Machining Tolerance Guide: What Each Process Can Hold
Achievable tolerances by process
| Process | Standard tolerance | Tightest practical | Cost note |
|---|---|---|---|
| CNC turning (diameter) | ±0.01 mm | ±0.005 mm, ground ±0.002 mm | Tighter needs finish passes and gauging |
| CNC milling | ±0.02 mm | ±0.005 mm on finish cuts | Fixture rigidity is the limit |
| Surface grinding | ±0.005 mm | ±0.002 mm | Flatness and finish add cost |
| Wire EDM | ±0.005 mm | ±0.003 mm | Slow on thick sections |
| Sheet metal bending | ±0.1 mm, ±0.5° | ±0.05 mm on small parts | Depends on tooling and material springback |
| Progressive-die stamping | ±0.05 mm | ±0.02 mm | Set by tooling build, not cycle time |
| Compression spring (wire) | wire ±0.02 mm, free length ±1% | wire ±0.01 mm | Set by coiling and stress relief |
| Extruded heat sink profile | ±0.1 mm, cut ±0.2 mm | ±0.05 mm machined | Profile tolerance from the die |
Common GD&T symbols
| Symbol | Name | What it controls |
|---|---|---|
| ⏥ | Flatness | How flat a surface is, no datum needed |
| ∥ | Parallelism | Surfaces or axes kept parallel to a datum |
| ⊥ | Perpendicularity | Squareness to a datum |
| ◎ | Position | True position of a hole or feature |
| ⌓ | Concentricity | Axis of a feature relative to a datum axis |
| Ⓜ | Maximum material condition (MMC) | Bonus tolerance when a feature is at its largest |
| ⌖ | Total runout | Combined wobble over a full rotation |
Surface roughness (Ra) reference
| Process | Typical Ra (µm) |
|---|---|
| Rough turning | 3.2 - 6.3 |
| Finish turning | 1.6 - 3.2 |
| Fine turning / boring | 0.4 - 1.6 |
| CNC milling | 1.6 - 3.2 |
| Surface grinding | 0.2 - 0.8 |
| Lapping / polishing | 0.05 - 0.4 |
| Stamping (sheared edge) | 1.6 - 6.3 |
| Extruded aluminium | 1.6 - 3.2 |
| Anodized (matte) | 0.4 - 1.6 |
Notes for quoting
- A tolerance tighter than the standard column usually means extra setup, in-process gauging and a CMM report - budget for it.
- Try to keep all tight features on one datum; stacked tolerances multiply inspection cost.
- Specify roughness only where it matters. Calling Ra 0.4 everywhere can triple the finishing time.
Frequently asked questions
What tolerance can BQUQ hold on CNC machined parts?
Our standard process control is ±0.005 mm on critical features, with ±0.01 mm as routine. We machine and inspect against your drawing and ship an inspection report with every batch.
How tight can a stamped part be?
Progressive-die stamping holds about ±0.05 mm as standard and down to ±0.02 mm on critical features. The limit is set when the tool is built, so the tolerance should be agreed before the die is cut.
What Ra value is normal for a machined surface?
Rough turning is 3.2 to 6.3 µm, finish turning 1.6 to 3.2 µm, and ground surfaces 0.2 to 0.8 µm. Specify only the roughness the function needs.
Do you provide dimensional inspection reports?
Yes. Every production batch ships with an inspection report, and full CMM reports are available for parts with tight or GD&T tolerances. Ask for the report level you need in the RFQ.
Talk to an engineer
CNC machining · metal stamping · custom springs · heat sinks — ISO9001 source factory in Dongguan, China. ±0.005mm process control, quote within 12 hours.
Email sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com