Runout is the single biggest tooling-related problem in deep hole drilling — a µm-level error at the clamp becomes a fraction-of-a-millimetre deviation at the tip of a long drill. Shrink-fit and hydraulic chucks hold the line where collets cannot. This guide compares every holder family, quantifies what runout actually costs, and lays out the setup discipline that separates reliable deep holes from scrap.
Every error at the clamping point is amplified at the cutting end, and once the drill is embedded the hole simply follows the direction the tip first chose. An oversized, bell-mouthed, or broken hole is usually a runout story before it is a cutting-speed story.
| Defect | How Runout Causes It | Where You See It |
|---|---|---|
| Oversize hole | Off-axis rotation makes each margin cut an arc wider than the nominal diameter; the drill never seats centrally and walks off true. | Diameter consistently +0.02–0.05 mm on the first hole |
| Bellmouth entry | Tip whip at entry reams the mouth before the guide pads engage — and a bad start is then followed for the entire depth. | Entry 0.05–0.1 mm larger than mid-hole |
| Oval / eccentric hole | Unequal chip load on opposing cutting corners lets the tool cut deeper on one side, drifting the centre off-axis. | Roundness error on first inspection |
| Tapered hole | Progressive deflection changes the effective radius as the drill extends. | Diameter shrinks or grows along the bore |
| Drill breakage | Margins overload on one flute, vibration climbs, and a solid-carbide gundrill snaps almost instantly. | Broken tool at depth — the part is usually scrap |
| Poor finish / chatter | Unstable edge contact produces smear marks and coarse Ra. | Surface comparator fails; rework required |
Six families cover the practical spectrum. Read the runout column first — it is the single number that decides whether a holder belongs on a deep hole machine.
| Type | Runout (TIR) | Clamping Force | Vibration Damping | Max Coolant Pressure | Max Torque (Ø20 mm shank) | Balancing Grade | Cost |
|---|---|---|---|---|---|---|---|
| Shrink Fit | ≤ 0.003 mm | Highest | Poor (rigid connection) | Unlimited (sealed) | ~900 Nm | G2.5 @ 25,000 RPM | $$$ |
| Hydraulic Chuck | ≤ 0.003 mm | High | Excellent (30–50% vibration reduction) | Up to 150 bar (80 bar standard) | ~200 Nm | G2.5 @ 25,000 RPM | $$$ |
| ER Collet (precision) | 0.005–0.010 mm | Moderate | Moderate | Up to 100 bar (seal-dependent) | ~80 Nm | G6.3 @ 15,000 RPM | $ |
| ER Collet (standard) | 0.010–0.020 mm | Moderate | Moderate | Up to 80 bar | ~60 Nm | Not balanced | $ |
| Milling Chuck (Weldon / Whistle Notch) | 0.010–0.020 mm | High | Moderate | Up to 60 bar (seal-dependent) | ~500 Nm | Not balanced | $$ |
| Power Chuck (3-jaw) | 0.020–0.050 mm | Highest | Poor | Low (<30 bar) | — | Not balanced | $$$ |
Past 10×D these two dominate. They are close on runout; they diverge on damping, coolant sealing, and changeover logistics. Choose on rigidity versus vibration control, not on the last µm.
| Property | Hydraulic | Shrink Fit |
|---|---|---|
| Runout | ≤0.003 mm | ≤0.003 mm |
| Vibration damping | Excellent (30–50% reduction) | Poor (rigid) |
| Rigidity / clamping force | High | Highest — no moving parts |
| Tool life effect | Up to +40% from damping | Indirect — via stiffness |
| Coolant pressure | 80 bar std / 150 bar HD | Unlimited (no seals) |
| Changeover | Seconds | ~30 s plus heating/cooling |
| Operator skill | Low | Low — accuracy built in |
| Maintenance note | Check fluid level; a leak ruins grip and runout | Replacement bore after ~3,000 cycles |
| Best for | Finishing, reaming, vibration-prone work, gundrilling on machining centres | High-speed, heavy cuts, long overhang, ultra-high-pressure coolant |
HSK (Hollow Shaft Taper), per ISO 12164 / DIN 69893, is the preferred spindle interface for deep hole drilling on modern machining centres. The 1:10 taper delivers simultaneous face and taper contact — the most rigid and repeatable seat available.
| Type | Application | Key Features |
|---|---|---|
| HSK-A | Automatic tool changing, general machining | Drive slots at taper end, gripper grooves, central coolant (most common) |
| HSK-C | Manual tool changing | Same as HSK-A but without gripper grooves |
| HSK-E | High-speed machining (HSC) | Fully symmetrical, no drive slots — torque via friction grip; minimal unbalance |
| HSK-F | High-speed, large diameter | Similar to HSK-E but larger flange for higher torque capacity |
| Interface | Taper Ratio | Max Runout (New) | Rigidity | Best For |
|---|---|---|---|---|
| HSK (ISO 12164) | 1:10 hollow | 0.002–0.003 mm | Highest (face + taper) | High-speed deep hole drilling |
| BT / SK (ISO 7388 / DIN 69871) | 7:24 steep | 0.003–0.005 mm | Good (taper only) | Conventional gundrilling machines, moderate speeds |
| CAT (ANSI B5.50) | 7:24 | 0.005–0.010 mm | Moderate | Older machines, retrofit deep hole systems |
| Capto (ISO 26623) | 1:20 polygon | <0.003 mm | Very high (3-lobe polygon) | Turning centres with deep hole capability |
Runout must be budgeted across the whole chain — spindle, taper seat, holder, and tool shank. Add them up: a “perfect” holder can still land the system over budget.
Holder precision is worthless against a sloppy shank. The industry standard is h6 for all precision toolholding systems:
Published tool-life tests (BIG Kaiser / Modern Machine Shop, Ø3 mm drills in C55 steel) are the clearest demonstration anywhere that runout is the dominant tooling variable. Tool life criterion: 0.2 mm wear land.
| Measurement | At 2 µm runout | At 15 µm runout | Gain |
|---|---|---|---|
| Carbide drill life (Ø3 mm, blind 4D hole) | ~2,300 holes | ~1/3 of that | ~2.9× — roughly tripled |
| HSS drill life (Ø3 mm, blind 3D hole) | ~1,500 holes | ~1/2 of that | 2.1× |
| Through-coolant HSS drill life | Baseline | — | 1.6× (least sensitive) |
| Surface finish Ra (milling test) | ~0.5–0.6 µm | ~1 µm | Roughly halved |
| Surface finish Rz | ~3.3 µm | ~4.7 µm | — |
| Cost per hole (Ø3 mm carbide) | $0.27 | $0.80 | ~66% saving per hole |
Deep hole drilling is coolant delivery. Every drop of high-pressure coolant must reach the cutting edge — and none may leak past the holder. That single sentence rules out most collet setups for serious work.
| Holder | Through-Tool Coolant | Limit / Notes |
|---|---|---|
| Shrink fit | Unlimited pressure | No moving seals — the ultra-high-pressure choice |
| Hydraulic | 80 bar std, 150 bar HD | Fluid-sealed around the shank |
| ER collet (sealed) | Up to ~100 bar | Seal-dependent; leakage starves and washes out the edge |
| ER collet (standard) | Up to 80 bar | Coolant bypasses between collet and shank |
| Weldon / side-lock | Up to 60 bar | Set-screw seal unreliable |
A gundrill is a single-lip, asymmetric tool — the cutting edge sits off-centre by design and the tool is not self-starting. It needs a bushing or pilot hole for entry support, then it self-guides on its pads for the rest of the bore.
Runout is a discipline, not a purchase. These six steps catch the majority of problems before the first hole.
Any chip or contamination between taper surfaces causes runout and can score the spindle. A 0.01 mm chip on an HSK face is a 0.01+ mm runout event.
Check for nicks, wear, or galling before loading — damaged shanks transfer damage into the holder bore.
ER nut torques: ER16 = 100 N·cm, ER25 = 140, ER32 = 180, ER40 = 220. Never an air wrench. Under-tightening allows tool pullout; over-tightening distorts the collet.
Dropping damping performance usually means fluid loss — consult the manufacturer for the recharge procedure.
Indicator at the tip and at 100 mm extension. Catch drift before the first deep hole is cut.
ER collets lose accuracy with use — replace when a known-good test bar reads >0.015 mm TIR.
| Symptom | Prime Suspect | Fix |
|---|---|---|
| Oversize holes, every hole | Holder runout, worn spindle, oversized chuck | Measure system TIR; switch to shrink/hydraulic; verify h6 shank tolerance |
| Bellmouth entry only | Bad start, no bushing, off-square face | Spot-face the entry square; add a guide bushing or pilot hole |
| Oval / tapered hole | Unequal chip load, growing tip runout | Reduce overhang; re-check tip TIR; stiffen the holder |
| Tool life collapsed | Runout >0.010 mm, one margin overloaded | Cut runout; check edge symmetry after resharpening |
| Breakage at entry | Flooded free-spinning drill, whip | Turn coolant on only once engaged; use a pilot hole |
| Chatter / poor finish | Low-damping holder, long overhang | Switch to hydraulic for damping; shorten reach |
| Coolant pressure drop | Collet leak, worn seal | Sealed collet or shrink/hydraulic; inspect O-rings |
Match the holder to the depth. As L/D climbs, the runout budget tightens and the coolant pressure climbs with it.
| L/D Range | Recommended Holder | Spindle Interface | Target Runout | Coolant Requirement |
|---|---|---|---|---|
| < 5×D | ER collet (precision grade) | BT/SK or HSK | ≤ 0.010 mm | Standard coolant |
| 5×D–10×D | Hydraulic chuck | HSK preferred | ≤ 0.005 mm | Through-spindle, ≥50 bar |
| 10×D–20×D | Shrink fit or hydraulic | HSK (A or E type) | ≤ 0.003 mm | Through-spindle, ≥70 bar |
| > 20×D | Shrink fit + balanced assembly | HSK-E (high-speed symmetrical) | ≤ 0.003 mm | High-pressure system, ≥100 bar |