Your datum must be a real feature. A basic rule of GD&T is that the fabricator and inspector must use your datums for fixturing. It helps to visualize what this fixture looks like.
The width and length of a slot can be picked up by a cross shaped pin. This is necessary if you have a separate datum that prevents your part from rotating.
On your drawing, the width and the length of your slot would be two separate datums, i.e. you will need four datums to locate your part. Both datums need to be specified at MMC. Whenever I specify a slot, I make the length sloppy. Sloppy features are not good feature-of-size datums.
If you need an X/Y locating feature, you should make it round, and slot all the other sloppy features. This makes the GD&T on your drawings, and your fabricator's and inspector's lives, easier.
Another possibility is to use your slot to control rotation as well as X[ ]and[ ]Y. This will work if your slot has an accurate width and it is long. Otherwise, this is a bad idea. Your maximum angle error in radians is your maximum clearance divided by your slot length.
This will maybe confuse, but it is a critical distinction. A datum is a theoretical point, line (axis) or plane. A datum feature is a real, physical feature on the workpiece, which is used to establish the datum (point, axis or plane). The datum simulator would be the pin or key (in this case) or other representative component that would be used to represent the datum.
The OP is asking how to add a datum point. I am interpreting this as how to specify such a datum on a drawing. My cross-shaped pin will locate the centre of a closed slot, at MMC, with the limitations noted above.
I suppose my first paragraph is not well written. How about...
The datum point (which is theoretical), must repeatable; i.e. you must be able to use real datum features (physical features) to establish the point. An example of this would be a truncated cone, the truncated apex of which is for some reason desired as the datum point. Again, a datum must always be related back to datum features.
After some careful consideration, it appears that you may be where you are looking to be. That is to say that if you have your 3 mutually perpendicular planes, established by the slot length, width and another surface, these three planes will converge in a point. Is that not the point you are seeking?
I agree with staying sticking to the definitions of the standard for discussion of it !
An "imaginary point" would not be based on fact. It lacks factual reality.
A "theoretical point" would be the analysis of a set of facts in their relation to one another.
Relative facts would establish a "theroretical point".
I would stick to the standard's definition of "theoretical datum point" and/or (not to be confused with) a "datum target point". "Imaginary point" terminology does not lend itself to geometric relationships and seems to confuse rather than to help in discussion.
"Datum points" always seem to be the intersection of 2 perpendicular planes as in the case of a hole's centerline, and of course the intersection of 3 mutually perpendicular planes as in the case of a DRF and it's origin.
Interestingly too is the figure on page 15 of ASME Y14.5m 1994 which demonstrates the way slots are to be dimensioned. Fig (b) depicts the centerline of the slot with a leader of width and length dimensions. In this case I would say there is a "datum point" at the intersection of the slot's centerline represented by a 'single vertical' and a 'single horizontal' line, representing 2
perpendicular datum planes. This "datum point" could be simulated by a gage pin in contact with the datum feature (the physical slot). This is a common way to dimension slots.
Not to be argumentative, but, I believe that the intersection of 2 planes, perpendicular or NOT, results in a line, which I believe in theory contains an infinite number of points. The center of a circle, on a plane, would result in a point I believe. Or the tangency of a circle and a line or plane, likewise.
No one is arguing on whether a point can be created from planes, lines, circles, or otherwise. The question is how to measure to it or from it or how to fixture around it...and how can a single point within a slot possibly be functional anyway?. This is not a valid dimensioning scheme. Re-read what Jim wrote in the post above.
Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
![[thumbsup2]](/data/assets/smilies/thumbsup2.gif "[thumbsup2] [thumbsup2]")
