# Twist drills

Helical-flute drill geometry, point and helix angles, material-specific designs, chip evacuation, and worked feed and breakthrough calculations.

Reviewed: 2026-11-02

Tags: tooling, drilling, holemaking

In the example, a **10 mm drill** through **20 mm plate** needs **23.00 mm tip travel** with an ideal **118° point**, or **22.07 mm** at **135°**, for full-diameter breakthrough. At **1,000 rpm** and **0.10 mm/rev** it feeds **100 mm/min**. Check actual point, fixture clearance and breakthrough margin.

## Definition and geometry

Twist drills cut at the point and evacuate chips through helical flutes. RUKO notes that point thinning reduces the chisel region and feed force. [RUKO, eight characteristics of a twist drill, undated](https://ruko.de/en/knowledge/know-how/eight-characteristics-of-a-twist-drill/)

The **point angle** is the included angle at the drill tip; the **helix angle** describes the spiral flutes. They are distinct from cutting-edge clearance. Flute profile affects chip space, while core thickness affects stability. Guide margins support the drill against the hole wall. A split point or thinned web changes entry behavior without changing the nominal hole diameter. [RUKO geometry explanation](https://ruko.de/en/knowledge/know-how/eight-characteristics-of-a-twist-drill/).

**Engineering synthesis:** diameter, cutting-edge form, flute length, usable drilling depth, overall length and shank interface describe different constraints. A long tool does not necessarily have enough usable reach, and a matching shank diameter does not establish adequate clamping length, runout or coolant delivery.

## Geometry families and material scope

Gühring illustrates the traditional N, H and W groupings as follows. The angles belong to its published examples, not mandatory values for all products bearing those letters.

| Type | Example helix angle | Example point angle | Stated application tendency |
| --- | --- | --- | --- |
| N | 30° | 118° | General steel and cast-iron work |
| H | 15° | 118° | Hard, brittle materials |
| W | 40° | 130° | Soft, long-chip materials, including aluminium |

[Gühring, drilling tools, undated](https://guehring.com/uk/products/drilling-tools/).

The cutting material is another choice. Gühring discusses HSS/HSCO tools where toughness or less stable conditions matter, and solid carbide for productive machining with stable clamping. Its comparison does not establish that carbide is always the economical choice. A coating or cobalt designation alone is insufficient to select a drill for a particular alloy and condition. [Gühring drilling-tool comparison](https://guehring.com/uk/products/drilling-tools/).

As a bounded modern example, **Gühring RT 100 InoxPro** is a solid-carbide family for stainless steels and titanium alloys. Its dedicated page lists **3×D, 5×D and 7×D** versions, **3–20 mm** diameters, **HA shanks**, polished flutes and **Perrox coating**. The page ties those features to the named family. They do not describe every twist drill or establish interchangeability with any holder accepting the same nominal shank size. [Gühring RT 100 InoxPro, undated](https://guehring.com/uk/products/drilling-tools/solid-carbide-drill-bit-rt-100-inoxpro/).

## Chip evacuation and access conditions

Chip formation depends on work material, drill geometry, cutting data and coolant pressure and flow. Sandvik relates chip jamming to radial drill movement, poor hole quality and breakage. It favors internal coolant for deeper holes and long-chip materials, while external coolant can serve shallow holes with good chip formation. Its guidance also emphasizes short overhang, accurate holders and low runout. These relationships do not define a universal peck cycle or coolant pressure for all twist drills. [Sandvik, drilling tips, undated](https://www.sandvik.coromant.com/en-gb/knowledge/drilling/drilling-tips).

Entry and exit geometry matter separately from nominal depth. Sandvik's **solid-carbide** guidance identifies unequal cutting forces, vibration and additional wear at inclined entries or exits, and warns of edge chipping when opening pre-drilled holes. Those statements concern the drill class discussed; an HSS enlargement operation and a solid-carbide full-bore operation cannot automatically share the same pilot-hole practice. [Sandvik, irregular-surface drilling, undated](https://www.sandvik.coromant.com/en-gb/knowledge/drilling/irregular-surface-drilling).

A conventional pointed drill also leaves a non-cylindrical end region in a blind hole. A depth measured to the deepest point differs from the depth over which the full hole diameter exists. At a through hole, the tip can emerge before the outer cutting corners finish breaking through. The following ideal model makes that geometric distinction explicit.

## Worked example: feed, point length and breakthrough

For drilling, Sandvik relates peripheral cutting speed, spindle speed and penetration rate by `vc = π × D × n / 1,000` and `vf = fn × n`, using `D` in mm, `n` in rev/min, `fn` in mm/rev, `vc` in m/min and `vf` in mm/min. The feed-per-revolution value already describes total axial advance per revolution; it is not multiplied by two merely because a drill has two lips. [Sandvik drilling formulas, undated](https://www.sandvik.coromant.com/en-gb/knowledge/machining-formulas-definitions/drilling-formulas-definitions).

Start the hypothetical **10 mm drill** with its tip at the entry face of the **20 mm plate** and assume constant feed. Ideal point length for included angle `β` is:

`h = (D/2) / tan(β/2)`

| Calculated quantity | 118° ideal tip | 135° ideal tip |
| --- | --- | --- |
| Point length `h` | 3.00 mm | 2.07 mm |
| Tip travel for full-diameter breakthrough `20 + h` | 23.00 mm | 22.07 mm |

Both cases have `vc = 31.42 m/min` at the outer diameter and `vf = 100 mm/min`. The ideal constant-feed travel times are therefore **13.80 seconds** and **13.24 seconds**, respectively. Stopping after 20.0 mm of tip travel would leave the far-face opening incomplete in this ideal model.

These are geometry and motion calculations, not predictions of actual cycle time or hole quality. Real point grinds have a web, relief surfaces and edge preparation rather than a perfect cone. Approach, pecking, feed changes, dwell, retract, tolerances and breakthrough margin are excluded. Backside fixture clearance must accommodate the projecting tip as well as the process margin; the calculation supplies no safe margin.

## Coverage and limitations

A drill's nominal diameter alone does not establish finished diameter, roundness, straightness, surface finish or positional accuracy. Tool wear, setup, entry conditions and chip evacuation influence those outcomes. A hole needing subsequent sizing or finishing is a different process requirement from simply producing an opening.

The references establish tool concepts and selected product applications, not a universal drill capacity, material hardness limit, feed table, achievable tolerance or tool life. Exact alloy and condition, hole depth and bottom definition, holder interface, machine rigidity and coolant arrangement remain application-specific. The cited pages display no issue date in the content reviewed on November 2, 2026; copyright and crawl dates are not publication dates.

## Related reference

[Cutting tools and tooling references](../index.md)