Choosing Dimensions
Break a part into simple forms, define each feature’s size and location, and remove redundant requirements.
Ask what information is necessary to make this part
Ask what information is necessary to make this part
Start with the geometry, not with the dimension tool. Break the part into familiar forms: blocks, cylinders, holes, steps, slots and arcs.
- Size dimensions
- Describe the feature itself: a block’s length, width and thickness; a hole’s diameter; a slot’s width and length.
- Location dimensions
- Place that feature relative to other features or a chosen origin. A hole diameter alone does not say where the hole goes.
Locate a round hole by its center, not by a tangent edge that moves when the diameter changes. Choose reference surfaces that make sense for how the part fits and is checked. A formal datum has a specific meaning in geometric tolerancing; an arbitrary sketch origin is not automatically a datum.
Dimension a feature in the view that shows its shape
Dimension a feature in the view that shows its shape
Contour dimensioning keeps a feature’s dimensions with the view where its outline is clearest. Put a hole size on a view showing its circle. Put a step’s height where the step is visible. Use a section or another suitable view for an internal feature.
For the beginner drawings here, do not dimension to hidden lines: choose a view that exposes the feature. Keep the dimensions of one feature together when practical. See the Reading Drawings group in Engineering Graphics for projection, section views and view selection.
Build the dimension scheme in a deliberate order
Build the dimension scheme in a deliberate order
- Identify the simple forms and list the features that need size, location or orientation information.
- Choose the views that show those features clearly. Establish useful reference edges or surfaces.
- Add the main body sizes. Leave space for smaller feature dimensions nearer the outline.
- Add feature sizes and end conditions, such as a hole diameter and THRU.
- Locate the feature centers, steps and other boundaries from the chosen references.
- Check completeness, remove duplicate requirements, and arrange the lines for readability.
Do not add dimensions just because the CAD system offers them. Review each imported model dimension for purpose, view, placement and duplication.
Give each independent requirement once
Give each independent requirement once
Completeness means every feature is defined. It does not mean every possible distance is dimensioned. A feature may need several dimensions, but the same size or location requirement should not be repeated in another view or restated by a closed chain.
A closed chain gives all the segments and their total as controlling dimensions. Remove one link, or make a useful derived value a reference dimension. Which link to remove depends on what the part must do.
Why nominal arithmetic is not enough
At the nominal values a closed chain may add up perfectly. Real parts vary within permitted limits, so the extra requirement can conflict with otherwise acceptable sizes. Changing one value also risks leaving its duplicate unchanged. The theory belongs in Tolerances and Fits.