CNC Machining Tolerances: IT Grades, GD&T & Cost Guide

The ISO IT Grade System: What the Numbers Actually Mean

📊 Image Slot ② IT grade logarithmic tolerance curve IT1–IT16 — four colour zones: ultra-precision / precision / general / rough — tolerance values in μm

alt="ISO IT grade tolerance chart showing IT1 to IT16 tolerance values in micrometers for nominal diameter 18mm to 50mm range CNC machining precision guide"

Why does the same drawing look completely different to a machinist in Stuttgart and one in Ohio — and how does a standardized tolerance system prevent that? The International Tolerance grade system, defined under ISO 286-1, creates a universal language for dimensional accuracy that eliminates ambiguity across supply chains, countries, and manufacturing methods.

The IT system defines 20 tolerance grades from IT01 to IT18, each representing a specific allowable dimensional variation for a given nominal size. The grades are logarithmically spaced — each grade is approximately 1.6× wider than the previous — which means:

Moving from IT7 to IT6: tolerance tightens by ~37% — not just "one grade tighter"
Moving from IT9 to IT6: tolerance tightens by ~4× — a fundamental shift in process requirements
Moving from IT12 to IT6: tolerance tightens by >15× — from stamping specification to precision grinding

IT Grade Values (μm) by Nominal Size — The Reference Table

IT Grade	6–10mm	10–18mm	18–30mm	30–50mm	50–80mm	80–120mm
IT5	8μm	11μm	13μm	16μm	19μm	22μm
IT6	9μm	11μm	13μm	16μm	19μm	22μm
IT7	15μm	18μm	21μm	25μm	30μm	35μm
IT8	22μm	27μm	33μm	39μm	46μm	54μm
IT9	36μm	43μm	52μm	62μm	74μm	87μm
IT10	58μm	70μm	84μm	100μm	120μm	140μm
IT11	90μm	110μm	130μm	160μm	190μm	220μm
IT12	150μm	180μm	210μm	250μm	300μm	350μm

Table 1 — ISO IT grade tolerance values in micrometers by nominal size range (ISO 286-1)

IT Grades to Machining Processes: The Direct Mapping

IT Grade	Tolerance (25mm)	Achievable by	Typical Application
IT5	±6.5μm	Precision grinding, fine honing	Bearing bores, precision gauges
IT6	±6.5–8μm	Precision CNC + grinding	Precision fits, hydraulic bores
IT7 ⭐	±10.5μm	High-quality CNC turning/milling	H7/g6 bearing seats, standard precision fits
IT8	±16.5μm	Good CNC milling/turning	Close fits, general mechanical assemblies
IT9	±26μm	Standard CNC machining	General CNC work, standard fits
IT10	±42μm	CNC machining (standard)	Loose clearance fits, non-critical features
IT11	±65μm	CNC milling (rough)	Rough fits, bolt clearance holes
IT12	±105μm	CNC roughing, casting post-machine	Very loose fits, non-functional surfaces

Table 2 — IT grade to machining process mapping with applications

💡 Key Insight

IT7 is the precision crossover point. It represents the tightest tolerance reliably achievable on a well-maintained CNC machine without dedicated grinding or honing. It is also the tolerance class at which inspection cost begins to increase significantly — every IT7 feature typically requires additional verification beyond standard in-process checks.

CNC Tolerance vs Cost: The Curve Every Design Engineer Should Know

📈 Image Slot ③ Cost vs IT grade exponential curve — X-axis IT12→IT6, Y-axis 1.0×→22×, IT7 marked as cost-performance optimum point

alt="CNC machining cost versus tolerance grade chart showing exponential cost increase from IT12 to IT6 precision with IT7 marked as optimal cost performance point"

Has anyone on your team ever asked why the bolt clearance hole tolerance is tighter than the mating bearing bore? In practice, that drawing error occurs in approximately 30% of first-issue production drawings — and it represents real money spent on unnecessary precision.

The relationship between tolerance tightness and production cost is not linear — it is exponential at the tight end of the scale. The following cost index is based on real production data for CNC turned steel components at 100–500 piece volumes:

IT Grade	Machining Cost	Inspection Cost	Total Cost	What Drives It
IT12	1.0×	1.0×	1.0×	Standard roughing
IT11	1.1×	1.0×	1.1×	Standard CNC program
IT10	1.2×	1.1×	1.3×	Finish pass required
IT9	1.4×	1.2×	1.7×	Controlled finish + basic SPC
IT8	1.7×	1.4×	2.4×	Multiple passes + spot check gauging
IT7	2.2×	1.8×	4.0×	Process qualification + 100% gauging
IT6	3.2×	2.8×	9.0×	Grinding/honing + CMM FAI + SPC
IT5	5.0×	4.5×	22.5×	Precision grinding, temp-controlled measurement

Table 3 — Relative cost index by IT grade (100–500 pcs, CNC turned steel, 10–50mm diameter)

⚠ Cost Implication

Specifying IT7 instead of IT9 on a non-critical feature adds approximately 2.3× cost to that feature. A drawing with 12 features unnecessarily specified at IT7 instead of IT9 can increase total part cost by 40–60%. Every tolerance tighter than IT8 should have a documented functional justification.

IT Grades in Shaft/Hole Fit Systems: H7/g6, H7/p6, H7/k6

The IT grade system is the mathematical foundation of the ISO fits and limits system — the engineering framework for specifying how two mating parts interact. Understanding fits transforms tolerance specification from a guessing game into a precise engineering decision.

Fit Type	Definition	Typical IT Combination	Application
Clearance fit	Shaft always smaller than hole	H7/g6, H8/f7	Sliding fits, rotating shafts in plain bearings
Transition fit	May be clearance or interference	H7/k6, H7/n6	Locating fits, light press fits, keyed assemblies
Interference fit	Shaft always larger than hole	H7/p6, H7/s6	Force fits, press fits, permanent assemblies

Table 4 — ISO fit types and typical IT grade combinations

Is H7/g6 the right fit for your application — or have you defaulted to it because it was on the last drawing? H7/g6 provides free rotation with minimal play — appropriate for running bearings and sliding elements. For press-fit assemblies H7/p6 is typically correct. For locating pins that must be removable, H7/n6 or H7/k6. Each choice implies specific IT grades, process requirements, and cost.

GD&T and IT Grades: How Dimensional and Geometric Tolerances Work Together

📐 Image Slot ④ GD&T symbols chart — flatness ⏥ / circularity ○ / cylindricity ⌭ / perpendicularity ⊥ / true position ⊙ / concentricity ⊚ / total runout ↗ — with IT grade pairing guide

alt="GD&T geometric dimensioning and tolerancing symbols chart showing flatness cylindricity position concentricity symbols used with IT grade tolerances on CNC machined parts"

Most CNC machined parts require both dimensional tolerances (IT grade system) and geometric tolerances (GD&T). The interaction between these two systems is where many otherwise well-specified drawings fail to fully define functional requirements.

IT grades control size: shaft diameter, bore diameter, slot width, spacing between features
GD&T controls form, orientation, location, and runout: cylindricity, perpendicularity, true position, total runout

GD&T Symbols and Typical Values for CNC Machined Parts

GD&T Symbol	Controls	Typical CNC Value	Notes
⏥ Flatness	Surface flatness deviation	0.010–0.050mm	As-machined milled: ~0.020–0.050mm
○ Circularity	Roundness in one cross-section	0.005–0.020mm	Turned parts typically better than milled
⌭ Cylindricity	Roundness + straightness combined	0.008–0.030mm	Critical for bearing bores
⊥ Perpendicularity	Angular relationship to datum	0.010–0.050mm/100mm	Machined: typically 0.020–0.030mm
⊙ True Position	Feature location to datum	Ø0.050–Ø0.200mm	5-axis CNC achieves Ø0.050mm
⊚ Concentricity	Axis-to-axis location	0.005–0.020mm	Critical for rotating assemblies
↗ Total Runout	Combined location + form variation	0.010–0.050mm	Most critical for high-speed shafts

Table 5 — GD&T symbols, typical CNC machined values, and application notes

⚠ Design Rule

GD&T geometric tolerances should generally not exceed 30% of the corresponding dimensional (IT grade) tolerance on the same feature. Specifying IT9 dimensional tolerance (±26μm) with a flatness of 0.005mm (IT5 equivalent) is internally inconsistent — the geometric tolerance is tighter than the dimensional tolerance, forcing the supplier to hold the tighter of the two to ensure both are met.

Material-by-Material Tolerance Capability: What Each Material Can Actually Hold

🗂 Image Slot ⑤ Heat-map matrix: rows = materials (Al/SS/Ti/Brass/PEEK), columns = processes (milling/turning/grinding) — colour = achievable IT grade from green(IT6) to red(IT11+)

alt="CNC machining tolerance capability matrix showing achievable IT grades by material and process for aluminum stainless steel titanium brass and PEEK engineering plastic"

Does the engineering team always account for the fact that achieving IT6 in titanium costs roughly twice as much as IT6 in aluminum — and that some materials have a practical precision ceiling regardless of machine capability?

Material	CNC Milling	CNC Turning	With Grinding	Limiting Factor
Aluminum 6061-T6	IT7	IT6	IT5	Best machinability — benchmark
Aluminum 7075-T6	IT7	IT6	IT5	Similar to 6061; slightly higher strength
Stainless 303	IT8	IT7	IT6	Better than 316; sulfur improves machinability
Stainless 316L	IT8–IT9	IT7–IT8	IT6	Work hardening; thermal sensitivity
Mild steel 1018	IT7	IT6	IT5	Good machinability; thermal stability
Alloy steel 4140	IT7–IT8	IT7	IT6	Pre-hardening recommended before finishing
Titanium Ti-6Al-4V	IT8–IT9	IT8	IT6–IT7	Spring-back, tool deflection, thermal effects
Brass C360	IT7	IT6	IT5	Excellent machinability; low work hardening
POM (Delrin)	IT8–IT9	IT8	N/A	Hygroscopic; dimensions shift with humidity
PEEK	IT8	IT7–IT8	N/A	Good stability; higher stiffness than POM
Nylon PA66	IT10–IT11	IT9–IT10	N/A	Hygroscopic — dimensional instability
PTFE	IT11–IT12	IT10–IT11	N/A	Very soft; tool pressure distortion

Table 6 — Achievable IT grades by material and process in production CNC machining

🌡 Thermal Expansion Note

All tolerance specifications are measured at 20°C per ISO 1. For tight-tolerance aluminum parts (IT6–IT7), a 5°C ambient temperature variation causes dimensional change of approximately 0.001mm per 10mm of feature length — consuming 7–15% of an IT7 tolerance budget before any machining variation is considered. Temperature-controlled machining environments are required for stable IT6 production in aluminum.

Surface Treatment Impact on Dimensional Tolerance

Surface finishing processes applied after CNC machining consume tolerance budget. A part machined to IT7 that receives hard anodizing can exit the anodizing process at IT9 or worse on the coated surfaces.

Surface Treatment	Layer / Side	Effect on IT7 (25mm)	Post-Treatment IT	Action Required
Passivation	0μm	None	IT7 maintained	None
Black oxide	1–2μm	Negligible	IT7–IT8	Note for ultra-precision only
Type II Anodize	5–25μm	Moderate	IT9–IT10	Pre-compensate dimensions
Type III Hard Anodize	12–50μm	Significant	IT10–IT11	Pre-compensate, specify 8H/8g
Electroless Nickel 25μm	25μm	Significant	IT9–IT10	Pre-compensate; specify 7H/7g
Zinc Plate 12μm	12μm	Moderate	IT8–IT9	Pre-compensate precision features
Powder Coating	50–150μm	Destroys precision	IT12+	Mask ALL precision features

Table 7 — Surface treatment dimensional impact on IT grade for precision CNC parts

Over-Tolerancing: The Most Expensive Habit in Mechanical Engineering

What percentage of tolerance specifications in a typical production drawing are tighter than they need to be? Across professional design reviews, the consistent answer is 35–55% of all linear tolerance callouts are at least one IT grade tighter than the function requires.

DFM Tolerance Audit: 5 Questions for Every Tight Tolerance

01

Does this tolerance directly determine a functional outcome?

Clearance, fit, sealing, alignment, or interchangeability. If not — loosen it to IT10–IT12.

02

Identify the Required Fit Type

Clearance (H7/g6), transition (H7/k6), or interference (H7/p6). The fit type determines the IT grade for both hole and shaft.

03

Verify Material Capability

Check the material against the tolerance capability matrix. Ensure the required IT grade is stably achievable in production — not just demonstrated once.

04

Account for Surface Treatment

If the part will receive any coating, calculate the pre-coat machining dimension. Add drawing note specifying which dimensions apply before or after treatment.

05

Run Tolerance Stack-Up Analysis

Tolerances contributing less than 10% of the stack-up budget can typically be relaxed 1–2 IT grades without any functional consequence.

Most Common Over-Tolerancing Patterns and Corrections

Tolerance Pattern	Over-Specified	Correct Specification	Cost Savings
Clearance bolt holes	±0.05mm (IT8)	±0.15mm (IT10–IT11)	−35% on feature
Non-mating flat surfaces	IT8 on milled face	IT10–IT12 + Ra 1.6μm	−40% on feature
Non-critical part length	±0.05mm	±0.25mm (IT12)	−50% on feature
Thread-adjacent dimensions	IT7	IT9–IT10	−30% on feature

Table 8 — Common over-tolerancing patterns with corrected specifications and cost savings

Tolerance Inspection Methods: Matching Instrument to IT Grade

Does your supplier's inspection method match the tolerance class they're claiming to hold? Using a standard micrometer (accuracy ±0.004mm) to verify an IT6 tolerance of 0.013mm is insufficient — measurement uncertainty alone consumes 30% of the tolerance budget.

IT Grade	Tolerance (25mm)	Appropriate Instrument	Meas. Uncertainty
IT5–IT6	6.5–13μm	CMM, high-precision air gauge	±0.001–0.002mm
IT7	21μm	CMM, precision bore gauge, air gauge	±0.002–0.004mm
IT8	33μm	Bore gauge, precision micrometer	±0.004–0.008mm
IT9	52μm	Standard micrometer, calibrated caliper	±0.005–0.010mm
IT10–IT11	84–130μm	Digital caliper	±0.010–0.020mm
IT12+	≥210μm	Digital caliper, tape	±0.020mm+

Table 9 — Required inspection instruments by IT grade with measurement uncertainty

📏 Measurement Rule

Per ASME B89.7.3.1, measurement uncertainty should not exceed 25% of the tolerance band. For IT7 at 0.021mm total tolerance, maximum allowable measurement uncertainty is 0.005mm — requiring calibrated CMM or precision air gauge, not a standard workshop micrometer. For IT6 and tighter, temperature-controlled measurement environment (20°C ±1°C) is required.

Great Light's Tolerance Capabilities and Quality Assurance

Great Light's CNC machining service is built around verified tolerance delivery. Every order specifies the required IT grade, inspection method, and acceptance criteria before the first chip is cut. All measurement equipment is calibrated to BIPM-traceable standards through accredited calibration bodies.

Feature Type	Standard	Precision	Advanced
Turned diameter	IT8–IT9	IT7	IT6 (with grinding)
Milled profile	IT9–IT10	IT8	IT7
Bored / reamed hole	IT8	IT7	IT6
Thread pitch diameter	6H / 6g	4H / 5h	Per customer spec
Surface flatness	0.05mm/100mm	0.025mm/100mm	0.010mm/100mm
True position	Ø0.1mm	Ø0.05mm	Ø0.025mm (5-axis)

Table 10 — Great Light certified tolerance capabilities by feature type

Documentation provided with every applicable order: First Article Inspection (FAI) CMM report · SPC charts for IT7 and tighter on repeat orders (Cpk ≥ 1.33) · Calibration certificates for all measurement equipment · ISO 9001:2015 Certificate of Conformance · Pre-coat and post-coat dimensional verification reports.

Frequently Asked Questions

What IT grade does standard CNC machining achieve?+

IT9–IT10 is the standard for general CNC milled features. IT7–IT8 is achievable with controlled finish passes on well-maintained machines — the production standard for precision mechanical components. IT6 requires additional operations (precision boring, reaming, or grinding) and is not achievable from standard CNC milling alone. IT5 requires dedicated precision grinding in a temperature-controlled environment.

What is the difference between IT grade and dimensional tolerance?+

IT grades are a standardized classification per ISO 286-1 that map to specific tolerance values by nominal size. For a 25mm feature: IT7 = 0.021mm total tolerance, IT9 = 0.052mm, IT12 = 0.210mm. Specifying IT7 is functionally identical to specifying ±0.0105mm — IT grades are a standardized shorthand ensuring consistency across suppliers and countries.

What is H7/g6 and when should I use it?+

H7/g6 is the most common clearance fit — a shaft in a bearing bore or bushing that must rotate or slide freely with minimal play. For a 25mm shaft: hole = 25.000–25.021mm (H7), shaft = 24.993–25.006mm (g6), resulting clearance = 0.006–0.034mm. Use H7/g6 for plain bearing shafts and sliding fits. Use H7/p6 for interference/press fits. Use H7/k6 for locating fits.

Why does an IT6 part cost so much more than IT9?+

The cost difference is approximately 5–9× on affected features. This premium exists because: additional machining operations (fine boring, reaming, grinding) are required; inspection escalates to 100% CMM verification; rejection rates increase and must be priced in; temperature-controlled machining environments add overhead. The cost is not just "tighter machining" — it is a fundamentally different production process.

Can I specify IT6 on aluminum CNC parts?+

Yes. IT6 is achievable in aluminum with the right process combination. For turned features, IT6 is achievable through controlled finishing passes on a precision CNC lathe. Milled features typically require additional precision boring or reaming. All IT6 work in aluminum requires temperature-controlled machining — aluminum expands at 23.6μm/m/°C, and a 3°C shop temperature variation causes 7μm expansion on a 100mm feature, consuming 50–100% of an IT6 tolerance budget.

How do GD&T tolerances relate to IT grades?+

IT grades control feature size. GD&T controls shape, orientation, and position. Both are needed to fully define a precision part. As a practical guide: GD&T geometric tolerances should be set at 25–50% of the corresponding IT grade tolerance. If a bore is specified at IT7 (0.021mm total), its cylindricity should be no tighter than 0.005–0.010mm.

What is the tightest tolerance achievable in titanium Ti-6Al-4V?+

In production CNC machining, IT7–IT8 is the practical limit for most titanium features through turning and milling. IT6 in titanium requires grinding and careful thermal management — with a cost premium typically 2.5–4× higher than IT6 in aluminum. Key challenges: high spring-back under cutting forces, very low thermal conductivity (6.7 W/m·K vs 167 for aluminum), and work hardening tendency at elevated cutting temperatures.

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📋 Image Slot ⑥ Precision CNC parts + CMM inspection report side-by-side — IT6/IT7 tolerance verification — or 60s CMM inspection process video

alt="Precision CNC machined parts with CMM inspection report showing IT6 IT7 tolerance verification at ISO 9001 certified manufacturing facility"

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