Ever paid for ±0.005mm when ±0.05mm would've worked?

I see it all the time. Drawings come in with a blanket tolerance block that says ±0.005mm across every single dimension. On a 300mm-long aluminum part, that's not precision — that's overkill. And overkill costs real money. Let's break down what CNC machining tolerances actually mean for your parts, your lead times, and your bottom line.

What Does ±0.005mm Actually Look Like?

Let's put it in perspective. A human hair is about 0.05mm thick. ±0.005mm is ten times thinner than that. It's roughly the width of a single bacteria. To hold that repeatably, your CNC machine needs thermal compensation, a rigid setup, and fresh tooling.

Here's the thing most buyers miss: holding ±0.005mm on a 10mm diameter shaft is routine. Holding it on a 500mm-long frame across different material batches? That's where things get interesting. Thermal expansion alone can shift your part by 0.01mm for every 10°C change.

The Tolerance Cheat Sheet

• Ra 6.3 μm – Roughing pass level.

• Ra 3.2 μm – Standard machining finish.

• Ra 1.6 μm – Fine machining.

• Ra 0.8 μm – Precision finish.

• Ra 0.4 μm – High-polish finish.

• Ra 0.2 μm – Mirror-like finish.

My rule of thumb: if you can't tell me why a dimension needs tight tolerance, default to standard and save money.

What Actually Affects Tolerance Holding?

Machine condition, material stability, and fixture design. A 15-year-old CNC won't hold tight tolerance. Aluminum 6061 moves with temperature. Thin-walled parts deflect under cutting pressure.

GD&T vs Block Tolerance

Drawings with tight block tolerance everywhere drive machinists crazy. Use selective GD&T callouts instead. Typically saves 15-25% on complex parts.

How to Specify Tolerances Without Overpaying

• Loose block tolerances on title block

• Tight callouts only on critical features

• Use GD&T symbols

• Specify at 20°C standard temperature

Common Tolerance Mistakes

• Tight tolerance on non-mating surfaces

• No tolerance callout at all

• Thread callouts without tolerance class

Got a drawing you're not sure about? Send it over for a free DFM review.