The most common tool-selection decision in deep hole drilling. A twist drill is cheap and fast on shallow holes; a gundrill is a self-piloting, single-lip tool that drills straight, round, fine-finished bores at depth-to-diameter ratios a twist drill cannot reach. Where the crossover happens — and what it costs in cycle time and tooling — decides the economics of the hole.
The first decision on almost every deep hole job is tool class: a conventional two-flute twist drill or a single-lip gundrill. Both remove solid metal, but they are engineered around completely different constraints. The twist drill is cheap, familiar, and fast on shallow holes; the gundrill is self-piloting, high-pressure-cooled, and built for depth, straightness, and finish.
Per CTE Magazine, any hole with a depth-to-diameter ratio (L/D) above 4:1 is generally considered “deep” — and that is exactly where the two tools start to diverge. The practical question is not “which is better” but “which constraint governs: depth, quality, or cycle time?”
| Tool | Best At | Weak At |
|---|---|---|
| Twist drill | Short holes (≤10×D), high feed rates, low tool cost, general-purpose drilling | Chip evacuation beyond critical depth, straightness, wandering, flute clogging |
| Gundrill | Long holes (>20×D), straightness, roundness, fine finish, single-pass quality | Low feed rates, high-pressure coolant infrastructure, tool cost, small-shop setup |
A twist drill removes metal with two (or more) symmetrical cutting edges and augers chips out along spiral flutes. That works well until the flute fills: beyond the “critical depth” — commonly around 4×D (CTE Magazine) — heat and chip accumulation stop a standard twist drill from drilling in one pass. CTE describes chip evacuation as the main difficulty of deep hole drilling.
| Twist Drill Type | Practical Depth (L/D) | Notes |
|---|---|---|
| Conventional flutes (crankshaft form) | ~4:1 | Peck to clear chips; pecking adds cycle time and re-cuts chips |
| Through-coolant, conventional flutes | 7–10:1 | Coolant pushes chips up the flutes, fewer pecks |
| Parabolic flutes | 10–15:1 | Wider flutes hold more chip volume in one pass |
| Through-coolant, parabolic | 15–17:1 | Best conventional twist configuration |
| Specialized deep-hole twist (WALTER TITEX DIN 1869) | 16–85:1 | Engineered web, coolant channels, deep flute design |
| Solid carbide, internal cooling | ~20:1 (to ~30:1 modified) | High rigidity but brittle; large overhang risk |
A gundrill is a single-lip, self-piloting tool: one cutting edge cut eccentrically off the drill centerline, no central web to generate thrust, and a straight V-shaped flute running the full length of the tube (UNISIG, Baucor). The shank is alloy steel tubing with a 110°–120° Vee-flute formed to the center; the flute area is roughly 22–26% of the hole area — more chip room than a twist drill.
High-pressure coolant (1,000 psi recommended, 250 psi the absolute minimum for adequate evacuation) is pumped through an internal channel in the tool — a round bore about ¼ of the drill diameter — directly onto the cutting edge. It cools and lubricates the tip, then flushes chips back out through the external V-flute. The twist drill instead carries chips up its spiral and can clog at depth.
Behind the cutting edge, gundrills carry guide pads that bear against the freshly cut wall. The pads stabilize the tool, force the edge to cut a true circular path, and burnish the hole as they pass — which is why a gundrilled bore is straight, round, and fine-finished in a single pass (SHIN-IL, CNCCookbook).
CNCCookbook’s guide frames the crossover cleanly by depth-to-diameter ratio. These are practical starting bands, not hard limits — material, diameter, and hole quality shift them.
| Depth (L/D) | Recommended Tooling | Comment |
|---|---|---|
| Up to 5:1 | Standard twist drill | No coolant needed in most materials; chips clear fine |
| 5:1 to 10:1 | Through-coolant or high-performance twist drill | Coolant-fed carbide keeps the flutes clear |
| 10:1 to 20:1 | Parabolic-flute or specialized deep-hole twist drill | High-performance geometry; some jobs push to ~20×D |
| 20:1 and greater | Gundrill | Single-pass; dedicated machines reach 100:1, 200:1, even 400:1 |
Below 20:1, a gundrill is still the right call when quality governs: tight straightness or diameter control, exceptional surface finish, or a functional bore that would otherwise need reaming or honing as a secondary operation. Eliminating secondary finishing often pays for the gundrill setup.
This is the widest gap between the two tools. SHIN-IL’s comparison is blunt: a twist drill is not a precision tool even when made to close tolerances, and its symmetrical design cannot neutralize the passive force asymmetries from grinding, so it drills slightly curved holes. The gundrill’s single-lip construction with guide pads “forces the edge to cut in a true circular pattern.”
| Parameter | Twist Drill | Gundrill |
|---|---|---|
| Diametral tolerance | IT11–IT14 (typ. ±0.002″) | IT7–IT9; down to 0.01 mm; ±0.0005″ |
| Straightness | Wanders / drifts with depth | Self-piloting; runout ≤0.001 in/in |
| Surface finish (Ra) | Rough; ~5–9 μm on steel | Ra 0.4–3.2 μm in a single pass |
| Roundness | ≥40 μm in deep tests | <5 μm typical; guide-pad burnishing |
| Secondary ops | Often ream or hone | Usually none required |
On pure penetration rate, modern twist drills win at shallow-to-moderate depth. A coolant-fed carbide twist drill can be 5–6× more productive than a gundrill (CTE/UNISIG), with feed rates high enough that machine burden rate — not tool cost — dominates. The gundrill makes up for its slower feed by never stopping to peck and by hitting the finish in one pass.
| Parameter | Gundrill | Twist Drill (carbide, TSC) |
|---|---|---|
| Cutting speed (steel) | 40–120 m/min (carbide tip) | 80–180+ m/min |
| Feed rate | 0.01–0.16 mm/rev | 50–100% higher than gundrill |
| Penetration | “A few inches a minute” typical | Up to 70 in/min in cast iron (contested, but high) |
| Chip removal | Continuous, hydraulic | Flute flow; pecks below critical depth |
| HSS variant | 35–70 m/min | Lower speeds, cheaper tooling |
In 316 stainless at 20×D depth, one comparison (Jimmytool) showed a gundrill at 4.2 minutes per hole versus 1.8 minutes for a modern custom carbide through-coolant drill — a 57% cycle-time reduction and roughly 3× tool life, worth about $12,400 per year on that job. At this depth the quality crossover still favors the gundrill for tight prints, but it is not automatic.
The sticker price tells only part of the story. The correct comparison is total cost per hole: tool price × replacements, regrinding, cycle time × machine burden rate, secondary operations, and scrap from quality failures. Both Jimmytool and CTE make this point explicitly.
| Cost Driver | Twist Drill | Gundrill |
|---|---|---|
| Initial tool cost | $40–$200 typical; ~$500+ for long-length carbide | $150–$1,000+ depending on length & head |
| Reconditioning | Regrind needed; week-plus turnaround means extra inventory | Reground and re-tipped 15–20 times (UNISIG) |
| Cycle time | Fast feed, but pecks below ~4×D add time | Slower feed, no pecks, one-pass finish |
| Secondary ops | Often ream / hone | Usually none |
| Infrastructure | Flood or moderate TSC | High-pressure coolant (250–2,000 psi), bushing or pilot |
Divide hole depth by diameter. Under 10:1 the twist drill is favored; over 20:1 the gundrill dominates; 10–20:1 depends on the next questions.
Under ~12.7 mm (0.500″) and deep, gundrills are usually the only practical option. Over ~75 mm, BTA takes over from gundrills.
If straightness, roundness, IT9 or tighter tolerance, or Ra ≤3.2 μm are specified, go gundrill — twist drills will not hold them at depth.
Do you have high-pressure coolant, a bushing or pilot provision, and the fixturing? If not, the twist drill (or a service shop) wins by default.
Estimate total cost per good hole: cycle time × burden rate, tooling, regrinds, secondary ops, and scrap. Let that decide, not the tool sticker price.
L/D ≤10:1, standard tolerance, no high-pressure coolant, low volume, general purpose.
L/D 10–20:1, moderate quality, through-coolant available, feed-rate driven.
L/D >20:1, tight straightness and finish, small diameters, functional bores, one-pass quality.
Large diameters where gundrills slow down — see the BTA and method-selection guides.
| Myth | Reality |
|---|---|
| “Gundrills are always slower” | Feed is lower, but gundrills never peck and finish in one pass — on deep, tight-tolerance bores the total cycle is often faster than a pecked twist drill plus reaming. |
| “Twist drills cannot drill deep holes” | Through-coolant parabolic and specialized designs reach 15–17×D (some 16–85×D) — but quality degrades and pecking returns. |
| “Deep hole drilling starts at 10:1” | Definitions vary: CTE calls >4:1 deep; patents and academic work use 10:1–15:1. The tool crossover is what matters, not the label. |
| “Gundrills are too expensive” | Reground 15–20 times, they amortize well; deleting reaming and honing plus scrap often flips the cost comparison. |
| “Higher pressure is always better for a twist drill” | Twist drills need about half the gundrill’s pressure (~500 psi); over-pressure can blow chips and damage flutes. |
| “A pilot hole fixes gundrill entry completely” | Pilots (2–3×D, H8) prevent walk, but bushing and bore alignment still govern straightness at extreme L/D. |