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Session 9: Tolerance Concepts

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Session 9

Tolerance Concepts

A Question We Keep Asking

Does all variation matter the same?

Today's answer: no. Here's the vocabulary for exactly how much is okay.

Reading a Real Spec Sheet

A real spec doesn't just give one number.

Reading a Real Spec Sheet

It gives a target and an allowed range around it.

Nominal

The target, intended value.

A bolt labeled 1/4 in is nominally 1/4 in.

Tolerance

How far from nominal is still acceptable — usually written as ±.

Min and Max

  • Minimum = nominal − tolerance
  • Maximum = nominal + tolerance

Worked Example

Spec: 0.250 in ± 0.005 in

Worked Example

  • Nominal: 0.250 in
  • Tolerance: ±0.005 in

Worked Example

  • Minimum: 0.250 − 0.005 = 0.245 in
  • Maximum: 0.250 + 0.005 = 0.255 in

Any part measuring between 0.245 and 0.255 in passes.

Why Tolerance Exists

Remember Session 6: even careful measuring has natural variation.

Tolerance is that variation, planned for on purpose — instead of pretending every part comes out exactly nominal.

Two Things to Remember

Tighter tolerance isn't automatically "better."

A tighter tolerance means harder and more expensive to make — you only spec it that tight when the part actually needs it.

Two Things to Remember

Tolerance is a range, not a guarantee of "perfect."

A part measuring exactly at min or exactly at max still passes — it's not almost failing.

Next Time

Session 10: The Mismatch Activity

Two parts, each within their own tolerance. Do they actually fit together?

'F' → Fullscreen

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Build

Step 1: Nominal + Tolerance → Min/Max

For each spec, calculate the minimum and maximum:

  1. 0.500 in ± 0.010 in
  2. 12.00 mm ± 0.05 mm
  3. 1.750 in ± 0.002 in

Step 2: Min/Max → Nominal + Tolerance

For each pair, work backward to find the nominal value and the tolerance:

  1. Min: 9.95 mm, Max: 10.05 mm
  2. Min: 0.370 in, Max: 0.380 in

Step 3: Spec Sheet Reading

A drawing calls for a shaft: 6.00 mm diameter, ±0.02 mm. Identify the nominal diameter, the tolerance, and the min/max a passing shaft could measure.

Step 4: Which Part Needs It Tightest?

Three parts on the same shop project. Rank them from tightest tolerance needed to loosest, and give a one-sentence reason for each:

  1. A hole that a bearing has to press into
  2. The overall outside length of a mounting bracket
  3. The width of a cosmetic cover panel

Then answer the real question: why not just spec all three at the tightest value and be safe?

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Checkpoint

Before you move on, explain out loud (or in writing, in your own words): if a tolerance is tightened — the ± number gets smaller — do min and max get closer together or farther apart? Why?

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Reflection

Answer these on your own. Reflections are individual, even when the rest of the session was team work.

  1. Think back to Session 6's caliper data. Was the spread in the class's measurements bigger or smaller than a typical manufacturing tolerance like ±0.005 in? What does that tell you about hand-measurement versus machine-made parts?
  2. Session 10 introduces two parts that are each within their own tolerance but might not fit together. Can you guess why that might happen, even though both parts “passed”?
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Submit

Submit your tolerance calculations and your reflection.

Activity Complete