What Tolerances Do You Really Need?
The fundamentals of parts sourcing — dimensions, tolerances and materials — can be surprisingly complex, especially when the parts are small. Metal Cutting takes a look at some of the challenges of sourcing small parts in order to make more informed decisions about specifications and the vendors that will meet your needs.
Tolerances affect product design, manufacturing, and quality control, making them critical to the successful performance of an end product. Therefore, determining tolerances and the amount of acceptable variation in cut parts is a critical component to the cost of quality. While it may be tempting to assume that the tightest tolerance possible should be your default choice, it’s important to strike a balance between excessively cutting costs and implementing unnecessary requirements.
Customers sometimes ask us why a part is so expensive, not realizing that extending tolerances out just one more decimal point can increase the cost by a factor of two or three. That is because tighter tolerances require greater care in fabricating and inspecting in order to ensure accuracy. The tighter the tolerance, the more likely it will be that you will have to add the cost of the following into your part:
- Greater frequency of quality inspections
- More sophisticated or specialized inspection equipment
- Lower yields of final acceptable parts
- Higher scrap rates
- More raw material use due to parts fallout
In addition to having an impact on the quality of parts and assemblies, how you specify tolerances affects what tools and machines will be used and the cutting technique that will be employed — which in turn also affects costs. You’ll need to ask yourself: does the economical method utilize machines with the necessary nominal tolerance capabilities? If not, your tight tolerance could result in expensive production methods.
The three magic words
There are standards in place for different industries, with tolerances recommended based on factors such as function, material, machine, and process. And there are handbooks, technical guides and fact sheets that provide the default tolerances for a variety of products and processes. In fact, most drawings will name a default tolerance under the heading “Unless Otherwise Specified,” indicating that unless a special tolerance value is given, use the default tolerance, (varying by the other convention of how many values are presented to the right of the decimal point, even if those values are zero).
Accepting the default tolerances without understanding whether or not they’re truly necessary, or simply not noticing them at all, can needlessly drive up costs. Here are just a few typical examples:
- A extra decimal of tolerance is specified not for a technical reason — that is, not because the particular part needs that level of precision — but because that is “the way it has always been done.”
- In a project with a number of components requiring a tolerance of ±0.001 because of the way they must fit together, a separate part is automatically specified using the same tolerances, even though the part does not interact with the others and may or may not need the same tight tolerance.
- In an effort to keep mechanical drawing labels consistent to the same decimal point, a zero is accidentally misplaced, labeling a tolerance ±0.001 instead of ±0.010.
- Two parts that failed to fit together correctly are re-specified at tighter tolerances — but the original failure was due to the parts being cut at two different loose tolerances.
Do I really need ± 0.001 vs ± 0.01 (or ± 0.1)?
Undoubtedly there are times when tight tolerances are absolutely mandatory — when the difference between a product that works and one that fails means dimensional tolerances expressed in decimal places that go much further than you initially expected. In these scenarios small variations in each part’s dimensions can multiply, especially with complex assemblies, resulting in tolerance buildup and unacceptable variations in the intended design.
The good news is that there are many instances where looser tolerance may be perfectly acceptable. If a tolerance is tighter than you really need, you’ll end up paying for it. But by carefully considering the end use, where the part is going, and what the part needs to do before specifying a tight tolerance, you can distinguish between critical and non-critical tolerances, and minimizing your cost of quality.
For example, one customer said they needed “a simple part”: a solid with a length and a diameter specified at a tight ±0.000X tolerance. However, in probing for more information, Metal Cutting discovered that the part needed a big corner radius, with no sharp edges — an interaction that would put the four-decimal diameter at risk. In addition, the part needed a smooth surface finish that would require mechanical polishing, which in turn would remove material and affect the spec’d tolerance.
Fortunately, having uncovered these critical details, we were able to work with the customer and adjust the specs to make a part that works for its intended use.
It may seem like common sense, but before you commit to a tight tolerance, consider:
- How does the part function in its end use?
- What tolerances are important for the environment in which the part will be used?
- Does the part interact with any other part or parts? If not, you might not need such a tight tolerance. If so, you need to be aware of the potential for tolerance accumulation.
- If the same part has different attributes requiring tolerances — such as both a diameter and a radius — which is the more critical dimension?
- As in the customer example above, is there a very tight tolerance that may be countered by some interaction within the part and therefore, not be practical?
- Are you willing to test a slightly out-of-spec part to see if a looser tolerance will work for your application?
Choosing the proper tolerances can help you to ensure product quality, ease of manufacturing, and time to market. If you don’t know or are unsure about what tolerances are right for your application, don’t worry — your vendor can walk you through the decision points to help you identify what is mission critical and determine the proper tolerance for your parts. Just ask us!