15.5 mm drill size
A 15.5 mm metric drill is 0.6102 in (15.500 mm). It is the cut tap, 75% thread drill for M18x2.5.
- Decimal inch
- 0.6102
- Millimeters
- 15.500
- Nearest 1/64
- 39/64" (+0.0009)
- Type
- metric drill
Nearest equivalents in other systems
Speeds and feeds: in mild steel an HSS drill starts at about 560 RPM and 0.0109 in/rev (6.10 IPM); in 6061 aluminum about 1,560 RPM.
15.5 mm drill speeds and feeds by material
Jobber-length twist drill, D = 0.6102 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.
| Material | HSS SFM | HSS RPM | Carbide SFM | Carbide RPM | Feed (in/rev) | HSS IPM | Carbide IPM |
|---|---|---|---|---|---|---|---|
| Aluminum (6061) | 250 | 1,560 | 600 | 3,760 | 0.0104 | 16.2 | 39.1 |
| Brass / free-machining | 150 | 940 | 400 | 2,500 | 0.0086 | 8.08 | 21.5 |
| Low-carbon steel (1018) | 90 | 560 | 300 | 1,880 | 0.0109 | 6.10 | 20.5 |
| Alloy steel (4140, annealed) | 70 | 440 | 250 | 1,560 | 0.0117 | 5.15 | 18.3 |
| Stainless 304 | 40 | 250 | 150 | 940 | 0.0089 | 2.23 | 8.37 |
| Gray cast iron | 80 | 500 | 250 | 1,560 | 0.0092 | 4.60 | 14.3 |
| Titanium 6Al-4V | 25 | 160 | 100 | 630 | 0.0052 | 0.83 | 3.28 |
| Plastics (acetal) | 300 | 1,880 | 500 | 3,130 | 0.0064 | 12.0 | 20.0 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 560 RPM × 0.277 mm/rev = 155 mm/min.
Worked example: 15.5 mm drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.6102 ≈ 560 RPM.
- Feed for a 0.610 in drill in mild steel ≈ 0.0109 in/rev.
- IPM = 560 × 0.0109 = 6.10 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 1.831 in (46.5 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 3.051 in (77.5 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 4.882 in (124 mm) | G83 full retract | 0.5-1 × D, shrinking with depth | Retract to clear chips; reduce feed ~10-20% past 5×D. |
Taps that use a 15.5 mm drill
- M18x2.5 — cut tap, 75% thread · tap speeds and feeds
Nearest drills
- 39/64": 0.6094 in / 15.478 mm (-0.0009 in)
- 15.75 mm: 0.6201 in / 15.750 mm (+0.0098 in)
- 15.25 mm: 0.6004 in / 15.250 mm (-0.0098 in)
- 5/8": 0.6250 in / 15.875 mm (+0.0148 in)
Common questions
- What is a 15.5 mm drill in decimal and mm?
- 0.6102 in, or 15.500 mm; nearest fractional drill: 39/64" (0.6094 in, -0.0009 in); nearest number / letter drill: Letter Z (0.4130 in, -0.1972 in).
- What RPM for a 15.5 mm drill in steel?
- In low-carbon steel (1018) an HSS 15.5 mm drill (0.6102 in) runs about 560 RPM at 90 SFM; a carbide drill about 1,880 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a 15.5 mm drill?
- About 0.0109 in/rev in mild steel, 0.0104 in/rev in aluminum and 0.0089 in/rev in 304 stainless. At 560 RPM that is 6.10 IPM in steel (IPM = RPM × IPR).
- What RPM for a 15.5 mm drill in aluminum?
- 1,560 RPM with HSS (250 SFM) or up to 3,760 RPM with carbide (600 SFM), feeding about 0.0104 in/rev.
- What tap uses a 15.5 mm drill?
- M18x2.5 (cut tap, 75% thread).
Nearby drill sizes
Sources
- Machinery's Handbook, 31st ed., Speeds and Feeds for Drilling, Reaming and Threading (Tables 17-23) — HSS and carbide drilling SFM (rounded).
- RedLine Tools (Productivity Inc.) catalog p. 340, Cobalt & HSS Drills Speeds & Feeds — feed per rev by drill diameter and material (middle of the catalog range).
- ASME B94.11M twist drills: number (#1-#80) and letter (A-Z) drill sizes; drill size reference.
Material feed multipliers (aluminum 1.3×, brass 1.1×, 4140 0.85×, stainless and titanium 0.7× the mild-steel feed) are JobLine estimates. Peck guidance is JobLine shop practice. Use your drill maker's data first.
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