Tolerances in CNC machining — how much does accuracy cost?

Two parts that look identical can differ in price by tens of percent simply because one has tight tolerances across the whole drawing and the other only where necessary. Accuracy is valuable, but it is not free — every tightening of a tolerance costs machine time, measurement and sometimes a separate operation.

This post explains where the cost of accuracy comes from and how to specify tolerances so that you pay for precision only where it decides the part's function. If you want to see the full list of price drivers, pair it with the article how much does a CNC part cost.

A tolerance is a cost, not just a dimension

A tolerance defines how far the actual dimension may deviate from the nominal value. A 20 mm dimension with a tolerance of ±0.1 mm and the same dimension with ±0.01 mm look almost identical on the drawing, but in machining they are two different levels of difficulty.

A narrower tolerance means a smaller margin for error, so the machine runs slower, the tool must be in better condition, and the part has to be measured more often and more accurately. The more such dimensions there are, the longer the cycle time and the higher the inspection cost — and that translates directly into the price.

What drives the price up with tight tolerances

A tight tolerance rarely costs in just one place. Most often it raises the cost at several stages at once:

• Slower machining — smaller feeds and additional finishing passes lengthen the cycle time,
• More accurate measurement — a micrometer or a coordinate measuring machine instead of a calliper, sometimes a report and first-article inspection,
• An additional operation — very tight dimensions often require grinding after turning or milling,
• A higher scrap risk — the narrower the tolerance, the more easily a part falls out of range and goes to rework or scrap,
• Better fixturing — stable clamping, and sometimes temperature control, to keep the dimension repeatable.

None of these costs is bad in itself — the problem appears when you incur them on dimensions that do not need to be that accurate at all.

Accuracy grades and the general tolerance

Not every dimension requires an individual tolerance. Dimensions without a callout are covered by the general tolerance, most often per the ISO 2768 standard, in classes from fine to coarse (designated by letters, e.g. f, m, c). For most free dimensions the medium class is entirely sufficient and costs nothing extra.

Individual tolerances and fits (e.g. H7/g6) are added where parts must fit together. The level of accuracy is conveniently described by IT grades — the lower the number, the tighter and the more expensive.

The ranges given are indicative — the real accuracy is determined for the specific part, material and machine. What matters is the proportion: going down a grade or two can require an entirely different technology and cost.

Surface roughness costs too

Accuracy is not only the dimension but also the smoothness of the surface, described by the roughness parameter Ra. A lower Ra means a smoother surface, but usually more finishing passes or a separate operation.

A surface after standard turning or milling (indicatively around Ra 1.6–3.2) is sufficient for most applications. Smoothness in the order of Ra 0.8 and below — typical for sliding surfaces or seals — most often requires grinding or lapping. It is worth ordering it where the surface genuinely works, not across the whole part out of caution.

Which tolerances are truly critical

The key to a sensible cost is separating functional dimensions from free ones. A tight tolerance is worth reserving for:

• fits and diameters that mate with other parts,
• surfaces for bearings, bushings and seals,
• assembly dimensions and datums on which the geometry of the assembly depends,
• sliding and guiding surfaces.

The rest of the dimensions — outlines, chamfers, free dimensions — can stay in the general tolerance. The note "everything ±0.01 mm" is convenient for the designer but expensive in production and usually unnecessary. How to specify these requirements sensibly in the documentation is described in the article how to prepare a drawing for a quote.

Accuracy also depends on the material — material selection affects how easily and repeatably a dimension can be held during machining.

Decision framework: where to tighten, where to let go

Before you send a drawing for a quote, go through it dimension by dimension:

Summary

Accuracy in CNC machining is a cost decision, not a default setting. Most dimensions fit comfortably within the general tolerance, and tight tolerances and low roughness are worth reserving for surfaces that genuinely work. Such a drawing is cheaper to produce and faster to quote, with no loss of the part's function.

Not sure which tolerances are necessary? Send the drawing to Nomatec — we will advise what can stay in the general tolerance and where precision is essential, and we will select the technology: turning, milling or grinding for the most critical dimensions.