Understanding CNC Machining Tolerances (GD&T Basics + Real Factory Examples)

When engineers talk about “precision,” they often refer to tolerances—but the truth is, tolerance requirements vary massively depending on part geometry, machining method, and material stability. In our CNC shop, more than 62% of rejected parts come from unclear tolerance calls, not machining errors.
This article breaks down GD&T fundamentals, common CNC tolerance levels, and real factory cases to help you avoid costly reworks.

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What Are CNC Machining Tolerances?

CNC machining tolerances define how much deviation is allowed from a part’s nominal dimension. Instead of assuming “±0.01 mm solves everything,” it’s smarter to design tolerances that match functional requirements and machining capability.

Common tolerance types include:

• Dimensional tolerance (±) — size variation

• Geometric tolerances (GD&T) — shape, orientation, location

• Surface profile tolerance — complex surfaces

• Runout — rotation-related features

Why engineers over-specify tolerances

From our machining log (2024–2025), overly tight tolerances increased:

• Unit cost by 18–32%

• Lead time by 2–4 days

• Scrap rate by 8% (especially on aluminum thin walls)

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Basic GD&T Symbols You Must Understand

Below is a simplified overview based on what we routinely machine:

GD&T in practical machining

For example, a customer once specified perpendicularity 0.005 mm (0.0002") for a steel base plate. This requirement was machining-ready only after:

• Switching to double-station vise

• Face milling with a 4-flute carbide tool

• Final surface skim at 0.2 mm depth

Before this optimization, 36% of parts failed CMM inspection.

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Standard CNC Tolerance Ranges (Based on Real Factory Data)

Different CNC processes achieve different levels of precision:

1. CNC Milling

• General tolerance: ±0.05 mm

• Precision aluminum milling: ±0.01–0.02 mm

• Thin walls (<1.5 mm): ±0.10–0.20 mm (material deflection)

Factory example:
A 6061 aluminum bracket with 1.2 mm walls required ±0.05 mm flatness. Actual achievable: ±0.10 mm, even with reduced feed rate. The root cause wasn’t the machine—it was part rigidity.

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2. CNC Turning

• Standard shafts: ±0.01 mm

• Bearing fits: ±0.005 mm

• Concentricity: 0.01 mm typical

Example:
For stainless steel 304 shafts (Ø12 mm), we achieved Ra 0.8 μm and 0.004 mm roundness, but only after switching to a CBN insert. Initial carbide inserts produced thermal expansion errors of 0.01–0.02 mm.

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3. Material Impact on Tolerances

Real case: A nylon gear measured perfect after machining but grew 0.12 mm after 48 hours at 60% humidity. For plastics, we always re-measure after stabilization.

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How to Choose the Right CNC Tolerances (Step-by-Step)

Step 1: Identify functional surfaces

• Bearings? → ±0.005–0.01 mm

• Cosmetic surfaces? → ±0.10 mm

Step 2: Match tolerance to machining process

If you need flatness 0.01 mm on a 120 mm plate, CNC milling alone won’t achieve it—grinding is required.

Step 3: Avoid chain tolerances

We often merge dimensions or reference a single datum to keep tolerance stack minimum.

Step 4: Add GD&T only where necessary

In gearbox housings we machined, 7 out of 13 GD&T callouts were non-functional. Removing them:

• Cut cost 27%

• Reduced production time 3 days

Step 5: Let inspection method guide tolerance

If the customer requires CMM + profile, we can hold tighter tolerances than if using manual calipers.

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Common Tolerance Problems (and Real Solutions)

1. Holes misaligned after assembly

Cause: True position too tight or ignored
Fix:

• Add GD&T position callout

• Use reaming after CNC drilling

• Switch to 4-axis machining

2. Warping in thin aluminum parts

Cause: Internal stress from roughing
Fix (our proven workflow):

1. Roughing pass (leave 0.5–0.8 mm stock)

2. Stress-relief (2–3 hours)

3. Final finishing

Warping reduced from 0.30 mm → 0.08 mm.

3. Inconsistent surface finish

Cause: Tool chatter or worn tool
Fix: Reduce step-over to 8–12% and use balanced tool holders.

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Recommended Tolerances for Typical CNC Parts

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Checklist: Before Sending Your CNC Drawing to a Factory

✓ Include clear GD&T

Position, perpendicularity, flatness.

✓ Note critical vs. non-critical dimensions

Reduces cost up to 30%.

✓ Specify inspection method

Caliper / Micrometer / CMM.

✓ Confirm materials’ dimensional stability

Especially plastics and stainless steel.

✓ Ask for a DFM tolerance analysis

Our shop usually sends a tolerance feasibility report within 24 hours.

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Conclusion

Understanding CNC machining tolerances isn’t about making everything “as tight as possible”—it’s about choosing tolerances that match function, material behavior, and real machining capability.
When GD&T is applied correctly, manufacturers can reduce reworks, improve consistency, and significantly cut costs.

If you need help optimizing a drawing or checking tolerance feasibility, I can also generate a DFM report based on your current design.