Question about: Basic dimension or Theoretically exact dimension (TED)

[jeffmech]

Hello, can someone confirm me below theory referred from a website related to TED which they mentioned? I'm still having doubt...
"A basic dimension or Theoretically exact dimension (TED) is a numerical value used to describe the theoretically exact size, profile, orientation, or location of a feature or datum target and is not affected by individual or general tolerance. It represents ideal case (or true position) and have no tolerance and must appear associated with geometrical tolerance shown in a feature control frame. In figure 1, basic 21,75 and 15 locates true position of hole diameter 9 of which its location varies in the limit of cylindrical zone of diameter 0,05 with respect to datum A, B, C. At maximum material condition, the maximum pin size limit of 8,95 is permitted to pass though. So a gage system can be used to verify the part whether it meet design intent satisfy above conditions or actual measured location need to perform."
See the detail & help me:




Thanks
P/S: I'm from Quality Assurance dept

[Kelly_Bramble]

Yes, for the most part the explanation is correct and might be utilized in many GD&T training classes– however.

The provided definition is a mashup of ASME (ANSI) and ISO dimensioning and tolerancing standards. The term “Basic Dimension” is per ASME Y14.5-2009 or earlier standards and “Theoretically Exact Dimension TED” and the terminology used in ISO 1101 or derivative standards.

Be aware that the definition is an academic one mostly derived from the ISO 111 standard. Though a TED or Basic Dimension is defined as “perfect” in practice we know nothing is perfect nor is manufacturing likely to create a perfect part or feature.
In practical terms when establishing a TED in the real world we use gauging or tooling tolerances. The generally accepted practice is that the gauge is manufactured 5x more accurate than the total tolerance spread requiring measure for mm and 10X more accurate than the total tolerance spread requiring measure for inch units.

So, in the case of the position tolerance defined in your example, when the hole features is as-built at MMC size (dia. 9,0 – tolerance) we have a 0,05 diameter tolerance cylinder oriented and located at the TED’s. Therefore, when manufacturing or designing the functional gauge to measure the location of the 9,0 mm hole the location of the MMVC holes would be 5% more accurate then 0,05. See ANSI B4.4M.

[jeffmech]

Thanks Kelly Bramble for your information and clarification, it's useful for my practice on understanding a recent sample received. It may come to my notice that, is it possible to use TED or basic for location thread hole position? See my picture inserted...

[Kelly_Bramble]

It may come to my notice that, is it possible to use TED or basic for location thread hole position? See my picture inserted...

Yes, a position tolerance and associated TED dimension may be applied to locate threaded features.

Within the image shown, one is using a dimension origin leader end from zero. This is not correct usage when using position tolerance and TED's. You should have an arrow on both ends of the leader extension line.

Your questions are very fundamental and in conclusion you should acquire a relevant GDT Training or reference book and study it if you are to learn the proper application of dimensioning and Tolerancing to ASME Y14.5-2009 or ISO 1101 - 2009.

[jeffmech]

Thanks, I do appreciate your help!
P/S: the dimension in the picture I think they use ordinate dimension type

[Kelly_Bramble]

the dimension in the picture I think they use ordinate dimension type

I believe that "ordinate dimension" terminology is a CAD word. In ASME Y14.5-2009 a limit dimension can be related to an origin feature by substituting the extension line arrow with an circle as you have illustrated above. I'm not aware that a basic dimension (TED) can be used with the dimension origin symbology though I suspect this is possible.

See ASME Y14.5-2009, paragraph 2.6.1 and Figure 2-5 as well as paragraph 3.3.17. The dimension is called "Dimension Origin Symbol"