Table of Contents
If your CNC supplier asks which dimensions are critical, there is usually a good reason. A drawing may place tight limits on every feature, even though only a few surfaces control the fit, seal, movement, or alignment. That can raise the quote without improving the final part.
Reading this guide will help you choose practical CNC machining tolerances, understand common industry references, and see how material, geometry, setup, and inspection affect the result. It also explains when tighter control is worth paying for and what information your drawing should provide before machining begins.
Standard CNC Machining Tolerances
There is no single default tolerance followed by every machine shop. The value depends on the supplier, material, feature size, part geometry, finishing, and inspection plan. Individually marked dimensions and GD&T requirements also override general drawing tolerances.
General CNC machining tolerances often fall around ±0.005 in for unspecified metal dimensions, while plastic parts may require wider allowances. Closer tolerances may be available for selected features, but the actual capability depends on the material, geometry, setup, process, and inspection method.
CNC Machining Tolerance Chart
The figures below are useful during early design work. Final values still need to be agreed upon for the actual part.
| Requirement | Metric Reference | Inch Reference | Notes |
| General metal dimensions | ±0.127 mm | ±0.005 in | Common public supplier reference |
| General plastic dimensions | ±0.254 mm | ±0.010 in | Material movement needs review |
| Closer machined dimensions | ±0.051 mm | ±0.002 in | Should be identified on the drawing |
| Tighter critical features | Project review | Below ±0.002 in | May need special processing or inspection |
A CNC machining tolerance chart always needs context. A value that is straightforward on a short aluminum diameter may be difficult on a large plastic plate, a long shaft, or a thin stainless steel wall.
Tolerance is also different from machine accuracy and repeatability. Tolerance is the allowed range on the drawing. Accuracy describes how closely the result reaches the target, while repeatability describes how consistently the process produces similar results.
ISO and ASME Standards
ISO 2768-1 is widely used for linear and angular dimensions without individual tolerance callouts. It allows ordinary dimensions to follow a stated general tolerance class instead of placing a separate value beside every feature.
As of July 2026, ISO 2768-1:1989 remains published, while its replacement, ISO 2768 Edition 2, is under publication. ASME Y14.5-2018, reaffirmed in 2024, remains the current ASME reference for GD&T. Drawings should identify the standard, edition, units, and tolerance class being used.
Four Ways to Specify CNC Tolerances
Machined-part drawings commonly communicate allowable variation through the following four formats.
Bilateral Tolerance
A bilateral tolerance permits variation on both sides of the nominal dimension. The positive and negative allowances may be equal or different.
It works well when movement in either direction will not interfere with the part’s function.
Unilateral Tolerance
A unilateral tolerance places the permitted variation on only one side of the nominal size.
This is useful when a feature may safely become smaller but must not become larger, or the other way around.
Limit Tolerance
Limit dimensions state the highest and lowest acceptable values directly. Inspectors can compare the measured result against those limits without calculating the deviation.
Geometric Tolerance
GD&T controls form, orientation, location, and runout. Common controls include flatness, parallelism, perpendicularity, position, profile, cylindricity, and runout.
A clear datum structure can communicate how features must relate to one another without forcing every coordinate dimension into a narrow plus-or-minus range. ASME Y14.5 provides the recognized US framework for applying and interpreting these controls.
CNC Milling vs Turning Tolerances
Milling and turning both produce precise components, but each process handles different types of features. Neither is automatically more accurate in every situation.
CNC Milling Tolerances
CNC milling tolerances often apply to faces, pockets, slots, hole positions, and features spread across several sides of a part. Related features are usually easier to control when they share a datum and can be completed in one setup.
Deep pockets may cause tool deflection. Thin walls can move during cutting or spring back after unclamping, while multiple setups add more chances for locating error.
CNC Turning Tolerances
CNC turning tolerances commonly focus on diameter, roundness, cylindricity, shoulder position, and runout. Coaxial features machined in one clamping are often easier to keep aligned.
Long shafts may bend or vibrate, and thin rings can distort. Reversing the part for a second setup also introduces a new locating step, so turning should not be assumed to be tighter than milling across the board.
What Affects CNC Machining Tolerances?
The final result depends on more than the machine model. Material behavior, geometry, tooling, setup, temperature, and inspection all play a part.
Material and Part Geometry
Aluminum, stainless steel, brass, titanium, and engineering plastics respond differently to cutting forces and heat. The specific grade and heat-treatment condition matter as well. Plastics may move with temperature, stainless steel can work-harden, and stressed stock may shift as material is removed.
Thin walls, deep cavities, large plates, long shafts, and unsupported features are more likely to deflect or distort. Tight limits on these areas should be reviewed alongside the cutting and clamping plan.
Machine, Tooling, and Setup
Machine condition sets the foundation, but tool runout, edge wear, cutting parameters, fixture rigidity, clamping pressure, and datum selection determine how well that capability reaches the part.
Setup planning matters too. Related features are generally easier to control when the workpiece does not have to be removed and relocated. A multi-axis machine can reduce setups, but it cannot compensate for an unstable part or an unclear datum system.
Temperature and Inspection
Dimensions change with temperature. The machine, tool, workpiece, workshop, and inspection room may all be at slightly different temperatures, which becomes more important as the permitted range narrows.
The measuring method must suit the requirement. Calipers may be adequate for general dimensions, while bores, positions, profiles, and geometric relationships may require micrometers, gauges, or a CMM. A tolerance that cannot be measured reliably cannot be accepted reliably.
How Tight Should CNC Tolerances Be?
Tighter is not always better. A useful tolerance protects fit, function, interchangeability, sealing, alignment, or safety. Anything beyond that may only restrict manufacturing.
Reasonable Machining Tolerances
Bearing seats, mating diameters, sealing faces, locating holes, press fits, slip fits, and datum surfaces often require closer control.
Clearance holes, outer profiles, and nonmating surfaces can usually follow a general drawing tolerance unless they affect assembly or appearance. Even in medical, aerospace, or automotive work, the requirement should be tied to the function of a specific feature rather than applied equally across the whole part.
Why Tight Tolerances Cost More
As the acceptance range becomes smaller, the machinist has less room to manage tool wear, material movement, setup variation, and temperature. Costs often rise on both the production and inspection sides.
Machining and Setup Cost
Tight tolerance CNC machining may call for stronger fixtures, slower finishing passes, extra tool compensation, more tool changes, machine warm-up, or secondary processing.
Scrap risk rises as well. A dimensional shift that would be harmless under a general tolerance may reject the same part under a narrow limit. Public machining guides from Protolabs and Xometry both note that tighter tolerances can increase machining, setup, inspection, and rejection costs.
Inspection and Quality Control
CNC machining with tight tolerances may require CMM programming, dedicated gauges, first-article inspection, in-process checks, dimensional reports, or more frequent sampling.
Before ordering tight tolerance CNC machining services, agree on how the feature will be measured, which standard controls acceptance, and what documentation must be delivered. Different measurement methods can otherwise produce disagreements even when both sides are working carefully.
How to Specify Tolerances on a CNC Drawing
A good drawing identifies what affects the product and leaves reasonable manufacturing freedom elsewhere.
Control Critical Features
Apply close tolerances to dimensions that control assembly, sealing, bearings, movement, alignment, or safety. Let ordinary dimensions follow the general tolerance in the title block.
This keeps the shop focused on the features that matter and avoids paying for precision that does not improve the product.
Add Datums, GD&T, and Surface Finish
Use datums to show how the part should be located and measured. Apply GD&T where form, orientation, or position matters more than a simple dimensional limit.
Surface roughness is a separate requirement. A dimension may be acceptable while the surface is still unsuitable for a seal, bearing, or cosmetic face. Coatings and plating should also be considered because they may change final dimensions.
Send Complete Manufacturing Files
A useful machining review normally requires:
- 3D CAD model and matching 2D drawing
- Material, grade, and heat treatment
- Units and general tolerance standard
- Critical dimensions and GD&T
- Surface finish and coating requirements
- Quantity and repeat demand
- Inspection and reporting requirements
- Relevant assembly information
A 3D model alone may not communicate the functional tolerances of a complex part. HingTung’s CNC machining services support custom metal and engineering-plastic components through design review, machining, finishing, and inspection.
FAQs
What are the four types of tolerance?
Four common formats are bilateral, unilateral, limit, and geometric tolerance. Bilateral and unilateral tolerances control dimensional variation around a nominal size. Limit dimensions state the maximum and minimum directly, while geometric tolerances control form, orientation, location, and runout.
What is the tightest CNC tolerance?
There is no single value that applies to every part. The achievable tolerance depends on the material, feature size, geometry, machining process, setup, temperature, and inspection method. Very close requirements may need grinding, honing, EDM, special fixtures, or controlled metrology.
What are reasonable tolerances for machining?
Reasonable tolerances are wide enough for stable production but close enough to protect the part’s function. General dimensions can follow a drawing or supplier standard, while bearings, seals, locating features, and critical mating dimensions should be specified separately.
Are CNC turning tolerances tighter than milling tolerances?
Not necessarily. Turning can control coaxial diameters efficiently in one setup, while milling may be better for planar relationships and complex feature positions. Part stiffness, material, tool access, fixture design, and the number of setups matter more than the process name alone.
Conclusion
Good CNC machining tolerances focus tight control on the dimensions that affect fit, sealing, movement, alignment, and safety. The rest can usually follow a suitable general tolerance, making the part easier to produce and inspect.
For injection molding, CNC machining, sheet metal fabrication, or related manufacturing services, contact HingTung with your drawings, material, finish, quantity, and quality requirements. Our engineering team can review the design, identify manufacturing risks, recommend a practical process, and prepare a quotation based on the actual project.