Machining Method to Tolerance Grade Matrix
Which IT band each cutting process holds in routine production: turning, milling, drilling, boring, reaming, broaching, grinding, honing, lapping and polishing, mapped against IT1 to IT13 with the surface roughness that comes with each. Use it to check whether a tolerance on your drawing needs a second operation before you send the RFQ.
Process to IT grade chart
Reading the chart: each bar spans the IT grades the process holds routinely on a healthy machine, in production quantities, without heroic inspection. Darker means finer. Grinding and honing reach grades turning and milling cannot, which is why a drawing that combines a milled body with a ground bore costs more than either process alone.
The same matrix as a table
Click any value to copy it. Ra values are what each process reaches together with the stated grade; a process can be pushed finer on Ra, but size and finish usually have to be paid for separately.
| Process | Economic IT range | Typical Ra (um) | Notes |
|---|---|---|---|
| Turning, external, semi-finish | IT9 to IT11 | 3.2 - 6.3 | Heavy passes ahead of a finishing cut |
| Turning, external, finish | IT7 to IT9 | 1.6 - 3.2 | Default for shaft seats and shoulders |
| Turning, diamond (fine) | IT5 to IT7 | 0.2 - 0.8 | Light cuts, sharp carbide or diamond tooling |
| Facing, finish | IT7 to IT9 | 1.6 - 3.2 | Diameter changes across the face stay inside one setup |
| Boring, finish | IT7 to IT9 | 1.6 - 3.2 | Hole size, not hole position |
| Boring, diamond (fine) | IT5 to IT7 | 0.2 - 0.8 | Precision bores in light alloys |
| Drilling | IT11 to IT13 | 6.3 - 12.5 | Pilot for reaming or boring when closer bands are needed |
| Reaming | IT6 to IT9 | 0.8 - 3.2 | Corrects size; does not correct position |
| Milling, semi-finish | IT8 to IT10 | 3.2 - 6.3 | |
| Milling, finish | IT7 to IT9 | 1.6 - 3.2 | Five-axis setups remove re-clamping error between faces |
| Broaching | IT6 to IT8 | 0.8 - 1.6 | Keyways, splines and other internal profiles |
| Cylindrical grinding | IT5 to IT7 | 0.2 - 0.8 | Works hardened parts |
| Surface grinding | IT5 to IT7 | 0.2 - 0.8 | Flat and parallel datum faces |
| Honing | IT4 to IT7 | 0.1 - 0.4 | Bores; corrects geometry, not position |
| Lapping | IT1 to IT5 | 0.025 - 0.1 | Flatness and seal faces; quantity dependent |
| Polishing | IT5 to IT7 | 0.012 - 0.1 | Improves Ra far more than size |
How to use this when sourcing
Three checks that catch most quoting surprises. First, a tolerance a band or two finer than the base process usually means one added operation, not a new supplier. Second, tight size on a bore plus tight position to another feature means the fine cutting step must happen in the same setup, which is a five-axis or bore-and-stand conversation. Third, geometry (flatness, runout, cylindricity) is carried by grinding and honing far better than by fine turning, so state it on the drawing instead of tightening size alone. Our GD&T reference covers the symbols; the tolerance page covers the IT bands and a fit calculator.
Benchmark for buyers: at the 80 to 120 mm size band, KGL's standard commitment of plus or minus 0.005 mm sits at about IT4, and our conditional five-axis grade of plus or minus 0.002 mm with 100 percent CMM verification sits below it. Both are inside what grinding and fine turning reach in the chart above; the quote states which process carries each tolerance so the routing is visible before you order.
Free quick-reference card (PDF)
A two page printable card: IT tolerance bands by size, general tolerances for untoleranced dimensions, this process to grade matrix and the Ra series. Enter your work email and the download opens immediately; no signup sequence, no drip campaign. Your address is used once to send the file link and for nothing else.
Where each accuracy band is bought
The matrix tells you which process can hold which grade. These pages are where that process is quoted, run and verified.
Process pages behind each row
- CNC millingThe milled accuracy band, including what changes with tool reach and wall height.
- CNC turningTurned diameters and the roundness that goes with them.
- 5-axis CNC machiningPositional accuracy across one-setup work, and where it stops being economical.
- Surface and cylindrical grindingThe band that sits below routine cutting and what it costs to get there.
- Wire EDMSharp corners and hardened material the cutting tools cannot reach.
Verification of the achieved band
- Inspection and QCWhich instrument proves which grade, and the report you get.
- Tolerance standardsHow the quoted tolerance is stated when a process cannot hold the drawing alone.
- Equipment listMachine inventory behind the accuracy claims on this page.
Industry contexts
- AerospaceWhere an IT grade is a certification requirement, not a preference.
- Medical devicesWhere process capability has to be shown, not just achieved.
Have a feature that sits at the edge of one of these bands? Send the drawing and the tolerance question with the drawing and we will answer against it.
Related reference pages
Sources
| Standard | Covers |
|---|---|
| Machining handbook, economic accuracy tables (p. 2-209 area, tables 2.3-25 to 2.3-28) | IT ranges per process, transcribed and rounded to the grades a CNC shop quotes in production |
| Machining handbook, process vs Ra chart (p. 2-249) | Cross-check of the Ra values paired with each process |
| GB/T 1800.1 / ISO 286-1 | Definition of the IT tolerance grades themselves |