How Do Manufacturing Tolerances Work?

Credit: CC BY-SA 2.0 BY RICHARD MASONER / CYCLELICIOUS

In July 2017, a rollercoaster ride in Ohio failed dramatically, killing one person and injuring seven others. The cause of the accident? Corrosion.

Roller coasters must operate and withstand outdoor conditions for long periods of time, which is why a range of factors go into designing and producing the final products. One of the factors is manufacturing tolerance. Depending on the intended application of the parts, this could include natural phenomena like corrosion.

Why even bother?

Most modern manufacturing processes and equipment can’t produce parts exactly according to specifications. There is always a variance between the intended dimensions and the actual delivered parts. These complex parts then require complex processes before their concept becomes reality. But manufacturing these precise parts is not always so precise and for this reason, the idea of an “ideal” part is just that...an idea. The leeway permitted between the design and the actual product is called manufacturing tolerance, essentially giving some wiggle room in final product production.

According to a definition from an industry standard, ASME Y14.5M, tolerance is “the total amount a specific dimension is permitted to vary. The tolerance is the difference between the maximum and the minimum limits.”

Tolerances define the upper and  lower limits. This range is called the tolerance band. For example, if an ideal part is to be 0.5 mm +/- 0.1mm, any resulting product between the range of 0.4 to 0.6 mm will be acceptable; the rest will be rejected.

Where is manufacturing tolerance particularly necessary?

Manufacturing tolerance is necessary when operating conditions can be unpredictable and extreme, or if a part is expected to perform for decades on end without much degradation. The physical parts themselves might change shape naturally. Railroad tracks, for example, have thermal expansion due to high summer temperatures factored into their design.

Similarly, systems with complicated geometries and interlocking curved parts need tight enough manufacturing tolerances to ensure that they will continue to work as intended even with all of their individual part variances adding up. The same holds true for systems subject to high pressure or temperatures, or parts made from composite materials with different processing techniques such as metal sheet stamping or injection molding, which may vary by different amounts. If manufacturers don’t closely follow the required tolerance bands, products may have costly failures when put into production.

Standards and Best Practices

The International Organization for Standardization (ISO) or the American Society of Mechanical Engineers (ASME) set standards for manufacturing tolerance. Geometric dimensioning and tolerancing (GD&T), which account for the geometry of interlocking parts, is also an important technique. This prevents the problem of over-tolerancing. Sometimes designers might add a standard tolerance factor to each part, but once you take into account a series of interlocking parts, the net tolerance accumulates (“tolerance stack”), making the effective tolerances too large.

Production processes use  International Tolerance grades (IT grades) which help engineers determine the tolerance needed for a given part. For example, injection molding might be an economical way of mass-producing parts, but its assigned IT grade might be too high, persuading designers to use a different process if it cannot deliver the kind of tight tolerances they’re looking for.

Too much of a good thing?

The flip side of over-tolerancing is that designers often err on the side of caution and add tighter tolerance factors than needed. Manufacturing parts with precise tolerances can be challenging and requires specialized equipment and labor. It also causes more rejected parts and could get expensive.

Design is all about tradeoffs and high precision comes with a high price tag. Unless your design absolutely requires tight tolerances, looser tolerances can lower your manufacturing costs.

Kick it into action

Given that parts can have varying tolerances, they should ideally be tested under a wide range of operating conditions (extreme heat, cold, corrosion etc.) before being produced on a mass scale.

In manufacturing, controlling your tolerances means controlling your prices. Finding the balance will allow for mass-scale economical delivery of parts that still meet the required quality.