Tight Tolerances Have A Job To Do. Give Each One A Reason

By Author: PiPrecision Co Ltd
Total Articles: 2
Comment this article

## Begin with the failure you want to prevent

Take a small sensor mount. It holds a sensor against a reference face and bolts to a machine frame. If the sensor sits too high, it may miss its target. If the outside edge moves slightly but still clears the cover, the equipment may work exactly as intended.

Those two dimensions deserve separate decisions. One controls function. The other may only define a convenient shape.

Before entering a tolerance, ask: what happens at the largest and smallest permitted values? If the answer is unclear, more decimal places will not solve the design question.

## Review the assembly, not only the individual part

Variation can accumulate across several components. A bracket, spacer, and mounting surface may each meet their own drawing while the final assembly falls outside its required position.

A tolerance analysis examines that relationship. A simple worst-case review considers how permitted extremes combine. Other methods may be appropriate when their assumptions about variation are justified.

The important point is to do the analysis before choosing ...
... which component must carry the tightest requirement. The easiest dimension to edit in CAD is not always the best place to control the assembly.

## Size and geometry answer different questions

A hole diameter tells you about size. It does not, by itself, fully describe where the hole belongs relative to the mounting surfaces. Likewise, controlling thickness does not necessarily communicate the flatness needed for a mating face.

Geometric dimensioning and tolerancing provides a language for these relationships. The [ASME Y14 standards](https://www.asme.org/codes-standards/y14-standards) describe the rules and symbols used in this form of product definition.

Use the system deliberately and identify the applicable standard on the drawing. Adding symbols without a functional datum scheme can create confusion rather than precision.

## Ask how the requirement will be measured

A tolerance is also an inspection commitment. If the critical feature sits deep inside a narrow pocket, the supplier needs a practical way to reach and measure it.

Ask what equipment, fixturing, and part condition the measurement requires. Thin or flexible parts may behave differently when clamped than when released. Coating and temperature can also affect how results should be interpreted.

An early inspection discussion can reveal a problem while a design change is still easy to make. It may also show that the intended requirement can be expressed more clearly.

## Compare the cost of specific decisions

Instead of asking for a general discount, ask the supplier which requirements drive additional operations or inspection effort. Then review those requirements with the designer.

For the sensor mount, the team might preserve a carefully controlled locating interface while using a less restrictive requirement on the external profile. That is a deliberate allocation of precision, not a blanket reduction in quality.

Any change still needs engineering approval. Cost alone does not establish whether a looser tolerance is acceptable.

## Leave the next person a clear drawing

After the review, update the drawing and revision. Remove outdated notes, resolve conflicting limits, and make the acceptance condition explicit.

Write down why the most demanding characteristics matter in the supporting design record. That explanation can help when a future engineer is asked to reduce cost or investigate an assembly issue.

For another practical view of the relationship between geometry, setup, and inspection, see [PiPrecision’s CNC milling accuracy guide](https://piprecision-cnc.com/blogs/cnc-milling-accuracy).

The strongest drawing is not the one with the smallest numbers. It is the one that makes the necessary precision clear enough to manufacture, measure, and use.

PiPrecision Co Ltd is a rapid manufacturing specialist focused on delivering high quality prototypes and low-to-medium-volume production parts to varied customers across multiple industries and countries. If your product is made from plastic, metal, or composite materials there is a very good chance that we have a rapid manufacturing solution for you. When it comes to rapid prototyping services speed is important but effective communication with the client and flexibility to their project needs are just as important to the ultimate success of the project. [Submit your quote request](https://piprecision-cnc.com/quote) to start your free project consultation.