How to Specify CNC Machining Tolerances: A Practical Guide for Engineers

Written by the engineering team at Robocon CNC Pvt Ltd, Pune, India — ISO 9001:2015 certified CNC manufacturer.

One of the most common causes of expensive CNC machined parts is over-tolerancing. Engineers specify tight tolerances on every dimension as a precaution, without realising that +/-0.005 mm on a non-critical surface costs three times more than +/-0.05 mm. This guide explains how to tolerance your drawings correctly so you get the accuracy you need, at the cost you want.

General vs. Critical Tolerances

Every engineering drawing should have a general tolerance note that covers all dimensions not individually called out. This note handles 80–90% of your part. Only dimensions that actually matter functionally — bearing seats, press-fit bores, sealing surfaces — should carry individual tolerances.

The most widely used general tolerance standard in machining is ISO 2768. It defines four grades:

For most precision CNC parts destined for mechanical assemblies, a title block note of ISO 2768-m or ISO 2768-f covers the majority of dimensions perfectly well.

When to Use GD&T

Geometric Dimensioning and Tolerancing (GD&T) should be used when the geometric relationship between features matters — not just their size. Common cases:

• Flatness — for sealing surfaces, mating faces, or CMM datum planes
• Perpendicularity / Parallelism — for bores that must align with a reference surface
• True Position — for bolt hole patterns that must align with a mating part
• Concentricity / Runout — for rotating shafts where balance matters
• Cylindricity — for precision bearing bores where roundness is critical

GD&T eliminates ambiguity that +/-0.01 mm linear tolerances create. A bore at +/-0.005 mm diameter is useless if its position is wrong by 0.3 mm.

Tolerance and Cost: The Direct Relationship

Every tighter tolerance step roughly doubles inspection time. Here is a realistic cost multiplier table based on our shop floor data:

Practical Rules for Tolerancing Your Drawing

• Put ISO 2768-m in the title block. This sets sensible defaults for everything not called out individually.
• Only apply individual tolerances to functional surfaces: bore diameters that accept shafts or bearings, sealing faces, datum surfaces for assembly.
• On threads, don't tolerance the diameter — specify the class (6H for internal, 6g for external). Thread class carries the tolerance.
• If you need flatness, say so with a flatness callout — not by tightening all linear tolerances on that face.
• For press fits and running fits, use ISO fit notation: H7/p6 for press, H7/g6 for running. Your machinist knows exactly what to hit.
• Ask your machinist for DFM feedback before finalising. At Robocon CNC, we flag over-toleranced dimensions in every quote at no charge.

What Tolerances Can Robocon CNC Achieve?

Our standard capability on Mazak Variaxis and Makino VMC centres:

• Linear dimensions: +/-0.01 mm standard, +/-0.005 mm on critical features with full CMM verification
• Flatness: 0.01 mm over 200 mm length
• Bore diameter (H7 class): consistently achievable on all our machines
• Surface finish: Ra 0.8 μm as-machined, Ra 0.4 μm with fine finishing, Ra 0.2 μm with grinding