CNC machining tolerances · 8 min read
ISO 2768, IT tolerance grades and how tolerances drive machining cost — everything you need to specify correctly on your drawing and order CNC parts with confidence.
Published: · CNC Turkey
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A tolerance in CNC machining defines the permissible upper and lower limits of dimensional deviation. For example, 50 mm ± 0.05 mm means the part must fall between 49.95 mm and 50.05 mm to be accepted.
When no tolerance is stated on a drawing, a general tolerance standard applies — most commonly ISO 2768. This is why clarifying which tolerance class applies is critical in every CNC part procurement.
ISO 2768 defines four general tolerance classes for dimensions where individual tolerances are not stated: f (fine), m (medium), c (coarse), v (very coarse). For most CNC machined parts, class m or f is used.
IT tolerance grades (IT5 through IT14) define the permissible dimensional error for a given diameter or length range. IT6 and IT7 are common for precision fits and bearings; IT9–IT11 are widely used in general engineering applications.
Tighter tolerances require longer machining time, more frequent measurement stops and sometimes additional finishing operations. This directly raises unit cost. As a general rule, widening a tolerance from IT6 to IT9 can reduce the relevant operation's cost by 20–40%.
When specifying tolerances, decide by function: specify tight tolerances only on surfaces and dimensions that are critical for assembly or operation. Applying general tolerance (ISO 2768-m) on non-functional surfaces is advantageous for both cost and lead time.
Tolerance and surface roughness are related but distinct concepts. Dimensional tolerance defines how much a dimension may vary, while Ra (arithmetic mean roughness) measures how smooth the surface is.
As a general guide: Ra 1.6 µm is standard for machined surfaces; Ra 0.8 µm suits precision and sliding surfaces; Ra 3.2 µm is appropriate for rough-machined or non-functional surfaces. Specify both requirements explicitly on your drawing.
The most common mistake is sending a drawing with dimensions that have no stated tolerance. In this case, the supplier cannot know which standard to apply, leading to incorrect cost estimation at the quotation stage.
Best practice: include a general tolerance block on the drawing (e.g. ISO 2768-m), then add individual tolerances only to critical dimensions. This approach is both clear and cost-optimised.
FAQ
With CNC turning, IT7 (approximately ±0.013 mm for a 50 mm diameter) is achievable under standard production conditions. IT6 requires dedicated setup and additional measurement. Geometry, material, heat-treatment condition and process all influence what is achievable.
IT6 and IT7 refer to International Tolerance grades defined in ISO 286. IT6 for a 50 mm diameter means approximately 16 µm tolerance band; IT7 means approximately 25 µm. These values vary with diameter range — consult the standard tables for exact values.
No. Tolerance should be selected by function. Specifying unnecessarily tight tolerances raises cost, extends lead time and in some cases increases rejection risk. Quality starts with selecting the right tolerance — not the tightest one.
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