CNC Machining Tolerances: What Tolerances Can You Realistically Hold?

Every CNC machining drawing carries a tolerance, and every tolerance carries a price. Hold a dimension to ±0.05 mm and the part is routine. Hold the same dimension to ±0.005 mm and it can cost several times more — not because the machine is better, but because the whole process around it has to change. Knowing what a CNC machine can realistically hold, and exactly where the cost steps up, is the difference between a quote you can defend and one you have to renegotiate.

This guide covers the tolerances our precision CNC machining service holds every day, why tighter tolerances cost more, and how to specify them on a drawing so you pay only for the precision you actually need. If you are comparing suppliers, it also shows which questions separate a capable CNC machining service from one that simply quotes the lowest price.

What Is a Machining Tolerance?

A tolerance is the amount a dimension is allowed to vary from the nominal value on the drawing. A bore specified as 20 mm ±0.05 mm is acceptable anywhere between 19.95 mm and 20.05 mm. The tolerance is not a target — it is the band the part must fall inside to be accepted.

Two things follow from that definition. First, a tighter tolerance means a narrower band, which means the machinist has to work closer to the centre and check more often. Second, an untoleranced dimension is not free of tolerance — it falls back on a general tolerance standard such as ISO 2768-m or ISO 2768-f, which the drawing should state.

Standard cnc machining tolerances

The table below shows what our machines hold as standard, and what they can hold when a job is set up specifically for it. These are the numbers to compare against when you read a quotation.

For general machined features with no tolerance called out, we work to ISO 2768-m: ±0.1 mm for lengths up to 6 mm, ±0.2 mm up to 30 mm, ±0.3 mm up to 120 mm and ±0.5 mm up to 400 mm. If your drawing needs finer general tolerances, specify ISO 2768-f and we will quote accordingly.

What Tolerances Can CNC Machining Realistically Hold?

The honest answer is that a well-maintained CNC machine can hold ±0.01 mm on a feature all day, and ±0.005 mm on a good day with the right setup. What it cannot do is hold ±0.001 mm on a production run of ten thousand parts, at speed, without an inspection step after every operation.

Three things decide whether a tight tolerance is realistic:

• Feature geometry. A bored hole or a flat face is easy to hold. A thin wall, a deep pocket or a long unsupported bore will deflect, and no amount of machine accuracy will change that.
• Material. Aluminium cuts and holds tolerance predictably. Stainless steel and titanium move as the tool wears and the part heats, so the same tolerance costs more.
• Temperature and fixturing. A part measured at 25 °C and used at 60 °C will not hold the same dimension. Rigid fixturing and controlled temperature are part of the tolerance, not separate from it.

If a feature needs better than ±0.005 mm, the usual answer is a secondary operation — grinding, lapping or honing — rather than trying to cut it in one pass.

Why Tighter Tolerances Cost More

A tolerance is a manufacturing instruction, and a tighter instruction costs more in four predictable places:

• More operations. A ±0.01 mm bore may need roughing, semi-finishing and finishing as three separate passes with a measurement between each.
• Slower cutting. Finishing passes take lighter cuts and run at lower feed rates, so machine time per part goes up.
• More inspection. Tight features are usually checked on a CMM rather than a calliper, which adds cost per part and per batch.
• Higher scrap. The narrower the band, the more parts fall outside it — and the more material, machine time and inspection you lose with them.

The practical rule is to tolerance the features that matter and leave the rest on the general standard. A drawing where every dimension carries ±0.01 mm is not a more precise drawing — it is a more expensive one.

How Tolerance Interacts with Surface Finish

Tolerance controls how close a dimension is to nominal; surface finish controls how smooth the surface is. They are specified separately, but they are produced together. A fine surface finish needs a light finishing pass, and that same pass is usually what brings a dimension into a tight band.

That is why a callout such as ±0.01 mm with Ra 0.8 costs more than the two requirements priced separately — the surface finish and the tolerance have to be achieved in the same operation. If you need a fine finish for appearance only, say so: a cosmetic finish on a non-critical face is far cheaper than a functional finish on a sealing face.

Reading a cnc tolerance chart

A CNC tolerance chart maps a size range to an allowed deviation. A row that reads "over 30 to 120 mm: ±0.3 mm" means any dimension in that range may vary by 0.3 mm either way under the general standard. Charts are read together with the process: the same 50 mm dimension carries ±0.3 mm as a general tolerance, ±0.05 mm as a standard machined tolerance, and ±0.01 mm as precision work.

The chart is also where the cost of a tight tolerance becomes visible. Moving a single 50 mm dimension from ±0.3 mm to ±0.01 mm changes it from a general milling pass into a finishing operation with a CMM check behind it. When a drawing asks for tight tolerance cnc machining on every dimension, that is usually a sign the tolerances were copied from a template rather than chosen for function. cnc machining accuracy is worth paying for on the features that mate; everywhere else it is cost with no benefit.

How to Specify Tolerances on Your Drawing

Most tolerance problems start on the drawing, not on the machine. Four habits keep a drawing manufacturable:

• State the general tolerance standard — ISO 2768-m or ISO 2768-f — in the title block, so untoleranced dimensions are defined.
• Tolerance only the dimensions that matter. If a feature has to mate with another part, tolerance it. If it is clearance or cosmetic, leave it on the general standard.
• Use GD&T where position matters. A true-position callout on a bolt pattern tells the machinist what actually has to be right, instead of stacking linear tolerances that over-constrain the part.
• Name the datum. A tolerance without a datum is measured from wherever the inspector happens to start, which is not a specification.

Frequently Asked Questions

What is the standard tolerance for CNC machining?
For general machined features we work to ISO 2768-m, which is ±0.1 mm up to 6 mm, ±0.2 mm up to 30 mm and ±0.3 mm up to 120 mm. Our standard machining tolerance on a toleranced feature is ±0.05 mm, and ±0.01 mm on precision work.

Can CNC machining hold ±0.01 mm?
Yes, routinely, on a rigid part in a free-cutting material. It becomes difficult on thin walls, deep pockets and long unsupported bores, where deflection rather than machine accuracy sets the limit.

Why does a tighter tolerance cost more?
Because it adds operations, slows the cut, requires more inspection and raises the scrap rate. The machine itself is rarely the bottleneck — the process around it is.

Do I need a tolerance on every dimension?
No. Put the general standard in the title block and tolerance only the dimensions that are functionally critical. That keeps the drawing clear and the price down.

What file do you need to quote a tolerance-critical part?
A 3D model (STEP, IGES or X_T) plus a 2D drawing that shows the tolerances, datums and surface finishes. If you only have a model, tell us which features are critical and we will review the design with you.

Conclusion

Tolerance is a cost decision as much as a design decision. Our standard CNC machining holds ±0.05 mm, precision work holds ±0.01 mm, and tighter than that is usually a secondary operation rather than a tighter setup. Specify the general standard in the title block, tolerance only what matters, and check the result with a first article inspection — that is how you get the precision the part needs without paying for precision it does not. Send us your drawing and our engineers will review the tolerances, flag anything that will drive cost, and quote the whole job within 24 hours. Contact us to start your project.