Engineering tolerance defines how much a manufactured dimension is allowed to vary from the nominal design value. No machining process creates a perfect mathematical part, so tolerances tell the manufacturer what variation is acceptable and what must be rejected.
For CNC parts, good tolerancing is not about making every dimension extremely tight. It is about controlling the features that affect fit, motion, sealing, strength, or alignment while keeping non-critical dimensions practical. This balance improves quality and keeps machining cost under control.
Basic Tolerance Terms
| Term | Meaning |
|---|---|
| Nominal dimension | The target size shown on the drawing |
| Upper limit | The largest acceptable size |
| Lower limit | The smallest acceptable size |
| Tolerance zone | The total allowed variation between limits |
| Fit | How two mating parts interact after manufacturing |
| Datum | A reference feature used for measurement or assembly |
Types of Engineering Tolerances
Dimensional tolerances control size, such as length, width, diameter, or depth. Geometric tolerances, often shown through GD&T, control shape, orientation, position, and runout. General tolerances apply when individual dimensions are not specifically called out.
- Bilateral tolerance allows variation in both directions, such as +/-0.05 mm.
- Unilateral tolerance allows variation in only one direction, such as +0.02/-0.00 mm.
- Limit dimensions show the upper and lower values directly.
- GD&T controls relationships such as flatness, parallelism, profile, true position, and runout.
Fits Between Mating Parts
| Fit Type | How It Behaves | Example |
|---|---|---|
| Clearance fit | There is always space between mating parts | Shaft rotating inside a bushing |
| Transition fit | May have slight clearance or slight interference | Accurate locating dowels |
| Interference fit | Parts press together with force | Pressed bearing or permanent sleeve |
Using ISO 2768 and General Tolerances
Many drawings use a general tolerance standard such as ISO 2768 for dimensions that do not require special control. This keeps drawings readable and avoids unnecessary tight callouts. Critical features should still have specific tolerances or GD&T notes when function demands it.
How to Choose Practical Tolerances
- Start with the function of the feature, not the machine capability.
- Tighten only dimensions that affect assembly, motion, sealing, or performance.
- Consider material movement, heat treatment, plating, anodizing, and other finishing steps.
- Discuss inspection method for tight or complex tolerances before production.
- Use datum structures that reflect how the part actually assembles.
If coating thickness matters, coordinate tolerance planning with the surface treatment process. If complex GD&T is involved, confirm that the quality team can measure the feature reliably.
Tolerance and Cost
Tighter tolerance usually increases cost because it may require slower machining, special tooling, more stable fixtures, extra inspection, or secondary finishing. A realistic tolerance strategy helps suppliers quote accurately and reduces back-and-forth during manufacturing review.
Related MTSPi Resources
FAQ
Should every dimension have a tight tolerance?
No. Only functional or assembly-critical features should be tightly controlled.
What is GD&T used for?
GD&T controls geometry and relationships between features, such as position, flatness, parallelism, and runout.
Can surface finishing change tolerances?
Yes. Plating, anodizing, coating, and heat treatment can change dimensions or part behavior, so they should be considered early.
