Machining tolerances and ISO 286 fitsA right-sized tolerance
costs less than a tight one.
ISO tolerancing standards are precise, but often misread. This guide calculates your fits, clears up common misconceptions and shows what our equipment holds in production.
ISO 286 fit calculator
Enter a nominal diameter and the hole and shaft tolerance classes. Deviations are those of ISO 286-2; the result also shows our equipment suited to the tightest tolerance.
Covers diameters from 0.5 to 180 mm and the common positions. For K, M, N, P holes and grades outside the table, read the value in ISO 286-2; we check it when reviewing your drawing.
Tolerancing: eight costly misconceptions
Each of these points comes up regularly in the drawings we receive. Correcting them before the request for quote avoids skewed quotes and parts that conform to the drawing but not to the design intent.
- “IT7 is always the same precision.”
- The IT grade widens with diameter. IT7 is 12 µm at Ø 5, 18 µm at Ø 15 and 25 µm at Ø 40 (ISO 286-1). A dimension is always read with its nominal diameter.
- “h7 means ±0.01.”
- The letter gives the position of the zone, the number its width. h7 is unilateral: from 0 to −18 µm at Ø 15. For a centered zone, use js7: ±9 µm.
- “A diameter tolerance guarantees the form.”
- By default, each requirement is independent (independency principle, ISO 8015). A Ø 10 ±0.01 can be out of round by more than 0.01. To link size and form: the envelope requirement Ⓔ, or an explicit form tolerance.
- “ISO 2768-mK covers everything else.”
- Part 2 of ISO 2768 (classes H, K, L for geometry) has been replaced by ISO 22081 (2021). And general tolerances only apply to what the drawing does not tolerance: they do not replace a functional dimension.
- “Ra 0.8 describes the surface.”
- Ra is an average: it hides peaks and isolated scratches. For a bearing surface or an implant, also specify Rz (or Rmax) and the evaluation length. The current framework is ISO 21920 (2021), which replaces ISO 4287.
- “The dimension will be measured, so it will be held.”
- Measurement uncertainty uses up tolerance (ISO 14253-1). A 10 µm tolerance measured with 2 µm uncertainty leaves only a 6 µm zone of proven conformity. Hence our metrology at 1.5 µm (CMM) and ±1 µm (Keyence).
- “Tighter means a better part.”
- Cost rises faster than precision. Going from IT9 to IT6 can double or quadruple cycle and inspection time, with no functional gain if the dimension is not functional.
- “The dimension holds at any temperature.”
- The reference temperature is 20 °C (68 °F, ISO 1). A 10 mm PEEK part grows by nearly 5 µm for a 10 °C difference: on a polymer, measurement is only meaningful after stabilization.
Machine capability: what our equipment holds in production
Width of grades IT5 to IT9 at Ø 10 mm, and the machines whose stated capability (twice the precision on a dimension) fits within the grade. Below that, the dimension requires a secondary operation: grinding, honing or dedicated finishing.
Indicative capabilities, confirmed part by part during industrialization (IQ/OQ/PQ) and capability studies.
- IT5 6 µm Fanuc Robocut, Chiron FZ 08K S Magnum
- IT6 9 µm Fanuc Robocut, Chiron FZ 08K S Magnum, DMG Mori NTX 1000
- IT7 15 µm All equipment, including DECO Swiss-type lathes
- IT8 22 µm All equipment, including DECO Swiss-type lathes
- IT9 36 µm All equipment, including DECO Swiss-type lathes
The cost of precision: tolerance right to pay less
- Identify the functional dimensions. A fit, a seating face or a seal deserves a tight grade. The rest can fall under general tolerances.
- Start from ground bar stock. Our bars stocked in h7, h8 or h9 avoid re-machining an outside diameter if it can stay as supplied.
- Prefer a form tolerance to a tight size tolerance. A 5 µm cylindricity on a Ø h8 costs less than a Ø h5, and better states what the function needs.
- Match surface finish to function. Ra 0.4 everywhere lengthens finishing; target contact and sliding surfaces.
- State the delivery condition. Dimensions before or after heat treatment, anodizing or passivation: the process plan is not the same.
Send your drawing: we point out the dimensions that cost without serving, before quoting. If needed, the Origin 3D engineering office reworks the dimensioning with you.
Send a drawing See the material data sheets