Tolerance Cost Impact (2026)
|
Tolerance Class |
Tolerance Range |
Cost Premium |
Typical Application |
|
ISO 2768-m (Medium) |
±0.1 to ±0.5 mm |
0% (baseline) |
General machining — most common default |
|
ISO 2768-f (Fine) |
±0.05 to ±0.2 mm |
+5% to +15% |
Mechanical assemblies |
|
ISO 2768-extra fine |
±0.02 to ±0.1 mm |
+20% to +40% |
Precision mechanical |
|
±0.025 mm |
— |
+30% to +50% |
Bearing seats, sealing |
|
±0.01 mm |
— |
+50% to +100% |
Aerospace, semiconductor |
|
±0.005 mm or tighter |
— |
+100% to +300% |
Optical, metrology standards |
ISO 2768 Explained
ISO 2768 is the international standard for general tolerances on linearly and angularly dimensioned features. It's the easiest way to specify "default tolerance" without marking every dimension.
Tolerance Classes
ISO 2768 has two parallel classification systems:
|
Class |
Length Tolerance |
Angular Tolerance |
Use |
|
m (Medium) |
±0.1 to ±0.5 mm |
±1° to ±2° |
General machining — most common |
|
c (Coarse) |
±0.2 to ±1.0 mm |
±2° to ±4° |
Welded assemblies |
|
v (Very coarse) |
±0.5 to ±3.0 mm |
— |
Sheet metal, sand casting |
Length Tolerance Tables (ISO 2768-m Medium)
|
Length Range (mm) |
Tolerance (mm) |
|
3 – 6 |
±0.1 |
|
6 – 30 |
±0.2 |
|
30 – 120 |
±0.3 |
|
120 – 400 |
±0.5 |
|
400 – 1000 |
±0.8 |
|
1000 – 2000 |
±1.2 |
|
2000 – 4000 |
±2.0 |
For most custom CNC parts, ISO 2768-m is the right default. It produces tolerance ranges that are easily achievable on standard CNC machines without special measures.
When to Use Each Class
ISO 2768-m (Medium) — Default for ~80% of custom CNC parts. Encompasses tolerances easily achievable on any 3-axis CNC machine.
ISO 2768-f (Fine) — For precision mechanical assemblies where parts must fit together consistently.
ISO 2768-c (Coarse) — For welded or brazed assemblies where post-weld machining will reset tolerances.
Custom tolerances — Override ISO 2768 on specific dimensions where tighter or looser is needed.
ASME Y14.5 GD&T Explained
ASME Y14.5 is the US standard for Geometric Dimensioning and Tolerancing (GD&T). Where ISO 2768 controls size, GD&T controls form, orientation, location, and runout — the geometric relationships between features.
When GD&T Is Necessary
Use GD&T when:
Position of a hole or feature matters more than its size
Concentricity or coaxiality between features must be controlled
Perpendicularity or parallelism between surfaces is critical
Flatness or cylindricity must be guaranteed regardless of size tolerance
The part has functional datums that other features reference
For simple rectangular parts where dimensions are independent, ISO 2768 is sufficient. For complex parts with interrelated features, GD&T is essential.
Common GD&T Symbols
|
Symbol |
Name |
What It Controls |
|
∥ |
Parallelism |
Parallel between two features |
|
○ |
Circularity (Roundness) |
Cross-section roundness |
|
⌭ |
Cylindricity |
Combined roundness + straightness + taper |
|
⌖ |
Position |
True position of a feature |
|
↗ |
Angularity |
Angle between two features |
|
⌒ |
Profile of a line/surface |
3D form of a contour |
|
↘ |
Runout |
Circular runout when rotated |
|
▲ |
Total runout |
Full indicator movement when rotated |
|
⌭ |
Circular runout |
Single-circle indicator movement |
|
⏣ |
Concentricity/Coaxiality |
Shared center axis |
GD&T vs +/- Tolerances Example
A part has 4 mounting holes that must align with mating features:
With +/- tolerances:
Hole 1: 25.00 ±0.10 mm
Hole 2: 25.00 ±0.10 mm
Hole 3: 25.00 ±0.10 mm
Hole 4: 25.00 ±0.10 mm
→ Worst case: ±0.20 mm misalignment between any two holes (chained tolerance)
→ If you also need them aligned within ±0.05 mm, you're stuck — the +/- approach can't express it
With GD&T:
4x Ø5 holes, position tolerance ⌖0.05 mm to datum A-B
→ All 4 holes guaranteed within ±0.05 mm of true position, regardless of individual size
→ Much clearer specification, easier to inspect, no chained tolerance issue
Combining ISO 2768 and GD&T
Best practice for most custom CNC parts:
State "ISO 2768-m" in the title block or general note
Override with specific +/- tolerances where tighter is needed
Add GD&T callouts for functional interfaces (perpendicularity, position, runout)
Example drawing notes:
GENERAL TOLERANCES (UNLESS OTHERWISE SPECIFIED):
LINEAR DIMENSIONS: ISO 2768-m
ANGULAR DIMENSIONS: ISO 2768-m
SURFACE FINISH: Ra 3.2 μm
CRITICAL FEATURES (SEE CALLOUTS):
- All Ø8 H7 holes: position ⌖0.05 to A-B
- Top face: flatness 0.05 mm
- Bearing seat Ø35: perpendicularity 0.02 to A
This gives the machinist clear guidance on what's critical and what's not, and avoids over-specifying the entire drawing.
Real Cost Impact Example
A buyer submitted a bracket with 47 dimensions all marked ±0.05 mm:
|
Approach |
Quote Result |
|
ISO 2768-m default + ±0.05 on 6 critical features |
$115/pc (-38%) |
|
ISO 2768-m default + ±0.05 on 2 critical features |
$62/pc (-66%) |
The bracket functioned identically — only the over-specified tolerances were relaxed.
Common Tolerance Mistakes
"Just to be safe" tight tolerances — every decimal place tighter than necessary costs 10–30%
Chained tolerances — never specify tolerances that accumulate; use GD&T datums
Mixing metric and imperial — creates conversion errors and ambiguous specifications
Specifying tolerances on stock material — you can't machine tighter than the stock
Ignoring datum references — features need a clear reference frame for inspection
Inspection Considerations
Each tighter tolerance class requires progressively more sophisticated inspection:
|
Tolerance |
Inspection Method |
|
±0.05 mm |
Calipers, height gauge, micrometers |
|
±0.025 mm |
Micrometers, dial indicators, optical comparator |
|
±0.01 mm |
CMM (Coordinate Measuring Machine) |
|
±0.005 mm or tighter |
CMM with certified probes, in temperature-controlled environment |
If your drawing requires ±0.01 mm, the inspection alone adds $20–$100/piece (depending on feature count) for CMM time.
Frequently Asked Questions
What is the default tolerance for CNC machining?
ISO 2768-m (medium) is the standard default for most custom CNC parts. It produces tolerance ranges easily achievable on any 3-axis CNC machine without special measures.
Is GD&T required for CNC parts?
No — GD&T is only required when geometric relationships matter more than absolute dimensions. For simple rectangular parts, ISO 2768-m with selective +/- overrides is sufficient. For complex parts with interrelated features, GD&T is essential.
What is the tightest tolerance CNC can hold?
Standard CNC machines can reliably hold ±0.025 mm. Precision CNC machines in temperature-controlled environments can hold ±0.005 mm or tighter, but at significant cost. Beyond ±0.005 mm, you need grinding, lapping, or specialized metrology.
What is the difference between ISO 2768 and ASME Y14.5?
ISO 2768 controls linear and angular dimensions (size tolerances). ASME Y14.5 controls geometric features (form, orientation, position, runout). They are complementary — many drawings use both.
How much does tighter tolerance cost?
Each step tighter typically adds 15–30% to cost. Going from ISO 2768-m to ±0.05 mm adds 5–15%. Going from ±0.05 mm to ±0.025 mm adds 20–40%. Going from ±0.025 mm to ±0.01 mm adds 30–50%.
Do I need CMM inspection?
Only for tolerances ±0.05 mm or tighter. For looser tolerances, calipers and micrometers are sufficient and faster.
What is H7 tolerance?
H7 is an ISO fit tolerance class for holes. H7 = +0.000 / -0.025 mm for Ø10 hole, scaling with diameter. Common for bearing seats and precision fits. Specify H7 on the drawing, and the machinist will produce to ISO 286-1 fit class.
Should I use ISO or ASME on my drawing?
Either is fine — both are recognized globally. ISO 2768 is more common in EU/Asia; ASME Y14.5 is more common in US. Yuqing works with both seamlessly. Pick the standard your engineering team is most comfortable with.
