CNC Machining Tolerances

What a tolerance actually is

A tolerance is the amount a dimension is allowed to vary from its nominal value and still be acceptable (source 1). A Machine Design article quotes the ASME Y14.5M definition: the total amount a dimension is permitted to vary, the difference between its maximum and minimum limits (source 5). No real feature comes out at exactly the number on the print, so the print says how much variation the part can have and still work. Write a dimension as 1.000 in ±0.005 in and any part that measures between 0.995 in and 1.005 in passes. That is a 0.010 in band: the plus side and the minus side added together. A print can also show the same band as upper and lower limits, 1.005 in and 0.995 in; both ways define the same range (source 5).

Tolerance is about the band, not the target. A part at 1.004 in is just as good as one at 1.000 in if your callout is ±0.005 in. The tighter the band, the less room there is for variation, and published guides tie that to what a part costs (sources 2 and 5). Most of the skill in a good drawing is putting the band where it matters and leaving it loose everywhere else.

Standard machining tolerances and what they cover

For most custom parts, ±0.005 in is the typical machined tolerance (source 5): outside dimensions, hole locations, slot widths, general block sizing. It suits most brackets, plates and housings, and if a dimension has no reason to be tighter, it is the right place to leave it. Under ASME Y14.5 every dimension carries a tolerance, either directly or through a general note in the title block (source 3), so that note is what covers any dimension with no tolerance of its own.

Critical turned diameters are typically ±0.001 in. Put that on the features that mate, such as a bore that takes a bearing or a shaft that slips into a mating part. A sensible block default plus a handful of features called out closer covers most custom parts. Laser-cut parts have their own tolerance class, and laser cut tolerances covers it; the figures on this page are for machined parts.

Tighter tolerances and how they’re reached

Tight bands often come from fits. Press, locational and close-running fits are named by hole and shaft tolerance pairs such as H7/p6 and H8/f7 (source 2), and whether a feature needs one is a question about what it has to do. On a milled feature, a callout tighter than the general tolerance is quoted from the print.

The tightest bands come from finishing processes rather than a milling cut: ground features are quoted to ±0.0001 in, and wire-EDM and other finished features down to ±0.0002 in. Grinding is used primarily to finish surfaces and suits very hard materials better than regular machining does (source 8), and wire EDM is used to make punches, tools and dies from hard metals that are difficult to machine by other methods (source 7). Precision grinding and wire EDM each have a page. If a part lives in this range across several features, it belongs in the precision machined components conversation, where the plan accounts for those steps up front. The forming and EDM punches page covers hardened tool-steel parts that call for this kind of tolerance.

Why tighter tolerances cost more

A narrower band leaves less room for error, and it shows up in the quote. A 2023 Machine Design article on designing parts for CNC machining lists increased scrap, additional fixturing, special measurement tools and longer inspection as the added costs of tighter tolerances (source 6), and the Wikipedia entry on engineering fits says more accurate fits are more expensive to produce (source 2). Temperature is part of it: a part’s size changes with temperature, and industrial lengths are defined at 20 °C (68 °F) (source 4), so a callout in the ten-thousandths leaves little room for a part that is warmer or cooler than that when it is checked.

That is why over-tolerancing is a cost question and not a quality one: where the function does not need the band, it adds to the quote without adding to the part. The machining cost page covers the other drivers.

How to spec tolerances without overpaying

The common drawing habit to avoid is over-tolerancing: blanket-tightening every dimension when only one bore, one mating face or one locating feature actually needs it. Menin’s Machine Design article puts it plainly: unnecessarily tight tolerances make parts more expensive, and there is no reason to apply ±0.0002 when ±0.002 will do (source 5). The fix is simple: set a sensible default block tolerance (±0.005 in is a good starting point), then tighten only the features that mate, seal or locate. Over-tolerancing is one of several drawing habits that affect a quote, and design choices that raise a quote walks through the others.

Spec well by being explicit. Call out your critical dimensions by name on the print, use standard fits for holes and shafts instead of inventing numbers (source 2), and let the loose features stay loose. If you are not sure where the line is, the free design review that comes with every quote flags a callout that is tighter than the part needs, and whether to change it is your call. Send your files, any type, with STEP and PDF ideal, and you will get a quote in 24-48 hours.

Sources

All accessed 4 October 2026. Where a figure here differs from your print, the print governs.

1. Wikipedia, "Engineering tolerance": en.wikipedia.org/wiki/Engineering_tolerance (tolerance is the permissible limit or limits of variation in a physical dimension).

2. Wikipedia, "Engineering fit": en.wikipedia.org/wiki/Engineering_fit (clearance, transition and interference fits; hole-basis designations such as H7/h6, H8/f7 close-running and H7/p6 press fit; "more accurate fits will be more expensive to produce").

3. Wikipedia, "Geometric dimensioning and tolerancing": en.wikipedia.org/wiki/Geometric_dimensioning_and_tolerancing (ASME Y14.5 is one of the standards that define GD&T; form, orientation, location and run-out controls including position, perpendicularity and circular run-out; every dimension must have a tolerance, applied directly or from a general tolerance block or note).

4. Ted Doiron, "20 °C: A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements", Journal of Research of the National Institute of Standards and Technology, vol. 112, no. 1, February 2007: pmc.ncbi.nlm.nih.gov/articles/PMC4654601 (a part dimension changes with temperature because of thermal expansion; since 1931 industrial lengths have been defined as the size at 20 °C).

5. Stefan Menin, "Avoiding Problems with Dimensional Tolerances", Machine Design, updated 13 February 2023: machinedesign.com/3d-printing-cad/article/21831626/avoiding-problems-with-dimensional-tolerances (the ASME Y14.5M definition of tolerance as the difference between the maximum and minimum limits; limits and a plus-or-minus amount define the same range; "Unnecessarily tight tolerances make parts more expensive"; no reason to apply ±0.0002 when ±0.002 will do).

6. Greg Paulsen, "Top Tips on Designing Parts for CNC Machining", Machine Design, 8 May 2023: machinedesign.com/automation-iiot/article/21265541/top-tips-on-designing-parts-for-cnc-machining (tighter tolerances add costs due to increased scrap, additional fixturing and special measurement tools, and increased inspection times).

7. Wikipedia, "Electrical discharge machining": en.wikipedia.org/wiki/Electrical_discharge_machining (wire EDM is used to make punches, tools and dies from hard metals that are difficult to machine with other methods).

8. Wikipedia, "Grinding (abrasive cutting)": en.wikipedia.org/wiki/Grinding_(abrasive_cutting) (normal grinding is used primarily to finish surfaces; usually better suited to very hard materials than regular machining).