Select tolerances around function, manufacturing capability and inspection feasibility. Apply demanding limits where they matter and avoid unnecessary cost on non-critical features.
Example: 20 ± 0.05 mm. Useful for dimensions that allow equal deviation above and below the nominal value.
Examples: 20 +0.02/0 mm or 20 +0/−0.02 mm. Useful where clearance or interference requirements permit deviation in one direction.
Example: 19.98–20.02 mm. States the permitted minimum and maximum directly for precision fits and other critical features.
Specify the general tolerance standard and grade for dimensions without individual tolerance callouts. Without a defined rule, suppliers may interpret the same drawing differently. Geometric tolerances require their own applicable specification.
In the title block or technical notes:“General linear and angular tolerances: ISO 2768-1, class m.”Alternatively, specify the applicable national standard and grade. Dimension critical features individually.
| Linear dimensions (mm) | Fine f: Reference | Medium m: Reference | Coarse c: Reference |
|---|---|---|---|
| 0.5 – 6 | ±0.05 | ±0.10 | ±0.20 |
| >6 – 30 | ±0.10 | ±0.20 | ±0.50 |
| >30 – 120 | ±0.15 | ±0.30 | ±0.80 |
| >120 – 400 | ±0.20 | ±0.50 | ±1.20 |
| >400 – 1000 | ±0.30 | ±0.80 | ±2.00 |
The IT grade defines tolerance width for a nominal size. A fit designation such as H7/g6 combines tolerance positions and grades for a hole and shaft. Correct selection supports interchangeability and consistent assembly.
| Nominal Diameter | IT6: Reference | IT7: Reference | IT8: Reference | Engineering Note |
|---|---|---|---|---|
| Ø20 mm | ≈ 0.013 mm | ≈ 0.021 mm | ≈ 0.033 mm | For a given grade, the tolerance width generally increases with nominal size. |
| Ø50 mm | Depends on the nominal-size range | Depends on the nominal-size range | Depends on the nominal-size range | Use the applicable ISO 286 tables or a verified tolerance calculator. |
Use geometric tolerances when function depends on form, orientation or relative position. Select the controls and datum references under the drawing's specified ISO or ASME standard; terminology and permitted controls differ.
Surface roughness affects friction, sealing, fatigue and coating adhesion. Select Ra or Rz requirements for the application and process. Roughness values alone do not define gloss, transparency or overall cosmetic quality.
| Roughness Ra (μm) | Indicative Surface Texture | Typical Process Route | Common Applications |
|---|---|---|---|
| Ra 3.2 | Visible machining texture | General CNC machining | Non-cosmetic structural surfaces |
| Ra 1.6 | Finer machining texture | Finishing passes and optimized toolpaths | General cosmetic or mating surfaces, subject to function |
| Ra 0.8 | Smooth surface texture | Precision finishing or polishing | Selected sealing or sliding surfaces, subject to application review |
| Ra 0.4 (and lower) | Fine surface texture; mirror appearance is not defined by Ra alone. | Precision finishing, grinding, lapping or polishing | Demanding contact or cosmetic surfaces, with additional finish criteria where needed |
Variation accumulates through an assembly's dimensional chain. Stack-up analysis helps explain why individually conforming parts may still fail to assemble or function as intended.
Evaluates the extreme combination of component tolerances for a defined dimensional chain.
Estimates combined variation statistically when process distributions and independence assumptions are appropriate.
Use this table for early design discussion, subject to part-specific confirmation. Material, stiffness, thermal effects, fixturing and inspection affect achievable tolerances. Review critical dimensions with an engineer before quoting.
| Manufacturing Process | ≤100 mm (reference) | 100–500 mm: Reference | ≥500 mm (reference) | Key influencing factors |
|---|---|---|---|---|
| CNC Machining | ±0.10 mm | ±0.20 mm | ±0.30 mm or wider, subject to review | Thin-wall distortion, fixtures, temperature, toolpaths and inspection |
| Injection Molding | ±0.10–0.30 mm | ±0.20–0.50 mm | Wider tolerances; requires review | Resin shrinkage, wall thickness, warpage, mold temperature and process parameters |
| Sheet Metal Fabrication | ±0.10–0.20 mm | ±0.15–0.30 mm | Requires review | Springback, bend radius, hole spacing, datums and flat-pattern strategy |
| Vacuum Casting | ±0.20–0.50 mm | ±0.30–0.80 mm | Tight tolerances require part-specific review. | Resin, silicone mold wear, batch consistency and finishing |
Tighter tolerances can require additional operations, fixtures, temperature control, tool-wear management and inspection, while increasing scrap risk. Focus demanding requirements on critical features to manage total cost.
Apply these checks during design review for CNC-machined, molded, fabricated and cast components.
For initial discussions, ±0.10 mm is a common reference for general structural dimensions. Confirm the applicable tolerance with our team for your part. Tighter tolerances depend on geometry, material, fixturing, temperature control and inspection requirements, and can increase cost and lead time. Specify tight tolerances only on critical features.
Specify the general linear and angular tolerance standard and grade in the drawing notes or title block, for example ISO 2768-1 class m or an applicable GB/T 1804 grade. Define geometric tolerances separately under the drawing's specified standard. Dimension critical mating, sealing and locating features individually.
Class m denotes medium general tolerances for the dimensions covered by ISO 2768-1. Permitted deviations depend on nominal size ranges. It applies when specified on the drawing and does not replace individual limits on critical features.
A lower IT grade number corresponds to a narrower tolerance band for a given nominal size. It generally requires more demanding machining and inspection. Reserve tighter grades for critical fits and functional features.
H7/g6 specifies an ISO 286 clearance fit between a hole and shaft. H7 identifies the hole tolerance position and grade; g6 identifies the shaft tolerance position and grade. Look up the actual limits for the nominal diameter in the applicable ISO 286 tables.
Use geometric tolerances when function depends on form, orientation or relative position, such as position, perpendicularity or runout. Apply the controls permitted by the drawing's specified standard and define the datum reference frame and inspection method.
Ra 1.6 μm is a lower average roughness than Ra 3.2 μm. Select the finish for each surface's functional and cosmetic needs. Lower Ra may require additional machining or polishing, and does not by itself define gloss or appearance.
Worst-case analysis evaluates the extremes of a dimensional stack. Root-sum-square (RSS) analysis estimates variation statistically under appropriate process and independence assumptions. Define the assembly datums, stack and risk requirements before selecting a method.
Tighter tolerances may require slower machining, specialized fixturing, temperature control, more frequent inspection and additional process steps. Specify the tightest requirements only where function demands them, with clear datums and inspection criteria.
Provide a 3D CAD file, material, quantity, finish, appearance requirements, critical dimensions and tolerances, datums, required surface roughness, target delivery date and a description of the part’s function or assembly. Include a dimensioned 2D drawing where needed.
