CNC machining tolerances: what they mean (and why they cost money)
CNC machining tolerances define the allowable variation from a nominal dimension on your drawing. In practice, tolerance is the “permission” a machinist has to hit your part size while accounting for tool deflection, material movement, heat, fixturing, and measurement uncertainty. The tighter the tolerance, the more operations, inspection steps, and process control are required.
If you are sourcing precision CNC machining for custom parts, getting tolerances right is one of the fastest ways to reduce cost and lead time without compromising function.
Quick tolerance glossary (plain English)
- Nominal: the target number on the drawing (e.g., 10.00 mm).
- Limit dimension: two numbers that define max/min (e.g., 9.98–10.02 mm).
- Plus/minus tolerance: variation around nominal (e.g., 10.00 ±0.02 mm).
- Unilateral tolerance: variation allowed only one direction (e.g., 10.00 +0.00 / -0.05 mm).
- Geometric tolerancing (GD&T): controls shape, orientation, and location (e.g., position, flatness) rather than only size.
- Datum: the reference feature used to measure other features.
- Tolerance stack-up: how multiple tolerances combine in an assembly.
Common “default” tolerances in CNC machining
Many shops quote a standard tolerance for general features and then treat tighter callouts as exceptions. A common pattern for general machining is around ±0.10 mm (±0.004 in) for non-critical features, with tighter tolerances applied only where needed.
However, “default” is not universal. The best practice is to specify a general tolerance standard (such as ISO 2768) or provide a general tolerance note on the drawing, then add specific callouts only for critical dimensions.
ISO 2768 explained (when you don’t want to tolerance every feature)
ISO 2768 is widely used for general tolerances on dimensions and geometry when individual tolerances are not specified. It helps reduce drawing clutter and avoids over-tolerancing every non-critical dimension.
In many CNC part drawings, you will see a note like:
“ISO 2768-mK unless otherwise specified.”
This means:
- m is the class for general dimensional tolerances.
- K is the class for general geometrical tolerances.
- Specific features can still override the general tolerance with their own callouts.
Using ISO 2768 correctly is one of the simplest ways to speed up quoting and prevent unnecessary inspection time.
GD&T basics for CNC machined parts (what buyers should know)
GD&T (Geometric Dimensioning and Tolerancing) is often the difference between a part that “measures right” and a part that actually assembles correctly. For example, a hole can be the right diameter but still be in the wrong location. GD&T controls that.
Common GD&T callouts in custom CNC parts:
- Position: controls the location of holes/features relative to datums (critical for assemblies).
- Flatness: controls how flat a surface is (important for sealing surfaces).
- Perpendicularity: controls squareness to a datum (important for mating features).
- Parallelism: keeps surfaces aligned (important for sliding or clamping features).
- Runout: controls how a rotating feature “wobbles” (shafts, bores, spindles).
If you are not using GD&T, you may be forced to specify very tight linear tolerances everywhere to “be safe,” which increases CNC machining cost without improving functional assembly.
Machining tolerance chart: what to expect (typical ranges)
Every supplier has a different capability window depending on machine rigidity, fixturing, part size, material, and inspection equipment. The table below is a practical starting point for machining tolerance expectations on most milled/turned parts.
| Feature type | Typical “standard” tolerance | Tight tolerance (needs justification) | Notes |
|---|---|---|---|
| Overall non-critical dimensions | ±0.10 mm | ±0.02 mm | Large parts or thin walls may require looser tolerances. |
| Hole diameter (reamed/boring) | ±0.03 mm | ±0.01 mm | Reaming/boring improves size consistency vs drilling alone. |
| Hole position (assembly holes) | 0.10–0.20 mm true position | ≤0.05 mm true position | Use GD&T position with datums to communicate intent. |
| Flatness (mating surface) | 0.10 mm | 0.03 mm | Surface finish, stress relief, and clamping strategy matter. |
| Threaded holes | Per standard | Per standard | Threads have their own tolerance classes; focus on position and depth. |
Tip: If your drawing uses “tight tolerances everywhere,” ask yourself: which dimensions are truly functional? Moving non-critical dimensions back to standard tolerance is a common way to reduce quote price and shorten lead time.
What makes tight tolerance machining expensive?
Tight tolerance machining isn’t just about “better machines.” It usually triggers a chain of process upgrades:
- More setups and finishing passes: leaving stock for a final pass after stress is relieved.
- Slower cycle times: conservative feeds/speeds to control deflection and heat.
- Special tooling: reamers, boring heads, stabilized end mills, probing cycles.
- Temperature control: measuring and machining at stable temperature.
- Higher inspection load: more frequent checks, CMM reports, SPC data.
- Higher scrap risk: one drift can scrap the part late in the process.
How to choose tolerances: a practical method
Use this approach when you want a high-performing part without paying for unnecessary precision:
- Identify functional interfaces: mating faces, press fits, bearing seats, alignment holes.
- Translate function into measurable controls: GD&T position for hole patterns, flatness for sealing, runout for rotating parts.
- Leave non-functional features “standard”: cosmetic edges, outside profiles, clearance pockets.
- Control assembly, not individual dimensions: specify datums and patterns to reduce tolerance stack-up issues.
- Ask for feedback: a good CNC supplier will suggest tolerance relaxations that keep function intact.
Press fits, slip fits, and hole/shaft tolerances (common trap)
Fits are often the most sensitive tolerance area in CNC machining because they combine material choice, surface finish, and dimension control. If you need a bearing seat or a dowel pin fit, don’t just specify a tight diameter tolerance—define the fit intent.
Practical guidance:
- Slip fit: easier assembly; allow clearance; specify surface finish if needed for sliding.
- Transition fit: minimal clearance; consistent alignment; may need controlled assembly force.
- Press fit: interference; requires material and temperature considerations; risk of distortion on thin walls.
If possible, provide a fit class or the mating part’s nominal size so the machinist can validate the tolerance stack realistically.
Inspection and reporting: when you need a CMM report
For many production and high-reliability applications, buyers request a CMM inspection report or a first article inspection (FAI). This is especially common with:
- tight positional tolerances on hole patterns
- complex freeform surfaces
- critical datums and multi-feature relationships
- regulated or audited supply chains
- repeat production where process drift must be controlled
If you request a CMM report, specify which dimensions/features must be reported (or provide a ballooned drawing). Reporting everything on a dense drawing can add significant time and cost.
FAQ: CNC machining tolerances
What tolerance is “standard” for CNC machining?
Many CNC shops treat around ±0.10 mm (±0.004 in) as a general standard for non-critical milled/turned features, but it depends on part size, material, and geometry. The best practice is to include a general tolerance note (for example ISO 2768) and tighten only the features that matter.
When should I use GD&T instead of tight linear tolerances?
Use GD&T when assembly depends on relationships between features: hole patterns, perpendicular faces, bearing bores, or any situation where “location/orientation” matters more than the raw dimension. GD&T often reduces cost because it targets what truly controls function.
How do tolerances affect CNC machining lead time?
Tighter tolerances typically increase lead time because they require more careful process planning, additional finishing passes, and more inspection checkpoints. If you can relax tolerances on non-critical features, you often get a faster quote and a shorter delivery.
Need help optimizing tolerances for custom CNC parts?
If you share your drawing and the functional requirements (fit, sealing, alignment), a good supplier can propose a tolerance strategy that keeps performance while lowering cost. For more CNC machining guides and manufacturing tips, visit jingoucnc.com.
