ISO Tolerance Grades (IT Grades) Explained for CNC Machining
The ISO tolerance grade system (IT grades) provides a standardized framework for specifying achievable dimensional accuracy on machined parts. UTEC Industrial provides precision CNC machining services for large and oversized industrial components in the Pacific Northwest, with in-house heat treatment and induction hardening integrated into the machining workflow. This article explains what IT grades are, how they translate to specific tolerance values, and which grades are achievable by which machining processes.
What is an IT grade and what does the number mean?
IT grades (International Tolerance grades) are defined in ISO 286-1 and represent standardized levels of dimensional accuracy for linear features — bores, shafts, and other dimensions where mating or fit requirements must be controlled. The numbering system runs from IT01 (tightest, sub-micron precision) to IT18 (loosest, for rough raw material). The IT number corresponds to a tolerance magnitude that increases with both the grade number and the nominal feature size — IT7 at a 50mm nominal diameter represents a tighter absolute tolerance than IT7 at a 500mm nominal diameter, but both represent the same relative precision for their respective size ranges. The grade spacing follows an approximately 1.6× ratio between consecutive grades: IT8 has approximately 1.6× the tolerance of IT7, and IT7 has approximately 1.6× the tolerance of IT6. In practical machining terms, each step change in IT grade represents a meaningful difference in the process capability and cycle time required to hold that tolerance (ISO 286-1:2010).
What are the common IT grades for CNC machined parts and their typical tolerance values?
The working range for CNC machining is IT5 through IT11. For a 50mm (approximately 2-inch) nominal dimension: IT5 = 0.011 mm (0.00043 inches) — precision boring, precision turning with temperature control; IT6 = 0.016 mm (0.00063 inches) — fine boring, precision CNC turning; IT7 = 0.025 mm (0.00098 inches) — standard precision boring and reaming, achievable routinely on a well-maintained CNC lathe; IT8 = 0.039 mm (0.00154 inches) — standard CNC turning and boring without special measures; IT9 = 0.062 mm (0.00244 inches) — general-purpose CNC turning with normal tolerancing; IT10 = 0.100 mm (0.00394 inches) — rough boring, turning to general tolerances; IT11 = 0.160 mm (0.00630 inches) — rough machining, first operations. For context: a human hair is approximately 0.070 mm (0.003 inches) in diameter — an IT7 tolerance at 50mm is less than half a hair's diameter. The practical implication: specifying IT5 on a feature that only needs IT8 costs money and schedule without providing functional benefit. Use the loosest IT grade that satisfies the fit and function requirement (ISO 286-1:2010; Machinery's Handbook, 31st ed., Industrial Press, 2020).
Which machining processes achieve which IT grades?
Process capability and IT grade have a well-established relationship. Bandsaw cutting achieves IT14–IT16 (±1–3mm) — a roughing operation, not a precision process. CNC turning (standard) achieves IT7–IT9 routinely — this is the range where a well-maintained CNC lathe operates without special measures. Fine CNC turning with reduced feed, sharp tooling, and temperature-stable setup achieves IT6–IT7. Precision boring achieves IT6–IT7 as standard, and IT5–IT6 with fine tooling and temperature control. Reaming achieves IT6–IT7 for bore diameter on standard reamers. Grinding achieves IT5–IT6 as standard; precision cylindrical grinding reaches IT4–IT5. Honing achieves IT4–IT6 for bore finish and size. Lapping achieves IT3–IT5 for the highest precision applications. CNC plasma cutting achieves IT12–IT14 — a cutting process, not a finishing process. The key practical implication for procurement: a drawing calling for IT6 on a bore feature means the shop must plan for a fine boring or grinding operation, not a standard turning pass. A drawing calling for IT9 is achievable in standard CNC turning without special measures. UTEC's CNC lathes and boring mill handle IT7–IT8 as production standard, with IT6 achievable on precision work (ISO 286-1:2010; ASM Handbook, Vol. 16, ASM International, 1989).
How do IT grades relate to the ANSI B4.1 fit classes used in US practice?
US engineering drawings frequently use ANSI B4.1 fit designations — RC (running clearance), LC (locational clearance), LT (locational transition), LN (locational interference), FN (force fit) — rather than ISO IT grades. The fit classes in ANSI B4.1 correspond approximately to IT grades as follows: RC1/RC2 (precision running fits) correspond to IT5–IT6; RC3/RC4 (standard running fits) correspond to IT7; LC1/LC2 (close locational fits) correspond to IT6–IT7; LN2/LN3 (medium locational interference) correspond to IT7; FN2/FN3 (medium force fits, used for press-fit and thermally-installed axle fits) correspond to IT6–IT7. The conversion is approximate — ANSI B4.1 and ISO 286 use slightly different fundamental deviation systems — but for specifying machined parts, either system produces comparable dimensional requirements when the same fit class intent is specified. UTEC's machinists work in inch dimensions and use ANSI B4.1 fit classes for bore-to-axle specifications on crane wheels; the shop is equally capable of working from ISO 286 tolerance codes on metric drawings (ANSI B4.1-1967, R2019).
How do feature size and IT grade interact to produce actual tolerance values?
IT tolerance values scale with nominal feature size — the same IT grade represents a larger absolute tolerance on larger features. For IT7: at a 10mm nominal (0.39-inch shaft), IT7 = 0.015 mm (0.0006 inches). At 50mm nominal (2-inch shaft), IT7 = 0.025 mm (0.001 inches). At 250mm nominal (10-inch shaft), IT7 = 0.046 mm (0.0018 inches). At 500mm nominal (20-inch shaft), IT7 = 0.063 mm (0.0025 inches). The scaling reflects the manufacturing reality that larger features have inherently larger absolute variations — thermal growth is proportional to size, machine deflection is proportional to the load moment arm, and measuring instrument uncertainty scales with range. For crane wheels, this means that an IT7 bore specification on a 4-inch bore (±0.0010 inches) represents a tighter manufacturing challenge than an IT7 bore on a 10-inch bore (±0.0018 inches) — both are labeled IT7, but the absolute tolerance is 44% tighter on the smaller bore. When specifying tight-tolerance features on large parts, use IT grade notation rather than absolute tolerances to ensure the specified accuracy is appropriate for the feature size (ISO 286-1:2010).
What IT grade should be specified for different functional applications?
The grade should match the functional requirement, not the desire for precision. Bearing fits (shaft or bore contacting a rolling element bearing): the bearing manufacturer specifies the required fit and tolerance — typically k5 or m5 for shaft interference fits, and H7 for bore clearance fits, per bearing installation guidelines. Follow the bearing manufacturer's specification directly; do not substitute IT grades without verifying against the bearing catalog. Gear mesh fits: IT6–IT7 for standard gear bores. Press-fit assemblies: IT7–IT8 on the mating parts, using ANSI B4.1 LN or FN fit classes to define the actual interference. Running clearance fits: IT7–IT9 depending on speed and load. General machined dimensions (not a mating fit): IT9–IT11 is typically adequate — these tolerances are achievable in standard CNC machining without special measures and add no cost beyond the machining operations already required. Specifying IT7 on a feature that needs only dimensional consistency (not a precision fit) wastes the customer's money and the shop's setup time without providing any functional benefit (ANSI B4.1-1967, R2019; Machinery's Handbook, 31st ed., Industrial Press, 2020).
How does UTEC verify that bore and OD features meet specified IT grades?
Verification depends on feature size and tolerance grade. For bores in the IT7–IT9 range at diameters under 4 inches: plug gauges (go/no-go gauges matched to the tolerance limits) are the fastest and most reliable production tool. For bores in the IT6–IT7 range or requiring actual size documentation (not just go/no-go): bore gauges or inside micrometers read to 0.0001-inch resolution. For large bores (6–24+ inches): inside micrometers in the appropriate size range, with measurement at multiple cross-sections and two depths to check cylindricity and taper. For OD features: outside micrometers, bench micrometers, or CMM contact probing. All measurements are taken after the part has thermally stabilized to ambient temperature — a part that has just been machined may be 10–40°F above ambient, producing a reading that reflects the hot dimension rather than the room-temperature dimension the drawing specifies. UTEC documents actual measured dimensions on critical features (bores, tread OD, tread width) on the shipping inspection record, providing the customer with the as-shipped dimensional data to verify fit before installation.
- Machining Tolerances: What to Specify and What They Cost — practical tolerance guidance
- Clearance, Transition, and Interference Fits — applying IT grades to fit selection
- Dimensional Inspection Methods for CNC Parts — how tolerances are verified
References
- ISO 286-1:2010: Geometrical Product Specifications — ISO Code System for Tolerances on Linear Sizes. ISO.
- ANSI B4.1-1967 (R2019): Preferred Limits and Fits for Cylindrical Parts. ASME/ANSI.
- Machinery's Handbook, 31st ed. Industrial Press, 2020.
- ASM International. (1989). ASM Handbook, Volume 16: Machining. ASM International.
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