Tips for Application of Tolerances

There are six main reasons why it is important to always apply tolerances:

  1. Improved fit and functionality of the part – if multiple parts are going to be assembled, tolerancing ensures that they will fit and function well.
  2. Improved aesthetics of the product – to avoid obvious gaps between parts, or ensure two parts are flush when assembled, both parts’ dimensions need to be controlled.
  3. Accounting for error – when defining tolerances from the start, it is less likely that parts will have to be remade.
  4. Manufacturing is more cost-effective – by tolerancing, a product will only be as precise as it needs to be. This way, only the materials, manufacturing tools, and labour this is needed will need to be paid for.
  5. Products get to market faster – mismeasurements and inconsistent products increase time to market, tolerancing prevents this by clearly communicating all needed information.
  6. Reduced manufacturing ambiguity and complexity – if what is needed isn’t specified, an unusable product may be produced. Tolerancing removes all ambiguity from the manufacturing process.

Does this mean you should add the tightest tolerances possible to every dimension? The answer is: definitely not. Generally, the tighter the tolerance the higher the cost. This is because it takes more time, skilled labour, and expensive machinery to create the product with the higher degree of precision and accuracy. The difficulty is deciding which tolerances matter most and how tight or loose those tolerances need to be.

While tolerances are important, it is just as important to apply them correctly. Below are some tips on how to do exactly that.

Keep Tolerances as Large as Functionally Possible

In general, tolerances of ±0.1 mm are expected and achieved from today’s CNC mills. To provide some context, an average human hair is also around 0.1 mm in diameter. However, just because you can hold tolerances smaller than a human hair doesn’t mean you need to.

The engineer or designer should strive to keep tolerances as large as possible while preserving the function of the part. Small tolerances can increase cost in the manufacturing, inspection, and tooling of parts. Tight tolerances are sometimes necessary, but it’s important to keep them in perspective.

Understand the Manufacturing Process Being Used

Different manufacturing processes require — and can achieve — different tolerances. For example, when manufacturing rubber parts, the moulding process is generally able to achieve higher tolerances than the extrusion process.

Consider The Material Being Used

Different types of materials have differing levels of accuracy, consistency, and shrinkage rates. For example, it is significantly easier and cheaper to achieve tight tolerances for metal parts than it is to achieve the same tight tolerances for rubber parts. We go into more detail about this in the following article.

Be Sure to Consider How the Part Will Be Assembled

One of the most important considerations when applying tolerances is fit. This is how shafts fit into bearings or bushings, motors into pilot holes, and so on. Depending on your application, you might want a clearance fit to allow for expansion due to heat, a sliding fit for better positioning, or an interference fit for holding capability.

Be Aware of Tolerance Stacking

This is when tolerances are based on points that have tolerances of their own, as is the case in Figure 1.

Tolerance stacking

Figure 1: Tolerance stacking.

Although every length dimension has the same tolerance, the maximum tolerance between surfaces A and B can be as large as ±0.4 in Figure 1 or as low as ±0.1 in Figure 2. This is due to the way the dimensions and tolerances have been specified.

Ordinate dimensioning

Figure 2: Ordinate dimensioning.

Be Careful When Applying Tolerances to a Radius or Diameter

A radius’ tolerance will double when measured as a diameter, so if care is not taken, this may lead to tolerances that are looser or tighter than intended.

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