CNC tolerance calculator: is your milling tolerance achievable?
Enter a size, a tolerance, a feature type, a material and the number of setups and get a verdict: achievable, tight, not with that many setups, or grinding needed. Next to it are the ISO 2768-1 class, the cost increase and thermal expansion.
Opens without an account 3 times a day. A free account removes the limit.
- Classes f, m, c, vfine, medium, coarse, very coarse in ISO 2768-1
- 6 materialsfrom brass to hardened steel
- 3.5×cost increase of class f over class v
- Setupseach adds an error of the same order
Milling tolerances: what it checks
This CNC tolerance calculator is a free online tool from EduFacturing that checks whether a tolerance you plan to put on a drawing is achievable when milling on a machining center and what it will add to the cost of the part. It does not turn H7 into microns: that is the job of the separate ISO 286 fits tool.
It is shown for one setup, for the worst case and by the RSS method. The calculation runs in your browser.
ISO 2768 class and the cost of a tolerance
Gives a verdict on the tolerance
Achievable, tight but achievable with process control, not with the planned setups, or not by milling alone.
Shows the ISO 2768-1 class and the cost
It classifies the tolerance as f, m, c or v for the size and says how many times more expensive it is than class v: 1.3× for c, 2× for m, 3.5× for f. Anything tighter than f is extrapolated linearly.
Checks thermal expansion
It works out how much the size changes when the temperature moves by 5 K in a shop and by 1 K in an air-conditioned room, and warns if the temperature alone eats the tolerance.
Calculates a dimension chain
Each link carries its own ± tolerance. The result is the worst case (the sum), the statistical RSS and the ratio between them.
When you need to check a tolerance
A tolerance on a drawing is a decision about money: a tighter one costs more and is not always reachable.
A designer before the drawing
Checks whether a tolerance on a hole in aluminum is achievable in one setup.
A buyer of parts
Decides which sizes to leave in the general class before asking for a quote.
How the achievable tolerance is calculated
The base is the ISO 2768-1 table, and factors sit on top of it. What is achievable in one setup is the base times the feature factor and the material factor.
Base by size
The size picks the size range, for example over 30 to 120 mm, and the base is the class f tolerance for it: ±0.15 mm. Above 2000 mm there is no class f and an individual tolerance is needed.
Factors
Feature: hole 0.7, external size 1.0, depth 1.3, distance between axes 1.6. Material: from 0.85 for brass to 1.6 for hardened tool steel.
Setups
With N setups the worst case is N times the single value, and RSS is √N times.
Verdict
Below ±0.01 mm: grinding. A tolerance not smaller than the worst case: achievable. Not smaller than the single value with several setups: fewer setups. Otherwise: tight, with process control.
| Applies to | Formula | Meaning |
|---|---|---|
| Achievable in one setup | t = t_f · k_feature · k_material | t_f is the class f tolerance of ISO 2768-1 for the size |
| Several setups | worst case = N · t; RSS = √N · t | N is the number of setups |
| Cost increase | v = 1×, c = 1.3×, m = 2×, f = 3.5× | tighter than f: the f multiplier · t_f / tolerance |
| Thermal expansion | Δd = size · α · ΔT / 1,000,000 | size in mm, α in µm/(m·K), ΔT = 5 K in a shop and 1 K in an air-conditioned room |
| Dimension chain | worst case = Σ t_i; RSS = √(Σ t_i²) | the ratio is worst case to RSS |
What is different from a table
Setups are counted
A second setup is a new independent source of error: in the worst case the error doubles instead of being halved. RSS grows only √2 times.
Temperature is a separate check
At a small tolerance, temperature alone can eat it even when the part is clamped perfectly. A 200 mm part in S235 steel changes by 0.0117 mm at 5 K.
Material is not only cutting force
POM cuts easily but has a factor of 1.2 because of its expansion of 110 µm/(m·K), more than 9 times that of S235 steel, and because its internal stresses relax.
Achievable values, the material and feature factors and the cost increase are a summary of shop practice, not a standard and not a quote. The base and the cost increase follow published guides (Xometry, Protolabs, Fictiv). The standard parts are the ISO 2768-1 table and the 20 °C reference temperature of ISO 1:2016. Treat the result as a basis for a talk with the shop.
The tool does not account for a particular machine, tool or fixture. Series size does not change the numbers: for a large series, over 100 pieces, and a tolerance below ±0.3 mm it only shows a tool wear warning.
Questions about CNC tolerances
What are the general tolerances of ISO 2768-1 for a size from 30 to 120 mm?
Class f (fine) is ±0.15 mm, class m (medium) ±0.3 mm, class c (coarse) ±0.8 mm and class v (very coarse) ±1.5 mm. For 6 to 30 mm the same classes are ±0.1, ±0.2, ±0.5 and ±1.0 mm.
What tolerance does the tool show for milling a 100 mm part?
For an external size in S235 steel in one setup: ±0.15 mm, the class f base. With two setups the worst case is ±0.3 mm. For a hole in the same part the factor is 0.7, so ±0.105 mm.
Does a tighter tolerance cost more?
Yes. Against class v: class c about 1.3 times, class m 2 times, class f 3.5 times. These are summarized published ranges, not a quote, and the real price at a tight tolerance usually rises faster.
When does temperature alone eat the tolerance?
By Δd = size · α · ΔT / 1,000,000 a 200 mm part in S235 steel (α = 11.7) changes by 0.0117 mm at 5 K. That is more than ±0.01 mm from temperature alone, before any cutting.
What is the difference between worst case and RSS tolerance?
The worst case is the sum of the tolerances (Σ t_i), and RSS is √(Σ t_i²), which is smaller.
Is the tool free?
Yes. Without an account it opens 3 times a day, and with a free account there is no limit.
The milling tolerance checker is free. Pro opens the rest
Pro costs 15 EUR a month and opens 45 Pro tools.
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Check your tolerance
Opens without an account 3 times a day. A free account removes the limit.