ONE-DIMENSIONAL TOLERANCE ANALYSIS AND TOLERANCE STACKUP

Audio: One-dimensional tolerance analysis and tolerance stackup – part 2

In my previous article, One-dimensional tolerance analysis and tolerance stackup – part 1, we learned what tolerance analysis is and the basic steps to calculating tolerance stackup. In this article, we will show you the influence of different dimensioning techniques on tolerance accumulation and show you the report format that you could use to document your analysis.

This article will not consider Geometrical Product Specification (ASME standard name: Geometric dimensioning and tolerancing – GD&T). This topic will be addressed separately.

Introduction

In the previous article, we already learned the basics of tolerance stackup analysis, but we are far from mastery of this topic. There are many other things still to be learned.

For the beginner level, I would say that two more things are essential to learn: how does dimensioning method influences tolerance stackup and how to communicate results with others (report format). Understanding this will give you a strong foundation for the tolerance analysis’s more complex examples and topics. Also, you will be able to use the knowledge from this and the previous article immediately to start building up your experience. The more examples you do, the faster you will internalize the basics, and you will be ready to move on to other related topics.

Influence of dimensioning methods on tolerance stackup

Let us now examine how the tolerancing methods are affecting tolerance accumulation. In the picture below, we can see the example of three different methods used for specifying component dimensions (more about the dimensioning you can read in Engineering drawing – dimensioning).

As I said, the component in all three examples is identical, and the only difference is the dimensioning method used. Therefore, we will analyze each method and determine how each of these methods is influencing our overall tolerances.

Parallel dimensioning

We are interested in calculating the distance between points “A” and “B.” When performing the tolerance stackup analysis with parallel dimensioning, we have two dimensions contributing to the tolerance accumulation.

The missing dimension is 15 ± 0.6 mm.

Chain dimensioning

When performing the tolerance stackup analysis with chain dimensioning, we have four dimensions contributing to the tolerance accumulation between points “A” and “B.”

The missing dimension is 15 ± 0.9 mm.

Combined dimensioning

When we combine parallel and chain dimensioning and perform the tolerance stackup analysis, we have three dimensions contributing to the tolerance accumulation between points “A” and “B.”

The missing dimension is 15 ± 0.7 mm.

Conclusion on the influence of dimensioning methods on tolerance stackup

From our calculation above, we have the following results:

Parallel dimensioning: the missing dimension is 15 ± 0.6 mm.

Chain dimensioning: the missing dimension is 15 ± 0.9 mm.

Combined dimensioning: the missing dimension is 15 ± 0.7 mm.

From the above results, we can clearly see that the dimensioning method influences our tolerance buildup. The chain dimensioning method contributes the most to the tolerance accumulation, followed by combined dimensioning. The least influence of the tolerance buildup has the parallel dimensioning.

With that in mind, when dimensioning different features on the components, it is necessary to be aware of the methods we use and how they influence the overall tolerances of our parts. Unfortunately, there is no one-fit-it-all rule on when to use which method. The best way to learn this is through deliberate practice. Create as many drawings as possible, and be aware of the dimensioning technique you are using. Then, consider and study how these dimensioning techniques influence your tolerance stackup. Remember, the theory is useless if not applied in real life.

Tolerance stackup report

We want to make sure that we are communicating the results of our stackup analysis without any ambiguity. Therefore, it is advisable to use the same template for all your stackup reports. By template, I mean a predefined form with all relevant information written in it. The template you will use for reporting depends on the company, project, or product. There are no hard-defined rules for creating a template as long as the template and the documentation process are adequately defined. There are a few reasons why I am suggesting creating a report template:

Every report will have the same crucial information written in it regardless of the report’s author.
Once people have learned how your report looks and what it represents, they will no longer be distracted by the form of your document, but they will be focused on the content and results of the report.
You and your company look more professional. If every report contains the same basic information regardless of the author, it gives a positive impression to everyone you are working with.

We can communicate the results in different ways, and I prefer to use an Excel worksheet as a working document and share results as a pdf. If you use particular software for stackup analysis, you can probably use a template that goes with that software or create your template. Later, I will share what I think is important in the tolerance analysis report.

The reasons that I am using the Excel worksheet for the tolerance stackup reports are:

easy to create templates based on my project requirements,
templates are easily reusable and adjusted to different problems,
setting up formulas once and instantaneous recalculation of any variable change,
in one document, it is possible to have multiple sheets (each sheet representing a different revision),
possibility of locking the document to prevent any unauthorized change,
easy to export PDF files,
in general, great online support for software usage,
excel is in wide use, and it is pretty affordable (price-wise).

What to consider when creating the stackup report

When creating the stackup report (or any other report), we always have to keep in mind that there will be people who will read it, who are not as close to the topic as we are. For that reason, we have to ensure that we document everything with as many details as possible. So let us now discuss what every excellent report should have:

Part number, revision, and name: we have to define on which part the stackup analysis was performed. Revision is also important due to the possibility of the revision change – it is possible that with the change of the part revision, the stackup analysis is no longer valid.
Stackup report details: It is more than advisable to specify the stackup number, date of creation, and document revision. The stackup number can be defined as part of your document management process on the company level, and some parts of it can be the same for all of the reports (for example, stackup report numbers start with 300-…).
Responsible persons: you want to write the name of the report’s author when the report was created. Then, whoever will look into the report later on, can contact the author in case they have some questions. Also, I would suggest you add the name of the person who checked the report after you. It is usually defined in the document management process on the company level that all official documents should be checked and signed.
Objective of the stackup analysis: in as few possible words, you want to communicate an ambiguous and straightforward message about the purpose of the stackup analysis and what you want to accomplish with it. This gives both the author and the reader a clear statement about what we are doing here.
Dimension details: each dimension used in the stackup analysis has a reason why it is there. It is pretty simple to identify them when doing the stackup analysis on one part. But it is not that easy when you have an assembly with many different dimensions. So I would suggest that you write down the part number and revision for each dimension number. If it is assembly, write down the item number and a short description of the dimension.
Calculation part: of course, you want to include your calculation in your report. I would also suggest adding the picture with the numbered dimensions in your report.
Notes: include any relevant notes for proper conducting of the stackup analysis. Notes can be written in any form: as plain text, bullet points, numbered lists, etc.
Assumptions: you can write what assumptions you made to conduct the stackup analysis. For example, it was assumed that parts are correctly aligned, etc. This is an important part of the stackup report, and I suggest you write any assumptions you made, no matter how small it is.
Suggested action: You finished your stackup analysis, and now what? This is the place where you want to write further steps. For example, change the nominal value of dimension 5 to xy, tolerance to x.xy, etc.

The tolerance stackup report template

I created a stackup report template that you can download for free (as an Excel workbook) based on the points above. The first page contains all the relevant textual and numerical information related to the stackup analysis. The second page is reserved for the picture of the stackup problem with all the relevant information. The black frame around the report represents the edge of the A4 paper. For an example of how this report should look populated, keep reading.

Tolerance stackup report – beam trolley example

For the purpose of this exercise, I designed a simple manual beam trolley. The beam trolley is designed to be used on an HEB 100 standard wide flange beam.

As part of this exercise, we want to determine that we do not have interference between the beam trolley wheel and the beam in the worst-case tolerance scenario. Furthermore, we will create a stackup report for this problem.

In my previous article, One-dimensional tolerance analysis and tolerance stackup – part 1, I explained how to do stackup analysis. We will not go step by step. Instead, we will focus only on the stackup report. Here is what the stackup report for this problem looks like:

We have a two-page report with all the relevant data written in it. The picture itself does not have to be so detailed (I would recommend it), but at least I would add a picture with the dimensions, dimension numbers, and balloons with the item number (for assemblies).

Documenting calculations like this is beneficial in case that something goes wrong in the future. You can always pull out your report and explain what you did and when. The proof that you forwarded this report to the person who should do the next step will make an iron case for you. In this way, you are protecting both yourself and the company from any damage that could occur if this report was not taken into account.

Closing words

With this article, together with part 1, we covered the basics of tolerance analysis and tolerance stackup. This can serve all the purposes that do not include Geometrical Product Specification (ASME standard name: Geometric dimensioning and tolerancing – GD&T). Like everything else, to become comfortable with this analysis, you need to practice and build your experience. Just be patient, and keep grinding.

Now you have an excellent overview of one-dimensional tolerance analysis and tolerance stackup. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

Do you like what you read? Then, share it with your friends, colleagues, and on your social media channels. And if you want to become a part of the Newtonians, make sure to subscribe to our newsletter!