Gundrilling is a single-lip, external chip-removal process, and its parameters follow different rules than a twist drill. Cutting speed comes from the material and the tool; feed comes from the drill diameter; both fall as hole depth (L/D) climbs; and high-pressure coolant is the fourth variable that makes the whole system work.
Gundrill parameter selection is four linked numbers. Cutting speed Vc (m/min) comes from the workpiece and tool material; spindle speed follows from tool diameter; feed is set per revolution; and the two combine into linear feed rate and metal removal rate.
The four levers move predictably. Learn these relationships and you can reason about any table before you read it.
This page is the gundrill-specific companion to the general Feeds & Speeds Tables guide, which covers the same relationships for BTA and ejector drilling. For material hardness, tool group, and machinability lookups, pair this page with the Material Parameters Quick Reference.
Before reading a table, classify the workpiece into one of four difficulty paths. Each path sets the ballpark; the Speed by Material table below then narrows it to a range.
Aluminum, brass, copper, magnesium — start high: Vc 80–200+ m/min, feeds near the top of the range.
Low- and medium-carbon steel — Vc 60–150 m/min on carbide, feeds mid-range.
Tool steel, hardened steel, 250 HB+ — Vc 30–80 m/min, cut feed roughly 30–50%.
Ti-6Al-4V, Inconel 718 — Vc 15–40 m/min; keep feed up to avoid work hardening.
Cutting speed is governed primarily by tool material and workpiece hardness. Carbide-tipped and solid carbide gun drills dominate production; HSS gun drills are a fallback and run at 35–70 m/min regardless of workpiece (Drillstar Cutting Tool). The table below gives carbide starting ranges by material group.
| Material Group | Hardness HB | Carbide-Tipped Vc (m/min) | Solid Carbide Vc (m/min) |
|---|---|---|---|
| Non-alloy steel <0.25%C | ~125 | 120–180 | 100–140 |
| Non-alloy steel ≥0.25%C | ~190 | 100–150 | 90–120 |
| Low-alloy steel (annealed) | ~200 | 60–80 | 60–70 |
| High-alloy steel, tool steel | ~200 | 60–80 | 60–70 |
| Hardened steel | ~325 | 40–60 | 30–50 |
| Ferritic/Martensitic stainless steel | — | 40–80 | 40–50 |
| Austenitic/Duplex stainless steel | — | 30–60 | 30–40 |
| Gray cast iron | — | 70–100 | 60–90 |
| Aluminum alloy (wrought) | — | 80–160 | 100–200 |
| Aluminum alloy (cast, high Si) | — | 60–120 | 80–140 |
| Copper alloy / Brass | — | 80–150 | 80–140 |
| Bronze (high-tensile) | — | 50–90 | 50–80 |
| Nickel-based superalloy (Inconel) | — | 15–30 | 10–20 |
| Titanium alloy (Ti-6Al-4V) | — | 20–40 | 15–30 |
| Magnesium alloy | — | 100–200 | 100–200 |
| Thermoplastics / Composites | — | 40–100 | — |
Feed per revolution is limited by the rigidity of the tool–workpiece–machine system, chip evacuation, and surface quality. Feed rises with drill diameter: larger drills have a stiffer shank and a bigger chip flute. A research reference (Smartlathe / IPLeiria gundrill thesis) confirms the same pattern — for steel, roughly 0.010–0.025 mm/rev at Ø8 mm climbing to 0.035–0.040 mm/rev at Ø19 mm.
| Material | D=3mm | D=5mm | D=10mm | D=20mm | D=30mm |
|---|---|---|---|---|---|
| Low-carbon steel | 0.005–0.010 | 0.010–0.020 | 0.020–0.040 | 0.050–0.080 | 0.100–0.160 |
| Alloy steel (annealed) | 0.005–0.010 | 0.010–0.020 | 0.020–0.040 | 0.050–0.080 | 0.100–0.140 |
| Alloy steel (hardened) | 0.004–0.008 | 0.008–0.018 | 0.015–0.030 | 0.040–0.070 | 0.080–0.120 |
| Stainless steel (austenitic) | 0.004–0.008 | 0.008–0.015 | 0.020–0.030 | 0.050–0.070 | 0.100–0.150 |
| Cast iron (gray/ductile) | 0.006–0.012 | 0.012–0.025 | 0.030–0.050 | 0.070–0.120 | 0.150–0.220 |
| Aluminum alloy | 0.020–0.040 | 0.040–0.080 | 0.070–0.130 | 0.150–0.280 | 0.300–0.500 |
| Copper alloy / Brass | 0.015–0.030 | 0.030–0.060 | 0.050–0.100 | 0.100–0.200 | 0.200–0.400 |
| Titanium alloy | 0.003–0.006 | 0.006–0.012 | 0.010–0.025 | 0.030–0.060 | 0.060–0.100 |
| Nickel superalloy | 0.002–0.005 | 0.005–0.010 | 0.008–0.020 | 0.020–0.050 | 0.040–0.080 |
As hole depth increases, reduced rigidity, harder chip evacuation, and coolant pressure drop along the bore force both cutting speed and feed down. Multiply the base Vc and feed from the tables above by the factors below for your L/D; interpolate linearly between entries.
| L/D Ratio | 3D | 5D | 8D | 10D | 15D | 20D | 30D | 50D | 75D | 100D |
|---|---|---|---|---|---|---|---|---|---|---|
| Speed factor Klv | 1.00 | 0.85 | 0.70 | 0.60 | 0.50 | 0.40 | 0.35 | 0.28 | 0.22 | 0.18 |
| Feed factor Klf | 1.00 | 0.80 | 0.60 | 0.50 | 0.40 | 0.35 | 0.30 | 0.25 | 0.20 | 0.16 |
Gundrilling is only as fast as its chip evacuation, and evacuation is driven by coolant pressure and volume. The rule is inverse: small holes want high pressure and low flow; large holes want lower pressure and high flow. Values below are published gun-drill coolant settings (carbideanddiamondtooling.com; Star-SU troubleshooting).
| Hole Diameter | Ideal Pressure (psi) | Min Pressure (psi) | Flow @ Ideal (GPM) |
|---|---|---|---|
| Ø3.2 mm (0.125 in) | 1500 | 500 | 1.0 |
| Ø4.7 mm (0.187 in) | 1150 | 400 | 1.6 |
| Ø6.4 mm (0.250 in) | 925 | 350 | 2.5 |
| Ø9.5 mm (0.375 in) | 675 | 300 | 4.5 |
| Ø12.7 mm (0.500 in) | 525 | 250 | 7.0 |
| Ø15.9 mm (0.625 in) | 450 | 200 | 10.0 |
| Ø19.1 mm (0.750 in) | 400 | 175 | 14.0 |
| Ø25.4 mm (1.000 in) | 300 | 150 | 20.0 |
| Ø31.8 mm (1.250 in) | 250 | 125 | 28.0 |
| Ø38.1 mm (1.500 in) | 200 | 100 | 36.0 |
Flow rule of thumb for sizing pumps and nozzles (Rotem): GPM = 30 × (nozzle diameter in inches)² × √(pressure in psi). Keep coolant temperature at 20–22°C and below 50°C maximum, and filter to ~20 μm. Full system design lives in the Deep Hole Drilling Coolant System Guide.
Two worked examples covering the two ends of the gundrill spectrum: a medium steel at 30:1 and a small superalloy bore.
Alloy steel (annealed), mid-range: 70 m/min.
Ø15 mm, interpolated between 10 mm and 20 mm: 0.030 mm/rev.
Klv = 0.35, Klf = 0.30.
Vc = 70 × 0.35 = 24.5 m/min. Feed = 0.030 × 0.30 = 0.009 mm/rev.
n = 24.5 × 1000 / (π × 15) = 520 RPM. Vf = 520 × 0.009 = 4.7 mm/min.
T = 450 / 4.7 = 96 minutes per hole — typical for a 30:1 gundrilled bore.
Ni superalloy, coated carbide: Vc ~20 m/min; feed at Ø6 mm ~0.007 mm/rev.
Klv = 0.40, Klf = 0.35.
Vc = 20 × 0.40 = 8 m/min. Feed = 0.007 × 0.35 = 0.0025 mm/rev.
n = 8 × 1000 / (π × 6) = 424 RPM. Vf = 424 × 0.0025 = 1.1 mm/min.
Ø6 mm wants ~925 psi ideal pressure (see table) — mandatory for a work-hardening superalloy; drop to ~50 mm of depth before resharpening.
T = 120 / 1.1 = 109 minutes, plus coolant hold during retract.
Published studies give concrete optimized set-points you can use as launch values — note how cutting-fluid pressure is often the lever that moves surface roughness most:
| Material | Optimized parameters | Result |
|---|---|---|
| 304 stainless | 1,270 rpm, feed 0.02 mm/rev, oil 3 MPa (30 bar) | Stable chip control |
| Oxygen-free copper (TU1) | 47.1 m/min, feed 0.023 mm/rev, fluid 2.1 MPa (21 bar) | Ideal C-type chips, smooth evacuation |
| TC4 titanium (dry) | 20 m/min, feed 0.08 mm/rev | Minimum temperature and work hardening |
Chip shape is the fastest feedback loop in gundrilling. Consistent, well-formed chips mean balanced speed, feed, and coolant; every abnormal shape points at a specific parameter error (Mastercut Tool, Heuletool, and Ceratizit troubleshooting references).
| Chip Appearance | What It Means | Action |
|---|---|---|
| Short, segmented / C-shaped chips | Balanced parameters, good chip breaking (304 SS study) | Keep as-is; log for the process sheet |
| Long, stringy / continuous band chips | Feed too low for the material | Increase feed within the tool range |
| Whitish chips with long tails | Speed and feed both too low, low cutting temperature | Raise speed and/or feed; inspect for BUE |
| Long spiral chips from point dwelling | Feed so low the point dwells in place | Increase feed to break the spiral |
| Powder / dust chips | Edge rubbing instead of cutting (plastics, soft metals) | Increase feed; verify edge sharpness |
| Entangled, multi-strand rolls | Chip-removal failure — coolant too low or edge worn | Raise pressure/flow; resharpen; check flute packing |
| Mistake | Why It Fails | Fix |
|---|---|---|
| Underfeeding on titanium / superalloys | Rubbing work-hardens the bore; tool chips | Hold feed, lower speed instead |
| Skipping the L/D correction | Deep holes run at 3D parameters — flute packs, drill twists | Always multiply by Klv and Klf |
| Coolant pressure too low for diameter | Chips pack in the flute; pipe twists and snaps | Match table above; verify at the drill tip |
| Rapid feed at entry | No support yet — instant breakage | Cutting feed only; ramp in over 2–3 mm |
| Bushing not in contact with the part | Gun drill is not self-starting; it wanders or breaks | Hold bushing against entry face; check alignment |
| Wrong bushing clearance | Too tight binds, too loose lets the drill whip | Clearance +0.003 to +0.008 mm over drill OD |
| No feed reduction at break-through | Exit burr, edge chipping, breakout | Reduce feed ~50% over the last 2–3 mm |
| Retracting at speed | Chips and coolant disturb the finished bore; tool stress | Drop to ≤50 RPM before retract; keep coolant on |
| Running too-high Vc on titanium | Every 10% of speed costs 30–50% of tool life | Stay conservative; speed is the wear dial |
| Ignoring chip shape | Problems are detected by tool breakage, not inspection | Read the first chips every hole or every setup |