The unsung hero of every straight, on-size deep hole. A precision sleeve at the entry face supports the drill against deflection, holds spindle-to-hole concentricity, seals high-pressure coolant, and steers chips into the flute. Choosing the right type, material, clearance and alignment determines hole straightness, surface finish and tool life before the first chip is cut.
In deep hole drilling the tool is supported at the spindle and, once cutting, by its own guide pads riding the hole wall. At the moment of entry it has nothing to hold it — and that is the instant the hole is decided. A deflection here becomes a bellmouth, an oversize bore, a wandering axis or a snapped tool. The guide bushing is the fixed, precisely sized sleeve that constrains the tool within thousandths of a millimetre of the spindle axis at the cutting zone.
Constrains the drill near the cutting zone and prevents lateral wander and whip during entry penetration — hole centering to 0.0005 in is achieved by drilling through a bushing.
Maintains concentricity between the spindle axis and the hole axis over the full stroke. A 0.02 mm TIR alignment error at entry becomes millimetres of deviation at depth.
Contains high-pressure coolant at the hole entrance — critical in BTA/STS single-tube systems where external coolant is the only chip-transport driver.
Directs chips into the flute or chip trough and keeps the annular chip-clearance gap open, preventing packing, torque spikes and deflection.
| Failure mode without a bushing | Mechanism | Result |
|---|---|---|
| Bellmouthing | Drill deflects on an uneven or off-square entry face | Oversize, tapered first several diameters |
| Walking / wander | Excessive bushing ID clearance lets the drill slip over the entrance face | Random concentricity, bell-mouth formation |
| Drill breakage at entry | No entry constraint combined with rapid feed | Snap on engagement, expensive recovery |
| Guide-pad wear | Radial load carried by the drill pads instead of the bushing | Short pad and tool life, poor straightness |
Deep hole drilling separates bushings by where they sit in the system, not just by mounting style. Entry, intermediate and exit bushings cover the workpiece; a rotary bushing covers rotating-part machines; the pressure-head bushing is unique to single-tube BTA/STS.
| Type | Where it sits | Job | Used with |
|---|---|---|---|
| Entry bushing | At the workpiece face, clamped in the fixture or pressure head | Guides the drill at start, seals coolant, defines hole position | All methods — gundrill, BTA, ejector |
| Intermediate bushing | Along the drill body, between the machine and the entry | Supports long, slender drills and suppresses whipping | Gundrill at L/D > 40:1 |
| Exit (back-up) bushing | At the far face / breakthrough point | Supports the tool at exit, prevents breakout edge damage | BTA and gundrill through-holes |
| Rotary guide bushing | Rotates with, or is driven against, the part | Anchors and seals the bushing by cutting a seating groove in the face | Gun/annular drilling on boring mills, lathes, HBM |
| Pressure-head bushing | Integral with the BTA pressure head | Guides the tool and seals coolant against the workpiece face | BTA / STS single tube |
| Type | Application | Accuracy | Relative cost |
|---|---|---|---|
| Headless press-fit | Light-duty, low-volume, simple through-holes | Moderate (±0.01–0.02 mm) | $ |
| Headed press-fit | General purpose; the head provides a positive depth stop | Good (±0.008–0.015 mm) | $ |
| Fixed renewable | Production; replaceable drill bushing in a steel retainer | Excellent (±0.005–0.01 mm) | $$ |
| Slip-fixed renewable | Quick-change; several bushing IDs for different drill sizes in one fixture | Very good (±0.008–0.012 mm) | $$$ |
| Material | Hardness | Wear life | Best use |
|---|---|---|---|
| Hardened tool steel (O1, A2, D2) | 58–62 HRC | Baseline | General purpose; good wear resistance at moderate cost |
| Tungsten carbide (C2, C6) | High HRA | Up to 50× steel | High volume; holds tight tolerance over long runs |
| TiN-coated steel | 2,000–2,500 HV surface | Reduced friction | Sticky materials; improves chip flow and galling resistance |
| Nitrided alloy steel (4140, 4340) | 60–65 HRC case | Good | Large-diameter bushings; balance of cost, toughness, wear |
| Gun-drill bushings (GDI / GDE) | ~62 HRC (Rockwell C62) | High, quick-change | Extended-length gun drilling; sleeve-mounted IDs swap without disturbing the fixture |
| Plastic / form bushings (whip guides) | Non-marring | — | Supporting long drill bodies between bushing and workpiece |
For gun drilling, purpose-made bushings such as the GDI (sleeve-mounted) and GDE (headed “Eldorado” style) types slide in and out of a fixture sleeve, so a worn ID is replaced in seconds without disturbing the fixture — and different drill sizes share the same fixture hole.
Two different clearances are in play and both matter: the guide clearance between the bushing ID and the drill OD (governs guidance and tool life), and the chip clearance at the entry (governs chip evacuation). Confusing them is the most common setup error.
| Rule | Value | Note |
|---|---|---|
| Recommended fit | H6/g6 or G6/g6 | A G6 bushing bore consistently gives the best balance of tool guidance and tool life |
| Gun-drill production clearance | +0.003 to +0.008 mm (+0.0001 to +0.0003 in) | If clearance leaves this band, replace the guide bush |
| Jig bushing ID tolerance | +0.0000/+0.0002 in; metric 3–27 mm +0.000/+0.005 mm | Over nominal drill size |
| Drill tolerances (Sandvik) | DC h5, shank g6 | Hole IT8, surface Ra 0.1–3.0 µm |
| Clearance too tight | — | Over-constrains the drill; catastrophic breakage on engagement |
| Clearance too loose | — | Drill “walks” over the entrance face; bellmouth and oversize holes |
For normal drilling (L/D up to 20) use a bushing 1.5–2.5× the drill diameter long. For deep drilling (L/D above 20) extend it to 2–3× the drill diameter to add anti-deflection support.
| Drill diameter (mm) | Body diameter (mm) | Overall length (mm) |
|---|---|---|
| 0.8–1.099 | 3 | 9 |
| 2.7–3.399 | 6 | 12 |
| 6.1–8.099 | 12 | 16 |
| 12.1–15.099 | 22 | 28 |
| 22.1–26.099 | 35 | 36 |
| 35.1–42.099 | 55 | 56 |
Order the bushing by the drill diameter (DC). The table scales proportionally for larger gun-drill and BTA sizes.
Proper installation directly affects bushing life and hole quality. Rushing this step produces every downstream defect the bushing is meant to prevent.
Clean and deburr the housing bore; it must be within 0.005 mm of the specified diameter.
Guide bushings are designed for interference fits of 0.008–0.020 mm (0.0003–0.0008 in), depending on diameter and housing material.
Arbor press only — never a hammer. Impact loading fractures carbide bushings and distorts steel. A thin film of lithium grease on the OD prevents galling and eases removal.
After pressing, check that the bushing ID is round and in tolerance with a bore gauge — excessive interference can collapse the ID below the lower limit.
Mount a test indicator on the spindle and indicate the bushing ID at the entry face and at 1× bushing length deep. Total indicated runout (TIR) must be within 0.02 mm (0.0008 in).
Register the bushing plate with dowels for repeatable location within 0.01 mm. Clamp the workpiece against the bushing face — the bushing must not be separated from the entry surface — and engage at cutting feed, not rapid.
| Method | Bushing role | Sealing | Entry guidance |
|---|---|---|---|
| Gundrill (single lip, external chip removal) | Bushing at entry on dedicated machines; pilot hole on machining centers and lathes | Coolant delivered through the tool; bushing seals the entry where the face is uneven | Pilot hole Ø tolerance G8 with tool Ø m7; use a pilot hole for depths ≥ 16×D |
| BTA / STS (single tube, internal chip removal) | Bushing mounted in the pressure head, pressed against the workpiece | Bushing seals against the workpiece; the pressure head is the high-pressure seal between workpiece and drill bush | Bushing ensures the tool starts at the correct location and correct hole diameter |
| Ejector / DTS (twin tube, Venturi) | Entry bushing in the fixture; outer tube must extend > 5 mm into the bushing, gap < 1 mm | No pressure head and no face seal — coolant returns through the inner tube | Bushing supports the drill at entry on conventional machines |
On dedicated gun-drilling machines a guide bushing is always used. When gun drills run on machining centers or lathes, a pilot (start) hole is usually cheaper and simpler than mounting a bushing — installing bushings is time-consuming, and bushing ID tolerances are hard to control on small parts.
Beyond the entry bushing, long drills need support along their length or they whip, vibrate and cut oversize. For holes exceeding 40× diameter in gundrilling, or 800–1,000 mm in BTA drilling, intermediate supports are mandatory.
| Support | Spacing | Pads | Alignment |
|---|---|---|---|
| Whip guide (form bushing) | Every 40×D for gundrilling | Plastic / form, non-marring | Aligned to spindle within 0.03 mm TIR |
| Steady rest | Every 800–1,000 mm for BTA | Adjustable nylon or bronze | Aligned to spindle within 0.03 mm TIR |
Inspect guide bushings regularly: measure the ID with a bore gauge and compare to nominal. Replace the bushing when the ID has worn more than 0.02 mm from nominal size, or when witness marks, scoring or bellmouthing are visible. A worn bushing consistently produces oversize holes and accelerates drill guide-pad wear. Keep replacement bushings in stock for every active drill size to minimise downtime.
| Symptom | Likely cause | Fix |
|---|---|---|
| Oversize / poor concentricity | Bushing-drill clearance out of the +0.003 to +0.008 mm band | Replace the guide bush |
| Bellmouth / enlarged entry | Guide bush separated from the workpiece entry surface | Clamp the part against the bushing; eliminate the gap |
| Wandering / walking start | Bushing ID too loose; drill slips over the entrance face | Tighten clearance, replace the bush |
| Drill bending in the hole | Excessive bushing-to-spindle concentricity error; excessive feed; wrong guide pads | Realign within 0.02 mm TIR, reduce feed, check pad count (2 pads vs 3) |
| Drill breakage at entry | Wrong bushing shape, rapid feed, loose workpiece clamp | Correct bushing, cutting feed, tighten the clamp |
| Short tool life / whipping | Improper clearance, whipping, concentricity error | Replace bush, add a whip guide, realign |
| Guide-pad wear | Radial load carried by drill pads instead of the bushing | Longer bushing, correct clearance, better alignment |