How to calculate and put dimensions in a right way?
Hi,
Usually, when I design a part, I follow my sense when I need to add dimensions. In my opinion, this is absolutely WRONG and unprofessional because the way that the designer chooses can effect on the manufacturing processes; and hence, we end up with a part that does not match the design . Therefore, I would like to learn the concept of putting dimensions in a right way. Please have a look on the attached picture, it is a very simple example that can explain my problem. I will be more than grateful if anyone can give me links of videos, lectures or online courses for beginners. Also, after learning the concept of putting dimensions, I would like to learn how to calculate tolerances in a right way.
Not: as I mentioned above, the example that I attached is very simple. For me, I would choose case (2): however, I followed my sense to make this choice, but I have no idea why I did that.
Thank you in advance...
4 Answers
As you said, it really depends on the desired outcome.
Many times a CAD model may be dimensioned differently than how it will be documented in a drawing. That goes against the "look how fast you can import dimensions and make a drawing" demo which the sales guys will show you, but that is the difference between modeling and documenting.
I agree the case 2 method you show is the better option. In case 2 all dimensions are based from a common location (it could be considered a datum).
In case 1 all features are defined by a preceding feature. If one feature is wrong, the others will then be wrong.
Chaining dimensions in such a way could also end up forcing features to be manufactured in a certain way so they can be correctly measured.
There are right and wrong ways to define dimensions, but the rules tend to change depending on the desired outcome, and whether you are modeling, or documenting.
It might be worth looking online for some tutorials to get a feel for the "best practices" which will tend to remain constant through different applications.
The first step in dimensioning any part in the best possible way is to have an understanding of how it will be made. Too often away in our warm air conditioned drawing offices we forget about the business end of the operation, the workshop, where the real work is done. Going out into the works, seeing how things are made, talking to the skilled men, learn the functions and limitations of the machines. Our aim is to dimension to suit best the needs of the ones who will make the part.
Hi,
Check out the presentation this may useful for you.
Thanks.
Mazin,
In reply to the comment you left on my response above:
As Lawrie suggested, one of the first places to start is with a good sense of how the part will be produced. Until the designer understands the manufacturing process, there will always be disagreements regarding what the print says, and what is produced.
At the same time, the manufacturer should understand the function and use of the part. Receiving a drawing or file with no additional input limits the role they can play. If they fully understand the product they are asked to produce, they will be able to offer valuable advice to make the job easier, cost less, and perform better. They are experts at building stuff.
Can you narrow down the scope of what you are looking for to a particular topic? Do you have two parts meant to fit together, but they don't because to tolerance issues?
Do you have plated parts which no longer fit after plating?
Do you have molded parts which do not work because of material shrinkage?
I'm sure there is a solution to the issue, but a broad "these are ALL the rules you need to follow to make a good drawing" is likely beyond the scope of what can be learned from these posts.
