Sr GDTP Y14.5-2009 Exam Review Ch-8 Part-I Sep2021

[metrologic]

Sr GDTP Y14.5-2009 Exam Review Ch-8 Part-I Sep2021

Hi Everyone! I'm up to section 8, Tolerances of Profile. As you may recall, I'm working through the Y14.5 standard in preparation for upgrading my GDTP certification from the tech level to the senior level. Here are some questions I had from this section:


Q1. 8.2.1.2 Profile of a Line. Frequently I see line profile tolerances applied directly to the entire silhouette outline of a drawing view. However, in many of these applications the outline does not describe a true profile with constant cross-section. Nor is the part in question similar to a tapered wing. Rather, this application is analogous to applying a line profile to the outline of a person posing for the cover of a Van Damme action movie --they are quite complicated injection molded shapes. Is this a legitimate use of line profile tolerancing? The intention appears to be control of the derived shadow outline of the part, and not cross-sections of the part surface.

Q2. 8.3.1 Uniform Tolerance Zone. "Where a profile tolerance encompasses a sharp corner, the tolerance zone extends to the intersection of the boundary lines. See Fig. 8-12." For some reason I've always thought profile tolerance zones were conceptually defined using a swept ball method. And hence the sharp external boundary shown in figure 8-12 would end up being a rounded external boundary. Where am I getting this swept ball vision from? Is that from ISO?

Q3. 8.3.1 Uniform Tolerance Zone. More of a comment than a question. Profile tolerance zones provide boundaries that follow the geometric shape of the true profile in which the actual surface must lie. For example, if a surface were controlled with a 1mm tolerance zone equally disposed about the true profile, the manufactured surface must not deviate beyond boundaries 0.5mm offset normal to either side of the true profile. However, there is no requirement that a given point on the actual surface be within 0.5mm from the true profile. And there is no requirement that a given point on the true profile be within 0.5mm from the actual surface. Profile boundaries around sharp corners allow for the actual surface to take rounded short cuts. In these cases the distance between the true profile and the actual surface may depart significantly from what one may expect based on the profile tolerance alone. This isn't really new or surprising, but these rounded corner examples can be taken much further. Given a sufficiently large enough profile tolerance, all manner of detailed features present on the true profile may be acceptably rounded or filled in, totally absent from the manufactured surface. Not sure how I feel about this. For some reason composite profiles and max radius seem like bandaids in this context to me.

 

[Burunduk]

I'll start with Q1.
Drawing creators tend to think that any outline generated by a drawing view can be controlled by profile of a line, but it's not quite so. The 8.2.1.2 sub-para. defines that:

"Each line element tolerance zone established by the profile of a line tolerance requirement is two dimensional (an area) and the tolerance zone is normal to the true profile of the feature at each line element."

While most understand and remember the bit about the tolerance zone being two-dimensional, many seem not to notice or mind the part of the definition which states that the tolerance zone has to be normal to the true profile of the considered feature.

What are the implications of a tolerance zone that lies on a 2D plane that needs to be normal to the "true profile"?
First thing that has to be considered is the meaning of a "true profile":

"A digital data file or an appropriate
view on a drawing shall define the true profile. A true profile is a profile defined by basic radii, basic angular
dimensions, basic coordinate dimensions, basic size dimensions, undimensioned drawings, formulas, or mathematical data, including design models" (8.2).

In other words, if the considered feature surface on the 3D CAD model or its dimensioned representation on a 2D drawing is the true profile, then you need to have 2D planes that cut the surface along line elements, in normal directions to that surface. On each of these planes you sample the actual line element which has to fit between the two lines forming the tolerance zone. Features with complex shape, curved and twisted enough, will not have a normal plane or a collection of normal planes. Instead, this kind of surface has an infinite number of local normal vector directions. Heck, I'm not even sure that the example the standard itself gives - "tapered wing of an aircraft", suits for this application, considering the "normal to the true profile" direction of the tolerance zone and assuming the tolerance is applied all-around. The Aerospace experts around here should comment on that.
Van Damme on the action movie poster may cause the same problem, as you rightfully noted.

 

[Belanger]

Drawing creators tend to think that any outline generated by a drawing view can be controlled by profile of a line, but it's not quite so. The 8.2.1.2 sub-para. defines that:

"Each line element tolerance zone established by the profile of a line tolerance requirement is two dimensional (an area) and the tolerance zone is normal to the true profile of the feature at each line element."

I recently got into an interesting discussion about using profile of a line on the sharp edge of a fluted drill bit. Though it's not 2-D (it's helical), would the statement above allow for such an application? I don't mean to deviate from the main topic of the OP, but I have my own feelings about such an application.

 

[Burunduk]

Belanger,
Excellent example for the exact issue I was addressing. This is THE type of geometry that doesn't fit with what the text of the standard requires for the establishment of the tolerance zone.
What are your feelings about it?

 

[Belanger]

When the question was posed to me, I initially thought that profile of a line might be plausible. But I looked at the exact verbiage in the standard and it seems pretty strict that we're limited to 2-D geometry, so profile of a surface would be the way to go. But to avoid confusion, I did recommend that they add a small note to clarify.

 

[Burunduk]

Belanger,
What would the profile of a surface apply to? The V shaped twisted sharp edge itself or the surfaces that form it by their intersection?

 

[Belanger]

Although the intent was to follow the path of the curve, the profile of a surface tolerance was applied to one face/side of the cutting edge. (If it were applied to the sharp edge itself, we'd get into all sorts of issues such as that raised by the OP.)

 

[Burunduk]

Do you mean it was applied to the "relief areas"?

 

[Belanger]

Sorry -- not the cutting edge in that sense. It was a designated portion of the fluted areas.

 

[Burunduk]

Interesting.
The cutting edge is formed by the intersection of the relief areas and the flute. If the cutting edge is to be controlled by profile tolerances indirectly, both the end of the flute and the reliefs have to be controlled by profile of a surface.

 

[metrologic]

@Belanger, feel free to head off on any tangent you want.

@Burunduk, that makes sense. I can't have a 2D tolerance zone that is always normal to the true profile of a complex surface. It seems like "true profile" is overloaded by Y14.5 though. For example, even a line profile on an axial view of a cone feature would be an incorrect application.

 

[3DDave]

They appear to have revised the "profile of a line" section significantly. It was poorly defined in '2009. Very poorly defined.

 

[Burunduk]

metrologic,
On your Q2,
Yes, the swept ball profile tolerance zone is from ISO.