When specifying tolerances, the most common dilemma engineers face isn’t *how* to apply them, but rather *how tight* they should be.
“Is IT7 sufficient for this hole? Would IT6 be safer?” Questions like these stem from a lack of intuitive understanding regarding the precision levels and machining conditions associated with specific IT grades.
If tolerances are too tight, machining costs rise and prototyping success rates drop; if they are too loose, parts may fail to assemble correctly or malfunction. Finding the level of precision that is “just right” requires a clear understanding of what IT grades actually entail.
This article clarifies IT grades IT6 through IT9 using concrete figures, focusing on practical engineering implications rather than textbook theory.
I. Fundamentals of the IT Tolerance Grade System
IT stands for “International Tolerance.” The GB/T 1800 series of standards defines 20 tolerance grades, ranging from IT01 to IT18; the lower the number, the higher the precision.
Precision part machining typically utilizes grades IT5 through IT10. This article focuses on IT6 through IT9, as these four grades cover the practical requirements of the vast majority of precision parts.
Tolerance values in the IT system increase as the nominal size increases—for instance, the tolerance value for an IT7 grade differs between a 30mm hole and a 100mm hole. This highlights the key difference between the IT system and specifying a fixed value like ±0.01mm: IT tolerances represent a relative precision standard (proportional to size), whereas fixed values represent an absolute precision standard.
Below are the tolerance values for grades IT5 through IT10 across several common size ranges (unit: μm = 0.001mm):
Example: For a φ25mm hole, the tolerance value corresponding to IT7 is 21μm (0.021mm). When specified as H7, the tolerance zone is +0.021/0 (lower deviation 0, upper deviation +0.021mm). II. IT6 — High-precision grade for precision fits
Tolerance range (for a φ25mm hole): 13 μm (0.013mm)
Tolerance zone (H6): +0.013/0
Manufacturing method:
IT6 represents the high end of precision CNC milling/turning or the standard level for precision grinding.
To consistently achieve IT6, the following are required:
Precision 5-axis machines or high-precision CNC lathes (repeatability ≤ ±0.003mm)
Temperature-controlled workshop (23 ± 1°C)
100% inspection using a Coordinate Measuring Machine (CMM)
Manufacturing cost: Approximately 40%–80% higher than IT7 (due to longer processing times, stricter process control, and higher scrap rates)
Typical applications:
Scenarios requiring IT6:
· Mating holes for rolling bearings (bearing outer ring to housing bore: IT6; bearing inner ring to shaft: IT5–IT6)
· Mating valve bores and spools in precision hydraulic components
· Critical mating surfaces in precision instruments (e.g., precision transmission gear journals)
· Locating holes in precision fixtures
Scenarios not requiring IT6 (using IT6 would be wasteful):
· Clearance fits in general mechanical assemblies
· Bolt clearance holes
· Dimensions of cosmetic/aesthetic parts
Key criteria: Is there precision sliding or rolling motion, and are there high requirements for motion accuracy or sealing? → IT6 is appropriate.
General static connections or coarse-precision transmissions → IT7 or IT8 are sufficient.
III. IT7 — Mainstream grade for precision machining
Tolerance range (for a φ25mm hole): 21 μm (0.021mm)
Tolerance zone (H7): +0.021/0
Manufacturing method:
IT7 represents the standard precision level for precision CNC machining centers. It can be consistently achieved by 5-axis machining centers in good condition and by high-precision 3-axis machining centers. Grinding is not required; precision milling/turning combined with process control is sufficient. Machining cost: Approximately 20%–40% higher than IT8
Applicable scenarios (the H7/h6 fit is the most common in engineering):
· General precision shaft-hole fits (a mainstream choice for transition and clearance fits)
· Motor shaft and coupling fit: shaft φ20h6, hole φ20H7
· Precision lead screws and nuts: bearing bore IT7
· Hydraulic cylinder piston fits
· Mating surfaces of most precision instrument parts
· Mounting holes for speed reducers in robot joints (strict positional accuracy requirements, but IT7 hole diameter tolerance is usually sufficient)
· Mounting holes for FPV UAV motor assembly components (strict positional accuracy requirements, but IT7 hole diameter tolerance is usually sufficient)
Most common practical application of IT7:
Hole specified as H7 (+T/0) and shaft as h6 (0/-T), resulting in a clearance fit ranging from 0 to +0.034 mm (for φ25 mm); suitable for most applications requiring assembly/disassembly while minimizing wobble.
IV. IT8 — The tolerance grade engineers should use most frequently
Tolerance range (φ25 mm hole): 33 μm (0.033 mm)
Tolerance zone (H8): +0.033/0
Machining method:
Standard operation on precision CNC machining centers; no special process controls or constant-temperature environments required (a 5°C temperature difference affects the dimension of a φ25 mm aluminum alloy part by 0.003 mm, which is <10% of the 0.033 mm tolerance—an acceptable level). Machining cost: 20%–40% lower than IT7; offers the best cost-performance ratio among precision grades.
Applicable scenarios (situations often assigned IT7 “just to be safe,” even though IT8 is perfectly adequate):
· Mating holes for general mechanical parts (standard assembly rather than high-precision transmission)
· Pin holes (locating pin fits; IT8 provides sufficient precision)
· Pilot holes for threading (diameter of the hole before tapping; IT8 is sufficient)
· General bearing holes (housing holes for rolling-element bearings in low-precision applications)
· Gearbox housing holes (non-precision transmission)
· Standard mating surfaces for transition fits
A question engineers should ask themselves: “If I change the tolerance of this surface from IT8 to IT7, what functional difference will it make?” If you cannot identify a specific functional benefit, IT8 is likely sufficient, and the extra cost is unnecessary.
V. IT9 — General tolerances for non-mating surfaces and free dimensions
Tolerance range (for a φ25mm hole): 52 μm (0.052mm)
Tolerance zone (H9): +0.052/0
Machining method:
Standard 3-axis machining centers and conventional CNC lathe operations; no precision process control required.
Applicable scenarios:
· Non-precision mating holes (clearance holes for bolts, retaining ring grooves, and other non-precision features)
· General mounting holes (clearance holes for bolts; usually 0.5–1mm larger than the bolt; IT9 is perfectly adequate)
· Non-mating external dimensions (free dimensions such as total part length or overall width)
· Rough machining dimensions
Application of general tolerances:
If the drawing title block states “Untoleranced dimensions per GB/T 1804-m,” the “m” (medium) class roughly corresponds to IT14. Any dimension on the drawing without an individually specified tolerance is treated according to this general tolerance standard. When engineers want dimensions without specified tolerances to be treated according to the IT9 standard, they can note the following in the title block:
“Unspecified hole tolerances: H9; unspecified shaft tolerances: h9”
VI. Relationship between IT grades and machining processes
Cost factor explanation: IT8 serves as the baseline (1.0); IT7 is approximately 1.5–2.5 times more expensive; IT6 is 2.5–4 times more expensive; grades IT5 and finer require grinding, with costs 4–6 times higher or more.
VII. Relationship between fit designations and tolerance grades
The IT grade itself defines only the width of the tolerance zone, not its position (i.e., whether it is shifted upwards or downwards).
Fit designations (e.g., H7, h6) define both:
Tolerance grade (e.g., IT7)
Tolerance zone position (H = basic hole, lower deviation = 0; h = basic shaft, upper deviation = 0)
Common fit combinations:
Practical principles for tolerance annotation
Apply strict tolerances only to functionally critical surfaces:
Mating surfaces, sealing surfaces, motion-guiding surfaces → IT6–IT7
Static connection mating surfaces → IT7–IT8
Mounting holes, non-mating surfaces → IT8–IT9
Unspecified dimensions → General tolerances (H9/IT9 or coarser)
Select tolerance grades for mating surfaces in pairs:
Typically, the hole’s precision grade is one level lower than the shaft’s (as holes are harder to machine); e.g., shaft IT6, hole IT7
Common configuration on a single drawing:
Critical mating hole: IT7 (H7)
General mounting hole: IT9 (H9) or general tolerance
Overall dimensions: General tolerance
Kzron specializes in small-batch precision part machining, supporting materials such as metals, plastics, ceramics, and rare metals. We serve industries including robotics, drones, semiconductors, medical technology, custom automation, research institutes, and universities; we are ISO9001:2015 certified. No minimum order quantity (MOQ), 5–10 day delivery, and precision within ±0.01mm. If you have questions regarding tolerance specifications, feel free to send your drawings for consultation; our process engineers will provide recommendations on the same day.
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