Tolerances are where engineering intent meets manufacturing reality. Specify them too loosely and your parts won't fit or function. Specify them too tightly and you'll pay significantly more — sometimes for precision you don't actually need.
This guide will help you understand what tolerances are achievable with CNC machining, how different processes compare, and how to specify dimensions in a way that gets you the precision you need without inflating your costs.
A tolerance defines the permissible range of variation for a given dimension. When a drawing calls out a shaft diameter of 10.00 mm ±0.02 mm, it means any part measuring between 9.98 mm and 10.02 mm is acceptable. That ±0.02 mm is the tolerance band — a total range of 0.04 mm.
Tolerances exist because no manufacturing process is perfect. Every cut introduces tiny variations from tool wear, thermal expansion, material inconsistencies, and machine positioning accuracy. The job of a good machinist and a well-maintained machine is to keep those variations within your specified tolerance window.
Different CNC operations achieve different levels of precision. Here's what you can typically expect:
| Process | Standard Tolerance | Achievable (Tight) |
|---|---|---|
| CNC Milling | ±0.05 mm (±0.002") | ±0.01 mm (±0.0005") |
| CNC Turning | ±0.025 mm (±0.001") | ±0.005 mm (±0.0002") |
| Swiss-Type Turning | ±0.013 mm (±0.0005") | ±0.005 mm (±0.0002") |
| CNC Grinding | ±0.005 mm (±0.0002") | ±0.001 mm (±0.00005") |
| EDM (Wire) | ±0.01 mm (±0.0004") | ±0.003 mm (±0.0001") |
| Drilling | ±0.05 mm (±0.002") | ±0.025 mm (±0.001") |
| Reaming | ±0.013 mm (±0.0005") | ±0.005 mm (±0.0002") |
| Threading (cut) | ISO 6H/6g class | 4H/4h class |
These numbers assume properly maintained equipment, appropriate tooling, and competent operators. A shop running well-calibrated Swiss-type CNC lathes — like the fleet at KING HAN — will routinely hit the tighter end of the turning tolerances.
Raw machine capability is only one piece of the puzzle. Several real-world factors influence what tolerance you can reliably hold in production:
Free-machining materials like brass (C36000) and aluminum (6061-T6) are easier to hold tight tolerances on because they cut cleanly with minimal tool deflection. Stainless steels (especially 316 and 17-4PH) work-harden and generate more heat, making tight tolerances harder. Titanium and Inconel are more challenging still.
Thin walls flex under cutting forces. Long, slender shafts deflect. Deep bores are harder to hold round. The geometry of your part directly impacts what tolerances are practical. A 0.5 mm wall with ±0.01 mm tolerance is exponentially harder than a solid 10 mm shaft at the same tolerance.
Smaller features are generally harder to measure and harder to machine consistently. A ø1.0 mm cross-drilled hole at ±0.02 mm is a very different challenge than a ø10 mm hole at the same tolerance.
Machine shops that control ambient temperature (typically 20°C ±2°C) and use coolant effectively will achieve tighter tolerances more consistently. In Swiss-type turning, the guide bushing helps by maintaining workpiece rigidity regardless of thermal expansion along the bar.
Holding tolerance on 10 parts is very different from holding it across 10,000 parts. Tool wear, material lot variation, and machine drift all accumulate during long production runs. Shops that practice in-process SPC (statistical process control) maintain tighter distributions over time.
This is the part most engineers underestimate. The relationship between tolerance and cost is not linear — it's exponential.
Moving from ±0.05 mm to ±0.025 mm might increase part cost by 10–20%. But moving from ±0.025 mm to ±0.005 mm could increase it by 50–100% or more, because it requires:
The golden rule: only tolerance what matters. If a surface is non-functional (no mating part, no sealing surface, no critical fit), leave it at standard machining tolerance. Reserve tight callouts for the features that genuinely require them.
Set a reasonable general tolerance in your drawing's title block (e.g., ±0.05 mm for machined dimensions) and only call out specific tighter tolerances on individual features that need them. This tells the machinist exactly where to focus attention.
Ask yourself: "What happens if this dimension is at the extreme of its tolerance range?" If the answer is "nothing," the tolerance is probably tighter than it needs to be. If the answer is "the assembly won't fit" or "the seal will leak," that feature deserves a tighter callout.
Geometric dimensioning and tolerancing (GD&T per ASME Y14.5 or ISO 1101) is far more precise than ± linear tolerances when you need to control concentricity, perpendicularity, true position, or runout. A good CNC shop will understand and work from GD&T callouts.
Before finalizing a drawing, share your design intent with your machining partner. An experienced shop can often suggest tolerance adjustments that save money without compromising function. For example, they might recommend a reaming operation to hit a bore tolerance instead of a more expensive grinding step.
Specifying a tolerance is only meaningful if it can be reliably measured. If you call out ±0.003 mm on an internal feature that's physically difficult to probe, you'll end up paying for custom fixturing or accepting uncertain measurements. Make sure your tolerances are measurable and verifiable.
For cylindrical fits (shafts and holes), ISO tolerance grades provide a standardized framework:
Swiss-type CNC lathes routinely deliver IT6–IT7 quality on turned diameters, which is one reason they're favored for precision components.
Understanding CNC machining tolerances helps you design better parts, choose the right manufacturing process, and communicate effectively with your machining partner. The key takeaways:
At KING HAN, our 26 Swiss-type CNC lathes are calibrated and maintained to consistently deliver precision in the ±0.005–0.013 mm range for turned features. If you need precision parts and want to discuss what tolerances are realistic for your application, contact us for a consultation.