Reference · Tolerances
A working reference for calling out tolerances that are precise enough to function and loose enough to stay affordable. Use these tables before you dimension a drawing.
Most of a machined part's cost is decided on the drawing, not the shop floor. A tolerance one grade tighter than necessary can multiply machining time, so the goal is to call out precision exactly where the part needs it and nowhere else. These are the references we use when we review your drawing.
ISO 2768 general tolerances (linear, mm)
| Nominal size | f (fine) | m (medium) | c (coarse) |
|---|
| 0.5 – 3 | ±0.05 | ±0.1 | ±0.2 |
| 3 – 6 | ±0.05 | ±0.1 | ±0.3 |
| 6 – 30 | ±0.1 | ±0.2 | ±0.5 |
| 30 – 120 | ±0.15 | ±0.3 | ±0.8 |
| 120 – 400 | ±0.2 | ±0.5 | ±1.2 |
What each IT grade means for process choice
| IT grade | Typical tolerance band | Realistic process |
|---|
| IT6 | very tight | Grinding / EDM / fine boring |
| IT7 | tight | Precision milling/turning, often finish-ground |
| IT8 | medium-tight | Standard CNC milling & turning |
| IT9–IT10 | general | CNC roughing, laser, general fabrication |
Surface finish (Ra) you can specify
| Ra (µm) | How it is reached | Typical use |
|---|
| 3.2 | As-machined, roughing | Non-critical surfaces |
| 1.6 | Standard CNC finish | Most machined faces |
| 0.8 | Finishing pass | Sealing / sliding surfaces |
| 0.4 or better | Grinding / lapping / multi-pass EDM | Bearings, optical, sealing |
How to use this
- Set the title-block default to ISO 2768-m and tolerance individual features only where function requires.
- Match the IT grade to the cheapest process that can hold it — don't specify IT6 if IT8 works.
- Call surface finish only on faces that seal, slide or show; leave the rest as-machined.
Apply these on your milled, turned and EDM parts.
Frequently asked questions
What tolerance should I put on my drawing?
Use ISO 2768-m (medium) as the default general tolerance, then add tighter callouts only on features that mate, seal or locate. Tolerancing every dimension tightly is the single most common way buyers overpay for machining.
What is the difference between IT grades?
IT grades describe how tight a tolerance is for a given size. Machining routinely reaches IT7–IT9; IT6 and finer usually needs grinding or EDM. Smaller IT number = tighter tolerance.
What surface finish is standard?
As-machined parts are typically Ra 1.6 µm. Ra 0.8 µm is achievable with finishing passes, and Ra 0.4 µm or better needs grinding, lapping or multi-pass EDM.