Twist Drill Point Angles by Material

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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