How do multiple tolerances conflict

Avoid the blame game for part tolerances

Tolerance is one of the basic elements of small parts procurement and has an impact on the optimization of manufacturability. Specifying tolerances requires a balance between cost and the need for accuracy.

By distinguishing critical and non-critical tolerances, the cost of overdesigning parts can be avoided.

This means that when you have a part with different attributes that require tolerances (such as diameter and radius), you need to consider which dimension is more critical.

This last point, however, raises another question: what happens when the same part has multiple conflicting tolerances?

What are conflicting tolerances?

The term "conflicting tolerances" can be used to refer to any given number of tolerances, which together make no sense at all.

For example, a drawing of an electronic component may show two conflicting tolerances on component spacing: the first tolerance is boxed and uses a general drawing tolerance of three decimal places, ±5μm (0.0002"); Next to it is a cumulative pitch (four single pitches) with a tolerance of ±50μm (0.002").

Risk of tolerance conflicts

Mechanics hate such conflicting tolerances because they force manufacturers to choose between conflicting requirements.

If tolerances are programmed into the machine, it can cause the tool to crash and seriously damage the delicate positioning function. This can require expensive repairs or parts replacement and result in production delays, missed deadlines and higher costs.

In addition, for more complex components, the impact of conflicting dimensions can be multiplied, leading to tolerance buildup and unacceptable changes in the expected design.

When multiple tolerances need to be corrected

When multiple tolerances conflict in this way, it is usually a matter of incomplete dimensions or human error.

Fortunately, an alert and knowledgeable technician can spot discrepancies and know to go back to customers or other sources for more information - to determine intent and to make corrections in specifications to resolve any conflicts.

Loose and tight tolerances

Another type of conflicting tolerance is actually a very common situation: when a single part has loose tolerances that are easy to satisfy as well as strict tolerances in another dimension.

For example, you might have a loose tolerance of ± 0.01" (0.254 mm) for length and a very strict tolerance of ± 0.001" (0.0254 mm) for verticality, making the part more difficult to produce.

Can you achieve strict tolerances in one dimension and a cheaper, looser tolerance in the other? The reality is that you may not be able to implement both specified in a single part.

This is because the rule of thumb is that tighter tolerances are the tolerances that must guide production -- determining the types of machines and tools to be used, and ultimately driving the cost of parts.

Therefore, you must look at the parts as a whole and consider all tolerances to get a true understanding of the process and cost.

For example, very large corner radii can reduce the diameter because parts need to roll over for a long time to reach the radius. This will require tighter tolerances and eliminates the cost-effectiveness of specifying a looser overall od.

When multiple tolerances require compromise

Often, these conflicting tolerations require an acceptance of compromise. Such as:

You may need to specify tighter length tolerances to maintain tight verticality.

For radius and diameter, you may need to compromise on diameter to get a more critical size (minimum end radius).

Using our previous example of loose length and tight verticality, you may need to make a choice:

Does the perpendicularity really need to be that tight?

or

Do all tolerances, including length, need to be tighter to achieve the desired perpendicularity?

Adjust for multiple tolerances for optimization

Keep in mind that choosing the right tolerances helps ensure proper part performance, ease of assembly, and ultimately product quality. When multiple tolerances conflict, the challenge is to correct or compromise to avoid issues that will cost you time, money, and quality.

The good news is that tweaking a tolerance slightly (or a few tolerances) can help you ensure that the most critical dimensions are met to achieve the end goal.

By identifying the most challenging tolerances and determining the best way to achieve them, you can clearly understand the processes that need to be used as well as the costs.

In addition, knowledgeable manufacturing partners can help you make informed decisions and understand the trade-offs that often must occur when you have a part with multiple tolerances.

Please contact us at zhang@pride-cnc.com