GD&T - True Position - "Boundary" under the feature control frame



true position question

I have a print that has [Position .012 (m)A B C] of an oblong hole, which is no problem. There is the word "Boundary" under the feature control frame. What does that mean? Thank you.

Al Dyer

Welcome Jager,

Boundry is another word for ---Range---



position question

How does that apply to the position of my oblong hole? Is it something that doesn't even need to be on the print? Is it just normal position calculations?


Fully vaccinated are you?
GD&T is almost a Mystical Art. Ive seen a lot of screwy prints. I saw one with a datum on a curved surface. I know it all sounds easy (think in 3 D) but I rarely find anyone who truely understands GD&T to the point where they can take the CAD file and mark up a print with GD&T info.

I would appreciate any comments and help form those familiar with GD&T and prints. The more details the better.

Al, you said "Boundry is another word for ---Range---". Details, please, if you can.

Al Dyer

Marc, I'm surely no pro on this topic but the definitions of "boundry" and "range" seem to be synonymous(sp)

Both consider that there is a finite area. And I have seen neither in actual usage.


Multiple values:

I feel that the various multiply valued coordinates you have referred to are of several distinct types. I would argue that the "representative" or "midpoint" value is the principal coordinate value. This one should always exist, and it is this which must be monotonic (or at least ordered) if it is one-dimensional.

GDT distinguish three kinds of subsidiary coordinates:

(1) Boundary (section 21). We group the upper and lower boundaries into one variable for tidiness and ease of access.

(2) Component (section 18). These are for cases where the coordinate values are tuples, such as for the hybrid pressure-sigma vertical coordinate. However, an ordinary principal coordinate value must still be provided, for ordering the axis.

(3) Associate (section 19). Associate values are additional information, or extra ways of labelling the points, such as your "lev_label".

I think these are all truly different. Moreover, component and associated coordinate values can have boundaries, and associated coordinates can have components. For this reason, rather than coordinates = "lon lat (lev_upper lev_lower lev_midpoint lev_label)";

I think that it would be better to specify

:coordinates="lon lat lev";

and provide the boundary, component and associate coordinates by attaching them to lev, the representative or midpoint value. That makes for a simpler and clearer definition of the coordinate system, and it shows that the other information really is subsidiary to lev.

I think we are converging on a listing of the various "types" (meanings) we want to have for coordinates:

1) "point", "representative point", or "principal value".
2) "boundry" or "range"
3) "label", "nominal" or "associate"

Independently is the possibility that a coordinate value is specified by a tuple, eg (year,month,day,daysec): in this case we can represent such a value along a single axis………………

Any up to date experts out there?


Bill Ryan - 2007


You should only see "Boundary" employed with an elongated (oblong) hole which requires more tolerance in one direction than the other, and, therefore, has two positional callouts. When the positional callout is the same in both directions, the term should not be used. You still calculate the Virtual Condition Boundary in the same manner.




Thanks. I see that figuring position of an elongated hole would possibly be different than a round hole since there is a length and a width of the hole (I think!). I'm going to try this and see what happens. It looks like the center of the hole should be in the same place both directions. Any more replys would be welcome! Thank you all.

Bill Ryan - 2007


Something didn't sit right with my first response, so I'll try to get it right this time.

From Lowell Foster (Geo-metrics IIIm):

If the need is to control location of a noncylindrical or symmetrical size feature's center plane of the actual mating envelope, within a tolerance zone relative to datum planes, a center plane or axis... the "Boundary" notation is not used.

If the need is to control location of a noncylindrical or symmetrical size feature, as based upon a tolerance zone at the boundary periphery or about the true position center planes... the "Boundary" notation is applied.

Sorry about the original "misinformation.

Hope that helps explain :bigwave:




Since the hole is symmetrical (just a cleanly punched, slotted hole), I suppose I could measure to the center of the hole and not have to worry about "boundary". Does that make sense?
The two Basic dimensions that go to the hole both go to the center, not edges.
You have been helpful.


Take the nomenclature off the print.

Too many engineers and designers have no idea how to use the basic GD&T features.

I once had a GM engineer put a 43 mm tolerance on the third point of -A- Datum and could not understand why the layout came put so badly.

One of my engineers thought it was great to have a lateral tolerance of 23 in one direction. He said it was cool to be different.
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