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Typical Achievable Tolerance by Manufacturing Process (IT Grade Table)

Before you write a tight tolerance on a drawing, check whether the intended process can hold it economically. This table maps common processes to the ISO IT grade they usually achieve and the width of the tolerance band on a typical 25 mm feature.

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Process	Family	Typical IT grades	Tolerance band at 18 to 30 mm (mm)	Planning note
Lapping and superfinishing	Abrasive	IT4 to IT6	0.006 to 0.013	Used for gauges and sealing faces
Honing	Abrasive	IT5 to IT7	0.009 to 0.021	Bore size and cylindricity
Cylindrical grinding	Abrasive	IT5 to IT7	0.009 to 0.021	Shafts, journals, bearing seats
Surface grinding	Abrasive	IT5 to IT7	0.009 to 0.021	Thickness and flatness
Wire EDM	Thermal	IT5 to IT7	0.009 to 0.021	Hard materials, punches, dies
Sinker EDM	Thermal	IT7 to IT9	0.021 to 0.052	Cavities and ribs
Reaming	Hole making	IT6 to IT8	0.013 to 0.033	After drilling or boring
Precision boring	Hole making	IT6 to IT8	0.013 to 0.033	Fine boring heads, stable setup
Broaching	Hole making	IT7 to IT9	0.021 to 0.052	Splines, keyways, shapes
Drilling	Hole making	IT10 to IT12	0.084 to 0.21	Size varies with drill and material
Precision turning	Machining	IT6 to IT8	0.013 to 0.033	Stable bar, sharp tooling
General turning	Machining	IT7 to IT10	0.021 to 0.084	Typical CNC lathe work
CNC milling	Machining	IT7 to IT10	0.021 to 0.084	Depends on part rigidity and setups
Laser cutting, sheet	Sheet metal	IT11 to IT13	0.13 to 0.33	Depends on thickness
Punching and stamping	Sheet metal	IT11 to IT13	0.13 to 0.33	Tool wear drifts the size
Fine blanking	Sheet metal	IT7 to IT9	0.021 to 0.052	Specialist press and tooling
Press brake bending	Sheet metal	IT13 to IT14	0.33 to 0.52	Springback and bend allowance
Die casting	Casting	IT11 to IT13	0.13 to 0.33	Cast-in features only
Investment casting	Casting	IT10 to IT13	0.084 to 0.33	Shrink allowance in the tool
Sand casting	Casting	IT14 to IT16	0.52 to 1.3	Machine all mating faces
Hot forging	Forming	IT14 to IT16	0.52 to 1.3	Allow machining stock
Injection molding	Plastics	IT10 to IT13	0.084 to 0.33	Material and wall thickness drive shrink
Metal 3D printing, powder bed	Additive	IT11 to IT13	0.13 to 0.33	Machine critical interfaces
Polymer extrusion 3D printing	Additive	IT13 to IT15	0.33 to 0.84	Machine variation is significant

These are planning ranges drawn from common IT grade guidance and the standard IT values for the 18 to 30 mm size range. Real capability depends on machine, tooling, material, part geometry and measurement method, so confirm with a Cpk study before committing a tolerance.

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How to use the table

Choose the feature you need to control and decide the tolerance band. Then filter the table by family or process and see whether your tolerance sits inside the typical range. If the tolerance is tighter than the process range, expect higher cost, a secondary operation such as grinding or reaming, or lower yields. If it is looser, check whether a cheaper process would work. The band is for one size range, so scale with the IT grade table for larger or smaller features.

Filter by Family to compare alternatives for one feature
Treat the lower IT number as a premium capability
Export the list for a design review or supplier discussion

How IT grades work

ISO 286 defines tolerance grades IT01 to IT18, where the tolerance width grows with both the grade number and the nominal size. For sizes over 18 up to 30 mm, IT6 is 13 micrometers, IT7 is 21, IT8 is 33, IT9 is 52, IT10 is 84, IT11 is 130, IT12 is 210, IT13 is 330 and IT14 is 520. Each step up is roughly a 1.6 times increase. That is why moving from IT8 to IT7 often adds a finishing operation.

Worked example

A 25 mm bore needs a 0.02 mm tolerance band to seat a bearing. The table shows that drilling cannot hold it, boring and reaming reach IT6 to IT8, and honing or grinding gets there reliably. A good route is drill, rough bore, finish bore, then hone only if the process study shows the finish bore falls short. This keeps the expensive operation to the cases that need it.

Common mistakes

Applying one tight tolerance to the whole drawing is the most common cause of avoidable cost. Another error is assuming a casting or forging holds the tolerance of the finishing process, instead of leaving machining stock on mating surfaces. Teams also forget that measurement uncertainty must be small compared with the tolerance, a rule of thumb being at most one tenth of the band.

Frequently Asked Questions

What IT grade does CNC milling usually achieve?

General CNC milling normally holds IT7 to IT10, depending on part rigidity, tool wear, fixturing and temperature. Tighter results are possible with careful finishing passes and in-process probing, but they add cycle time and inspection cost.

Why are casting tolerances so much looser?

Casting involves shrinkage, mold wear and variation in pour conditions, so dimensions scatter more than in machining. Sand casting is typically IT14 to IT16. Features that need tight limits are machined after casting, so leave machining stock on them.

Is this table a guarantee of capability?

No. It is a planning reference. Capability depends on your machine, tooling, material and measurement system. Use a capability study, such as Cpk across a representative run, to confirm a specific tolerance before promising it to a customer.

How do I convert an IT grade to a tolerance?

Use the ISO 286 table for the nominal size range. For 18 to 30 mm, IT7 is 0.021 mm and IT9 is 0.052 mm. The band is the total width, so you can split it symmetrically or place it unilaterally depending on the fit.

Netray can connect routing, work center capability and inspection results in your ERP, so planners can compare required tolerances with demonstrated capability before they release an order.

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