Gd and T

What on earth are you talking about?????????

Primary secondary and tertiary

First, Second, and Third.

Can u please explain me what does this mean.suppose a feature is given with all the 3 datums.which one should i consider?

What do I, Gd and T mean? What is the context? Is datum a level datum? In UK we have levels AOD = Above Ordnance Datum.

Codey

Dam it, I thought you were talking about Gin and Tonic but then noticed the d

I'll bet that Gin and Tonic, under the right conditions could also," allow the use of mating parts that would otherwise have been rejected if using conventional inspection techniques.

Who's on First

What's on Second

I don't know = 3rd

http://www.youtube.com/watch?v=sShMA85pv8M

sorry, just feeling a little witty this morning.

Its the X,Y and Z datum.

Ron

GD&T = Geometrical Dimensioning and Tolerancing ANSI Y14.5

The primary, secondary and tertiary datums define the precedence and are shown in call-out block from left to right

eg | | | Φ0.1| A| B| C|

Will show primary datum as A, secondary as B and Tertiary as C

The order of precedence means how the practical inaccuracies are addressed.

let us take two reference surfaces A and B, defined to be perpendicular to each other within 0.02mm and are primary and secondary data.

Obviously these two are going to have their flatness problems (errors) too.

The primary datum now you can consider as if the component is put on a ideal flat surface and made to contact the surface "A" with it on at least two points. Let us call this surface as D_A

Now put another surface exactly perpendicular to the surface in contact with A ie D_A (remember it is ideal surface and exactly perpendicular). Obviously the surface B can not now have contact with this surface D_B. At the best you can have one point in contact. D_B is the secondary datum.

These are easily generated on CMM.

There is a link you can go through to understand the concept.

http://www.meadinfo.org/2009/05/gd-datum-better-insight-asme-y145.html

It's OK, I'll take your word for it

Correct.

Geometric Dimensioning and Tolerancing allows the use of mating parts that would otherwise have been rejected if using conventional inspection techniques.