1. Which tolerance grade to use
| Tolerance grade | Typical applications | Matching process |
|---|---|---|
| IT01–IT2 | Gauge blocks, precision reference parts | Lapping, superfinishing |
| IT3–IT5 | Precision instruments, precision mating parts | Lapping, fine grinding, diamond boring |
| IT6–IT7 | Precision fits: bearing seats, gear bores, guide pillars and bushes | Grinding, fine turning, reaming, broaching |
| IT8–IT10 | General machined fits, ordinary connections | Turning, milling, drilling, boring |
| IT11–IT13 | Rough machining, stamping, non-mating cast/forged surfaces | Rough turning, stamping, die forging |
| IT14–IT18 | Free and non-mating structural dimensions | Gas-cut, cast or welded blanks |
How to choose: Choose the coarsest grade that still meets function The coarsest grade— each step up in accuracy can double manufacturing cost.
2. Tolerance calculator (IT fit tolerances · casting CT / moulding MT / stamping ST·FT process tolerances)
Enter "nominal size + fundamental deviation letter + grade", e.g. 21H7 (holes), 50f7 (shafts), 30p6 (shafts), 60K7 (holes). Applies to Φ0–500 mm; data per GB/T 1800.1.
Process tolerance converter (casting CT / moulding MT / stamping ST·FT)
Pick the standard → enter the basic size → choose the grade and the tolerance appears. Use this tool for blank and process dimensions; fits still use the IT calculator above. The five grade systems are unrelated and cannot be converted into one another (see section 6).
3. Standard tolerance values (excerpt, μm)
From GB/T 1800.1 (Limits and fits), common size ranges IT5–IT11:
| Basic size (mm) | IT5 | IT6 | IT7 | IT8 | IT9 | IT10 | IT11 |
|---|---|---|---|---|---|---|---|
| >3–6 | 5 | 8 | 12 | 18 | 30 | 48 | 75 |
| >6–10 | 6 | 9 | 15 | 22 | 36 | 58 | 90 |
| >10–18 | 8 | 11 | 18 | 27 | 43 | 70 | 110 |
| >18–30 | 9 | 13 | 21 | 33 | 52 | 84 | 130 |
| >30–50 | 11 | 16 | 25 | 39 | 62 | 100 | 160 |
| >50–80 | 13 | 19 | 30 | 46 | 74 | 120 | 190 |
| >80–120 | 15 | 22 | 35 | 54 | 87 | 140 | 220 |
| >120–180 | 18 | 25 | 40 | 63 | 100 | 160 | 250 |
| >180–250 | 20 | 29 | 46 | 72 | 115 | 185 | 290 |
Example of use:Φ50h7 → size range >30–50, IT7 = 25 μm → tolerance zone 0 / −0.025 mm; Φ50H7 → +0.025 / 0. Uppercase for holes, lowercase for shafts.
4. Hole-basis preferred fits (13 common pairs)
| Fit designation | Character | Typical applications |
|---|---|---|
| H11/c11 | Large clearance | Slow-running coarse fits such as hinges and flange joints |
| H9/d9 | Larger clearance | Plain bearings, pulleys on shafts, low-speed shafts |
| H8/f7 | Medium clearance | Plain bearings at moderate speed, slide guides |
| H7/g6 | Small clearance | Precision sliding: machine spindles, precision lead-screw guidance |
| H7/h6 | Location clearance | Removable location fits, sliding gear guidance |
| H7/k6 | Transition (towards clearance) | Accurate location, dismantlable: couplings on shafts, bearing outer rings |
| H7/n6 | Transition (towards interference) | Precision location, rarely dismantled: bronze worm-wheel rims |
| H7/p6 | Light interference | Location interference fit, press assembly |
| H7/s6 | Medium interference | Medium loads through interference: bearing inner ring on a shaft |
| H7/u6 | Heavy interference | High torque transmission, shrink-fit or hydraulic assembly |
Shaft-basis preferred fits (swap H↔h, e.g. F8/h7, K7/h6, S7/h6) suit setups based on a cold-drawn shaft, or one shaft mating with several holes in different fits.
5. General tolerances (GB/T 1804, linear dimensions)
| Tolerance grade | 0.5–3 | >3–6 | >6–30 | >30–120 | >120–400 | >400–1000 |
|---|---|---|---|---|---|---|
| f (fine) | ±0.05 | ±0.05 | ±0.1 | ±0.15 | ±0.2 | ±0.3 |
| m (medium) | ±0.1 | ±0.1 | ±0.2 | ±0.3 | ±0.5 | ±0.8 |
| c (coarse) | ±0.2 | ±0.3 | ±0.5 | ±0.8 | ±1.2 | ±2 |
| v (very coarse) | ±0.5 | ±0.5 | ±1 | ±1.5 | ±2.5 | ±4 |
- Units mm, applies to machined dimensions; castings and forgings follow their own standards (GB/T 6414 for casting dimensions).
- A note such as "GB/T 1804-m" near the title block means all untoleranced linear dimensions follow grade m.
- Untoleranced angles follow GB/T 11335; untoleranced geometric tolerances follow GB/T 1184 (they are neither marked on the drawing nor inspected separately).
6. Tolerance grades for castings, forgings, stampings and mouldings (each process has its own system)
Remember this first: Cast, forged, stamped and moulded parts Not marked with IT grades— each has its own national grade system (castings CT, stampings ST/FT, mouldings MT, die forgings F/E). Writing "IT12" on a casting drawing is wrong; wherever a fit is needed, leave machining allowance and let cutting establish the accuracy — blank tolerances only govern as-cast surfaces.
1) Castings: GB/T 6414, grades CT1–CT16 (smaller number = tighter).
| Casting method | Typical achievable grade | Note |
|---|---|---|
| Hand moulding, sand casting | CT11–CT13 | One-offs and small batches; volume production can gain 2 grades over single-piece work |
| Machine moulding, sand casting | CT8–CT10 | Batch and volume production of grey iron and cast steel |
| Lost foam / V-process | CT8–CT10 | Medium and large parts; lost-foam parts from this plant are accepted to this grade |
| Permanent mould casting | CT6–CT9 | Batch production of grey iron and non-ferrous parts |
| Low-pressure casting | CT6–CT9 | Aluminium wheels, housings |
| Investment (precision) casting | CT5–CT7 | Investment casting to near-net shape, complex small parts |
| Die casting | CT4–CT7 | Zinc, aluminium and magnesium alloys: small mating features can be cast directly |
CT tolerance excerpt (GB/T 6414, mm; the value is the total tolerance before symmetric distribution):
| Basic size (mm) | CT8 | CT9 | CT10 | CT11 | CT12 | CT13 |
|---|---|---|---|---|---|---|
| >16–25 | 1.2 | 1.7 | 2.4 | 3.2 | 4.6 | 6 |
| >25–40 | 1.3 | 1.8 | 2.6 | 3.6 | 5.0 | 7 |
| >40–63 | 1.4 | 2.0 | 2.8 | 4.0 | 5.6 | 8 |
| >63–100 | 1.6 | 2.2 | 3.2 | 4.4 | 6.0 | 9 |
| >100–160 | 1.8 | 2.5 | 3.6 | 5.0 | 7.0 | 10 |
- CT tolerance Draft angle excluded; for basic sizes ≤16 mm the CT13–CT16 values must be called out separately, and 2–3 grades can be gained.
- Callout example: untoleranced casting dimensions to GB/T 6414-CT12; mating surfaces (bores, spigots) carry their own machining allowance and finished tolerance.
- Inspection: judge as-cast surfaces to CT with calipers, templates or dedicated gauges; judge machined surfaces to the finished tolerance, and record the two separately.
2) Die forgings: GB/T 12362, normal grade F / precision grade E.
| Grade | Applies to | Key point |
|---|---|---|
| Normal grade F | Conventional die forging on hammers and presses | Look up the forging weight, parting-line class and material factor to determine length/width, thickness and mismatch tolerances |
| Precision grade E | Precision die forging, coining | Little allowance and tighter tolerances: forging temperature and mismatch must be tightly controlled, and tooling costs more |
- Forging tolerances include Mismatch (upper/lower die mismatch) and Residual burr; acceptance is judged item by item against the standard.
- Machining allowance on mating faces is typically 2–5 mm per side (depending on equipment and forging size); forging tolerances and casting CT values cannot be interchanged.
3) Stampings: GB/T 13914, flat stampings ST1–ST11 / formed stampings FT1–FT10.
| Process | Grade direction | Rule of thumb |
|---|---|---|
| Fine blanking | ST1–ST11, smaller is tighter | Fine blanking can reach ST1–ST3 |
| Ordinary flat blanking | Same as above | Usually ST6–ST9 |
| Bending / drawing / other forming | FT1–FT10, smaller is tighter | Drawn diameter is usually one grade tighter than height |
Flat stamping tolerance excerpt (GB/T 13914 Table 1, mm; basic size >3–10, material thickness >1–3):
| ST1 | ST2 | ST3 | ST4 | ST5 | ST6 | ST7 | ST8 | ST9 | ST10 | ST11 |
|---|---|---|---|---|---|---|---|---|---|---|
| 0.036 | 0.05 | 0.07 | 0.10 | 0.14 | 0.20 | 0.28 | 0.40 | 0.56 | 0.78 | 1.1 |
- Deviation direction rules (essential for acceptance):Hole (internal) Take the tolerance value with "+" and set the lower deviation to 0; Shaft (external) Take the tolerance value with "−" and set the upper deviation to 0; Hole spacing, edge distance, bend or draw height Use half the tolerance as ±.
- Callout example: GB/T 13914-ST6; for untoleranced formed features use GB/T 13914-FT6.
4) Injection mouldings: GB/T 14486, grades MT1–MT7 (smaller number = tighter).
Each grade has columns A and B: A is tighter (precision parts, fits), B is looser (free dimensions). Common values (column A, mm):
| Basic size (mm) | MT2 | MT3 | MT4 | MT5 |
|---|---|---|---|---|
| >3–6 | 0.12 | 0.14 | 0.18 | 0.24 |
| >10–14 | 0.16 | 0.18 | 0.24 | 0.32 |
| >18–24 | 0.20 | 0.22 | 0.32 | 0.44 |
| >30–40 | 0.24 | 0.30 | 0.42 | 0.56 |
| >80–100 | 0.38 | 0.52 | 0.72 | 1.00 |
| Typical materials | Specified tolerance grade | General tolerance (Columns A / B) |
|---|---|---|
| ABS, PC, PMMA, PS, PPO, PVC-U | MT2 | MT3 / MT5 |
| PA (nylon), POM, PET, PBT | MT3 | MT4 / MT6 |
| PP (standard) | MT4 | MT5 / MT7 |
| PE, soft PVC | MT5 | MT6 / MT7 |
- Moulding tolerances and Material shrinkage It is tied directly to the material: the same mould with a different material gives different dimensions. For precision parts prefer shrinkage-stable ABS/PC.
- PA moisture absorption and PP post-shrinkage both make dimensions drift —Acceptance must specify the measuring condition (usually measured 24 h after moulding); state it explicitly in the inspection procedure.
Quick reminder: Castings use CT, forgings F/E, blanking ST, forming FT, mouldings MT — five unrelated systems. A drawing callout must carry the standard number (e.g. GB/T 6414-CT12, GB/T 14486-MT3, GB/T 13914-ST6); a bare grade number without the standard is not a valid basis for acceptance.
7. Three-step table lookup
- Step 1: See which size range the basic size falls in (e.g. Φ50 → the >30–50 range).
- Step 2: Look up the standard IT value for the chosen grade → that gives the size of the tolerance zone.
- Step 3: Look up the fundamental deviation (H=0, h=0, other letters from the deviation table) to fix the zone position, then combine to get the upper/lower deviations.
Note: The table gives upper/lower deviations; accept against the inward-shifted acceptance limits (see chapter 9 of the Gauge Management Handbook, GB/T 3177).