Deep hole drilling is a global industry. A European machine builder specifies coolant pressure in bar, an American tool manufacturer rates gun drills in PSI, and a Japanese workpiece drawing lists hardness in HRC. Moving between these unit systems accurately is essential for specification writing, tool ordering, and process setup.
This article provides conversion tables for all common units used in deep hole drilling. Each table includes the conversion factor and a practical example relevant to deep hole drilling operations.
Length and Diameter Conversions Inch to Millimeter Inches (in) Millimeters (mm) Deep Hole Drilling Context 0.001 (0.001 in) 0.0254 Typical dial indicator resolution 0.005 0.127 Target spindle runout tolerance 0.010 0.254 Marginal runout — plan repair 0.015 0.381 Runout requiring immediate repair 0.020 0.508 Typical guide bushing clearance range 0.0394 1.0 1 mm drill diameter 0.0625 (1/16) 1.588 Small gun drill diameter 0.125 (1/8) 3.175 Common small drill 0.250 (1/4) 6.35 Common medium drill 0.375 (3/8) 9.525 — 0.500 (1/2) 12.7 — 0.750 (3/4) 19.05 — 1.0 25.4 1 inch drill diameter 2.0 50.8 BTA range 3.0 76.2 BTA range 4.0 101.6 BTA range 6.0 152.4 Large BTA 10.0 254.0 Large BTA 12.0 304.8 Large trepanning
Conversion formula:
mm = in × 25.4 in = mm / 25.4 Example: A 0.375 in gun drill = 0.375 × 25.4 = 9.525 mm. A 50 mm BTA drill = 50 / 25.4 = 1.969 in.
Common Tolerances in Inches and Millimeters Tolerance Description Inches Millimeters Spindle runout (excellent) 0.0002 0.005 Spindle runout (acceptable) 0.0004 0.010 Guide bushing clearance 0.0005–0.0010 0.013–0.025 Bed level (per meter) 0.0008 per meter 0.02 per meter Bore tolerance IT7 (25 mm) 0.0008 0.021 Bore tolerance IT8 (25 mm) 0.0013 0.033 Guideway straightness (3 m) 0.0004 0.01 Test bar parallelism (300 mm) 0.0004 over 300 mm 0.01 over 300 mm
Decimal and Fractional Inch to Millimeter Quick Reference Fraction Decimal (in) mm 1/64 0.0156 0.397 1/32 0.0313 0.794 3/64 0.0469 1.191 1/16 0.0625 1.588 5/64 0.0781 1.984 3/32 0.0938 2.381 7/64 0.1094 2.778 1/8 0.1250 3.175 9/64 0.1406 3.572 5/32 0.1563 3.969 3/16 0.1875 4.763 7/32 0.2188 5.556 1/4 0.2500 6.350 5/16 0.3125 7.938 3/8 0.3750 9.525 7/16 0.4375 11.113 1/2 0.5000 12.700 9/16 0.5625 14.288 5/8 0.6250 15.875 3/4 0.7500 19.050 7/8 0.8750 22.225 1 1.0000 25.400
Pressure Unit Conversions Pressure is the most frequently converted unit in deep hole drilling because coolant system specifications vary by country and manufacturer.
Pressure Conversion Factors From To Multiply By psi bar 0.06895 psi MPa 0.006895 psi kPa 6.895 psi kg/cm² 0.07031 bar psi 14.504 bar MPa 0.1 bar kPa 100 bar kg/cm² 1.0197 MPa psi 145.04 MPa bar 10 MPa kg/cm² 10.197 kPa psi 0.1450 kPa bar 0.01
Common Coolant Pressure Ranges Application psi bar MPa Flood coolant 40–150 2.8–10.3 0.28–1.03 Through-spindle coolant (standard) 300–500 20.7–34.5 2.07–3.45 High-pressure through-spindle 500–1,000 34.5–69.0 3.45–6.90 Gun drilling (small diameter) 1,000–3,000 69.0–207 6.90–20.7 Gun drilling (4–10 mm) 500–1,500 34.5–103 3.45–10.3 Gun drilling (10–25 mm) 300–800 20.7–55.2 2.07–5.52 BTA drilling (standard) 500–750 34.5–51.7 3.45–5.17 BTA drilling (high-pressure) 750–1,500 51.7–103 5.17–10.3 Deep hole boring 200–500 13.8–34.5 1.38–3.45 Hydraulic system (typical) 1,000–3,000 69–207 6.9–20.7
Conversion example: A BTA machine specification lists coolant pressure at 750 psi. In bar: 750 × 0.06895 = 51.7 bar. In MPa: 750 × 0.006895 = 5.17 MPa.
Flow Rate Conversions Volume Flow Rate From To Multiply By GPM (US gallons/min) L/min 3.7854 GPM L/s 0.06309 GPM m³/hr 0.2271 L/min GPM 0.2642 L/min L/s 0.01667 L/min m³/hr 0.06 m³/hr GPM 4.403 m³/hr L/min 16.667
Common Coolant Flow Ranges Application GPM L/min m³/hr Small gun drilling (1–6 mm) 1–5 3.8–18.9 0.23–1.14 Gun drilling (6–25 mm) 5–20 18.9–75.7 1.14–4.54 Small BTA (20–40 mm) 40–90 151–341 9.1–20.5 Medium BTA (40–80 mm) 90–185 341–700 20.5–42.0 Large BTA (80–150 mm) 185–300 700–1,136 42.0–68.1 Extra-large BTA (150–250 mm) 300–400 1,136–1,514 68.1–90.9
Conversion example: A BTA-60 machine requires 300 GPM coolant flow. In L/min: 300 × 3.7854 = 1,136 L/min. In m³/hr: 300 × 0.2271 = 68.1 m³/hr.
Force and Torque Conversions Force From To Multiply By lbf (pound-force) N 4.4482 lbf kgf 0.4536 N lbf 0.2248 N kgf 0.1020 kgf lbf 2.2046 kgf N 9.8067 kN lbf 224.8 kN kgf 102.0 klbf (kip) kN 4.448
Torque From To Multiply By lbf·ft N·m 1.3558 lbf·ft kgf·m 0.1383 lbf·in N·m 0.1130 N·m lbf·ft 0.7376 N·m lbf·in 8.851 N·m kgf·m 0.1020 kgf·m lbf·ft 7.233 kgf·m N·m 9.807
Typical Spindle Torque Ranges Machine Type Typical Torque Context Small gun drilling spindle 5–20 N·m (3.7–14.8 lbf·ft) < 10 mm diameter Medium gun drilling spindle 20–80 N·m (14.8–59 lbf·ft) 10–25 mm diameter BTA drilling spindle (50 mm) 200–500 N·m (148–369 lbf·ft) Medium BTA BTA drilling spindle (100 mm) 500–2,000 N·m (369–1,475 lbf·ft) Large BTA Large BTA / trepanning 2,000–10,000 N·m (1,475–7,375 lbf·ft) > 150 mm diameter
Conversion example: A BTA spindle rated at 1,500 N·m. In lbf·ft: 1,500 × 0.7376 = 1,106 lbf·ft. Anchor bolt torque of 700 N·m = 700 × 0.7376 = 516 lbf·ft.
Temperature Conversions °F = (°C × 9/5) + 32 °C = (°F − 32) × 5/9 K = °C + 273.15 Temperature Reference Table °C °F Deep Hole Drilling Context 0 32 Freezing point 10 50 Coolant chiller supply temperature 20 68 Typical shop ambient temperature 25 77 Coolant tank target temperature 30 86 Warm coolant, acceptable 35 95 Coolant approaching upper limit 40 104 Spindle housing maximum acceptable rise above ambient 45 113 Coolant — chiller needed 50 122 Coolant — investigate 55 131 Coolant — stop and investigate 60 140 Spindle housing alarm temperature 70 158 Coolant — system damage risk 100 212 Water boiling point
Conversion example: A coolant system maintains 25°C. In °F: (25 × 9/5) + 32 = 77°F. A spindle housing measures 120°F. In °C: (120 − 32) × 5/9 = 49°C — marginal condition.
Cutting Speed Conversions Surface Speed (Vc) From To Multiply By SFM m/min 0.3048 m/min SFM 3.2808
Common Cutting Speed Ranges Application SFM m/min Gun drilling — low-carbon steel 525–655 160–200 Gun drilling — alloy steel (annealed) 330–395 100–120 Gun drilling — stainless steel 165–330 50–100 Gun drilling — cast iron 260–395 80–120 Gun drilling — aluminum 525–720 160–220 Gun drilling — titanium 100–165 30–50 Gun drilling — high-temp alloys 65–165 20–50 BTA drilling — low-carbon steel 395–655 120–200 BTA drilling — alloy steel 230–425 70–130 BTA drilling — stainless steel 100–230 30–70 BTA drilling — cast iron 260–460 80–140 BTA drilling — aluminum 490–720 150–220
Conversion example: A cutting speed specification of 600 SFM for gun drilling carbon steel. In m/min: 600 × 0.3048 = 183 m/min.
RPM Calculation Summary Metric: RPM = (Vc × 1000) / (π × Dmm) Imperial: RPM = (SFM × 3.82) / Din Where D is drill diameter.
Quick RPM Reference (at 100 m/min / 328 SFM) Drill Diameter RPM at 100 m/min RPM at 328 SFM 5 mm (0.197 in) 6,366 6,366 10 mm (0.394 in) 3,183 3,183 20 mm (0.787 in) 1,592 1,592 50 mm (1.97 in) 637 637 100 mm (3.94 in) 318 318 200 mm (7.87 in) 159 159
Note: RPM is independent of unit system — the same RPM results from both metric and imperial formulas when the correct diameter is used.
Feed Rate Conversions Feed per Revolution From To Multiply By IPR (in/rev) mm/rev 25.4 mm/rev IPR 0.03937
Feed Rate (Linear) From To Multiply By IPM (in/min) mm/min 25.4 mm/min IPM 0.03937
Common Feed Ranges Application mm/rev IPR Gun drilling (small, < 6 mm) 0.02–0.08 0.0008–0.0031 Gun drilling (medium, 6–25 mm) 0.04–0.16 0.0016–0.0063 Gun drilling (large, > 25 mm) 0.08–0.28 0.0031–0.0110 BTA drilling (30–50 mm) 0.06–0.16 0.0024–0.0063 BTA drilling (50–120 mm) 0.08–0.25 0.0031–0.0098 BTA drilling (120–300 mm) 0.12–0.35 0.0047–0.0138
Conversion example: A BTA drilling feed of 0.012 IPR. In mm/rev: 0.012 × 25.4 = 0.305 mm/rev. A gun drilling feed of 0.10 mm/rev. In IPR: 0.10 × 0.03937 = 0.0039 IPR.
Penetration Rate (Feed Rate × RPM) From To Multiply By IPM mm/min 25.4 mm/min IPM 0.03937
Common penetration rates in deep hole drilling:
Application mm/min IPM Gun drilling — steel (10 mm) 200–400 7.9–15.7 Gun drilling — aluminum (10 mm) 300–600 11.8–23.6 BTA drilling — steel (50 mm) 50–80 2.0–3.1 BTA drilling — steel (100 mm) 30–60 1.2–2.4
Hardness Conversion Approximate Hardness Equivalents HRC HB (Brinell) HV (Vickers) Material Context 20 230 240 Low-carbon steel, annealed 25 255 270 Medium-carbon steel, normalized 30 285 300 Medium-carbon steel, Q&T 35 330 345 Alloy steel, Q&T 40 375 390 Alloy steel, hardened 45 425 440 Tool steel, hardened 50 480 500 High-hardness tool steel 55 545 570 Hardened tool steel 60 615 640 Maximum for most drilling
Material Hardness and Cutting Speed Adjustment Material Condition Hardness (HB) Speed Factor Soft/annealed < 150 1.2 Normalized 150–250 1.0 Q&T (medium) 250–350 0.8 Q&T (hard) 350–450 0.6 Hardened > 450 0.4
HB ≈ 2 × HRC + 60 (for HRC 20–40, approximate) HRC ≈ (HB − 60) / 2 (for HB 200–500, approximate) HV ≈ HB × 1.05 (approximate for steels) HB ≈ HV × 0.95 (approximate for steels) Important: These are approximate conversions. For specification-critical hardness values, use a published standard (ISO 18265, ASTM E140) rather than simplified formulas.
Surface Finish Conversion Ra Conversion (Roughness Average) μm (Ra) μin (Ra) Deep Hole Drilling Context 0.1 4 Precision burnished bore 0.2 8 Skived and burnished 0.4 16 Fine BTA drilling with reaming 0.8 32 Good BTA drilling 1.6 63 Standard deep hole drilling 3.2 125 Rough drilling, acceptable for many applications 6.3 250 Very rough, secondary operation needed
Conversion formula:
μin Ra = μm Ra × 39.37 μm Ra = μin Ra / 39.37 Typical Ra Values by Drilling Method Method Typical Ra (μm) Typical Ra (μin) Gun drilling 0.8–3.2 32–125 BTA drilling 1.6–6.3 63–250 Deep hole boring 1.6–6.3 63–250 Trepanning 3.2–12.5 125–500 Skiving (finish pass) 0.4–1.6 16–63 Roller burnishing 0.1–0.4 4–16 Honing 0.2–0.8 8–32
Conversion example: A print specification of 63 μin Ra maximum. In μm: 63 / 39.37 = 1.6 μm Ra. This is achievable with standard BTA drilling.
Parameter Imperial Formula Metric Formula Spindle speed (RPM) RPM = (SFM × 3.82) / Din RPM = (Vc × 1000) / (π × Dmm) Feed rate (linear) IPM = IPR × RPM mm/min = mm/rev × RPM Material removal rate MRR = (D × IPR × SFM × 12) / π (in³/min) MRR = (D × f × Vc × 1000) / 4 (mm³/min) Cutting power HP = (MRR × K) / 396,000 kW = (MRR × Kc) / 60 × 10⁶ Machining time T = L / (IPM × RPM) (min) T = L / (f × RPM) (min)
Where K = specific cutting force (psi), Kc = specific cutting force (N/mm²).
Parameter Imperial Formula Metric Formula Pump power HP = (psi × GPM) / 1714 kW = (bar × L/min) / 600 Flow velocity V = (0.321 × GPM) / A (ft/s) V = (16.67 × L/min) / A (m/s) Pressure drop (pipe) ΔP = f × (L/D) × (ρ × V²) / (2 × gc) ΔP = f × (L/D) × (ρ × V²) / 2
Parameter Formula Straightness deviation mm/m = measured deviation (mm) / measurement length (m) Taper (per side) Taper angle = atan((Dlarge − Dsmall) / (2 × L)) Roundness Deviation from true circle = max radius − min radius Concentricity Offset between two diameters = √(Δx² + Δy²)
Power and Force Approximations Parameter Rule of Thumb Thrust force (gun drilling) 50–150 N per mm of diameter (steel) Thrust force (BTA drilling) 100–300 N per mm of diameter (steel) Torque (gun drilling) 0.5–2 N·m per mm of diameter (steel) Torque (BTA drilling) 2–8 N·m per mm of diameter (steel) Specific cutting force (steel) 2,000–3,000 N/mm² Specific cutting force (aluminum) 700–1,000 N/mm² Pump heat to coolant 25–35% of input motor power
Common Conversion Constants Quantity Conversion 1 inch 25.4 mm exactly 1 foot 0.3048 m exactly 1 pound (mass) 0.4536 kg 1 US gallon 3.7854 L 1 gallon (UK) 4.5461 L 1 HP 0.7457 kW 1 kW 1.341 HP 1 psi 0.06895 bar 1 N·m 0.7376 lbf·ft π (pi) 3.14159
Summary Table Conversion Factor Example in → mm × 25.4 0.5 in = 12.7 mm psi → bar × 0.06895 750 psi = 51.7 bar psi → MPa × 0.006895 1,000 psi = 6.90 MPa GPM → L/min × 3.7854 100 GPM = 379 L/min SFM → m/min × 0.3048 500 SFM = 152 m/min IPR → mm/rev × 25.4 0.010 IPR = 0.254 mm/rev HRC → HB (approx) × 2 + 60 30 HRC ≈ 120 HB
FAQ How do I convert coolant pressure from PSI to bar for a deep hole drilling specification? Multiply PSI by 0.06895 to get bar. For example, 750 PSI × 0.06895 = 51.7 bar. For quick mental conversion, divide PSI by 14.5 to get bar (750 / 14.5 ≈ 51.7 bar). Conversely, multiply bar by 14.5 to get PSI (50 bar × 14.5 = 725 PSI). Common coolant pressures in deep hole drilling: 500 PSI ≈ 34.5 bar, 1,000 PSI ≈ 69 bar, 2,000 PSI ≈ 138 bar.
What is the conversion between GPM and L/min for coolant flow? Multiply GPM by 3.7854 to get L/min. For example, 300 GPM × 3.7854 = 1,136 L/min. For quick estimation, multiply GPM by 3.8 (300 GPM × 3.8 = 1,140 L/min, close enough for specification work). Conversely, divide L/min by 3.7854 to get GPM.
How do I convert inches to millimeters for drill diameters? Multiply inches by 25.4 to get millimeters. This is an exact conversion (1 inch = 25.4 mm by international agreement). Common drill sizes: 0.5 in = 12.7 mm, 1.0 in = 25.4 mm, 3.0 in = 76.2 mm. For fractions, convert to decimal first: 3/8 in = 0.375 in × 25.4 = 9.525 mm.
What is the conversion between SFM and m/min for cutting speeds? Multiply SFM by 0.3048 to get m/min. For example, 600 SFM × 0.3048 = 183 m/min. For quick mental conversion, divide SFM by 3.28 (600 / 3.28 ≈ 183 m/min). Conversely, multiply m/min by 3.28 to get SFM (180 m/min × 3.28 ≈ 590 SFM).
How do I convert between IPR and mm/rev for feed rates? Multiply IPR (inches per revolution) by 25.4 to get mm/rev. For example, 0.005 IPR × 25.4 = 0.127 mm/rev. Conversely, divide mm/rev by 25.4 to get IPR (0.10 mm/rev ÷ 25.4 = 0.00394 IPR). Common deep hole drilling feeds: 0.004 IPR ≈ 0.10 mm/rev, 0.008 IPR ≈ 0.20 mm/rev, 0.012 IPR ≈ 0.30 mm/rev.
