How are tolerance limits defined? The previous articles in our Intro to Tolerances series have answered the questions, “What is a tolerance?” and “Why use tolerances?” This third and final article in the series explores how tolerance limits are determined and what types of tolerances exist.
How are tolerances determined?
As a rule of thumb for design, tolerances should be kept as large as possible while maintaining the desired function of the part. We want to ensure that the part will function correctly, and at the same time, we want to produce the part in a cost-efficient way. Tight tolerances increase manufacturing costs. If tight tolerances are used when they are not functionally required, money is wasted.
So how do we know what the tolerances need to be when designing a part? There are several ways to determine tolerance limits, including: knowledge of existing parts, engineering knowledge references, scientific principles (studies), and professional experience. We will explore each of these below.
Suppose we have a part that has been in production for several years. The assembly team has stable processes in place that have produced repeatable results over these years. Now, this part is being updated with a new feature. The existing knowledge from the production of this part can be used to ensure the tolerances on the new feature are manufacturable, and that the tolerances are not too tight or too loose.
Engineering Knowledge References:
There are many reference books available to help with tolerance selections. The most common one used is Machinery’s Handbook. Let’s say we want to know how to tolerance a hole for a close fit around a ½” bolt. In this case, you would reference Machinery’s Handbook and check out the “Clearance Holes for Inch Fasteners” Table. (An excerpt of this table is shown in Figure 1.) To determine the tolerance for the hole you are designing, find the row for a ½” nominal screw size. The table shows hole diameters for normal, close, and loose fits. Slide over to the “close” column, and you will find that the hole diameter limits for a close fit around a ½” bolt are 0.531” min and 0.538” max.
Scientific principles are any type of test run on a part to make sure that the part works as designed. These can include prototypes, studies, cycle tests, etc., and are an excellent way to test tolerances. When a new part is being designed, these tests reveal adjustments that may be needed and help ensure that your part will work as intended when it gets into production.
Professional experience is maybe the most common method of applying tolerances. Over time, you will learn what does and doesn’t work for your specific application. This applies not only to design, but to manufacturing as well. Manufacturing engineers and machinists will also come to understand what limits their machines can hold and how tight of a tolerance they can produce.
Types of Tolerances:
Though you are probably most familiar with dimensional tolerances, there are several other tolerance types as well. The drawing of the damping spring in Figure 2 includes four different types of tolerances, including dimensional tolerances, geometric tolerances, force or load tolerances, and test criteria tolerances. Dimensional tolerances are shown highlighted in green. These are giving the limits on diameter, thickness, and length. Geometric tolerances are highlighted in blue, and are indicating limits on perpendicularity. Force tolerance is highlighted in red – in this case giving a tolerance on the spring rate. The fourth tolerance type shown on this drawing is the test criteria tolerance, highlighted in purple. With these four types of tolerances on this drawing, it is important to understand that all we are doing is placing limits so that we know what is acceptable and what must be rejected.
Review of Tolerance Basics
Below is a short summary of what we have discussed in our Intro to Tolerances series. Links are included to each part of the series for more information.
1. Parts can never be made perfect, so to limit the variation we apply tolerances.
(Intro to Tolerances Part 1 – What is a Tolerance?)
2. A tolerance is the acceptable range of variation that still allows proper function.
(Intro to Tolerances Part 1 – What is a Tolerance?)
3. Tolerances are used to clarify part limits, convey critical dimensions, and ensure function or assembly with other parts. In some cases, these are legal requirements for a contract. **It is always important to have tolerances that are required for the function but not any tighter than necessary, as this adds additional cost.**
(Intro to Tolerances Part 2 – Why use Tolerances?)
4. Tolerances can be defined in many ways:
- Existing parts
- Engineering knowledge
- Scientific principles (studies)
- Professional experience
5. There are many types of tolerances:
- Dimensional (the most common tolerances you see)
- Performance based (like force requirements)
- Inspection/test criteria (such as specific tests we have on the part)
- Or any other type of limit that we are putting on this part.
Everything has a tolerance. Tolerances are defined in many ways and come in many forms. They are critical to having a part that functions as desired and is produced in a cost-effective way.
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