CNC Machining Tolerances Guide

I've been machining parts for 18 years. The single most common mistake I see on customer drawings is over-tolerancing — calling out ±0.005mm on features that functionally only need ±0.1mm. That's where your money goes.

Here's the thing: any half-decent CNC shop can hit ±0.005mm. The difference shows up when you need that tolerance across 5,000 parts with the same CMM reading on part 1 as on part 5,000. That's process control, not machine capability.

What Tolerance Costs — Real Numbers

We tracked 300+ jobs over 18 months to understand the real cost of tight tolerances. A 6061-T6 bracket block that costs $40 at ±0.1mm jumps to $55 at ±0.005mm and $80 at ±0.002mm. The drivers:

• More roughing passes — looser stock removal needs approach pass + semi-finish + finish. Each pass is a full machine cycle.
• Thermal stabilization — parts cool between passes. A warm part in the vice shifts 0.005mm at 5°C delta. We wait.
• Tool wear management — at ±0.005mm, we change inserts more often. Every 0.01mm of wear is 20% of your tolerance budget.
• CMM time — each part sits on the Zeiss CMM for 15 minutes. At $0.50/minute, that's real overhead.

Material Matters

Not all materials hold tolerance the same way. Here's what we see daily in our shop:

See our Aluminum Machining Guide and Stainless Steel Machining Guide for material-specific data.

GD&T and Tolerance Cost Impact

Geometric tolerances (GD&T) cost differently than dimensional tolerances. Here's what we see on the shop floor:

How to Spec Tolerances Without Wasting Money

After reviewing thousands of prints, here's practical advice:

1. Only tolerance critical features. Ask yourself: does this dimension affect function, assembly, or seal? If no, leave it at standard.
2. Use a general tolerance block. ISO 2768-mK (±0.1mm for linear, ±0.2mm for angular) for the title block. Then override only the few critical features.
3. Consider datum structure. Every tight tolerance needs a clear datum. Stacked tolerances without datum reference are the most common source of inspection failures.
4. Understand "statistical tolerance." If you're running 10,000 parts and need ±0.005mm on 95% of them, tell us. We'll set up SPC control. It's cheaper than 100% inspection.
5. Ask what's cheaper. A .5000 ±0.0005 hole reamed vs bored. Bored is faster but requires CMM re-check. I'll quote both and tell you the difference.

See also: GD&T for CNC Machining, DFM Guidelines

How We Verify Tolerances

We use a Zeiss CONTURA G2 CMM in a temperature-controlled room (20°C ±0.5°C). Every precision-tolerance part gets a full CMM report. Standard-tolerance parts get first-article CMM + spot-check caliper on subsequent pieces.

Our Zeiss has a maximum permissible error of 0.0015mm. That's 3x better than our tightest production tolerance, which gives us reliable data for every measurement. See our Quality & Inspection page for details.