GD&T tangent modifier with profile tolerance? 3

[JLS357]

Hi All. I’m a long-time reader of this forum but a first-time poster, and I’d really appreciate some GD&T help. Id’ like to use a tangent modifier with a profile tolerance if it’s permissible and if I can figure out how to properly apply it. I have a situation where the design is an assembly of a part with a cavity having another part inserted into it. See attached sketch. It’s a lot more complex than this, but this simplification should work to convey the situation.

The bottoms of the parts are machined flat prior to assembly, but due to stack-up the bottoms of the parts won’t be aligned, and the bottom of either part my be higher or lower than the other. For various reasons the bottom of the assembly can’t be machined, but the top and sides of the assembly get machined. What’s especially important is the surface profile of the top and sides relative to the lowest bottom surface. I’d like to define Datum A as the lowest surface, hence the thinking of using a tangent modifier.

In Y14.5 I don’t find an example of using a tangent plane with a profile tolerance, and besides that there is a problem with doing it. The tangent modifier is to be placed next to the value of the tolerance, so if used with a profile tolerance it appears that it would apply to all three datums, but I don’t want that. For obvious reasons a tangent datum would not apply to Datum B (Datum C is not shown for simplicity).

In this case I only want Datum A to be a tangent datum. Perhaps I’m better of just using notes to state what I’m trying to do. Thoughts and ideas would be appreciated.

 

[Belanger]

A datum is essentially the tangent plane (that is, when the datum feature symbol is applied to a nominally flat surface). Thus, you don't need to worry about all of that stuff -- you've already got it by merely hanging that datum triangle in Section A-A.
There is room for perhaps using the tangent plane modifier upon a profile tolerance. But it seems your concern is taking a tangent plane from the datum feature, in order to set up a profile tolerance.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Burunduk]

The tangent plane applies to the feature being controlled by the tolerance, not to the datum reference.
A plane datum established from a planar datum feature is always essentially a tangent plane.
So arguably if your datum feature A in the assembly is the two faces nominally aligned, the datum is established based on contact of the planar datum feature simulator at the 3 highest (outermost) points - so it's supposed to be contacting the part that gets to be lower than the other anyway. However unfortunately Y14.5 doesn't have assembly examples so it may be questioned. A note might be a good idea.

Edit: posted this before seeing John-Paul's reply so sorry if it may look like repeating the previous post a bit.

 

[3DDave]

There may be only one point or two points of contact, but that contact will be tangent. The fit of the simulator to the part is, per the latest Y14.5 version, some minimizing fit of some kind. For the drawing that doesn't matter.

If one mating surface is concave and the other convex, the fit won't match what the inspection reports might lead you to believe.

 

[JLS357]

Thank you all. I'm hearing essentially the same thing from everyone, and that is the datum plane is already understood to be tangent to the three outer-most points of the nominally flat surface. It appears I was trying to over-complicate a simple situation and had forgotten a basic principle.

Now that I'm straight on this, I don't see a case a tangent datum plant would be of benefit. The only example of it in Y14.5 is shown in Fig 6-18 and is where the tangent modifier is used with parallelism to a nominally flat surface whose datum plane is already tangent to the outer-most points on the surface. What would be the difference in outcomes if the tangent modifier were not used in this case?

 

[Belanger]

Without the T modifier, the parallelism tolerance would automatically control the form (flatness) to the same amount. So by using T, a designer can allow a somewhat bumpy surface to still have a mating part sit on top of it and feel parallel, even though the two parts don't mate flushly.
If it's a sealing surface, however, then not only is the high-point tangent plane required to be parallel, but the form is important to avoid leakage.
So like anything else, it comes down to the function of the part.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[JLS357]

Makes sense. Thanks!

 

[jassco]

Hi, JLS357:

If this assembly is mated with bottom surface in the next level assembly, then I would make bottom surface of the inserted part recessed. You need to consider size of datum features in selecting datum feature(s).

Best regards,

Alex