How to Specify CNC Machining Tolerances Without Blowing Up Prototype Costs

Tolerances are one of the most misunderstood specifications in machined component drawings. In CNC machining, tightening a dimension from ±0.1 mm to ±0.02 mm can dramatically affect cost and lead time. Yet many engineers apply tight tolerances across the board out of habit, not necessity. A more effective approach is to set generous general tolerances, then selectively tighten only features that affect fit, function, or assembly. This article offers practical guidance for making that distinction while still meeting your engineering requirements.

Start with ISO 2768. This international standard defines general tolerances for linear dimensions, radii, and chamfers. For most machined parts, specifying ISO 2768-m on the drawing is sufficient for non-critical features. This tells the machine shop that dimensions without an explicit tolerance can be held to a moderate, cost-effective range. For example, a one-millimeter chamfer or a 50 mm bolt-pattern spacing does not need a separate tolerance if the function allows a ±0.1 mm or ±0.3 mm general band. Reserve customized tolerance blocks for true functional interfaces, such as a bearing bore with a +0.02/−0.00 mm fit or a locating pin hole that must align with an assembly datum.

Material choice interacts strongly with tolerance capability. Aluminum 6061-T6 is an excellent, stable workpiece for achieving ±0.05 mm or tighter in most features. If higher strength is needed, 7075-T6 is also machinable, but residual stresses from heat treatment can cause movement during machining unless the supplier stress-relieves the blank. Stainless steel grades behave differently; 303 is a free-machining alloy that holds tight tolerances reliably, while 304 and 316 are more prone to work-hardening. That can increase tool wear and cycle time when you call out a close tolerance on a deep slot or a small drilled hole. Engineering plastics such as POM/Delrin expand more with temperature, so a claimed ±0.02 mm dimension on a long part may not be stable at wide temperature swings. For such materials, it is often better to design with slightly looser tolerances and to specify a controlled machining environment.

Surface finish and tolerance often go together, but they are not the same thing. A smooth finish of Ra 0.8 µm or better is achievable with standard carbide tooling and appropriate feeds and speeds, but it does not automatically guarantee a tight dimensional tolerance. For a precision bore, you might need both a Ra 0.8 finish and a circularity tolerance. Flatness on a large-machined face, however, is more dependent on clamping and stress relief than on surface roughness. If you require flatness within 0.01 mm on a 150 mm plate, mention it as a dedicated geometric control, not as an overall dimension tolerance. Also, avoid using coordinate tolerances for hole patterns because they create a square tolerance zone. Using GD&T position controls a pattern with a circular tolerance zone, which is more logical and often allows the machinist to work more efficiently while still meeting your functional needs.

Every tight tolerance callout forces the shop to reduce cutting parameters, perform intermediate probing, and add inspection steps. In a prototype run of one to ten parts, this can mean additional setup time for special tooling or fixtures, increasing both price and lead time. One production shop’s rule of thumb is that tightening a dimension from ±0.1 mm to ±0.025 mm might add 10–20% to machining time on that feature, although this depends on feature size and material. The real expense often comes from scrap risk and secondary measuring equipment, not just spindle time. By keeping standard turnings loose and specifying only the assemblies’ critical fits, you let your CNC machining partner use high metal removal rates, which reduces cost and shortens delivery.

A practical method is to apply ISO 2768 for all unspecified dimensions, then add a table of controlled features with explicit tolerance values and GD&T callouts. For example, set the bore for a bearing at +0.02/−0.00 mm, use positional tolerance for the bolt circle, and leave every other dimension based on the drawing’s general title-block tolerance. When quoting, ask your supplier to identify features that are unusually troublesome, such as hardened steel materials or very thin walls. Many shops, including precision job shops, can hold ±0.025 mm in 6061 aluminum reliably, but they may charge a premium because of increased measurement requirements. With a thoughtful, selective tolerance strategy, your parts will be easier to manufacture, your prototype costs will stay predictable, and your design will still perform in the field.