Why Basic Dimensions 2

[ozzy1]

Why use basic dimensions. Since these are theorectical in nature why use them? Many engineers I work with cannot explain a 'basic dimension' but they use them and then do not add a feature control frame. When confronted with this situation they tell us to use the block tolerances. It drives us QC guys nuts.

 

[dgallup]

You have to use basic dimensions to use GD&T controls like true position and profile. So there is a very real reason to use them. It is not correct to use basic dimensions and then just use title block linear tolerances. Somebody needs to go back to school.

 

[ctopher]

http://www.engineersedge.com/geometric_boundaries/basic-dimension.htm

http://www.answerbag.com/q_view/1884027

Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion

 

[KENAT]

See section 5.2.1.1 of ASME Y14.5M-1994 (or the equivalent section of the standard you work to) if you want to educate these folks.

Posting guidelines faq731-376

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

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

 

[pmarc]

ozzy1,

Think of controlling position of a hole which is in the center of a very simple rectangular plate with its three perpendicular planes assigned as datum features A, B & C.
The aim of basic dimensions is to unambiguously define how the positional tolerance zone of the hole is theoretically located and/or oriented relative to datums A, B & C whereas position tolerance value describes the size of a tolerance zone.

Now imagine that you have 8 holes in the plate also dimensioned by basic dimensions and controlled by position tolerance. This will create a set of perfectly located and oriented tolerance zones within which every axis of 8 holes has to be contained.

Similar concept can be used for profile and orientation controls.

I do not know if it helps, but at least it is not a definition from the standard.

 

[fcsuper]

dgallup,

You said, "Somebody needs to go back to school." I challenge this statement because I don't think this stuff is taught in school; which is part of the problem.

Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
Follow me on Twitter

 

[ctopher]

Matt,

I agree.

Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion

 

[ewh]

There is an exception to using basic dimensions without any control frame to define tolerance, when "established tooling or gaging tolerances apply." That's another can of worms.

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