Machining Tolerance Tables (General Tolerances, Fits, IT Grades)

A "tolerance" is the allowable variation in a dimension relative to the drawing's nominal size (the difference between the maximum and minimum permitted sizes). On the shop floor, tolerance is the standard for machining and inspection. This article compiles general tolerances, fits, IT grades, and how to read them on a drawing.

Basic terms

• Nominal size: the base dimension on the drawing
• Upper / lower deviation: the allowance on the plus and minus sides
• Tolerance: upper − lower deviation = the allowed range
• Example: φ20 +0.02 / −0.01 → max 20.02, min 19.99, tolerance 0.03

General tolerances (JIS B 0405 — linear dimensions, excluding chamfers)

Applied collectively to dimensions with no individual tolerance.

Nominal size range (mm)	Fine f	Medium m	Coarse c
0.5–3	±0.05	±0.1	±0.2
3–6	±0.05	±0.1	±0.3
6–30	±0.1	±0.2	±0.5
30–120	±0.15	±0.3	±0.8
120–400	±0.2	±0.5	±1.2
400–1000	±0.3	±0.8	±2.0

Representative values. Confirm exact values against JIS/ISO standards.

Fits (hole and shaft combinations)

The size difference between hole and shaft decides "clearance / interference."

Type	State	Use
Clearance fit	Always has clearance	Smoothly moving shafts
Transition fit	Clearance or interference	Location, light press
Interference fit	Always has interference	Fixing, press fit

• Hole basis (H): use the hole as datum, adjust with the shaft (common)
• Shaft basis (h): use the shaft as datum, adjust with the hole
• Common examples: H7/g6 (clearance), H7/k6 (transition), H7/p6 (interference)

IT grades (tolerance grades)

From IT01 to IT18; the smaller the number, the higher the precision (smaller tolerance). Each machining method targets a range of grades (grinding reaches fine; turning/milling are mid-range).

Reading tolerances on a drawing

• ± form: 50±0.1 → 49.9–50.1
• Separate upper/lower: φ20 +0.02 / 0 → 20.00–20.02
• Fit symbol: φ20 H7 → hole tolerance zone class H7
• Dimensions without a tolerance → general tolerance applies

Common mistakes

• Forgetting general tolerance, thinking "unspecified = free"
• Confusing hole basis and shaft basis
• An IT grade that doesn't match the process (targeting a grinding-level tolerance on a lathe)

FAQ

Q. What about dimensions with no tolerance? A. The general tolerance specified (a JIS B 0405 grade) in the title block applies. Check the title block.

Q. What do H7 and g6 mean? A. The letter is the tolerance zone position; the number is the IT grade (tolerance size). Uppercase = hole, lowercase = shaft.

Q. Is tighter tolerance always better? A. No. Tighter tolerances raise cost. Specify only within what function requires.

Conclusion

Tolerance is the agreement on "how much variation is allowed." Use general tolerances for the whole, and specify individual tolerances or fits only where function demands it.

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