Tolerances for Basic Dimensions 2

[Poeman]

How do you determine the tolerance of a basic dimension? I know that a basic dimension is the theoretically exact size of a characteristic, but in manufacturing an item, even these dimensions must have some type of tolerance. Yet, when I see drawings with basic dimensions, I never see them toleranced. I really want to understand this better.

 

[CorBlimeyLimey]

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[israelkk]

Basic dimension does not have tolerance. The actual manufacturing dimension should be the basic dimension plus/minus the geometric tolerance. The tolerance is given via geometric tolerance on the hole etc. It seems that you are not familiar with the concept of geometric tolerances. You should consult a book on the subjest.

 

[JStephen]

It seems to me that the tolerances are specified at some point because they are needed, not just to have them for the sake of having them. If no one knows what the tolerance is, or should be, you probably don't need them in the first place.

Suppose you're making Frisbees. And say, due to manufacturing slop, the weights vary by +/- 5%. What does it matter? It'll throw pretty much the same anyway, and there's no need to try to put a tolerance on it unless there's some massive problem.

But suppose you're making part A of something, and it has to connect to, or fit inside part B made my someone else. All the sudden, tolerances become important. Or you're making 500 little widgets that all assemble into some device. They have to fit together, and the idea of tolerances becomes useful.

I work in an industry that doesn't use geometrical tolerances. What we dimension is usually measured with a tape measure and cut with a torch, and that effectively sets the tolerances for us. Minor errors in cutting just mean additional welding time, but usually don't get the item rejected.

 

[diamondjim]

Basic dimensions are often used
as a reference dimension to show
the orginal centerline, plane,
and cannot be measured but are
theoretical in nature such as
a centerline of a bolt circle
diagram, pitch diameter, or
other surface which may often be
out in space and is not measurable.
A good example would be the pitch
diameter of a belt or pulley which
may lie outside of the part and
not measurable. Normally you use
basic dimensions to define something
you cannot measure.

 

[drawoh]

Poeman,

A basic dimension is an exact nominal dimension. It has no tolerance. The features are controlled by a geometric tolerance somewhere else on the drawing.

For example, if holes are located by basic dimensions, the hole specification itself will have a position tolerance attached to it. The position tolerance is NOT equivalent to a +/- dimension tolerance.

For another example, if an outline has basic dimensions, there ought to be a profile tolerance specifed somewhere.

JHG

 

[ewh]

Poeman,

drawoh has given you the best response so far. Basic dimensions are not usually used "to define something
you cannot measure." As drawoh said, basic dimensions are exact nominal dimensions, and as such have no tolerance. They are used in conjunction with datum features. No datums, no basic dimensions.

The tolerance on features located using basic dimensions are usually at the feature callout. To better understand the principles behind industry accepted geometric tolerancing, you should read ASME Y14.5M-1994 DIMENSIONING AND TOLERANCING. There are also other useful books on the topic (Lowell Foster first comes to mind), but be aware that there are practices out there which do not conform to the ASME standard (such as calling a centerline a datum).

I hope this helps point the way to a better understanding.

 

[MadMango]

I'd like to point out that the administrators have created a new forum.

forum1103

It's located in Engineering Methods> Drafting Standards & GD&T. Currently there are no other posts, I'm just drawing attention to it.

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[MintJulep]

Ok, I'll start.

First thread, thread1103-105379

in Forum1103