Dimensions, Tolerances, and Fits

Why Tolerances Matter in CNC Machining

No cutting tool produces a perfect dimension every time. Temperature, tool wear, vibration, and material variation all shift the actual size of a feature away from the designer's target. Tolerances define the permissible band of variation so that parts still assemble and function correctly. Understanding tolerances is essential for reading blueprints, setting up a CNC machine, and inspecting finished work — and it is directly tested in NIMS Machining credentials and Haas Certification.

Basic Tolerance Vocabulary

A nominal size (also called the basic size) is the theoretical target dimension stated on the drawing — for example, 1.500 in. The limits are the two extreme permissible sizes: the upper limit (largest acceptable) and the lower limit (smallest acceptable). The tolerance is the arithmetic difference between those two limits.

Tolerance = Upper Limit − Lower Limit

Maximum Material Condition (MMC)

MMC is the condition in which a feature contains the most material it is allowed to contain. For an external feature such as a shaft or pin, MMC is the largest permissible size. For an internal feature such as a bore or slot, MMC is the smallest permissible size. Think of it this way: more material means the feature is heavier and tighter.

Least Material Condition (LMC)

LMC is the opposite — the condition in which the feature contains the least material. For an external feature, LMC is the smallest permissible size. For an internal feature, LMC is the largest permissible size.

Methods of Stating Tolerances

Limit dimensioning states both limits directly on the drawing, one above the other. The upper limit is placed on top.

Example: 1.505 / 1.495 means the shaft must measure between 1.495 in. and 1.505 in.

Plus/minus (±) tolerancing states a nominal size with a single equal variation in both directions. A bilateral tolerance allows variation in both directions from nominal; a unilateral tolerance allows variation in only one direction.

• Bilateral: 1.500 ± 0.005 (limits: 1.495 to 1.505)
• Unilateral: 1.500 +0.000/−0.010 (limits: 1.490 to 1.500)

Angular dimensions follow the same rules — an angle callout of 30° ± 0°30′ means the actual angle may range from 29°30′ to 30°30′.

ANSI/ASME Fit Systems: B4.1 and B4.2

When two mating parts must fit together — a shaft into a bore — the designer selects a fit class that controls how much clearance or interference exists at assembly.

ASME B4.1 covers inch fits and defines three broad categories:

• Clearance fit — the shaft is always smaller than the bore; a gap always exists. Used for running and sliding assemblies.
• Interference fit — the shaft is always larger than the bore; force or heat is required to assemble. Used for press-fit hubs and bearings.
• Transition fit — tolerances overlap; the actual assembly may result in either slight clearance or slight interference. Used where accurate location is needed with easy disassembly.

ASME B4.2 covers the ISO metric preferred hole system (hole-basis) and shaft-basis system using tolerance grades (IT grades) and deviation letters. Common callouts such as H7/g6 indicate the tolerance grade and position for each mating feature.

To use a fit table, you need: (1) the basic size, (2) the fit class desired, and (3) the table column for upper and lower deviations. Apply those deviations to the basic size to get the actual limits, then verify with a tolerance stack — confirming that at MMC of both features, the correct condition (maximum interference or minimum clearance) is within specification.

Measuring Parts with a Micrometer

An outside micrometer measures external dimensions such as shaft diameters. Common ranges in the shop are 0–1 in. and 1–2 in. Safe and accurate measurement procedure:

1. Clean the part and anvil faces — chips or oil cause false readings.

2. Zero the micrometer using the standard or by closing on a clean gauge block.

3. Bring the spindle into light contact with the part; use the thimble ratchet or friction stop — never force the spindle.

4. Read the sleeve (main scale), then the thimble graduation, then the vernier line if present. Add the values.

5. PPE reminder: Burrs on machined parts can cut. Wear safety glasses at all times on the shop floor. Handle sample parts carefully; deburr before measuring if sharp edges are present.

6. Record the reading to the nearest 0.0001 in. and compare to the drawing limits to accept or reject the part.

A part is in tolerance when its actual size falls on or between the two limits (including the limits themselves). A part is out of tolerance when its actual size exceeds either limit, even by 0.0001 in. When in doubt, re-measure and, if still out, quarantine the part — never pass a borderline reading by guessing.