Learning Dimensioning and Tolerances

[Madison]

I'm going to be in the market for a job again very soon and would like to do some more CAD and design work if I can find it. I'm always very self-conscious about applying for these jobs, however, because my knowledge of dimensioning and tolerances is terrible. All of the design and CAD work I've done for FIRST led to parts that I was manufacturing myself, so critical attention to proper standards wasn't necessary.

What are good resources for learning more about proper methods for dimensioning of drawings? I don't expect that there's anything available online, but if there's anything, I'd love to see it. I'm also interested in books you may have used, standards guides and rules of thumb that might help me to seem a bit less like an idiot should I pursue something like this.

Thanks.

[meaubry]

M.
Many schools use textbooks by Cecil Jensen and Jay Helsel. They created a series called Engineering Drawing and Design, published by McGraw-Hill.
It's avaliable as hradcopy as well as CD-Rom.
I'm sure there are other alternatives available - this is the one I used when I taught the Drafting & Design classes.

Mike

[Tristan Lall]

A lot of companies will specify ASME Y14.5M-1994 for geometric dimensioning and tolerancing, and therefore desire that their designers know how to use it. The standard itself isn't a bad place to start, if you can get your hands on a copy at the library ($145, otherwise). Other members of the Y14 series are in less widespread use; have a look at the list at http://catalog.asme.org and see if there's anything that interests you.

The Dimensioning and Tolerancing Handbook by Paul Drake has fairly good diagrams and explanations of the standard GD&T concepts (this probably isn't one to buy just yet; try the library). Also, if you need real-world examples, Tec-Ease has a lot of tips. Also, the Eng-Tips forums have a section devoted to GD&T; it's probably worth looking into (though they don't particularly like students; present yourself as a young engineer, if asking questions).

[Molten]

In my drafting class we are currently using the Engineering and Design book. It has simple explanations for confusing questions. There are some sites online, they are hard to find though. I will see if I can find a certain site in particular that I personally use as a reference. If I find it I will post it later.

[Phil Ross]

To learn Inventor 11 I used the book that I found in my Tech Lab called

"Parametic Modeling with Autodesk Inventor R11" by Randy H. Shih. I learned a lot of the basics from this book, but the way you really need to learn it is from getting the basics from a book and then learning through experience.

to learn Autodesk Revit 9 I used: "Introduction to Commecial Design using Autodesk Revit Building 9" by Daniel John Stine. This book I found the same thing as I did with Inventor, the only way to really learn it is by messing around with different options and settings and most importantly, experience.

I hope this helps and Good Luck with the job thing.

[Carmine Rizzo]

Dimensioning and tolerancing cannot be taught completely from a book. You do have to know the basics, which can be provided through a book. This is something that has to be learned through experience. You have to know where the part is being applied as well as what the part is doing. You have to understand the constraints for the part and it’s interrelationship with other parts.

When parts are incorrectly dimensioned or if they are over tolerance, this could lead to added manufacturing costs. If questions arise in your dimensioning prior to fabrication at a GMP (Good Manufacturing Practices) Machine shop. They will call you for the answers and if you are unattainable then, they will charge you for the time the part is in the machine or if they pull the part, you will pay for set-up again. If the shop is questionable or make an assumption they will continue Fabrication and you will have to pay for a bad part or change your design.

My recommendation is intern at a machine shop and/or a machine building shop. Understand the interactions of Machinery and how the parts are fabricated. With the Book as well as the internship you will be on your way to understand the art of dimensioning and tolerancing

Carmine
Team 75 Mentor

[Bill Baedke]

From a person who has worked as an engineer and a machinist, I can say that one of the most important things on the drawing is the tolerance on each dimension. The tolerances convey to the person making the part a sense of the function of the part and where to pay the most attention. Tolerances also greatly affect the cost of the part. For example one might need a piece of 2 by 4 wood to use as a shim under a bookcase. The length that the board needs to be cut might have a tolerance of +/- 1/8" (or even +/- 1/2"). This would be reasonable and easy to cut with a hand saw. On the other hand if the tolerance were +/- .003" the board would have to be cut longer, then milled on both ends to hold the tolerance. What a waste of money. Seems silly, but I have seen far to many drawings with tight tolerances and then told later that the dimension didn't mean anything. Why not put it on the drawing?

Rule of thumb: Make every tolerance as large as you can to avoid extra cost and only make the functional tolerances (for instance a pin has to fit in a hole with a slip fit) as small as they need to be. Every engineer that approves a drawing should look at every dimension and question whether the dimension could be opened up, and every one who makes a drawing should do the same. A few minutes doing the job well will avoid much time and expense later.

The purpose of the drawing is to communicate, with as few words as possible, exactly what the part should be (in a universal language). An example would be a part with bores and counterbores on one axis. The person making the part needs to know the diameter of each counterbore and its coresponding depth. This information should be in the same location on the drawing (or as close as possible). This will avoid the manufacturer having to keep jumping from sheet 3 to sheet 21, and help eliminate making a mistake.

The designer/engineer may have spent weeks working on the parts for a new design so they are intimately familiar, but the people making the parts only have the drawings to work with. That drawing should tell them exactly what you want in a clear easy to understand way. Even the line thicknesses are important. The object lines (the part) are supposed to be thicker than the dimension lines so the part jumps out immediately and it is clear what the part looks like. I hope some of this helps. For most of us learning to put on paper what we have in our head is a difficult thing, which is always enhanced by experience, such as when people tell us that they didn't know what we meant. To improve that situation there should be a contact persons name and phone number on each drawing, then if there is a question, it can be resolved easily and quickly.

[Madison]

Thanks, everyone, for the advice.

I have a reasonable understanding of simple machining and practices and a pretty good idea of how drawings ought to be dimensioned, but I'm not familiar with the ANSI standards, so I'm just worried that I won't communicate my intent properly. I worked with several professional machinists on some robots several years ago and they did a good job of making me understand the importance of tolerances and dimensioning, but I haven't outsourced any parts in years, so I'm rusty. Moreso even than that, we use a paperless process at FRC488, so I haven't made drawings for anything in years. When I make sketches, I try to dimension them properly so that the kids on team see the proper way of doing things, but it's hardly as important as when a shop somewhere is doing the work.

[Michael Hill]

You should look at Fundamentals of Graphics Communications by Gary Bertoline. Purdue CGT classes use that textbook for drawing sketches and stuff like that. I strongly recommend it.

[meaubry]

Keep in mind that tolerances primary function is to define an acceptable amount of variation.

Yes, the dimensions and tolerances are needed to manufacture parts, but they are also used to manufacture the tools/tooling needed to produce the parts. There is a balance needed, when establishing the allowable variation you must consider the capability that can be achieved as well as the functionality that is required.

But, it doesn't stop with making the parts.
The dimensions are also used to determine if the parts are produced within the acceptable limits, prior to shipping and using the said parts. They become extremely important to quality and the PPAP process.

Dimensions and tolerances are used in Hi-Lo and Variation simulation models to determine potential outcomes when part variation occurs (and believe me variation will occur). They are extremely important to engineering and design analysis.

I agree that knowledge, when combined with real experience, is the ultimate in gaining the best overall understanding of dimensioning and tolerancing.

If done well, it can save time and money. If done poorly, it can be a nightmare. Spend the time to do it right - it will pay off in the long run.

[kramarczyk]

I've got to back Tristan up on this one. If you're looking for standards to comply with for employment Y14.5M-1994 is king. Not everybody uses GD&T, but the underlying principles are what eveything else is based from... go to the source.

Originally Posted by Tristan Lall

A lot of companies will specify ASME Y14.5M-1994 for geometric dimensioning and tolerancing, and therefore desire that their designers know how to use it. The standard itself isn't a bad place to start...

[Peter Matteson]

Originally Posted by kramarczyk

I've got to back Tristan up on this one. If you're looking for standards to comply with for employment Y14.5M-1994 is king. Not everybody uses GD&T, but the underlying principles are what eveything else is based from... go to the source.

I have to second this as well. We have had ASME Y14.5 GD&T classes several times when we bring in a batch contract draftsmen fro projects because it has become difficult to find people that have a solid background in it. I've seen many people apply the GD&T tags without understanding the true implication.

Another good practice to have is learning to do a tolerance study. I'm currently working with designers that do spreadsheets of all the tolerance stackups for clarity on making sure that transmission and shaft assemblies can never interfere and always work even if every thing is at Max or Min material condition.

[kramarczyk]

This should simplify the tolerancing study...

http://www.chiefdelphi.com/media/papers/1959