Although a mechanical part might appear to be straightforward when presented in a design drawing, it takes far more to create a dimensionally correct component. There are many different aspects to consider when designing a part, one of which is the tolerance of each individual feature.
What is a tolerance? A tolerance is a dimension specification indicating the allowed limit of variation from the stated dimension. For example, if we have a shaft that is designed to be 20 mm in diameter, there may be no need to produce all shafts to be precisely 20.000 mm. If we specify a tolerance, we allow a certain amount of variance to exist.
The objective here is not to avoid variance altogether. Instead, we are trying to manage the variance so that the part can still perform its intended function.
Is it feasible to produce parts with exact dimensions? It is difficult, if not impossible, to produce every component with exact dimensions due to inherent limitations in manufacturing. The cutting tool, for example, may become worn or dull, and thus change the size of the cut. Also, machines are never perfectly precise. Other examples include the fact that raw materials are sometimes not uniform and that temperature changes can occur during processing.
It would therefore be unreasonable and unnecessary to try to create every component to exact dimensions. Doing so would require an excessive amount of time and money without improving the performance of the component. A good engineer will use tolerances judiciously to ensure parts perform properly.
How does tolerance affect the fit of parts? The importance of tolerances becomes more evident when dealing with parts that interact with other components. Suppose that you need a shaft to slide through a hole. What do you want the shaft and the hole to look like? Perhaps you want them to move easily. Perhaps you want a tight fit that prevents movement altogether. Maybe you want the shaft to rotate in the hole, but you also want to keep the hole aligned with the shaft.
In order to achieve these different outcomes, you must choose appropriate dimensions and tolerances for the two components.
Different types of fits exist. In mechanical engineering, there are typically three kinds of fits that we encounter. A clearance fit exists when there is a gap between the two interacting parts, allowing for movement or rotation. A transition fit exists when either a small clearance or a small interference is present, depending on the specific dimensions of the parts. An interference fit exists when the components are designed to fit together such that the first component is slightly larger than the second. Sometimes an interference fit may require special methods of assembly, including heat or pressure.
We select the appropriate fit based on the function and application of the machine.
How do tolerances affect the cost of manufacturing? One of the things that you will discover as you gain experience in mechanical design is that the tighter the tolerances, the more expensive the manufacturing process is likely to be. It is often necessary to utilize advanced machinery or processes, additional inspection, slower production speeds, or specialized tooling in order to meet the required tolerances.
A balance is created between the desired accuracy and the cost of the manufacturing. While it is usually desirable to use the tightest tolerances necessary for your design, it does not necessarily make sense to use the tightest possible tolerance if doing so increases manufacturing costs unnecessarily.
The significance of quality control. Tolerances can help us to identify whether a manufactured part is within specifications. When a component has been manufactured, a measuring device can be used to determine whether it is in or out of tolerance. If the part is out of tolerance, an investigation can be launched to determine the cause of the error. The part may be rejected, repaired, or reused based on the results of the investigation. By monitoring the tolerances, we can ensure that parts manufactured in large quantities meet the required specifications.
Engineering tolerances in today’s world. Although modern computer-aided design and manufacturing (CAD/CAM) software can make the job of defining tolerances easier, it cannot eliminate the need for an experienced engineer to oversee the process. You need to know what is important and what is not important. You also need to know how your manufacturing process will impact your design.
The tolerance of a component may seem trivial, but it actually plays a significant role in ensuring that your components fit together, your machine operates correctly, and your products can be manufactured economically.
The Takeaway
The point of precision is not to make everything perfect. The point is to make everything as close to perfect as is needed for its intended function. Understanding tolerances and applying them appropriately is a fundamental skill in mechanical design and manufacturing.
