Twist Drill Point Angles by Material
Recommended twist-drill point angle, relief angle, and helix angle by material — from mild steel (118°) through stainless (135° split-point) to plastics and composites. Includes regrind guidance and special-purpose drill types.
Reference
The point angle is the included angle of the drill tip. 118° is the "standard" factory grind and works well on most carbon steels and cast iron. Harder / stickier materials want steeper points (135°+) with a split point so the chisel edge cuts instead of rubbing. Soft ductile metals and plastics like shallower angles (90-130°) to reduce grabbing. A split point reduces thrust force by 30-50% and eliminates the need for a centre-drill / pilot hole on steels.
Point angle by material
| Material | Point angle | Relief | Helix | Notes |
|---|---|---|---|---|
| Mild steel 1018 | 118° | 8-12° | 28-32° std | Standard HSS twist drill as shipped. |
| Medium carbon 1045 | 118-135° split | 8-12° | 28-32° | Split point helps on deep holes. |
| Alloy steel 4140 (annealed) | 135° split | 8-10° | 28-32° | Cobalt HSS or carbide recommended. |
| Alloy / hardened steel (Rc>32) | 135-140° split | 6-8° | 28-32° | Carbide required; reduce SFM 60%. |
| Tool steel (annealed) | 130-135° split | 8-10° | 28-32° | Cobalt HSS; high helix clogs. |
| Stainless 304 / 316 | 135° split | 8-10° | 28-32° std | Must feed constantly — dwell causes work hardening. |
| Stainless 303 (free-mach) | 118-135° | 8-10° | 28-32° | Easiest SS to drill; less aggressive point OK. |
| Cast iron (gray) | 118° | 10-12° | 28-32° | Dry machining; burn chip off surface. |
| Cast iron (ductile) | 130° split | 10° | 28-32° | Higher toughness than gray iron. |
| Aluminum (soft, 1100, 3003) | 130-140° | 12-15° | 35-45° high | Sharp tip; polished flutes; WD-40 or alcohol. |
| Aluminum (machining 6061, 7075) | 118-130° | 10-15° | 28-45° | Standard point OK; peck at depths >3 × diameter. |
| Brass (free-cutting 360) | 118° | 10-15° | Slow (12-18°) | Grind a flat on the cutting edge to prevent grabbing. |
| Bronze (aluminum or phosphor) | 118° | 10-15° | 28-32° | Rigid setup; brass can bite into the drill. |
| Copper (pure) | 118-140° | 12-15° | 35-45° high | Gummy; positive rake and sharp tool essential. |
| Titanium Ti-6Al-4V | 135-140° split | 6-8° | 28-32° | Low RPM, high feed; flood coolant always. |
| Inconel / Hastelloy | 135-140° split | 6-8° | 28-32° | Cobalt or carbide; aggressive feed, never dwell. |
| Plastics (Delrin, UHMW) | 90-120° | 12-15° | 9-15° slow | Slow feed; polished flutes to prevent melt. |
| Plastics (acrylic, PC) | 60-90° | 12-15° | Slow helix | Very slow feed; cooling (air blast or water mist); lead-in with centre drill. |
| Wood | 60-90° brad-point | — | High helix | Brad-point centres the bit; prevents tearout on exit. |
| Composites (CFRP, glass) | 125-140° PCD/carbide | 6-10° | Low 15-20° | PCD or solid-carbide; avoid fiber pull-out with dagger-drill or countersink pilot. |
| Rubber | 60-90° | 12-15° | Slow helix | Freeze part first; use hollow punches for cleaner holes. |
Point styles (geometries)
- Standard 118° conventional
- Conical tip with web at centre. Default factory grind. Needs centre drill or spot for accurate start.
- Split point (135°)
- Web is split with a secondary grind — creates two small chisel edges. Self-centres, cuts from first contact. Reduces thrust 30-50%.
- Notched point
- Single flat ground on one cutting lip to break chips in soft / stringy materials (brass, aluminium).
- Radius / full-radius
- Cutting lip is relieved to a curve. Holds size and finish better in high-volume production.
- Parabolic flute
- Wider, deeper flutes to evacuate chips in deep holes (> 5 × diameter).
- 4-facet point
- Four-facet ground; gives web-thinning naturally. Best DIY grind for hand-sharpened drills.
Relief angle — what it does
- Relief angle (lip relief) is the angle behind the cutting edge — prevents the drill heel from rubbing the hole.
- Small relief (6-8°) gives a stronger edge for hard materials; large relief (12-15°) is better for soft, grabby materials.
- Too much relief → weak edge, chatter; too little → rubbing, burns, slow cutting.
- Typical HSS drills ship at 8-12° relief. Regrind relief first, then the 118° / 135° point angle.
Helix angle
- Standard helix (28-32°)
- Default for most steels and cast iron. Balances chip evacuation with edge strength.
- Low / slow helix (8-18°)
- Brass, bronze, plastics. Lower helix = less aggressive cut → prevents grab / catch in brass.
- High helix (35-45°)
- Aluminum, copper, soft ductile materials. Faster chip evacuation; less torque.
- Straight flute (0°)
- Brass and thin sheet — zero helix prevents "walking" and pull-in.
Regrind guide
- When to regrind: chips change colour (bluing on steel = dull drill), hole goes undersize, or thrust force feels increased.
- Remove only enough material to reach fresh edge (0.005-0.015" at a time). Over-grinding heats the flute and destroys the temper.
- Check lip length: both cutting lips must be equal length and angle or the drill will cut oversize. Visual inspection or use a drill-point gauge.
- For split points: first grind the 135° point, then use a split-point grinder or manual notch-grinding jig to relieve the chisel.
- Web thinning: worn drills develop a thick chisel edge. Grind a secondary flat at ~30° behind the chisel to reduce thrust without changing the cutting angle.
- Stoned finish on the cutting lip — no grinder marks — extends life noticeably, especially on stainless and aluminium.
Special-purpose drill types
- Stub-length
- Flute length ≈ drill diameter. Rigid, minimal deflection; for CNC, sheet metal.
- Jobber-length
- Standard length. Good all-round choice.
- Taper-length
- Long flute for deep holes ~10× diameter. Weaker than jobber.
- Aircraft / long
- Very long reach for aviation / stacked sheet drilling.
- Centre drill
- Double-ended stub tip + body; makes a 60° V for lathe centres. Not a drill for production holes.
- Spot drill
- Single flute, 90° or 120° point; pre-dimples flat surfaces so a jobber drill starts on location.
- Step drill
- Conical with multiple diameters; for sheet metal. Self-deburrs and no reaming required.
- Core drill
- 3 or 4 flute, no centre cutting edge — for enlarging existing holes.
- Annular cutter
- Cuts only the rim → leaves a slug. 20× faster than twist drills in thick plate.
- Unibit / Uni-bit
- Brand for step drill; popular in electrical / HVAC sheet work.
- Masonry (tip carb.)
- Tungsten-carbide tip brazed on. Requires hammer action for concrete / brick.
- Glass / tile
- Spear-point carbide tip; slow RPM, water cooling, no hammer.
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