ASME Y14.5 True postion callout without datums 1

[PREV]

My question is if you can have (2) holes located from surfaces (non funtional) by +/- dimensions and have a basic dimension to each other and have a true positional callout on the holes to no datums.

 

[GDTGUY]

Prev,

The tolerance callout you describe (which mixes true position tolerancing with plus-or-minus tolerancing) was allowed under older versions of the dimensioning and tolerancing standards (ANSI Y14.5-1973 and earlier).

The intent of the callout was use the plus-or-minus dimensions to locate the pattern of holes and then specify the true position to maintain a tighter relationship of the holes within the pattern. The problems with this method is the same problems that always seem to come up when using plus-or-minus tolerances; dealing with square tolerance zones on a round hole, implied datums which allow multiple part set-ups, we can't invoke the MMC principle on the plus-or-minus dimensions to take advantage of additional hole clearance, etc.

The practice of mixing plus-or-minus dimensions with true position tolerancing was discontinued with the release of ANSI Y14.5M-1982. To acheive the same affect as the older method (i.e. - relaxed location of the pattern - tighter relationship of features within the pattern) composite positional tolerancing is the method used today.

Hope this helps you out.

GDTGUY

 

[ringman]

An opinion that I have held for some time is that it is permissible and oft times desireable to locate the Pattern Locating Tolerance Zone using a rectangular or square zone. The Feature Locating Tolerance could be subsequently located within this zone using a diametric zone.

This method would of course require datums for locating. A benefit is that it more directly controls the edge distance for the holes being located.

This tends to provide that oft, but sometimes misued phrase "Bonus Tolerance" with the square (PLTZ) being larger than the inscribed circle (FRTZ).