Need advice on giving tolerances on drawings for parts 1

[dhyperman]

I am a design process engineer. i used to work for Tool and die company. I was told to do the design and general dimensions, and the boys on the floor making the stuff would do the tolerance of fitting the things. I need some advice on specifying tolerances over distances and as in cut and protrusions. anything that could help please as its holding me back.....

I know that any tolerance on a protusion on a component say must be taken in to account over the over all dimesnion as in they small dimensions with tolerance must not exceed the overall diension and its tolerances. I need help anything to make me better at it.

 

[ctopher]

I can't tell you what tol to use without knowing the design. I suggest getting a copy of ASME Y14.5.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[MadMango]

As ctopher alludes to, if you are doing the design, then you should be the one that knows what tolerances your design will work with.

This question might be better placed in forum1103

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[ewh]

For tooling, in this case, only control the tolerances that are critical to the function of the part. Let the machinists determine how to achieve them. Most tooling does not call for the interchangability of individual components in the manner of finished parts, so too much GD$T can frustrate the machinists and drive costs way up. Why control the position of two mating pin holes when he's going to match drill anyway?

 

[mog69]

dhyperman
The first and best reference I ever had was BS 308 - I think it's now part 2 of the Standard that covers dimensioning and the effects of cumulative tolerances. As you say in your example, you can't have the cumulative sum of tolerances within a feature exceeding the overall tolerance of that same feature. You have to make the judgment (based on priority of function) which is the most important - the individual elements or the overall. There are so many different ways of detailing the dimensions and their tolerances, the final decision has to be yours. You could try looking at similar drawings already created within your Company to see how others have resolved the question. There might be a Company “customs and practice” in place that says “this is how we want it done” but more likely the answer will be evident when you look at the essential function of the component and how it interacts with other mating components. Sorry - no easy answer !

 

[KENAT]

308 was superseded by BS8888, which is a compilation of various EN, ISO and BS specs.

Depending on what country you're in will depend which spec you probably want to refer to. If US then Y14.5, in the UK the BS.