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.