSpecVault Engineering Data Team. (2026, October 28). Gun Drilling Coolant Pressure & Flow Rate Calculator. ToolSpecVault. https://toolspecvault.com/gun-drilling-coolant-pressure-flow-rate-calculator/