CNC Machining Tolerances: The Complete Guide for Engineers (2026)

Standard CNC machining tolerance is ±0.001 inch (±0.025 mm) for most linear dimensions and hole diameters. Tight tolerances of ±0.0005 inch (±0.013 mm) are achievable but increase cost by 30–60%. Understanding when to specify each level is one of the highest-leverage decisions you can make in part design.

Raj, a mechanical engineer at a robotics startup, initially sent us bracket design drawings with tolerances of ±0.005 mm for every dimension. He believed the tighter the better. The initial quote was $124 per part. Our engineers reviewed the CAD files and raised a simple question: which features actually mate with the other parts? Raj identified three key dimensions. We relaxed the tolerances of the other nine dimensions to standard tolerances. As a result, the quote was revised to $71 per part. Functionality remained the same, fit remained the same, and a 43% saving was achieved.

This guide shows you exactly how to make those calls. You will learn what standard and tight tolerances mean in practice, how material choice affects achievable precision, how tolerance decisions drive cost, and how to specify tolerances on your drawings so your parts arrive right the first time without paying for precision you do not need. For a more in-depth understanding of CNC machining services, please read the CNC Machining Guide.

Key Takeaways

• Standard CNC machining tolerance is ±0.001 inch (±0.025 mm) for most linear dimensions
• Tight tolerances of ±0.0005 inch (±0.013 mm) can increase part cost by 30–60%
• Aluminum 6061 can hold ±0.0002 inch; plastics like PEEK typically hold ±0.001 inch
• Only specify tight tolerances on features that mate with other parts or affect function
• Upload your CAD to Baetro for free tolerance optimization feedback with every quote

What Are CNC Machining Tolerances?

A tolerance is the permissible limit of variation in a physical dimension. In CNC machining, every dimension on your drawing, every hole diameter, every wall thickness, and every edge-to-edge distance has an ideal value called the nominal dimension and an acceptable range around it called the tolerance.

For example, if you specify a hole as 0.250 ± 0.001 inch, the actual machined hole can be anywhere from 0.249 to 0.251 inch and still be acceptable. That ±0.001 inch range is the tolerance zone. Without tolerances, manufacturers do not know how precise your part needs to be. And without the right tolerances, you might be paying for unnecessary precision or receiving parts that do not fit.

CNC machining tolerances matter for three reasons: fit, function, and cost. A shaft that must press into a bearing needs a tight tolerance. A mounting bracket that bolts to a frame does not. The trick is matching the tolerance to the actual requirement.

Standard CNC Machining Tolerances

General Tolerance Table

Most CNC machine shops quote with these standard tolerances unless you specify otherwise:

These values align with ISO 2768-m, the general tolerance standard for machined parts. They are the tolerances you get when you do not specify anything special. For the majority of mechanical parts, including brackets, housings, and structural components, these standard tolerances are sufficient.

When Standard Tolerances Are Enough

Standard tolerances work well for:

• Mounting brackets and plates
• Enclosures and housings
• Non-critical structural components
• Parts with generous clearances
• Prototypes where fit and function are being validated

If your part falls into these categories, resist the urge to specify tighter tolerances. You will save 20–40% on machining cost by accepting standard precision. At Baetro, our engineers review every CAD file that comes through our instant quote system and flag dimensions where standard tolerance is sufficient.

Tight Tolerance CNC Machining

What Qualifies as “Tight”

Tight tolerances generally refer to precision beyond the standard range:

• Linear dimensions: ±0.0005 inch (±0.013 mm) or better
• Hole positions: ±0.002 inch (±0.05 mm) or better
• Surface finish: Ra 32 μin (0.8 μm) or better
• Geometric tolerances: Concentricity, perpendicularity, and parallelism within 0.001 inch

These levels require slower machining speeds, more rigid workholding, and specialized inspection equipment. They are achievable on modern CNC centers but should be reserved for features that genuinely need them.

When You Actually Need Tight Tolerances

Specify tight tolerances for:

• Shafts and bearings requiring press fits
• Aerospace components (often ±0.0002 inch or better)
• Medical instruments with precisely-mated parts
• Optical mounting components
• Gears and transmission components
• Features that mate with purchased components

When Maria, an engineer at a medical device company, designed a stainless steel connector for a surgical instrument, the press-fit interface with a bearing required ±0.0005 inch to avoid looseness under load. That single tight dimension was justified. The mounting holes on the same part did not need the same precision. She specified standard tolerance for the holes and tight tolerance only for the bearing bore. The part functioned perfectly and cost 28% less than an earlier iteration where every dimension was tightened.

Cost Impact of Tight Tolerances

Tight tolerances increase cost in three ways. First, they require slower cutting speeds and more careful workholding, which extends machining time. Second, they demand inspection on a Coordinate Measuring Machine (CMM) instead of a caliper, adding inspection time and cost. Third, the scrap rate rises because more parts fall outside the acceptable range.

Expect tight tolerances to add 30–60% to your part cost compared to standard tolerances. On complex parts with multiple tight features, the premium can exceed 100%. The solution is simple: tolerate only what functionally matters.

For a deeper look at how tolerances affect your overall project budget, see how tolerances affect CNC machining cost.

How to Specify Tolerances on Your Drawing

Best Practices

Use Geometric Dimensioning and Tolerancing (GD&T) per ASME Y14.5 when multiple features relate to each other. GD&T controls not just size but form, orientation, and location. It is more powerful and often more economical than traditional plus-or-minus tolerances because it uses datum structures and bonus tolerances to reduce unnecessary precision.

Do not over-tolerance. Only specify tight tolerances where they matter. A mounting bracket that accepts an M6 bolt does not need ±0.0005 inch on the hole position. A shaft that must press-fit into a bearing does.

Your datum structure should reflect how the part is actually used. The surface that mates with another part is usually your primary datum. Match tolerance to the manufacturing process. A feature accessible to a milling cutter in a single setup can hold a tighter tolerance than one requiring repositioning.

Read our Design for Manufacturability guide for a full rules set.

Common Mistakes to Avoid

Specifying the same tolerance on every dimension is the most common mistake we see. It signals that the designer has not thought about which features matter. Using bilateral tolerances when unilateral tolerance is sufficient is another issue. If a hole must be no smaller than 0.250 inch but can be larger, specify 0.250 +0.002/-0.000 instead of ±0.001. That gives the machinist more flexibility and often reduces cost.

Ignoring thermal expansion in tolerance stack-ups is a frequent oversight. Aluminum expands roughly twice as much as steel per degree of temperature change. If your assembly includes both materials, account for differential expansion in your tolerance analysis. Finally, do not confuse surface finish requirements with dimensional tolerances. A part can be exactly the right size but have a rough surface, or vice versa. Specify both independently.

Reference ASME Y14.5 for the full GD&T standard, or ask our engineers to recommend the right datum structure for your part.

Material-Specific CNC Machining Tolerances

Different materials behave differently during machining. Thermal stability, machinability, and work-hardening characteristics all affect the tightest tolerance a given material can reliably hold.

Materials with poor thermal stability, primarily plastics and some aluminum alloys, are harder to hold tight tolerances because they expand and contract with cutting heat. A PEEK part machined at 20°C and measured at 25°C can shift by several thousandths of an inch. For tight-tolerance plastic parts, controlled-environment machining and extended cooling periods are essential.

Tolerance Stack-Up Analysis

When multiple parts assemble, their individual tolerances compound. A tolerance stack-up analysis predicts the worst-case assembly variation.

For example, if three parts each have a critical dimension with ±0.001 inch tolerance, the total variation in the assembly could be ±0.003 inch. If your design cannot accommodate that, you need to either tighten individual tolerances or add clearance.

There are three strategies to manage stack-up. First, tighten only the critical dimensions that directly affect the assembly interface. Second, add clearance or shim points in the design to absorb variation. Third, use GD&T bonus tolerances and maximum material condition modifiers to gain additional tolerance zone where function allows. Proper GD&T datum structures can reduce tolerance zone waste by 20–40% compared to traditional plus-or-minus tolerances.

Surface Finish vs. Dimensional Tolerance

Surface finish and dimensional tolerance are related but distinct. Dimensional tolerance controls how close the actual size is to the specified size. Surface finish controls how smooth or rough the surface is. A part can be exactly the right size but have a rough surface, or vice versa. Specify both independently.

Typical surface finish callouts for CNC machined parts include:

• Ra 125 μin (3.2 μm): Standard machined finish, visible tool marks
• Ra 63 μin (1.6 μm): Smooth machined finish, fine tool marks
• Ra 32 μin (0.8 μm): Fine finish, requires grinding or polishing
• Ra 16 μin (0.4 μm): Very fine finish, requires precision grinding

Do not specify a tight dimensional tolerance hoping it will improve surface finish. They are controlled by different process parameters. If you need both precision and smoothness, specify both requirements clearly on your drawing.

If you want to weigh the look, durability, and cost trade-offs of each option, our detailed guide to CNC machining surface finish breaks down when each finish is worth the extra spend.

Inspection and Verification Methods

The inspection method must match the tolerance level. Using the wrong tool wastes time or misses errors.

For standard tolerance parts, a digital caliper or micrometer is sufficient. When tolerances tighten to ±0.0005 inch or better, CMM inspection becomes necessary. CMM inspection adds 15–30 minutes per part and increases inspection cost by 3–5x over caliper checks.

At Baetro, every part receives dimensional inspection. Standard tolerance features are checked with calipers and micrometers. Tight tolerance parts are verified on CMM equipment, and full inspection reports are provided with every order.

Tolerance Optimization: A Real Example

Here is a before-and-after from our shop that shows the power of tolerance optimization.

Before: An aluminum 6061 bracket with 12 dimensions, all specified at ±0.005 mm. The part had four mounting holes, two locating pins, and a central bore. Every feature was treated as critical. Quote: $124 per part at quantity 10. Lead time: 8 days. Every dimension required CMM verification.

After: Our engineer reviewed the CAD file and identified three functionally critical features: the two locating pins that aligned the bracket to a mating plate, and the central bore that accepted a pivot shaft. Those three dimensions stayed at ±0.005 mm. The four mounting holes, wall thicknesses, and overall length were relaxed to standard ±0.025 mm. The bracket still bolted securely, still aligned perfectly, and still pivoted smoothly.

Revised quote: $71 per part at quantity 10. Lead time: 5 days. Only three features required CMM inspection; the rest were verified with calipers. Total savings: 43% with zero functional impact.

This is the kind of review we perform on every CAD file uploaded to our instant quote system. It costs nothing and often saves 20–40%.

FAQ

What is the standard tolerance for CNC machining?

Standard CNC machining tolerance is ±0.001 inch (±0.025 mm) for linear dimensions and hole diameters. Hole positions are typically held to ±0.005 inch (±0.13 mm), and angularity to ±0.5°. These values align with ISO 2768-m and are sufficient for approximately 80% of machined parts.

How tight can CNC machining hold tolerances?

CNC machining can hold tolerances as tight as ±0.0002 inch (±0.005 mm) on aluminum 6061 in ideal conditions. Stainless steel and titanium can reliably achieve ±0.0005 inch (±0.013 mm). Plastics like PEEK and ABS are generally limited to ±0.001 inch (±0.025 mm) due to thermal expansion during machining.

What is ISO 2768 and when does it apply?

ISO 2768 is an international standard that defines general tolerances for linear and angular dimensions without individual tolerance indications. The “m” class (medium) specifies ±0.001 inch for dimensions under 0.12 inch and scales from there. It applies when your drawing does not specify tolerances on individual dimensions.

How much do tight tolerances increase CNC machining cost?

Tight tolerances typically increase CNC machining cost by 30–60% compared to standard tolerances. On complex parts with multiple tight features, the premium can exceed 100%. The increase comes from slower machining speeds, CMM inspection requirements, and higher scrap rates.

What is the best way to specify tolerances on a drawing?

Use GD&T per ASME Y14.5 when features relate to each other. Tolerance only the dimensions that affect function or mating. Use standard tolerances for the rest. Match your datum structure to how the part is actually used. And always specify surface finish separately from dimensional tolerance.

How do material properties affect achievable tolerances?

Thermal stability is the key factor. Materials with high thermal expansion, like plastics and some aluminum alloys, shift dimensionally as cutting heat builds up. Stable materials like stainless steel and titanium hold tighter tolerances more reliably. Machinability also matters: gummy or work-hardening materials produce inconsistent results under aggressive cutting.

Conclusion

Getting tolerances right is one of the highest-leverage decisions in part design. Standard tolerances of ±0.001 inch are sufficient for 80% of CNC-machined parts. Reserve tight tolerances for features that genuinely need them, such as press fits, bearing bores, and mating interfaces. Every unnecessarily tight dimension adds cost, extends lead time, and increases inspection burden without improving function.

The five rules that matter most are: tolerance only what functions require, use GD&T for related features, match tolerance to material capability, account for stack-up in assemblies, and specify surface finish independently from dimensional tolerance.