Limit System: The Foundation of Dimension Control
Limit System: The Foundation of Dimension Control
Basic Size: This is the principal or nominal dimension from which variations are specified. For example, if you design a shaft to be Ø20mm, 20mm is your basic size.
Design Size: The size actually used in design, which may or may not match the basic size, depending on system and context.
Actual Size: The measured size of the finished part after manufacturing.
Tolerance: The difference between the upper and lower permissible limits. If a shaft is allowed to be between 19.8mm and 20.2mm, the tolerance is 0.4mm.
Limits: The two extreme permissible sizes (upper limit and lower limit) of a part. For a shaft, upper limit = basic size + tolerance, lower limit = basic size – tolerance.
Deviation: The difference between the actual size and the basic size. Actual deviation is the measured deviation, upper deviation is the difference between upper limit and basic size, and lower deviation is the difference between lower limit and basic size.
Allowance: The intentional difference in sizes between mating parts to achieve a desired fit (e.g., clearance or interference).
Limit System: Two common systems are basic hole (hole size is fixed, shaft varies) and basic shaft (shaft size is fixed, hole varies).
Fits: How Parts Come Together
Fits describe the relationship between two mating parts. There are three main types:
| Fit Type | Description | Application Examples |
|---|---|---|
| Clearance | Shaft smaller than hole—free movement | Hinge pins, bearings |
| Transition | Shaft can be slightly smaller, same, or slightly bigger than hole—some play, some press | Couplings, pulleys |
| Interference | Shaft larger than hole—requires force to assemble, forms a tight joint | Gears, bushings |
Each type is further divided into sub-categories (e.g., loose running, close running, press fit, forced fit) to fine-tune functionality and assembly requirements.
Tolerances of Form and Position
Tolerances are not just about size—they’re about shape, orientation, and location, too. This is where Geometrical Dimensioning and Tolerancing (GD&T) comes in.
Form Tolerance: Controls the shape of a feature (e.g., straightness, flatness, roundness, cylindricity).
Position Tolerance: Controls the location or orientation of a feature relative to a reference (e.g., parallelism, perpendicularity, angularity, position, concentricity, symmetry).
Tolerance Zone: The space within which the feature must lie—can be a cylindrical zone (when prefixed by ⌀), a space between two parallel lines, etc..
Indicating Geometrical Tolerances: On drawings, these are shown using feature control frames—rectangular boxes with symbols, tolerance values, and datum references.
Surface Roughness: The Feel of Quality
Surface roughness describes the texture of a machined surface, influencing wear, friction, and appearance.
Ra (Average Roughness): The arithmetic average of surface deviations from the mean line.
Rz (Mean Depth): The average of the highest peak to lowest valley in several sample lengths.
Indication: On drawings, surface finish is specified with symbols (e.g., √, numbers, or text like “Ra 1.6”).
Why It Matters: Smoother surfaces (low Ra) are better for moving parts, seals, and esthetics; rougher surfaces may be better for gripping or adhesion—but usually increase wear.
Standard Abbreviations and Symbols
Engineers use a universal language of abbreviations and symbols to keep drawings clear and concise. Here are a few examples:
| Abbreviation | Meaning | Symbol Example | Purpose |
|---|---|---|---|
| BSC | Basic size | – | Nominal dimension |
| LMC | Least material cond. | Ⓛ | Used in GD&T |
| MMC | Max material cond. | Ⓜ | Used in GD&T |
| ⌀ | Diameter | – | Indicates a circular zone |
| √ | Surface finish | – | Specifies Ra/Rz value |
Always refer to industry standards (ISO, ASME, DIN) for a complete list, as these are updated regularly.
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