🎯 IT GRADE · STRAIGHTNESS · Ra · ROUNDNESS

Tolerances & Quality

The achievable accuracy of deep hole machining is set by the method, the kinematics, the depth, and the setup — not by hope. Understand the tolerance envelope (IT grade), straightness, surface finish, and form control before you cut, and you can hold the print at the first article instead of the tenth.

IT6–IT11Diameter gradeAchievable by method

0.015mmStraightnessCounter-rotated gundrill / 400mm

Ra 0.4–1.6μm as-drilledTypical gundrill finish

0.003–0.050mm roundnessMethod & L/D dependent

PRINCIPLE

The Tolerance Envelope

💡 30-second summary: Deep hole accuracy is three independent envelopes — size (IT grade on the diameter), form (straightness, roundness, cylindricity), and surface (Ra roughness). A hole can hold an excellent diameter and still be useless if the axis drifts 1 mm at depth. Tolerance degrades as L/D ratio rises, and this relationship must be designed around at the drawing stage — not discovered at inspection.

IT6–IT11

ISO 286 grade

Size: diameter tolerance band in μm. Lower number = tighter hole. Degrades with depth.

0.03–0.5

mm/m drift

Form: straightness is the premium quality of deep hole drilling — and the easiest to lose to bad kinematics.

Ra 0.2–6.3

μm

Surface: as-drilled to finished. Feed, tool geometry and burnishing decide whether you need a secondary op.

Three levers that decide what you can hold

⚙️

Method & kinematics

Gundrill, BTA or ejector sets the ceiling; counter-rotation pushes straightness to the best values in class. Kinematics alone account for more than 60% of hole quality variability in controlled studies.

🌡️

Setup & depth

Spindle-to-bushing alignment, guide bushing clearance, coolant temperature and whip-guide spacing each eat tolerance linearly with depth. A 0.01 mm misalignment becomes 0.1 mm of deviation over 500 mm of hole.

⚠️ Design rule: If the print says IT6 on a deep hole, you have already chosen the machine, the method, the tooling and the finishing operation. Deep hole tolerance is a process decision made at design time, not a measurement decision made at inspection.

ACHIEVABLE ACCURACY

IT Grades by Method

The practical accuracy envelope for each process. Grades and finish are the sustainable production figures, not lab best-cases.

Method	Tolerance Grade	Surface Roughness Ra	Straightness	Typical Roundness
Gundrilling (tool rotates)	IT6–IT11	0.4–6.3 μm	~0.001 in/in (0.03 mm/m)	0.005–0.020 mm
Gundrilling (workpiece rotates)	IT7–IT9	0.4–3.2 μm	0.001–0.003 in/ft (0.08–0.25 mm/m)	0.003–0.015 mm
Gundrilling (counter-rotation)	IT6–IT8	0.4–1.6 μm	Best (0.0005 in/ft or 0.04 mm/m)	0.003–0.010 mm
BTA Drilling	IT8–IT10	0.4–3.2 μm	0.05–0.5 mm/m	0.010–0.050 mm
Ejector Drilling	IT9–IT11	0.8–3.2 μm	0.08–0.5 mm/m	0.015–0.060 mm
Pull Boring	IT7–IT9	0.4–1.6 μm	~0.001 in/ft (0.08 mm/m)	0.003–0.015 mm
Skive & Roller Burnishing	IT7–IT8	< 0.2 μm (to 0.05)	—	0.005–0.020 mm
Honing	IT5–IT7	0.1–0.8 μm	Can improve existing hole	0.002–0.010 mm
Reaming	IT7–IT9	0.8–3.2 μm	Follows existing hole	0.005–0.025 mm

✅ Manufacturers confirm these numbers: Ingersoll and ISCAR both cite IT7–IT9 hole tolerances for gundrilling with Ra 0.4–1.6 μm; botek reports IT7 diametric tolerance possible and achieved IT8 with a solid-carbide gundrill in a direct test — versus IT9–IT10 for a solid-carbide twist drill. Conventional twist drilling is the baseline: only IT9–IT12, roughly a 0.1–0.15 mm drift on a 90 mm deep hole where a gundrill drifts under 0.1 mm.

💡 BTA tooling split: Brazed BTA heads — ground after brazing — hold ±0.01 mm or better and finish to Ra 0.8–1.6 μm; indexable BTA heads hold ±0.02–0.05 mm and Ra 1.6–3.2 μm. If the diameter callout is the tight part of the print, pick braze-ground tooling.

IT GRADES

What an IT Grade Actually Means

ISO 286 defines the International Tolerance grade: the tolerance band in microns grows with nominal diameter, and each step tighter is a smaller multiplier of the basic tolerance unit i. IT7 = 16 i, IT6 = 10 i — IT6 is roughly 60% tighter than IT7 at the same size.

Nominal Diameter (mm)	IT6 (μm)	IT7 (μm)	IT8 (μm)	IT9 (μm)	IT10 (μm)	IT11 (μm)
3–6	8	12	18	30	48	75
6–10	9	15	22	36	58	90
10–18	11	18	27	43	70	110
18–30	13	21	33	52	84	130
30–50	16	25	39	62	100	160
50–80	19	30	46	74	120	190
80–120	22	35	54	87	140	220
120–180	25	40	63	100	160	250
180–250	29	46	72	115	185	290

💡 Worked example: A 40 mm hole at IT8 carries a tolerance band of 39 μm — e.g. 40.000 to 40.039 mm. Note what happens as the hole grows: at 200 mm the same IT8 band widens to 72 μm. A tolerance that was comfortable at 40 mm becomes demanding at 200 mm for exactly the same grade. Size the tolerance to the largest diameter in the family, not the smallest.

DEPTH EFFECT

Tolerance vs. L/D Ratio

Achievable tolerance degrades as hole depth increases. Use this as the rough planning guide — and assume the optimistic column only when the setup is textbook-perfect.

L/D Range	Gundrilling (optimal setup)	Gundrilling (typical)	BTA Drilling
10:1–30:1	IT6–IT8	IT7–IT9	IT8–IT9
30:1–60:1	IT7–IT9	IT8–IT10	IT8–IT10
60:1–100:1	IT8–IT10	IT9–IT11	IT9–IT11
> 100:1	IT9–IT11	IT10–IT11	Not recommended

These ranges assume aligned guide bushings, adequate coolant pressure, and correct feeds and speeds. Workpiece-rotating and counter-rotation configurations push to the tighter end of every band.

⚠️ Why depth eats tolerance: Drift compounds. A 0.01 mm spindle-to-bushing misalignment produces 0.1 mm of deviation over 500 mm of depth, and drill drift grows further at intersections, interruptions, and as tool wear changes the force balance. At L/D > 10:1, specialized method is mandatory — a 0.1° angular error produces about 1.75 mm of deviation over 1000 mm depth. That is not a tolerance problem; it is a geometry guarantee you must buy with the process.

STRAIGHTNESS

Straightness: The Deep Hole Premium

Straightness is what separates deep hole drilling from every other holemaking process. Per ISO 1101, the straightness tolerance is the diameter of the smallest cylinder that can contain the actual axis; ISO 12780 defines the measurement procedures and evaluation methods (least-squares LSM or minimum-zone MZM). The customary callout is written as “straightness 0.05 mm per 1000 mm” or “0.001 in/ft”.

Kinematics decides the ceiling

Who rotates decides how straight the hole gets. The general-purpose gundrill is self-piloting on its guide pads, but the kinematics layer still dominates:

Configuration	Relative straightness	Why
Tool rotates, workpiece fixed	Higher drift	Drill tube whip and any entry error propagate directly into the bore axis
Workpiece rotates, tool fixed	Less drift	Rotation averages the workpiece mass; the tool stays on axis
Counter-rotation (both)	Least drift	Opposite rotation cancels radial deviation — the reference setup for >100:1 straightness

A comparative study on brass (CuZn40Pb2) measured the counter-rotating arrangement at 40–66 μm straightness deviation versus 62.5–77.5 μm for a rotating tool on a fixed part and 55–83 μm for a fixed tool on a rotating part — and found the kinematic configuration accounted for more than 60% of the total hole quality variability, more than spindle speed or feed. Counter-rotation cut straightness deviation by up to 36% in the same study.

Published benchmarks

Application	Straightness / concentricity spec	Source
Medical titanium (Ø18mm, 400mm deep, single pass)	0.015 mm TIR over 400 mm depth	Mollart Engineering
Oil & gas downhole tooling	< 0.001 in/ft deviation (~0.025 mm / 300 mm); ±0.001 in/in diameter	Optgd gundrilling
Inconel 718 downhole equipment (up to 5 m deep)	1 mm per meter straightness required	ScienceDirect research
Gundrill vs twist drill, 90 mm deep hole	< 0.1 mm centerline drift (twist drill > 0.15 mm)	botek test
Typical 1000 mm gundrilled depth	< 1 mm core misalignment	Botech / IPROS
Counter-rotated gundrill system	0.005 in runout over 40 in depth	TechniDrill Model 100CR

⚠️ Entry is everything: An off-square start face is the #1 cause of drift in long bores. If the face is not square to the spindle, bell-mouth the entry with a spot-facing cutter before deep drilling. The drill must also start in a correctly sized pilot hole — pilot hole diameter tolerance per ISO F7 — through a guide bushing with 0.003–0.008 mm clearance.

💡 Counter-rotation speed split: A practical starting point is one-third of total speed from the workpiece and two-thirds from the tool, then tune per application. Counter-rotation also allows higher relative cutting speed at the tool tip, which buys tolerance and productivity at the same time.

SURFACE FINISH

Ra & Rz: As-Drilled vs. Finished

Roughness is the most inspected and least understood of the three envelopes. Deep hole drilling can leave a usable bore as-drilled — or it can be the entry point for reaming, skiving & burnishing, or honing when the print calls for sealing surfaces or fatigue-critical bores.

Ra 0.4–6.3

μm

As-drilled across all deep hole methods; Ra 0.2 achievable on a clean gundrill in good conditions

Ra 0.4–0.8

μm

Skive & roller burnish or honing result; burnishing adds compressive residual stress

Ra < 0.2

μm

Honing / roller burnishing best case (0.05–0.1 μm on good set-ups)

Finish progression for a precision bore

Operation	Typical Ra range	Role	Limitation
Deep hole drilling	Ra 0.4–6.3 μm	Rough through-hole / usable bore	Wide chips can scratch the wall; feed marks remain
Reaming	Ra 0.8–3.2 μm	Finishing an existing pilot or bore	Follows the existing axis — cannot fix drift or position
Skive & burnish	Ra 0.05–0.8 μm	True the bore round, then cold-work the surface	Needs stock; adds a dedicated pass
Honing	Ra 0.1–0.8 μm (to 0.025)	Best surface integrity & form control; L/D > 10 no problem	Cannot fix axis position; not for highly plastic non-ferrous or keywayed bores

✅ Why roughness matters: Fatigue life of holes decreases as surface roughness increases — a smoother bore (skive, burnish, or hone) yields longer fatigue life than the same hole as-drilled or EDM-finished. On hydraulic cylinders and pressure bores, finish and sealing go together: rollers also induce a compressive residual-stress layer that measurably extends service life.

⚠️ Ra vs. Rz: Some prints call out Rz (ten-point / peak-to-valley height) for sealing surfaces because a single deep scratch is invisible to Ra. For deep bores the parameter matters less than the measurement method — a contact profilometer stylus often cannot reach deep, small-diameter bores, so specify how and where roughness is measured, not just what number it must beat.

FORM CONTROL

Roundness & Cylindricity

Cylindricity is the greatest single contributor to the overall geometric accuracy of a deep hole — it bundles diameter consistency, roundness at every cross-section, and axis straightness into one number. It is also the hardest to verify because it needs helical scanning along the full depth.

Characteristic	Typical Achievable Range	Measurement Method
Roundness	0.003–0.050 mm (method and L/D dependent)	CMM, roundness tester, air gaging
Cylindricity	0.010–0.100 mm (increases with depth)	CMM with helical scanning
Concentricity (to external datum)	0.02–0.10 mm (workpiece-rotating: 0.01–0.05 mm)	Dial indicator, CMM
Perpendicularity (to face)	0.01–0.05 mm per 100 mm diameter	Dial indicator, CMM
Surface roughness (as-drilled)	Ra 0.4–6.3 μm	Profilometer, optical profiler

What drives form error — feed and speed rule

Controlled drilling studies put feed per revolution and spindle speed ahead of everything else. In PA6 alloy machining, feed contributed ~54% of roundness error, ~49% of cylindricity, ~74% of straightness, and ~40% of diameter error; in C45 steel, feed drove 81% of roundness error and 37% of cylindricity. In aluminum alloy, spindle speed contributed ~34% of roundness and ~48% of cylindricity error. High-speed, adequate-feed regimes consistently produce tighter form than slow, timid cuts.

⚠️ Lobing: Three- and five-lobed holes come from drill-tube vibration. Whip guides are the cure — support the drill tube every 40 × D maximum. Insufficient support spacing is a classic source of roundness and straightness scatter on long gundrilled holes.

QUALITY DRIVERS

What Actually Drives Tolerance

Every one of these converts directly into microns on the print. Attack the biggest, cheapest wins first — alignment, bushing fit, and coolant — before blaming the tool.

Factor	Typical Impact	Mitigation
Spindle-to-bushing misalignment	0.01 mm misalignment produces 0.1 mm deviation over 500 mm depth	Align to within 0.01 mm (0.0004 in) using laser alignment
Guide bushing clearance	Excess clearance (>0.03 mm) causes bell-mouth entry and axis deviation	Maintain drill diameter +0.02 mm clearance (0.003–0.008 mm per manufacturers)
Material non-uniformity	Hardness variation of ±20 HB causes measurable force fluctuation	Pre-drill or spot-face; consider pre-annealing for critical parts
Coolant temperature	10°C rise in coolant = 0.12 mm expansion per meter of workpiece steel	Install chiller; maintain 20–35°C range
Spindle runout	0.005 mm TIR runout produces oversize hole by 0.008–0.015 mm	Maintain spindle runout < 0.003 mm
Machine guideway wear	Progressive deviation with depth; more pronounced on older machines	Annual laser calibration; use counter-rotation to cancel effects
Coolant filtration	>30 μm particles cause guide pad scoring and diameter variation	Maintain 15–20 μm filtration for gundrilling
Whip guide support spacing	Insufficient support causes drill tube vibration and hole lobing	Support every 40 × D maximum

💡 Feed and speed strategy: Too low a feed is a form-error generator — it rubs instead of cutting, and the wide chips that come off rub the hole wall. Published gundrill guidance recommends oil with a viscosity of 8–15 mm²/s at 40°C; if running emulsion instead, cut feeds to 60–80% of the oil capability. Higher coolant pressure improves straightness by raising the first critical RPM of the drill shaft.

THERMAL EFFECTS

Heat & Thermal Expansion

Deep hole machining dumps heat into the workpiece, and the hole shrinks as it cools. Post-cooling dimensional change is a real, quantifiable effect — not an inspection artifact.

Material	Thermal Expansion Coefficient	Hole Diameter Change (40 mm bore, 30°C rise)
Steel (carbon/alloy)	11.5 × 10^-6 /°C	~0.014 mm shrinkage on cooling
Stainless steel (austenitic)	17.3 × 10^-6 /°C	~0.021 mm shrinkage on cooling
Aluminum (6061)	23.6 × 10^-6 /°C	~0.028 mm shrinkage on cooling
Titanium (Ti-6Al-4V)	8.6 × 10^-6 /°C	~0.010 mm shrinkage on cooling
Cast iron	10.5 × 10^-6 /°C	~0.013 mm shrinkage on cooling

💡 Design tip: Specify tolerance requirements as post-machining, at 20°C during the design phase. Deep holes experience thermal expansion from cutting heat during machining, and dimensions may shrink by 0.005–0.025 mm after cooling depending on material and bore diameter. Measure the bore after the workpiece returns to room temperature for final acceptance — and control coolant temperature, because a 10°C coolant swing moves steel 0.12 mm per meter of length.

SPECIFYING

Putting Tolerance on the Drawing

The drawing is where deep hole accuracy is won or lost. Over-tolerancing is the most common and most expensive mistake on deep hole prints.

Calling out the hole

What to write	Example	Notes
Diameter with ISO 286 fit	Ø40 H7	H7 = IT7 hole, basic-size zero lower deviation. Tighter than IT7 on a deep hole forces grinding or honing.
Straightness	Straightness 0.05 mm / 1000 mm	Per ISO 1101; cylindrical tolerance zone. Also written 0.001 in/ft on inch prints.
Roundness / cylindricity	Roundness 0.01 mm; Cylindricity 0.02 mm	Cylindricity implies roundness + straightness + diameter consistency along depth.
Surface finish	Ra 1.6 max (as-drilled)	Name the process stage — as-drilled vs. finished can differ by an order of magnitude.
Temperature datum	Verified at 20°C post-machining	Removes thermal ambiguity on tight bores.

Match the grade to the function

IT5–IT7 — precision engineering: bearing bores, spindles, sealing and sliding fits. Requires gundrill counter-rotation, honing, or a finishing pass; expect grinding-class effort.
IT8–IT9 — general engineering fits: bushings, clearance fits, cylinder barrels. Standard gundrilling and BTA with fine boring hold this comfortably.
IT10–IT11 — coarse / non-critical: structural holes, clearance for fasteners. Ejector and BTA drilling reach this without special effort.

⚠️ The cost of over-tolerancing: The tolerance-to-cost curve is sharply non-linear. Moving from IT9 to IT6 typically forces a change in production technology — from a single finishing operation to rough machining, stress relief, precision finishing and grinding, each extending cycle time and unit cost. Relax every feature that does not genuinely need the tight grade; tightening surfaces you do not need is where deep hole quotes explode.

VERIFICATION

Measuring the Hole

Deep holes defeat most standard gauging — the depth is the problem, not the diameter. Match the measurement method to what the print actually controls.

Method	Application	Accuracy	Notes
Air gaging	Diameter, roundness	±0.5–2 μm	Fastest production method; requires master rings
Plug gages (Go/No-Go)	Diameter verification	Depends on class	Simple pass/fail; no size data
Three-point bore micrometer	Diameter	±2–5 μm	Good for field measurement; workpiece must be stationary
CMM (Coordinate Measuring Machine)	Diameter, roundness, cylindricity, position	±1–5 μm	Comprehensive but slow for deep holes; limited depth in standard CMMs
Laser straightness system	Straightness	±2–10 μm/m	On-machine or post-process; measures axis deviation directly
Precision mandrel + indicator	Straightness, concentricity	±5–15 μm	Traditional method; mandrel must fit bore closely
Contact profilometer	Surface roughness	±0.01 μm Ra	Standard for Ra measurement; stylus may not reach deep bores
On-machine optical/laser probe	Diameter, roundness, straightness	±7 μm (straightness)	Emerging technology; measures during or immediately after machining

💡 Measure at the right temperature and the right stage: Acceptance checks on diameter and form must run after the workpiece cools to 20°C. Straightness in particular should be evaluated with a laser system or close-fitting mandrel along the full depth — a mandrel that is loose at one end and tight at the other is telling you about drift, not size. As holes get deeper and smaller, form measurement gets harder; plan the verification method at the same time you plan the process.

⚠️ A tolerance you cannot measure: Every tolerance on a deep hole print must be traceable to a method that can actually reach it. If the CMM cannot probe past 300 mm and the print calls cylindricity at 2000 mm depth, the tolerance does not exist yet — add an air-gage or laser capability, or relax the callout.

COST & CAPABILITY

Cost vs. Tolerance

The single most valuable planning rule in deep hole work: specify the loosest tolerance that still meets function. IT7 is the economic sweet spot for most precision bores — achievable with standard gundrilling and fine boring at moderate cost. IT6 demands grinding, honing, or counter-rotation with finishing, and roughly doubles to triples the cost per hole.

Grade demanded	Typical cost multiplier	What it forces
IT8–IT9	Baseline +20–40% over rough drilling	Fine boring, controlled feed, clean coolant
IT7	+60–100% over baseline	Precision finishing: counter-rotation, braze-ground BTA, or ream
IT6 and tighter	+200% or more	Grinding-class control or honing; hardened materials may need grinding outright

💡 Process capability: For Cpk ≥ 1.33 (the industry standard for production), the process spread (6σ) must be less than 75% of the tolerance band. If the print calls for IT8, the process must actually hold ~75% of that band in production — the nominal grade is not enough; the spread is what earns the Cpk.

💡 Where each step up buys you: From IT9 to IT6, deep holes favour grinding because of its rigidity at depth — but grinding adds a machine, coolant, and cycle time. Reserve IT6 for bearing bores, sealing surfaces and hardened materials (>45 HRC) where roundness within ~1 μm and finish below Ra 0.2 are genuinely required; everywhere else, IT7 with a clean gundrill or a skive & burnish pass is the money-saving answer.

Choosing the method from the print

🎯

IT6–IT7 needed→ Counter-rotation gundrill, hone, or skive & burnish

🔧

IT8–IT9 needed→ Standard gundrill, BTA, or pull boring

⚡

IT9–IT11 fine→ BTA or ejector drilling

🏭

Ra < 0.2 needed→ Skive & roller burnish or honing pass

✅ Final rule of thumb: Choose the method by straightness and finish first, diameter grade second — it is almost always cheaper to add a finishing pass for a tight diameter than it is to fix a drifting axis. And verify every claim with a test cut: manufacturers’ grades are achieved under recommended conditions, and your machine, material and coolant are the real test bed.

Tolerances & Quality

The Tolerance Envelope

Three levers that decide what you can hold

IT Grades by Method

What an IT Grade Actually Means

Tolerance vs. L/D Ratio

Straightness: The Deep Hole Premium

Kinematics decides the ceiling

Published benchmarks

Ra & Rz: As-Drilled vs. Finished

Finish progression for a precision bore

Roundness & Cylindricity

What drives form error — feed and speed rule

What Actually Drives Tolerance

Heat & Thermal Expansion

Putting Tolerance on the Drawing

Calling out the hole

Match the grade to the function

Measuring the Hole

Cost vs. Tolerance

Choosing the method from the print

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