Tolerances for the Shop Floor

Plain-English Summary

Understand practical tolerance meaning, nominal size, tolerance zones, decimal places, inspection reality, and first checks before machining, measuring, or accepting a part.

Why It Matters

Tolerances control fit, function, assembly, clearance, alignment, sealing, bearing life, motion, wear, safety, and quality. A part can look correct and still fail because a feature is outside tolerance.

Field Rule of Thumb

The dimension is not complete until the tolerance is understood. Ask what the nominal value is, the allowed variation, the unit, the feature controlled, the measurement method, and the feature function.

Core Concept

Example: 1.000 ± 0.005 inch means the acceptable range is 0.995 to 1.005 inch. Limit dimensions show the low and high values directly. Bilateral tolerances vary both ways; unilateral tolerances vary one way or unequally.

Worked Example

A shaft shown as 1.2500 ± 0.0005 inch has an acceptable range of 1.2495 to 1.2505 inch. A measured 1.2508 inch is only 0.0003 inch over the high limit, but it is still outside tolerance.

Common Mistakes

• Treating close as good.
• Ignoring title block tolerances.
• Assuming decimal places tell the whole story.
• Measuring with the wrong tool.
• Measuring the wrong feature.
• Forgetting temperature and cleanliness.

First Checks / Troubleshooting Flow

1. Confirm drawing revision and unit.
2. Identify the feature, nominal value, and tolerance.
3. Check notes and title block.
4. Confirm whether GD&T applies.
5. Select a measuring tool appropriate for the tolerance.
6. Clean and inspect the part.
7. Measure and compare against the actual allowed range.