my question is about base dimension or critical dimension

i want to know about base dimension or critical dimension
1. this is understood to me that in box value is important but can we explain it in tolerance. if a value is 100 in box what it means to achieve 100 or it can be accepted 99.999 or 100.001. any value which we can give to worker for achievements cause as we know no machine can achieve 100.000 dimension please help.

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3 Answers

I think you mean 'basic' dimension, which is often represented like the one you have highlighted in your attached image.

A basic dimension is the theoretically 'perfect' nominal location, size etc. of a feature. The tolerances which define the allowable boundaries of the feature are then called out elsewhere, perhaps in the title block when utilizing a general title block tolerance, or called out by pointing to the feature with a leader line, or under the dimension, it depends on what's being toleranced and your company standard methods of doing such.

A basic dimension does not mean a feature has no tolerance and must be perfect to be acceptable. If there is no tolerance indicated on the feature being defined with the basic dimension, or no tolerance in the title block, or no other way of indicating the amount of variation allowable, then the drawing is incomplete.

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I agree with Robert's answer. The title block should specify the tolerance of the dimensions. Also, each dimension will be shown with a number of decimal places.
The dimension you highlighted has three decimal places, so it needs to be more precise than a dimension listed with one or two decimal places. See the attached image for an example of a tolerance field on a title block.

A machine or man can certainly attain 100.000 accuracy. You'll pay more to hold a tight tolerance, but it is possible.

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I agree with Robert and Fred. I've always seen "BASIC" dimensions used with other GD&T dimensions, usually a True Position tolerances defines the tolerance range of the dimensioned feature.

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