basic dimension

[turboanden]

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.

 

[dingy2]

Yes, in my opinion, you can.

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.

http://www.qmsi.ca

 

[ewh]

Yes; as has been addressed in these fora recently, if no FCB is provided, your tolerance will default to gage maker tolerances (per Y14.5).

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

Given you're using 2768 are you working to ISO drawing standards?

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.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[pmarc]

Here are my remarks to your question as I understand it:

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.

 

[ewh]

As usual, I tend to disagree when absolutes are used...

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.

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." - [small]Robert Hunter[/small]

 

[pmarc]

ewh,
I agree. However my assumption was that OP is not asking about datum targets.

 

[Belanger]

Also, basic dims can be used without use of a datum if they set up a true profile that is applied all around.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[fsincox]

One of the things that happened as I tried to get my head around the ISO system was I came to realize that. if you remove the title block implied tolerances, all dimensions are really basic dimensions. Basic means "dimension without a tolerance" not "dimension in a box", that is just the symbolic means developed to represent a basic dimension when you do have title block tolerances.
Frank

 

[pmarc]

ewh, John-Paul,
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.

 

[fsincox]

What is the "EN" part of the spec stand for?
Frank

 

[dingy2]

I perceived the question as asking about a theoretical dimensions to an intersecting point and the EN (I assume an engineering notice) would cover the tolerance. That's it.

Dave D.

http://www.qmsi.ca

 

[pmarc]

EN is an abbreviation of European Standard. N in this case means "norm".

 

[ewh]

I try to avoid assumptions (and absolutes) whenever possible. I agree that the OP was probably not in regard to datum points (or profile), but it could have possibly been, and I answered accordingly. Your post effectively proclaimed that such situations never exist, when obviously they do.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[fsincox]

Thank you.
Frank