Machining Process Achievable Tolerance Grades

How close can turning, milling, drilling, grinding, honing, or lapping hold a dimension? The useful answer depends on both the operation and the whole manufacturing setup. This chart summarizes economic IT-grade ranges published by a precision machining supplier. Treat them as starting points for a design or quote, not as guaranteed limits for every shop, material, feature, or part size.

Machining process tolerance-grade chart

What an IT grade tells you

ISO 286 uses IT grades to describe the width of a dimensional tolerance band for a linear size. A smaller grade number means a tighter band. The grade alone does not set the upper and lower limits: the nominal dimension and the selected hole or shaft position also matter. A 50 mm feature and a 500 mm feature do not receive the same tolerance width just because both are called IT7.

The source chart describes economic production capability, rather than the most precise result a skilled operator could achieve once in a laboratory. Its ranges help compare process choices. They do not replace the tolerances on a drawing or a supplier’s process-capability evidence.

Choosing a process for the feature

Drilling is commonly used to make a starting hole. Reaming can improve a bore’s size and surface finish, while boring can adjust size and geometry. These operations do not automatically correct a misplaced hole. Position must be controlled through the setup and the relevant geometric tolerances on the drawing.

Turning and milling ranges overlap because the result depends on the operation, tool, machine, workholding, material, and feature geometry. A fine turning pass on a short, rigid shaft is a different case from a long, slender part. A milled face made in one setup is different from a face made after the part is re-clamped. Grinding, honing, and lapping are finishing operations used when the part design and production plan call for finer size or surface requirements.

Surface roughness and dimensional tolerance are related, but they are not interchangeable. A low Ra value does not prove that a diameter, flatness, or position requirement has been met. Specify the size tolerance and the surface finish separately when both matter, and define the datum and measurement method for critical features.

How to use the chart on a drawing

1. Identify the functional feature and its nominal size.
2. Choose the ISO tolerance class and hole or shaft deviation needed for the fit.
3. Compare that requirement with the supplier’s expected process range.
4. Ask the supplier whether an extra finishing operation, special setup, or inspection plan is required.

If the required band is tighter than the base process, the solution may be a finishing operation or a different sequence, not simply a more expensive machine. The supplier can confirm whether the feature can be held consistently across the expected quantity.

Common misreadings

Do not treat the grade ranges as universal promises. They are supplier guidance, and actual capability varies with nominal size, part geometry, material behavior, tool reach, thermal effects, machine condition, inspection equipment, and production quantity.

Do not use an IT grade as a complete fit callout. A fit also needs the correct tolerance positions for the hole and shaft, and the standard size band must match the nominal dimension. When tolerance stack-up affects assembly, calculate the full stack and confirm the chosen tolerances with manufacturing and inspection.

Sources

The process ranges and paired roughness examples are from KGL.MFG’s machining accuracy reference. The meaning and structure of the tolerance system are defined by ISO 286-1:2010, which ISO confirmed as current in 2026. ISO defines the IT system; it does not certify any particular shop’s process capability.