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How to Select Tolerance Grades in Mechanical Engineering
How to Select Tolerance Grades in Mechanical Engineering
Tolerance grade selection is critical for balancing performance, assembly reliability, and manufacturing cost. Higher precision is not always better—over-specifying tolerances drastically increases cost without proportional performance gains.
1. Basic Concepts of Tolerance Grades
Per ISO 286 and Chinese standard GB/T 1800, there are 20 standard tolerance grades from IT01 to IT18:
A smaller number means higher precision, smaller tolerance range, higher difficulty and cost.
A larger number means lower precision, bigger tolerance range, easier and cheaper machining.
Grade range Typical use
IT01–IT2 Ultra-precision: gauge blocks, reference masters
IT3–IT5 High-precision: instruments, aerospace key parts
IT6–IT9 General machinery: fits, spindles, gears
IT10–IT12 Household appliances, common structural parts
IT13–IT18 Non-critical dimensions: castings, weldments
2. Why Higher Precision Is Not Always Better
Cost surge: Raising a gear from IT7 to IT6 may increase machining cost by ~40% for only ~5% longer life.
Assembly issues: Excessively tight fits in bearing–shaft assemblies can cause mounting difficulty or damage.
Wasted effort: Non-mating surfaces rarely need IT6–IT8; loose grades work fine.
Rule: Choose the loosest grade that still satisfies functional and assembly requirements.
3. Scientific Selection Principles
Distinguish critical vs. non-critical dimensions
Critical fits (bearing, spindle, dynamic seal): higher grades (IT5–IT7).
Non-mating outlines: lower grades (IT10–IT18).
Match manufacturing capability
Grinding: IT01–IT5
Turning/milling: IT6–IT11
Casting/forging: IT14–IT16
Practical fit rules
For basic size ≤500 mm and grade ≥IT8: hole one grade looser than shaft (e.g., shaft IT6, hole IT7).
For grade 500 mm: same grade for hole and shaft.
4. Industry Application Examples
Aerospace: Key parts IT3–IT5 (e.g., turbine blades).
General machinery: Spindles and bearings IT6–IT7.
Home appliances: Shafts and housings IT9–IT12.
5. Matching Tolerance Grade and Surface Roughness
Higher precision requires finer surface finish (Ra, μm):
IT5: ~Ra 0.1 μm
IT6–IT7: Ra 0.2–0.8 μm
IT8–IT10: Ra 1.6–3.2 μm
IT11–IT13: Ra 6.3 μm
Mismatch causes poor fit, faster wear, or actual interference despite correct dimension tolerance.
6. Summary
Tolerance grade selection is a systematic decision:
Define functional needs.
Match machining process.
Coordinate with surface roughness.
Avoid over-specification to control cost.
The goal is optimal balance of function, quality, and cost.
Pub Time : 2026-04-01 09:26:49
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