basic dimension
basic dimension
(OP)
My I use basic dimension on a drawing to show a theoretically exact dimension, without til use of a datum and feature ?
In this case the basic dimension, will be given by 2 dimension controlled by EN 2768-1 m.
In this case the basic dimension, will be given by 2 dimension controlled by EN 2768-1 m.





RE: basic dimension
A basic dimension is a theoretical dimension and is used for theoretical center lines of features of size, surfaces, datum target locations, suspended centers and locations where a particular dimensions is confirmed.
Make sure that you EN provides a tolerance of location.
Dave D.
www.qmsi.ca
RE: basic dimension
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: basic dimension
What are you using the basic dimension for? If for use with position then per ASME stds it has to come either directly, or via a chain of basic dims, from a datum. ASMe Y14.5M-1994 para 5.2.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: basic dimension
1. You can use basic dimension on a drawing without using a datum and positional feature control frame indication only when somewhere on a drawing there is something mentioned about which part features are chosen as datums and what is general position tolerance allowed (either by defining specific value of tolerance or by referencing to a proper standard). These 2 requirements must be met together, otherwise basic dimension is meaningless.
2. It does not work in the way that if you specify basic dimension it can be substituted by a linear dimension (or 2 dimensions) with +/- tolerance(s), so in this case EN 2768-1 will not help you.
3. The only location tolerance that general tolerances are specified for in EN standards is symmetry. The standard which says about it is EN 2768-2. So if your case is not dealing with symmetrical relationship between features there is no general geometric tolerance that you could use.
4. And the general remark about using EN 2768 (-1 & -2) std.: "it applies to the dimensions of parts that are produced by metal removal or parts that are formed from sheet metal." So if your product is not manufactured with one of these two methods you should not use this standard.
RE: basic dimension
ASME Y14.5-2009 para 4.24.7: "If defined with basic dimensions, established tooling or gaging tolerances apply."
The OP may or may not be refering to datum targets, but precedent does exist for the use of basic dimensions WITHOUT reference back to existing datums.
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: basic dimension
I agree. However my assumption was that OP is not asking about datum targets.
RE: basic dimension
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: basic dimension
Frank
RE: basic dimension
I am sure we can find few more cases where basic dimensions are used without datum references, but these are quite specific situations and I do not think OP was asking about such details. My impression (maybe incorrect) was that the question is about very fundamental issue.
RE: basic dimension
Frank
RE: basic dimension
Dave D.
www.qmsi.ca
RE: basic dimension
RE: basic dimension
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: basic dimension
Frank