For most drilling, 118° is the right point angle and 135° is the exception you reach for when the material is hard, tough or prone to work-hardening. A 118° point cuts keenly and starts easily in wood, plastic, aluminum and mild steel. A 135° point is blunter, spreads the cutting load across a longer edge and, as a split point, centers itself instead of wandering, which is what stainless steel, cast iron and hardened alloys need. Angles below 100° belong to soft plastics and thin sheet, where an aggressive point grabs and tears on breakthrough. Those three figures cover almost everything you will drill, but point angle is only one of three angles ground into a twist drill, and the other two decide whether the bit clears its chips or packs up and burns.
What do the angles on a twist drill actually control?
A twist drill is a set of compromises, and each angle trades one property against another. The point angle is the included angle between the two cutting lips: shallower is keener and needs less thrust, but presents a longer, thinner edge that chips in hard material and snatches on breakthrough, while blunter is stronger and more stable but needs more push and makes more heat. The lip relief angle is the clearance ground behind each cutting edge so the heel does not rub; too little and the bit polishes the surface instead of cutting it, too much and the edge is unsupported and chips. General-purpose bits usually carry somewhere around 8 to 12 degrees of relief, toward the lower end for hard material. The helix angle is the twist of the flutes, which governs how fast chips clear and how hard the bit pulls itself into the work. Jobber-length bits are made to a dimensional standard, DIN 338, but that standard fixes lengths and diameters, not the grind, so two bits of identical size can behave completely differently in the same material.
Which point angle suits which material?
| Point angle | Best suited to | What it does |
|---|---|---|
| Under 100° | Acrylic and soft plastics, thin sheet, soft wood | Keen slicing action at low thrust; less grabbing and cracking on breakthrough |
| 118° | Mild steel, aluminum, wood, general workshop use | The standard grind on jobber bits; balances a sharp cutting action against edge strength |
| 135° | Stainless steel, cast iron, tool steel, tough alloys | Blunter, better-supported edge that resists chipping; usually supplied as a split point |
| 140° and above | Hardened and abrasive materials, solid carbide drilling | Maximum edge support at the cost of keenness; needs a rigid setup and steady feed |
Treat those as starting points rather than rules, because the condition of the work matters as much as the material. A blunter point is the safer choice on a curved surface or anywhere you cannot start a center punch. Bits are sold already ground for a purpose, so the practical decision is which sets to keep in the drawer: standard fully ground bits from the rotary cutting tools range for steel and wood, and a separate split-point set for stainless and cast iron rather than asking one set to do both.
When does a split point matter?
The chisel edge at the center of a conventional twist drill does not cut. It extrudes, pushing material sideways, and it accounts for most of the thrust you have to apply and most of the bit's tendency to wander before it bites. A split point thins the web and grinds two extra cutting edges through the center, so the drill cuts from the middle outward. It starts where you put it, often without a center punch. It needs noticeably less feed pressure, which matters in a hand drill and matters more in stainless, where heavy pressure at low speed is what glazes and work-hardens the surface. This is why 135° and split point are so often quoted together: a plain 135° point without the split is harder to start than the 118° bit it was meant to improve on.
What else changes how a bit cuts?
Coating changes how fast you can run a bit, not what angle it should be ground to. A surface treatment lowers friction and raises the temperature the edge tolerates, which buys speed and tool life in abrasive material, but it will not make a 118° point suitable for hardened steel.
Speed, feed and hole depth
Speed and feed do more damage than most people expect. Run too fast in stainless or titanium and the edge overheats and dulls in seconds. Feed too lightly in the same material and the bit rubs rather than cuts, work-hardening the surface into a skin the drill can no longer get through, which is why tough alloys want slower speed with firm, continuous feed. Feed too heavily in cast iron or brass and you chip the corners of the lips. Depth matters too: past roughly three times the bit diameter, chips stop clearing on their own, so peck the hole and withdraw the bit to clear the flutes rather than pushing on and packing them. That one habit breaks fewer small-diameter bits than any change of geometry.
Helix angle by material
| Helix | Also called | Suits |
|---|---|---|
| Roughly 10-22° | Slow or low helix | Brass, bronze, thin sheet and hard plastics, where a standard bit snatches and self-feeds |
| Roughly 25-35° | Standard helix | General purpose: steel, cast iron, wood, most workshop drilling |
| Roughly 35-45° | Fast or high helix | Aluminum, copper and soft plastics, and deep holes where chips must clear quickly |
How do you know a bit has been sharpened badly?
By the hole, not by the bit. If the two lips are not the same length, the drill turns about a center offset from its own axis and cuts a hole larger than its nominal diameter, often by a surprising margin. If the lips are equal in length but not in angle, only one of them cuts, the bit runs hot on one side and life drops sharply even though the hole may still measure correctly. Both faults are easy to create freehand on a bench grinder and both are worth checking with a gauge rather than by eye. The other tell is the chip: a correctly ground bit throws two equal helical chips, one from each flute, so one chip means one lip is doing the work. Measure a test hole with a caliper from the measuring equipment range before blaming the material, and retire any bit that is chipped rather than blunt, because a chipped lip will oversize every hole it drills. Replacements sit in the cutting tools range.